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| UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions. |
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Suspended ground floor condensation problem
I have installed rockwool insulation blankets under the floor and I am
now experiencing some worrying problems with condensation as the insulation itself gets very humid at times with water dripping onto the DPM laid on top of the underfloor (no concrete). The floor is made of 18mm plywood nailed on top of the joists. The insulation is placed between the joists. The crawling space is about 600-800mm deep. I am thinking of reducing the crawling space (and so the volume of air) so that the existing ventilation will be more efficient. Someone mentioned to install a vapour check membrane underside the plywood (floor). Will the above work? Any other suggestions? Thanks Alex |
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"The Natural Philosopher" wrote in message ... N. Thornton wrote: (Alex) wrote in message . com... I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the insulation itself gets very humid at times with water dripping onto the DPM laid on top of the underfloor (no concrete). The floor is made of 18mm plywood nailed on top of the joists. The insulation is placed between the joists. The crawling space is about 600-800mm deep. I am thinking of reducing the crawling space (and so the volume of air) so that the existing ventilation will be more efficient. Someone mentioned to install a vapour check membrane underside the plywood (floor). Will the above work? Any other suggestions? Thanks Alex remove the rockwool, that was a design error I'm afraid. More air bricks may help to some extent, but you still need to remove the rockwool as it prevents air movement round the wood. NT -------------------- I disagree. I would say the correct thing to do would be to isntall a vapour barrier above the insulation, and ensure decent insulation. Its no different from an insulated loft, upside down really. ------------------- I think that rockwool isn't the best insulation to use in this situation. I used 2" polystyrene slabs wedged between the joists. This was suggested and approved by my local Building Regs people and by the company who supplied the polystyrene. Building Regs suggested that there should be no gap between the insulation and the under-surface of the floor. The company who provided the polystyrene suggested that a 1/2" gap would be helpful in case of liquid spillage from above. The suggested 1/2" gap was to ensure that any spillage would drain away safely past the insulation. I'm not sure what 'The Natural Philosopher' intended when he said, ".............draught proof below it ............". It's actually important to maintain the existing ventilation (via the airbricks) under the insulation to prevent condensation / rot etc. Cic. |
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Cicero wrote:
"The Natural Philosopher" wrote in message ... N. Thornton wrote: (Alex) wrote in message . com... I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the insulation itself gets very humid at times with water dripping onto the DPM laid on top of the underfloor (no concrete). The floor is made of 18mm plywood nailed on top of the joists. The insulation is placed between the joists. The crawling space is about 600-800mm deep. I am thinking of reducing the crawling space (and so the volume of air) so that the existing ventilation will be more efficient. Someone mentioned to install a vapour check membrane underside the plywood (floor). Will the above work? Any other suggestions? Thanks Alex remove the rockwool, that was a design error I'm afraid. More air bricks may help to some extent, but you still need to remove the rockwool as it prevents air movement round the wood. NT -------------------- I disagree. I would say the correct thing to do would be to isntall a vapour barrier above the insulation, and ensure decent insulation. Its no different from an insulated loft, upside down really. ------------------- I think that rockwool isn't the best insulation to use in this situation. That I would not dispute. Its a vpuour barrier initself. Celotex even better. I used 2" polystyrene slabs wedged between the joists. This was suggested and approved by my local Building Regs people and by the company who supplied the polystyrene. Building Regs suggested that there should be no gap between the insulation and the under-surface of the floor. The company who provided the polystyrene suggested that a 1/2" gap would be helpful in case of liquid spillage from above. The suggested 1/2" gap was to ensure that any spillage would drain away safely past the insulation. I'm not sure what 'The Natural Philosopher' intended when he said, ".............draught proof below it ............". It's actually important to maintain the existing ventilation (via the airbricks) under the insulation to prevent condensation / rot etc. I menat to stop draights coming up through the cracks in the (rockwool) insulation. i.e. from teh bootom up, a berathable membarne, to stop darughts, then rockwool, then a DPM and tehn borads over teh joitss. Buit I like poolystrene better apart from teh fire hazrd. Celotex best of all, between the joists, taped down with the special tape and then boarded over. Cic. |
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#7
