[Roger Chapman]

On 27/02/2011 12:56, Andrew Gabriel wrote:
Interesting problem with loft condensation...

100 year old house, no water/plumbing in the loft.
Loft is lined and retiled, probably 40+ years ago.
Loft has always been bone dry, at least through to last summer.
Loft is well sealed from the house, but there's pretty much no
moisture source in the house anyway (far less than there would normally
be due to lifestyle of the occupant).

Last summer, various maintenance activities were carried out around the
loft and roof.

Some lead flashing was replaced, and that still seems to be fine - no
water penetration. (Prior to that, a small amount of rain water was running
down the chimney breast in the loft, but it wasn't actually causing any
problems, and loft was otherwise bone dry.) Chimney breast is now bone
dry, so that's fixed. Indeed, there's no sign of rain penetration anywhere
else either.

Another 4 inches of loft insulation were added, on top of the 2-3"
that have been there 25 years.

I replaced the facia and gutters along one side. There was no ventilation
there before (house has no soffits), and I spaced the new facia a little
bit off the wall plate timber to allow a little ventilation up between
them.

Whilst there was access to the chimney, I took opportunity to add more
ventilation in the form of air bricks in the gable end (one large, one
smaller), as the loft wouldn't meet modern ventilation requirements,
although as I said before, it had never got damp before.

Just checked it after the winter, and it's obvious that the rafters
have all been very wet over winter, which has never happened before.
It's spread across the whole roof, so it's condensation, not a leak.
Being an old house with untreated timbers, I can't let this continue.

So I'm trying to work out the cause, and it would seem to match
something I have hypothesised before, and this is what I think has
happened.

Before I did the work, there was a little loft ventilation, and little
loft insulation. Heat would leak from the house, warm the loft, and
consequently reduce the relative humidity of the (outside) air in the
loft so there was no chance of any surfaces lower than the dew point,
as the loft will be warmer than the outside air.

Now, the increased ventilation means more outside air is passing through
the loft. The increased insulation means the loft isn't being warmed.
On cold clear nights, exposed surfaces such as a roof radiate heat and
in the absence of any heat source, cool below the outside air temperature.
This will now drop below the dew point of the outside air, and form
dew/condensation on the inside of the roof, in the same situation you
get dew outside.

Does this sound plausible?

Yes. (But see below).

Any bright ideas on fixing it, apart from rolling up the loft insulation
and blocking off the new air vents? It seems to me such a roof is
dependent on heat leaking from the house.

Are you sure that there is no leakage of air from the main part of the
house into the loft. Your increased ventilation may have the affect of
providing a pressure difference between house and loft and an influx of
warm humid air into a cold space will certainly cause condensation.

There will be some slight transfer of heat to the loft however deep you
make the insulation so I have my doubts about the roof managing to
radiate down to below ambient. Grass frosts typically occur below 3C but
the grass is almost isolated from the ground and even with the limited
heat coming up from the earth air temperature above bare ground is much
closer to zero before that attracts a frost coating.

With a similar problem in the past I have used EasyLog data loggers to
establish the extent of the problem but that was transient rather than
continuous through the winter. (EL-USB-lite IIRC).

[Andrew Gabriel]

In article ,
Roger Chapman writes:

Does this sound plausible?

Yes. (But see below).

Any bright ideas on fixing it, apart from rolling up the loft insulation
and blocking off the new air vents? It seems to me such a roof is
dependent on heat leaking from the house.

Are you sure that there is no leakage of air from the main part of the
house into the loft. Your increased ventilation may have the affect of
providing a pressure difference between house and loft and an influx of
warm humid air into a cold space will certainly cause condensation.

There are no holes punched through the ceiling. The loft hatch
and ceiling isn't going to be completely air-tight of course, but
it's no worse than it was for previous 25 years. Nothing has changed
in the house either regards producing moisture.

There will be some slight transfer of heat to the loft however deep you
make the insulation so I have my doubts about the roof managing to
radiate down to below ambient. Grass frosts typically occur below 3C but
the grass is almost isolated from the ground and even with the limited
heat coming up from the earth air temperature above bare ground is much
closer to zero before that attracts a frost coating.