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"The Natural Philosopher" wrote in message ... Cicero wrote: "The Natural Philosopher" wrote in message ... N. Thornton wrote: (Alex) wrote in message . com... I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the insulation itself gets very humid at times with water dripping onto the DPM laid on top of the underfloor (no concrete). The floor is made of 18mm plywood nailed on top of the joists. The insulation is placed between the joists. The crawling space is about 600-800mm deep. I am thinking of reducing the crawling space (and so the volume of air) so that the existing ventilation will be more efficient. Someone mentioned to install a vapour check membrane underside the plywood (floor). Will the above work? Any other suggestions? Thanks Alex remove the rockwool, that was a design error I'm afraid. More air bricks may help to some extent, but you still need to remove the rockwool as it prevents air movement round the wood. NT -------------------- I disagree. I would say the correct thing to do would be to isntall a vapour barrier above the insulation, and ensure decent insulation. Its no different from an insulated loft, upside down really. ------------------- I think that rockwool isn't the best insulation to use in this situation. That I would not dispute. Its a vpuour barrier initself. Celotex even better. I used 2" polystyrene slabs wedged between the joists. This was suggested and approved by my local Building Regs people and by the company who supplied the polystyrene. Building Regs suggested that there should be no gap between the insulation and the under-surface of the floor. The company who provided the polystyrene suggested that a 1/2" gap would be helpful in case of liquid spillage from above. The suggested 1/2" gap was to ensure that any spillage would drain away safely past the insulation. I'm not sure what 'The Natural Philosopher' intended when he said, ".............draught proof below it ............". It's actually important to maintain the existing ventilation (via the airbricks) under the insulation to prevent condensation / rot etc. I menat to stop draights coming up through the cracks in the (rockwool) insulation. i.e. from teh bootom up, a berathable membarne, to stop darughts, then rockwool, then a DPM and tehn borads over teh joitss. ============= I understand now what you meant. In fact if you don't close the cracks in the insulation (whatever it may be) then there isn't much point in doing the insulation at all. Buit I like poolystrene better apart from teh fire hazrd. Celotex best of all, between the joists, taped down with the special tape and then boarded over. ====================== The Building Regs person I spoke to stated that the fire risk didn't bother him at all. I assume that he meant it isn't a great risk rather than it was me and not him at risk! Cic. |
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"Alex" wrote in message om... I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the insulation itself gets very humid at times with water dripping onto the DPM laid on top of the underfloor (no concrete). The floor is made of 18mm plywood nailed on top of the joists. The insulation is placed between the joists. The crawling space is about 600-800mm deep. I am thinking of reducing the crawling space (and so the volume of air) so that the existing ventilation will be more efficient. Someone mentioned to install a vapour check membrane underside the plywood (floor). Will the above work? Any other suggestions? The Rockwool needs to be placed firmly against the bottom of the floor so that no moist air can get there and allow condensation inside the Rockwool. A vapour sheet above the Rockwool will also help. However in any underfloor where insulation is added, it is often helpful to increase airflow by adding a humidity controlled extractor fan in front of one of the air bricks, and ensure there are adequate airbricks for fresh air to enter. |
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Hi
Another way of doing it would be to remove the rockwool, stuff it in a binliner and take it outside, then wrap/'bag' it in thick polythene tube or sheet stapled and sealed with tape, then replace it under the floorboards. Would then be much nicer to work with under the floorboards if it is wrapped in this way. cheers, Pete. |
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"Pete C" wrote in message news  Hi Another way of doing it would be to remove the rockwool, stuff it in a binliner and take it outside, then wrap/'bag' it in thick polythene tube or sheet stapled and sealed with tape, then replace it under the floorboards. Would then be much nicer to work with under the floorboards if it is wrapped in this way. Yes - but it may not insulate very well if it is compressed like this. Rockwool depends on being fairly 'fluffy' to work. |
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"Cicero" wrote in message ...
"The Natural Philosopher" wrote in message ... N. Thornton wrote: (Alex) wrote in message . com... I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the insulation itself gets very humid at times with water dripping onto the DPM laid on top of the underfloor (no concrete). The floor is made of 18mm plywood nailed on top of the joists. The insulation is placed between the joists. The crawling space is about 600-800mm deep. I am thinking of reducing the crawling space (and so the volume of air) so that the existing ventilation will be more efficient. Someone mentioned to install a vapour check membrane underside the plywood (floor). Will the above work? Any other suggestions? Thanks Alex remove the rockwool, that was a design error I'm afraid. More air bricks may help to some extent, but you still need to remove the rockwool as it prevents air movement round the wood. NT -------------------- I disagree. I would say the correct thing to do would be to isntall a vapour barrier above the insulation, and ensure decent insulation. Its no different from an insulated loft, upside down really. the difference is that the underfloor void is constantly wet. If or when any damp gets past that barrier, as IRL it will, it is then trapped, and rot will occur. The floorboards will be nailed thru the barrier, lots of holes. ------------------- I think that rockwool isn't the best insulation to use in this situation. I used 2" polystyrene slabs wedged between the joists. This was suggested and approved by my local Building Regs people and by the company who supplied the polystyrene. Building Regs suggested that there should be no gap between the insulation and the under-surface of the floor. The company who provided the polystyrene suggested that a 1/2" gap would be helpful in case of liquid spillage from above. The suggested 1/2" gap was to ensure that any spillage would drain away safely past the insulation. I'm not sure what 'The Natural Philosopher' intended when he said, ".............draught proof below it ............". It's actually important to maintain the existing ventilation (via the airbricks) under the insulation to prevent condensation / rot etc. Cic. expect rot. Dryness and air movement are the 2 things that avoid it. Regards, NT |