With a similar problem in the past I have used EasyLog data loggers to
establish the extent of the problem but that was transient rather than
continuous through the winter. (EL-USB-lite IIRC).

The house has temperature logging, but not in the loft. So, I just
added a sensor in the loft and I'll compare it with the outside sensor.
Having come out of the loft and closed the hatch, the loft temperature
is rapidly dropping towards outside temperature, but too early to know
where it will settle yet.

It looks like we might get a couple of nights at 0C this week, but not
with clear skys, but I'll see what readings I get.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

[Roger Chapman]

On 27/02/2011 18:41, Andrew Gabriel wrote:

snip

The house has temperature logging, but not in the loft. So, I just
added a sensor in the loft and I'll compare it with the outside sensor.
Having come out of the loft and closed the hatch, the loft temperature
is rapidly dropping towards outside temperature, but too early to know
where it will settle yet.

Temperature is only half the equation. Do you have a dehumidifer to put
up there for a bit to see how much water you can extract at say 80%
humidity?

[andrew]

Roger Chapman wrote:

Temperature is only half the equation. Do you have a dehumidifer to put
up there for a bit to see how much water you can extract at say 80%
humidity?

The hypothesis is that the roof is a good radiator and insulated from the
building and ground. Air passes through the roof space. As the air cools
faster than the surroundings barring those that are good radiators and not
able to conduct heat from the ground ( cars and windscreens being examples)
it approaches saturation. This saturated air passes into the roof space and
out the other vents but on the way it contacts the cold roof underside and
reaches its dewpoint. It's a small matter if there is only one air change
in the night, maybe 10 grams per m3 which would evaporate next day, but if
the space is well ventilated...

AJH

[David Robinson]

On Feb 27, 10:16*pm, andrew wrote:
Roger Chapman wrote:
Temperature is only half the equation. Do you have a dehumidifer to put
up there for a bit to see how much water you can extract at say 80%
humidity?

The hypothesis is that the roof is a good radiator and insulated from the
building and ground. Air passes through the roof space. As the air cools
faster than the surroundings barring those that are good radiators and not
able to conduct heat from the ground ( cars and windscreens being examples)
it approaches saturation. This saturated air passes into the roof space and
out the other vents but on the way it contacts the cold roof underside and
reaches its dewpoint. It's a small matter if there is only one air change
in the night, maybe 10 grams per m3 which would evaporate next day, but if
the space is well ventilated...

I've seen Andrew G propose his theory before, and it's a shame in a
way that it may have proven true in practice.

It doesn't seem like letting heat into the loft is seen as a good
thing. It doesn't seem like blocking up all the ventilation is seen as
a good thing. So how else can the loft surfaces be prevented from
acting like a car and its windscreen (and attracting condensation and
frost)?

Or, to put it another way - how come all modern houses (lots of
insulation, lots of ventilation) don't face this problem?

Cheers,
David.

[The Natural Philosopher[_2_]]

David Robinson wrote:
On Feb 27, 10:16 pm, andrew wrote:
Roger Chapman wrote:
Temperature is only half the equation. Do you have a dehumidifer to put
up there for a bit to see how much water you can extract at say 80%
humidity?
The hypothesis is that the roof is a good radiator and insulated from the
building and ground. Air passes through the roof space. As the air cools
faster than the surroundings barring those that are good radiators and not
able to conduct heat from the ground ( cars and windscreens being examples)
it approaches saturation. This saturated air passes into the roof space and
out the other vents but on the way it contacts the cold roof underside and
reaches its dewpoint. It's a small matter if there is only one air change
in the night, maybe 10 grams per m3 which would evaporate next day, but if
the space is well ventilated...

I've seen Andrew G propose his theory before, and it's a shame in a
way that it may have proven true in practice.

It doesn't seem like letting heat into the loft is seen as a good
thing. It doesn't seem like blocking up all the ventilation is seen as
a good thing. So how else can the loft surfaces be prevented from
acting like a car and its windscreen (and attracting condensation and
frost)?

Or, to put it another way - how come all modern houses (lots of
insulation, lots of ventilation) don't face this problem?