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#12
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N. Thornton wrote:
"Cicero" wrote in message ... "The Natural Philosopher" wrote in message ... N. Thornton wrote: (Alex) wrote in message . com... I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the insulation itself gets very humid at times with water dripping onto the DPM laid on top of the underfloor (no concrete). The floor is made of 18mm plywood nailed on top of the joists. The insulation is placed between the joists. The crawling space is about 600-800mm deep. I am thinking of reducing the crawling space (and so the volume of air) so that the existing ventilation will be more efficient. Someone mentioned to install a vapour check membrane underside the plywood (floor). Will the above work? Any other suggestions? Thanks Alex remove the rockwool, that was a design error I'm afraid. More air bricks may help to some extent, but you still need to remove the rockwool as it prevents air movement round the wood. NT -------------------- I disagree. I would say the correct thing to do would be to isntall a vapour barrier above the insulation, and ensure decent insulation. Its no different from an insulated loft, upside down really. the difference is that the underfloor void is constantly wet. If or when any damp gets past that barrier, as IRL it will, it is then trapped, and rot will occur. The floorboards will be nailed thru the barrier, lots of holes. so is an unventilated (cold) roof space. The key as you say is ventilation. No real problem with nailing through the DPM. Its not like its a pond liner. Its there to reduce teh progress of damp air to the point where natural ventilation can deal with it. ------------------- I think that rockwool isn't the best insulation to use in this situation. I used 2" polystyrene slabs wedged between the joists. This was suggested and approved by my local Building Regs people and by the company who supplied the polystyrene. Building Regs suggested that there should be no gap between the insulation and the under-surface of the floor. The company who provided the polystyrene suggested that a 1/2" gap would be helpful in case of liquid spillage from above. The suggested 1/2" gap was to ensure that any spillage would drain away safely past the insulation. I'm not sure what 'The Natural Philosopher' intended when he said, ".............draught proof below it ............". It's actually important to maintain the existing ventilation (via the airbricks) under the insulation to prevent condensation / rot etc. Cic. expect rot. Dryness and air movement are the 2 things that avoid it. Actually, its dryness pure and simple. Acchieved by keeping teh rain out, and not allowing condensatuoin to build up. Hence DPM and ventialtin. Regards, NT |
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#13
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snipped ------------ I disagree. I would say the correct thing to do would be to isntall a vapour barrier above the insulation, and ensure decent insulation. Its no different from an insulated loft, upside down really. the difference is that the underfloor void is constantly wet. If or when any damp gets past that barrier, as IRL it will, it is then trapped, and rot will occur. The floorboards will be nailed thru the barrier, lots of holes. snipped --------------- expect rot. Dryness and air movement are the 2 things that avoid it. ==================== I wouldn't describe the underfloor void in my house as "..............constantly wet.......". If anything it appears very dry - dry enough to encourage mummification. The problem is that the underfloor is so well ventilated that it creates a very cold floor especially in cold windy weather. I appreciate that there are houses in low-lying ground which do have damp ground surfaces, but it isn't universally the case. I would be very surprised if the insulation I've installed leads to any form of rot. Regards, NT |
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(xavier) wrote in message . com...
(Alex) wrote in message . com... I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the I think you have two problems he 1. There will be a vapour pressure from the living accommodation which is acting to force warm, moist air down via the permeable flooring. This is resulting in interstitial condensation and it's manifesting itself within the rockwool insulation. 2. This is exacerbated by a poor air circulation beneath the rockwool although even with a dramatic improvement in ventilation, problem 1 is likely to persist. (I don't understand or recommend your intended reduction in airspace) I don't think (as NT says) that rockwool is the ideal insulant here, probably Kingspan or somnething similar would be more appropriate. Even if you increase sub floor ventilation, you are still going to have the potential problem of the interstitial condensation and the only way you're likely to overcome this is by placing a vapour barrier (as your friend suggested) directly under the floor decking, between it and the rockwool. I guess this isn't really an option though (is it?) You must ensure that any damp rockwool is either dried out or removed as quickly as possible. If your joists achieve and retain a moisture content in excess of 20 percent you could easily develop a fungal problem. MY tuppenceworth would be to suggest you remove the rockwool and replace it with Kinspan or do away with it all together. Although its marginally 'helping' your fuel efficiency, its also having the detrimental affect of lowering the sub floor temperature and thus increasing the relative humidty. This will present you with an ongoing condensation problem... at last, someone who understands the issues. The short story is: remove the rockwool. NT |
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"The Natural Philosopher" wrote in message ... snip Buit I like poolystrene better apart from teh fire hazrd. Celotex best of all, between the joists, taped down with the special tape and then boarded over. I too was concerned about potential fire hazard when insulating under my wooden floor, so I got the manufacturer's technical dept number from wickes (was using standard wickes 50mm polystyrene insulation) and called them (can't remember the company name or number offhand, but they're a manufacturer based near Hull & I think they also make Jablite). I wanted to know whether I should be using insulation containing fire retarders. They reckon it isn't an issue for ground floors and that the standard stuff is perfectly safe & meets applicable regs. -- Richard Sampson mail me at richard at olifant d-ot co do-t uk |
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"N. Thornton" wrote in message om... (xavier) wrote in message . com... (Alex) wrote in message . com... I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the I think you have two problems he 1. There will be a vapour pressure from the living accommodation which is acting to force warm, moist air down via the permeable flooring. This is resulting in interstitial condensation and it's manifesting itself within the rockwool insulation. 2. This is exacerbated by a poor air circulation beneath the rockwool although even with a dramatic improvement in ventilation, problem 1 is likely to persist. (I don't understand or recommend your intended reduction in airspace) I don't think (as NT says) that rockwool is the ideal insulant here, probably Kingspan or somnething similar would be more appropriate. Even if you increase sub floor ventilation, you are still going to have the potential problem of the interstitial condensation and the only way you're likely to overcome this is by placing a vapour barrier (as your friend suggested) directly under the floor decking, between it and the rockwool. I guess this isn't really an option though (is it?) You must ensure that any damp rockwool is either dried out or removed as quickly as possible. If your joists achieve and retain a moisture content in excess of 20 percent you could easily develop a fungal problem. MY tuppenceworth would be to suggest you remove the rockwool and replace it with Kinspan or do away with it all together. Although its marginally 'helping' your fuel efficiency, its also having the detrimental affect of lowering the sub floor temperature and thus increasing the relative humidty. This will present you with an ongoing condensation problem... at last, someone who understands the issues. The short story is: remove the rockwool. Before doing something so drastic, I would make sure the underfloor ventilation is clear. If clear then add a few more air bricks and monitor. This is a far simpler, and less upheaval, than ripping out fitted insulation. |
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On Mon, 13 Sep 2004 22:33:43 +0100, "G&M"
wrote: "Pete C" wrote in message news Hi Another way of doing it would be to remove the rockwool, stuff it in a binliner and take it outside, then wrap/'bag' it in thick polythene tube or sheet stapled and sealed with tape, then replace it under the floorboards. Would then be much nicer to work with under the floorboards if it is wrapped in this way. Yes - but it may not insulate very well if it is compressed like this. Rockwool depends on being fairly 'fluffy' to work. Hi, No need to compress it, just wrap it fairly loosely. It needs to be compressed a little when pushed between the joists for a snug fit, I'd expect the effect of that on in it's insulation ability is pretty small. cheers, Pete. |
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#18
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"IMM" wrote in message ...
"N. Thornton" wrote in message om... (xavier) wrote in message . com... (Alex) wrote in message . com... I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the I think you have two problems he 1. There will be a vapour pressure from the living accommodation which is acting to force warm, moist air down via the permeable flooring. This is resulting in interstitial condensation and it's manifesting itself within the rockwool insulation. 2. This is exacerbated by a poor air circulation beneath the rockwool although even with a dramatic improvement in ventilation, problem 1 is likely to persist. (I don't understand or recommend your intended reduction in airspace) I don't think (as NT says) that rockwool is the ideal insulant here, probably Kingspan or somnething similar would be more appropriate. Even if you increase sub floor ventilation, you are still going to have the potential problem of the interstitial condensation and the only way you're likely to overcome this is by placing a vapour barrier (as your friend suggested) directly under the floor decking, between it and the rockwool. I guess this isn't really an option though (is it?) You must ensure that any damp rockwool is either dried out or removed as quickly as possible. If your joists achieve and retain a moisture content in excess of 20 percent you could easily develop a fungal problem. MY tuppenceworth would be to suggest you remove the rockwool and replace it with Kinspan or do away with it all together. Although its marginally 'helping' your fuel efficiency, its also having the detrimental affect of lowering the sub floor temperature and thus increasing the relative humidty. This will present you with an ongoing condensation problem... at last, someone who understands the issues. The short story is: remove the rockwool. Before doing something so drastic, I would make sure the underfloor ventilation is clear. If clear then add a few more air bricks and monitor. This is a far simpler, and less upheaval, than ripping out fitted insulation. doesnt really solve the problem. Mould wont grow when the woods sometimes wet but soon dries off again, and has airflow, ie as original design. When you put rockwool up it stops the airflow, so instead you get a steady averaged RH, high enough for rot to grow, and no ventilation, again encouraging mould. NT |
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#19
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"IMM" wrote in message ...