The frost or dew on a car disappears the moment the sun hits it: Its
really not a long term issue.

Internal misting happens more because the car is left with a sticky
steamy interior after last nights drive.. if a car is left for LONG
periods it doesn't suffer from internal condensation.

In short, any ventilated structure that is weather proof - a barn or a
shed- does not suffer rot until its inhabited and steamy breath raises
the RH.





Cheers,
David.

[Andrew Gabriel]

In article ,
Roger Chapman writes:
On 27/02/2011 18:41, Andrew Gabriel wrote:

snip

The house has temperature logging, but not in the loft. So, I just
added a sensor in the loft and I'll compare it with the outside sensor.
Having come out of the loft and closed the hatch, the loft temperature
is rapidly dropping towards outside temperature, but too early to know
where it will settle yet.

Temperature is only half the equation. Do you have a dehumidifer to put
up there for a bit to see how much water you can extract at say 80%
humidity?

I do have one, but they don't work at the current loft temp (6C) as
the evaporator ices up. I could perhaps put it upstairs with the
outlet ducted up into the loft as a measure to dry it out quickly.

I have some readings now. Last night and this morning, loft stays
2C higher than outside temperature. It's been continuously cloudly
over that period, so no radiative cooling or heating to speak of,
and no wind to speak of.

I varied upstairs temperature from 16.5C to 20.5C over night and it
made no difference to loft temperature, but this would probably need
running over a longer period to get a noticable effect.

This morning, I put 500W of heating in the loft to see what impact
that has on the temperature. That raises the loft by a further
4C, to 6C above outside. Very conveniently, the outside temperature
hasn't changed over this period, so the temperature rise should be
proportional to the heat loss, so that implies that I'm normally
losing about 250W through the new loft insulation. I don't have
equivalent measurements before doing the extra insulation, but I
would imagine it could well be many times that heat loss.

Another interesting observation. In the loft is a large metal tray,
4' x 3' (originally the side from a 19" rack). It was originally
supported above the new insulation, but the insulation has expanded
a bit since laying, and was now touching the bottom. There is a
little condensation on the bottom of this, which implies some
moisture is working it's way through the ceiling, which is lath
and plaster at that point, but not cracked. I still find it hard
to believe all the moisture in the loft is coming from the house
though.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

[george [dicegeorge]]

On 28/02/11 15:40, Andrew Gabriel wrote:
In ,
Roger writes:
On 27/02/2011 18:41, Andrew Gabriel wrote:

snip

The house has temperature logging, but not in the loft. So, I just
added a sensor in the loft and I'll compare it with the outside sensor.
Having come out of the loft and closed the hatch, the loft temperature
is rapidly dropping towards outside temperature, but too early to know
where it will settle yet.

Temperature is only half the equation. Do you have a dehumidifer to put
up there for a bit to see how much water you can extract at say 80%
humidity?

I do have one, but they don't work at the current loft temp (6C) as
the evaporator ices up. I could perhaps put it upstairs with the
outlet ducted up into the loft as a measure to dry it out quickly.

I have some readings now. Last night and this morning, loft stays
2C higher than outside temperature. It's been continuously cloudly
over that period, so no radiative cooling or heating to speak of,
and no wind to speak of.

I varied upstairs temperature from 16.5C to 20.5C over night and it
made no difference to loft temperature, but this would probably need
running over a longer period to get a noticable effect.

This morning, I put 500W of heating in the loft to see what impact
that has on the temperature. That raises the loft by a further
4C, to 6C above outside. Very conveniently, the outside temperature
hasn't changed over this period, so the temperature rise should be
proportional to the heat loss, so that implies that I'm normally
losing about 250W through the new loft insulation. I don't have
equivalent measurements before doing the extra insulation, but I
would imagine it could well be many times that heat loss.

Another interesting observation. In the loft is a large metal tray,
4' x 3' (originally the side from a 19" rack). It was originally
supported above the new insulation, but the insulation has expanded
a bit since laying, and was now touching the bottom. There is a
little condensation on the bottom of this, which implies some
moisture is working it's way through the ceiling, which is lath
and plaster at that point, but not cracked. I still find it hard
to believe all the moisture in the loft is coming from the house
though.

warm moist air can get between my floorbaords
then up the internal lathe and plaster walls
and then into the loft.
[g]

[Skipweasel[_4_]]

In article , lid says...
Internal misting happens more because the car is left with a sticky
steamy interior after last nights drive..