"N. Thornton" wrote in message om... (xavier) wrote in message . com... (Alex) wrote in message . com... I have installed rockwool insulation blankets under the floor and I am now experiencing some worrying problems with condensation as the snip MY tuppenceworth would be to suggest you remove the rockwool and replace it with Kinspan or do away with it all together. Although its marginally 'helping' your fuel efficiency, its also having the detrimental affect of lowering the sub floor temperature and thus increasing the relative humidty. This will present you with an ongoing condensation problem... at last, someone who understands the issues. The short story is: remove the rockwool. Before doing something so drastic, I would make sure the underfloor ventilation is clear. If clear then add a few more air bricks and monitor. This is a far simpler, and less upheaval, than ripping out fitted insulation. The situation here is as follows: There is a cold area (sub-floor space) with a warm area (living space) above. The vapour pressure differential will be (as is almost always the case) such that the vapaour pressure in the living space is greater than that in the sub floor space and thus moistened air will move from the living space to the sub floor space. Sandwiched between these two areas is the Rockwool and naturally there will be a temperature gradient across it - from virtually room temperature at the upper level to outside temperature at the lower level. Thus, as the warm air moves through the Rockwool it will cool and there will be a point within the rockwool where the dew point temperature is reached. At this point interstitial condensation will occur. (Don't forget that the sub floor space is now cooler than it was prior to the installation of the Rockwool and thus Dew Point is reached more often than before) To cure this problem the moisture contained in the warm air from the living space must be prevented from being carried through into the sub floor space. The only feasible way to achieve this is to insert a vapour barrier. To me this seems like a very difficult thing to achieve although I suppose that painting the floor with an impermeable surface could conceivably work and, if you really want to leave the Rockwool in place, then this is what you need to try first. Increasing the ventilation will probably not help the situation. It will make the sub floor space cooler if anything and thus will increase the temperature gradient across the rockwool. Its main effect will probably be to raise the dew point physically higher in the rockwool band (nearer the living area). The ventilation will have little if any effect on helping the moisture to evaporate harmlesslly. This problem is much more than theoretical as far as the OP is concerned because if it persists (and as winter approaches it will probably get worse) then his floor joists could be in imminent danger of fungal attack. It's one of those 'good ideas' that can have ramifications which are far worse than the problem you're trying to cure. It's lucky he noticed it, most people would have nailed the trap door down and forgotten all about it (until the floor fell in that is). Either remove the Rockwool or place an effective vapour barrier between the two areas. Anything else is tinkering  )Xav p.s. I'm working on the assumption that his UF ventilation was adequate prior to the Rockwool. He doesn't seem to have had this problem before then... |
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"xavier" wrote in message om... The situation here is as follows: There is a cold area (sub-floor space) with a warm area (living space) above. The vapour pressure differential will be (as is almost always the case) such that the vapaour pressure in the living space is greater than that in the sub floor space and thus moistened air will move from the living space to the sub floor space. Sandwiched between these two areas is the Rockwool and naturally there will be a temperature gradient across it - from virtually room temperature at the upper level to outside temperature at the lower level. Thus, as the warm air moves through the Rockwool it will cool and there will be a point within the rockwool where the dew point temperature is reached. At this point interstitial condensation will occur. (Don't forget that the sub floor space is now cooler than it was prior to the installation of the Rockwool and thus Dew Point is reached more often than before) To cure this problem the moisture contained in the warm air from the living space must be prevented from being carried through into the sub floor space. The only feasible way to achieve this is to insert a vapour barrier. To me this seems like a very difficult thing to achieve although I suppose that painting the floor with an impermeable surface could conceivably work and, if you really want to leave the Rockwool in place, then this is what you need to try first. Increasing the ventilation will probably not help the situation. It will make the sub floor space cooler if anything and thus will increase the temperature gradient across the rockwool. Its main effect will probably be to raise the dew point physically higher in the rockwool band (nearer the living area). The ventilation will have little if any effect on helping the moisture to evaporate harmlesslly. This problem is much more than theoretical as far as the OP is concerned because if it persists (and as winter approaches it will probably get worse) then his floor joists could be in imminent danger of fungal attack. It's one of those 'good ideas' that can have ramifications which are far worse than the problem you're trying to cure. It's lucky he noticed it, most people would have nailed the trap door down and forgotten all about it (until the floor fell in that is). Either remove the Rockwool or place an effective vapour barrier between the two areas. Anything else is tinkering )Xav p.s. I'm working on the assumption that his UF ventilation was adequate prior to the Rockwool. He doesn't seem to have had this problem before then... Xav that's a nice summary and explanation of this type of problem. the most frightening thing though, as you note, is the number of people who may well have done something similar and now have no idea what is going on down below. While this issue is well recognised when insulating between rafters in lofts, it doesn't get the same exposure when applied to the floor , although it is essentially the same problem. I've seen some plans for a couple of extensions that were drawn up with a similar set up and no mention of a vapour barrier as I recall. While I had intially apporached the architect who drew these up, to do my extension plans, he was ditched after a couple of meetings, and having given this matter a little more attention I may well have a quick word with some friends who used him for some work a while ago. As a consequence of Part L, of the regs people give greater consideration to the underfloor insualation but the emphasis is not always there that if you do this then the vapour barrier is essential, it needs eduction of both the building insectors and the builders, who need to ensure that care is taken to ensure an intact and well sealed membrane. cheers David |
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#21
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David wrote:
"xavier" wrote in message om... The situation here is as follows: There is a cold area (sub-floor space) with a warm area (living space) above. The vapour pressure differential will be (as is almost always the case) such that the vapaour pressure in the living space is greater than that in the sub floor space and thus moistened air will move from the living space to the sub floor space. Sandwiched between these two areas is the Rockwool and naturally there will be a temperature gradient across it - from virtually room temperature at the upper level to outside temperature at the lower level. Thus, as the warm air moves through the Rockwool it will cool and there will be a point within the rockwool where the dew point temperature is reached. At this point interstitial condensation will occur. (Don't forget that the sub floor space is now cooler than it was prior to the installation of the Rockwool and thus Dew Point is reached more often than before) To cure this problem the moisture contained in the warm air from the living space must be prevented from being carried through into the sub floor space. The only feasible way to achieve this is to insert a vapour barrier. To me this seems like a very difficult thing to achieve although I suppose that painting the floor with an impermeable surface could conceivably work and, if you really want to leave the Rockwool in place, then this is what you need to try first. Increasing the ventilation will probably not help the situation. It will make the sub floor space cooler if anything and thus will increase the temperature gradient across the rockwool. Its main effect will probably be to raise the dew point physically higher in the rockwool band (nearer the living area). The ventilation will have little if any effect on helping the moisture to evaporate harmlesslly. This problem is much more than theoretical as far as the OP is concerned because if it persists (and as winter approaches it will probably get worse) then his floor joists could be in imminent danger of fungal attack. It's one of those 'good ideas' that can have ramifications which are far worse than the problem you're trying to cure. It's lucky he noticed it, most people would have nailed the trap door down and forgotten all about it (until the floor fell in that is). Either remove the Rockwool or place an effective vapour barrier between the two areas. Anything else is tinkering )Xav p.s. I'm working on the assumption that his UF ventilation was adequate prior to the Rockwool. He doesn't seem to have had this problem before then... Xav that's a nice summary and explanation of this type of problem. the most frightening thing though, as you note, is the number of people who may well have done something similar and now have no idea what is going on down below. While this issue is well recognised when insulating between rafters in lofts, it doesn't get the same exposure when applied to the floor , although it is essentially the same problem. I've seen some plans for a couple of extensions that were drawn up with a similar set up and no mention of a vapour barrier as I recall. While I had intially apporached the architect who drew these up, to do my extension plans, he was ditched after a couple of meetings, and having given this matter a little more attention I may well have a quick word with some friends who used him for some work a while ago. As a consequence of Part L, of the regs people give greater consideration to the underfloor insualation but the emphasis is not always there that if you do this then the vapour barrier is essential, it needs eduction of both the building insectors and the builders, who need to ensure that care is taken to ensure an intact and well sealed membrane. Mm. Its the recommended way of keeping radon gas below danger levels too - suspended floor and contiguous DPM under it. Not many peple are buildng suspended timber floors at gruond floor level these days, but if I had to do it, I'd celotext the joists, and tape right over before laying to boards. Might even add a DPM in addition, tho the celotex tape has the same effect. I should say that when I demolihed my old house, there was a small pond under the suspended wooden floor, which led to a lot of uptake of water ino teh (non DPC'ed) brick chimneys. The floor was laid on fired clay plates on IIRC a couple of bricks, and tho there was no DPC, the rot was mainly in the timbers around the fireplace, not in general in the floor. Central heating had kept the boards dry enough. cheers David |
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#22
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"David" wrote in message ... As a consequence of Part L, of the regs people give greater consideration to the underfloor insualation but the emphasis is not always there that if you do this then the vapour barrier is essential, it needs eduction of both the building insectors and the builders, who need to ensure that care is taken to ensure an intact and well sealed membrane. Question to all - if one is going to put in a membrane, would it be better on top or underneath the floorboards ? Obviously underneath is required if you want to show the floorboards but if you have carpets or some other covering it seems to me that putting it on top of the floorboards would keep moisture properly out whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there ! Any thoughts ? |
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#23
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G&M wrote:
"David" wrote in message ... As a consequence of Part L, of the regs people give greater consideration to the underfloor insualation but the emphasis is not always there that if you do this then the vapour barrier is essential, it needs eduction of both the building insectors and the builders, who need to ensure that care is taken to ensure an intact and well sealed membrane. Question to all - if one is going to put in a membrane, would it be better on top or underneath the floorboards ? Obviously underneath is required if you want to show the floorboards but if you have carpets or some other covering it seems to me that putting it on top of the floorboards would keep moisture properly out whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there ! Any thoughts ? cant see it makes much difference really, I thik I'd put it safely underneath tho, from pure 'in case someone digs a screwdriver into it and pours a botle of wine over the top' ness :-) |
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#24
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"G&M" wrote in message ...