Or, in the case of many cars, 'cos the driver doesn't understand how to
set the fresh/recirculate lever.

Vauxhall seem to be to be particularly prone to this - the number of
Corsas and Astras you see driving along that look like someone's having
a sauna in 'em.

--
Skipweasel - never knowingly understood.

[The Natural Philosopher[_2_]]

Andrew Gabriel wrote:
In article ,
Roger Chapman writes:
On 27/02/2011 18:41, Andrew Gabriel wrote:

snip

The house has temperature logging, but not in the loft. So, I just
added a sensor in the loft and I'll compare it with the outside sensor.
Having come out of the loft and closed the hatch, the loft temperature
is rapidly dropping towards outside temperature, but too early to know
where it will settle yet.
Temperature is only half the equation. Do you have a dehumidifer to put
up there for a bit to see how much water you can extract at say 80%
humidity?

I do have one, but they don't work at the current loft temp (6C) as
the evaporator ices up. I could perhaps put it upstairs with the
outlet ducted up into the loft as a measure to dry it out quickly.

I have some readings now. Last night and this morning, loft stays
2C higher than outside temperature. It's been continuously cloudly
over that period, so no radiative cooling or heating to speak of,
and no wind to speak of.

I varied upstairs temperature from 16.5C to 20.5C over night and it
made no difference to loft temperature, but this would probably need
running over a longer period to get a noticable effect.

This morning, I put 500W of heating in the loft to see what impact
that has on the temperature. That raises the loft by a further
4C, to 6C above outside. Very conveniently, the outside temperature
hasn't changed over this period, so the temperature rise should be
proportional to the heat loss, so that implies that I'm normally
losing about 250W through the new loft insulation. I don't have
equivalent measurements before doing the extra insulation, but I
would imagine it could well be many times that heat loss.

Another interesting observation. In the loft is a large metal tray,
4' x 3' (originally the side from a 19" rack). It was originally
supported above the new insulation, but the insulation has expanded
a bit since laying, and was now touching the bottom. There is a
little condensation on the bottom of this, which implies some
moisture is working it's way through the ceiling, which is lath
and plaster at that point, but not cracked. I still find it hard
to believe all the moisture in the loft is coming from the house
though.

Are you saying that you have no vapour barrier underneath the
insulation? and that it's jut lath, plaster and rockwool there, alone?

[Jim K[_3_]]

On Feb 28, 4:35 pm, Skipweasel
wrote:
In article , says...

Internal misting happens more because the car is left with a sticky
steamy interior after last nights drive..

Or, in the case of many cars, 'cos the driver doesn't understand how to
set the fresh/recirculate lever.

Vauxhall seem to be to be particularly prone to this - the number of
Corsas and Astras you see driving along that look like someone's having
a sauna in 'em.

if with aircon - the driivers should check their aircon plenum
chambers (IME)

Jim K

[Skipweasel[_4_]]

In article 38772668-883d-4bd8-b6e0-fa81a6092d01
@z27g2000prz.googlegroups.com, says...
Or, in the case of many cars, 'cos the driver doesn't understand how to
set the fresh/recirculate lever.

Vauxhall seem to be to be particularly prone to this - the number of
Corsas and Astras you see driving along that look like someone's having
a sauna in 'em.

if with aircon - the driivers should check their aircon plenum
chambers (IME)

Mostly just plain ordinary ventilation - but switched off.

--
Skipweasel - never knowingly understood.

[harry]

On Feb 28, 5:06*pm, Tim Streater wrote:
In article ,
*The Natural Philosopher wrote:

Andrew Gabriel wrote:

[snip]

Another interesting observation. In the loft is a large metal tray,
4' x 3' (originally the side from a 19" rack). It was originally
supported above the new insulation, but the insulation has expanded
a bit since laying, and was now touching the bottom. There is a
little condensation on the bottom of this, which implies some
moisture is working it's way through the ceiling, which is lath
and plaster at that point, but not cracked. I still find it hard
to believe all the moisture in the loft is coming from the house
though.