"David" wrote in message ... As a consequence of Part L, of the regs people give greater consideration to the underfloor insualation but the emphasis is not always there that if you do this then the vapour barrier is essential, it needs eduction of both the building insectors and the builders, who need to ensure that care is taken to ensure an intact and well sealed membrane. Absolutely agree. IME Rockwool is an absolute sod for holding moisture and the wetter it gets, the better it gets at getting wetter - if you see what I mean! Question to all - if one is going to put in a membrane, would it be better on top or underneath the floorboards ? Obviously underneath is required if you want to show the floorboards but if you have carpets or some other covering it seems to me that putting it on top of the floorboards would keep moisture properly out ..... Any thoughts ? The object of all this is to stop moisture laden air getting into the sub floor space. If you put your dpm on top of the floor then this will work. The problem is that at some stage in the proceedings someone is going to come along and remove it (perhaps after you're long dead and gone) because they won't realise the purpose of it. The Rockwool still being there will then start the IC ball rolling! whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there ! Visible condensation won't normally occur in the subfloor space if it's ventilated adequately and the inclusion of a dpm separating it from the living spaces will reduce the risk even further (as indeed would another dpm over the actual ground itself). The Interstitial Condensation ocurring because of previously mentioned circumstances is the greater risk. xav |
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#25
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"xavier" wrote in message om... whereas putting it underneath would still allow condensation on the ?joists and underside of the boards - and then keep it there ! Visible condensation won't normally occur in the subfloor space if it's ventilated adequately and the inclusion of a dpm separating it from the living spaces will reduce the risk even further (as indeed would another dpm over the actual ground itself). The Interstitial Condensation ocurring because of previously mentioned circumstances is the greater risk. My suggestion was that since the Rockwool is pressing the DPM against the underside of the joists and floorboards, these won't be getting any ventilation and in fact are at quite some risk of damage. But this depends on whether moist air would condense on top of the DPM above the Rockwool or not ? Any thoughts ? |
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#26
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xavier wrote:
"G&M" wrote in message ... "David" wrote in message ... As a consequence of Part L, of the regs people give greater consideration to the underfloor insualation but the emphasis is not always there that if you do this then the vapour barrier is essential, it needs eduction of both the building insectors and the builders, who need to ensure that care is taken to ensure an intact and well sealed membrane. Absolutely agree. IME Rockwool is an absolute sod for holding moisture and the wetter it gets, the better it gets at getting wetter - if you see what I mean! Question to all - if one is going to put in a membrane, would it be better on top or underneath the floorboards ? Obviously underneath is required if you want to show the floorboards but if you have carpets or some other covering it seems to me that putting it on top of the floorboards would keep moisture properly out ..... Any thoughts ? The object of all this is to stop moisture laden air getting into the sub floor space. If you put your dpm on top of the floor then this will work. The problem is that at some stage in the proceedings someone is going to come along and remove it (perhaps after you're long dead and gone) because they won't realise the purpose of it. The Rockwool still being there will then start the IC ball rolling! whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there ! No. Not with insulation underneath. The boards would be warm and have an upwards direction for moisture to migrate. Visible condensation won't normally occur in the subfloor space if it's ventilated adequately and the inclusion of a dpm separating it from the living spaces will reduce the risk even further (as indeed would another dpm over the actual ground itself). The Interstitial Condensation ocurring because of previously mentioned circumstances is the greater risk. xav |
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#27
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G&M wrote:
"xavier" wrote in message om... whereas putting it underneath would still allow condensation on the ?joists and underside of the boards - and then keep it there ! Visible condensation won't normally occur in the subfloor space if it's ventilated adequately and the inclusion of a dpm separating it from the living spaces will reduce the risk even further (as indeed would another dpm over the actual ground itself). The Interstitial Condensation ocurring because of previously mentioned circumstances is the greater risk. My suggestion was that since the Rockwool is pressing the DPM against the underside of the joists and floorboards, these won't be getting any ventilation and in fact are at quite some risk of damage. But this depends on whether moist air would condense on top of the DPM above the Rockwool or not ? Any thoughts ? it won't condense. You need a markedly colder surface and that is the far sie of teh insulation only. Te rule is always 'warm and moist side, internal surace finish, then DPM, then insulation (these two may be the same as with e.g. celotex and its sealing tape, then cold side with ventilation' The point is that the thermal and moisture barriers need to be in the same plane or the DPM on the warm side of the insulation. |
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#28
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"The Natural Philosopher" wrote in message ... xavier wrote: "G&M" wrote in message ... "David" wrote in message ... As a consequence of Part L, of the regs people give greater consideration to the underfloor insualation but the emphasis is not always there that if you do this then the vapour barrier is essential, it needs eduction of both the building insectors and the builders, who need to ensure that care is taken to ensure an intact and well sealed membrane. Absolutely agree. IME Rockwool is an absolute sod for holding moisture and the wetter it gets, the better it gets at getting wetter - if you see what I mean! Question to all - if one is going to put in a membrane, would it be better on top or underneath the floorboards ? Obviously underneath is required if you want to show the floorboards but if you have carpets or some other covering it seems to me that putting it on top of the floorboards would keep moisture properly out ..... Any thoughts ? The object of all this is to stop moisture laden air getting into the sub floor space. If you put your dpm on top of the floor then this will work. The problem is that at some stage in the proceedings someone is going to come along and remove it (perhaps after you're long dead and gone) because they won't realise the purpose of it. The Rockwool still being there will then start the IC ball rolling! whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there ! No. Not with insulation underneath. The boards would be warm and have an upwards direction for moisture to migrate. Good point. And it saves me ripping down what I've already done under our floorboards with a mixture of Rockwool and Celotex. |
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#29
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snipped whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there ! No. Not with insulation underneath. The boards would be warm and have an upwards direction for moisture to migrate. Good point. And it saves me ripping down what I've already done under our floorboards with a mixture of Rockwool and Celotex. ============== Can you describe your mixture of Rockwool and Celotex? Is it substantially different from my version of insulation (described in an earlier part of this thread)? I installed 2" polystyrene boards between the joists and the difference is already noticeable. An added bonus is that the rooms are much quieter especially so the room fronting the road. I believe that Celotex would be even better than polystyrene but I don't understand why you would use the Rockwool as well. Cic. |
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#30
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"Cicero" wrote in message ... snipped whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there ! No. Not with insulation underneath. The boards would be warm and have an upwards direction for moisture to migrate. Good point. And it saves me ripping down what I've already done under our floorboards with a mixture of Rockwool and Celotex. ============== Can you describe your mixture of Rockwool and Celotex? Is it substantially different from my version of insulation (described in an earlier part of this thread)? I installed 2" polystyrene boards between the joists and the difference is already noticeable. An added bonus is that the rooms are much quieter especially so the room fronting the road. I believe that Celotex would be even better than polystyrene but I don't understand why you would use the Rockwool as well. Sorry to be confusing - they are separate areas. Celotex is used under the main living room and is stuffed tightly between the joists. Rockwool is used in a similar manner under the conservatory as that has underfloor heating and the Rockwool forms a good bed for the pipes to sit in whereas the usual manner of keeping foam in place by friction would probably not work and fall down every time the joists heated and cooled. Foam is definitely the right solution in your case. |
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#31
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"G&M" wrote in message ... "Cicero" wrote in message ... snipped whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there ! No. Not with insulation underneath. The boards would be warm and have an upwards direction for moisture to migrate. Good point. And it saves me ripping down what I've already done under our floorboards with a mixture of Rockwool and Celotex. ============== Can you describe your mixture of Rockwool and Celotex? Is it substantially different from my version of insulation (described in an earlier part of this thread)? I installed 2" polystyrene boards between the joists and the difference is already noticeable. An added bonus is that the rooms are much quieter especially so the room fronting the road. I believe that Celotex would be even better than polystyrene but I don't understand why you would use the Rockwool as well. Sorry to be confusing - they are separate areas. Celotex is used under the main living room and is stuffed tightly between the joists. Rockwool is used in a similar manner under the conservatory as that has underfloor heating and the Rockwool forms a good bed for the pipes to sit in whereas the usual manner of keeping foam in place by friction would probably not work and fall down every time the joists heated and cooled. Foam is definitely the right solution in your case. ============== Thanks for the explanation - knowledge for future reference. Cic. |
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#32
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"Cicero" wrote in message ...
"G&M" wrote in message ... "Cicero" wrote in message ... snipped whereas putting it underneath would still allow condensation on the joists and underside of the boards - and then keep it there ! No. Not with insulation underneath. The boards would be warm and have an upwards direction for moisture to migrate. Good point. And it saves me ripping down what I've already done under our floorboards with a mixture of Rockwool and Celotex. ============== Can you describe your mixture of Rockwool and Celotex? Is it substantially different from my version of insulation (described in an earlier part of this thread)? I installed 2" polystyrene boards between the joists and the difference is already noticeable. An added bonus is that the rooms are much quieter especially so the room fronting the road. I believe that Celotex would be even better than polystyrene but I don't understand why you would use the Rockwool as well. Sorry to be confusing - they are separate areas. Celotex is used under the main living room and is stuffed tightly between the joists. Rockwool is used in a similar manner under the conservatory as that has underfloor heating and the Rockwool forms a good bed for the pipes to sit in whereas the usual manner of keeping foam in place by friction would probably not work and fall down every time the joists heated and cooled. Foam is definitely the right solution in your case. ============== Thanks for the explanation - knowledge for future reference. Cic. Thank you all for your replies. There are still some points which need to clearify but I'll take care to examine carefully your advice first. In fact I think I'll install a vapour check and monitor the results...thanks god for that trap door! Meanwhile, it looks as the problem is not so nasty as I initially thought. The condensation problems seems to arise only on very hot humid days (e.g. last July). I had to crawl with mop and bucket to clear those poodles of stagnant water. Then I left the trap door open and the rockwool got completely dry in a couple of weeks. I want to see what is going to happen this winter when the central heating is on. Thank you very much P.S Those morons at Manchester Town Hall building controls...I contacted them before installing the insulation and they guarantee me there was no need for vapour check? |
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#33
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"Alex" wrote in message om... snipped P.S Those morons at Manchester Town Hall building controls...I contacted them before installing the insulation and they guarantee me there was no need for vapour check? How would they know. They're just people waiting for jobs in the more enjoyable traffic warden department next door. |
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