Are you saying that you have no vapour barrier underneath the
insulation? and that it's jut lath, plaster and rockwool there, alone?

Arrrhhh, are you saying there should be something? So you don't just put
the fibre-glass or whatever between the joists and run away?

--
Tim

"That excessive bail ought not to be required, nor excessive fines imposed,
nor cruel and unusual punishments inflicted" *-- *Bill of Rights 1689

The emulsion paint on the ceilng below usually acts s a sufficient
vapour barrier. Have you ventilation to shower cubicles ad kitchen
extractor hood?

[Terry Fields]

David Robinson wrote:


It doesn't seem like letting heat into the loft is seen as a good
thing. It doesn't seem like blocking up all the ventilation is seen as
a good thing. So how else can the loft surfaces be prevented from
acting like a car and its windscreen (and attracting condensation and
frost)?

Or, to put it another way - how come all modern houses (lots of
insulation, lots of ventilation) don't face this problem?

My 2003 house has breathable sarkin. I expect the roof is ventilated
but it isn't obvious how this is done. There's no sign of condensation
anywhere in the loft space, which has ~100mm fibreglass insulation.

I read an article many years ago in the New Scientist, that looked at
the deterioration of unused/abandonded stately homes. The article
concluded that the most significant destructive mode was humidity
cycling. Apparently, IIRC, cycling to less than about 45 percent
caused wood to shrink and paint to peel off, and above about 60
percent caused fungal spores to become active.

I'm certain the article concluded that the best way to keep the
humidity levels under some sort of control was to keep the interior 5
degC above the outside temperature. It just might be that thicknesses
of layers of loft insulation less than that currently recommended
might do just that, and be the price one pays for a damp-free loft
space.

I've never seen a calculation that estimated the water-vapour
diffusion rate from upstairs rooms at say 20 degC and 50 percent RH
into a loft at 5 degC and say 90 percent RH, but the OP's posting
suggests that it might be greater than desirable.

TF

[The Natural Philosopher[_2_]]

Tim Streater wrote:
In article ,
The Natural Philosopher wrote:

Andrew Gabriel wrote:

[snip]

Another interesting observation. In the loft is a large metal tray,
4' x 3' (originally the side from a 19" rack). It was originally
supported above the new insulation, but the insulation has expanded
a bit since laying, and was now touching the bottom. There is a
little condensation on the bottom of this, which implies some
moisture is working it's way through the ceiling, which is lath
and plaster at that point, but not cracked. I still find it hard
to believe all the moisture in the loft is coming from the house
though.

Are you saying that you have no vapour barrier underneath the
insulation? and that it's jut lath, plaster and rockwool there, alone?

Arrrhhh, are you saying there should be something? So you don't just put
the fibre-glass or whatever between the joists and run away?

Oh dear...well you CAN but then you get..wet rafters..

I think we know know why...the real discussion issue is 'what is the
simplest way to fix it?

..

[The Natural Philosopher[_2_]]

harry wrote:
On Feb 28, 5:06 pm, Tim Streater wrote:
In article ,
The Natural Philosopher wrote:

Andrew Gabriel wrote:
[snip]

Another interesting observation. In the loft is a large metal tray,
4' x 3' (originally the side from a 19" rack). It was originally
supported above the new insulation, but the insulation has expanded
a bit since laying, and was now touching the bottom. There is a
little condensation on the bottom of this, which implies some
moisture is working it's way through the ceiling, which is lath
and plaster at that point, but not cracked. I still find it hard
to believe all the moisture in the loft is coming from the house
though.
Are you saying that you have no vapour barrier underneath the
insulation? and that it's jut lath, plaster and rockwool there, alone?
Arrrhhh, are you saying there should be something? So you don't just put
the fibre-glass or whatever between the joists and run away?

--
Tim

"That excessive bail ought not to be required, nor excessive fines imposed,
nor cruel and unusual punishments inflicted" -- Bill of Rights 1689

The emulsion paint on the ceilng below usually acts s a sufficient
vapour barrier.

You are joking aren't you?

Drop a splash of water on any emulsion painted plaster..does it run off?
Nope. it soaks right in, unless you have a vinyl plasticky paint for
bathroom use.

[george [dicegeorge]]

On 28/02/11 18:17, The Natural Philosopher wrote:
Tim Streater wrote:
In article ,
The Natural Philosopher wrote:

Andrew Gabriel wrote:

[snip]

Another interesting observation. In the loft is a large metal tray,
4' x 3' (originally the side from a 19" rack). It was originally
supported above the new insulation, but the insulation has expanded
a bit since laying, and was now touching the bottom. There is a
little condensation on the bottom of this, which implies some
moisture is working it's way through the ceiling, which is lath
and plaster at that point, but not cracked. I still find it hard
to believe all the moisture in the loft is coming from the house
though.

Are you saying that you have no vapour barrier underneath the
insulation? and that it's jut lath, plaster and rockwool there, alone?

Arrrhhh, are you saying there should be something? So you don't just
put the fibre-glass or whatever between the joists and run away?

Oh dear...well you CAN but then you get..wet rafters..

I think we know know why...the real discussion issue is 'what is the
simplest way to fix it?

.
easiest - a lot more ventilation in the loft,
and kitchen extractor fixed.

Are there indoor plants adding to the moisture of cooking,
washing and breathing?

[g]

[Andrew Gabriel]

In article ,
The Natural Philosopher writes:

I think we know know why...the real discussion issue is 'what is the
simplest way to fix it?

Yes, but at two levels.

The first is what can I do quickly (like in half a day) to dry out
the loft and allow the rafters to start drying. I won't be able to do
a long-term fix until later in the year.

I might try something with the dehumidifier, such as having it upstairs
and discharging into the loft, but I need to do some measurements to
make sure it extracts enough moisture so it doesn't make the situation
worse, and I didn't have a humidity meter on me when I was there last
time. (Unfortunately it won't work in the loft as it's too cold for it.)

Second is what's a proper long term fix (which isn't too much work of
course;-) This requires proper diagnosis, since fixing the wrong
problem won't do any good.

It looks like moisture through the lath and plaster might be significant.
So, what's the best way to do this? I could probably roll back the
insulation to get to the laths (it runs between the joists, and then
another layer across the top of the joists).

Could I simply buy some of the thinner damp proof membrane and lay
this over the laths, or is there a better material? I have a large
roll of non-breathable sarking which might work for this.

Can it go up and over each ceiling joist? Cutting it to fit between
the joists would leave long unsealed stretches along the joist edges,
defeating the object of trying to seal. However, if it goes up and
over each joist, the joist tops will be sealed cold spots (with only
one layer of insulation over).

Another option is replacing the ceilings, but taking down two large
lath and plaster ceilings is not something I like the idea of, and
overboarding always seems to me like a bit of a bodge.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

[The Natural Philosopher[_2_]]

Tim Streater wrote:
In article ,
The Natural Philosopher wrote:

Tim Streater wrote:
In article ,
The Natural Philosopher wrote:
Andrew Gabriel wrote:
[snip]
Another interesting observation. In the loft is a large metal
tray,
4' x 3' (originally the side from a 19" rack). It was originally
supported above the new insulation, but the insulation has expanded
a bit since laying, and was now touching the bottom. There is a
little condensation on the bottom of this, which implies some
moisture is working it's way through the ceiling, which is lath
and plaster at that point, but not cracked. I still find it hard
to believe all the moisture in the loft is coming from the house
though.

Are you saying that you have no vapour barrier underneath the
insulation? and that it's jut lath, plaster and rockwool there, alone?
Arrrhhh, are you saying there should be something? So you don't
just put the fibre-glass or whatever between the joists and run away?
Oh dear...well you CAN but then you get..wet rafters..

Well I don't because I've never done it. I never investigated what the
builders put under the 4" they installed 15 years ago in our old house,
or what there is under the 10" in the new extension here. What should I
be expecting to find?

they might have laid some old tesco bags under it for starters.:-)

[The Natural Philosopher[_2_]]

Andrew Gabriel wrote:
In article ,
The Natural Philosopher writes:
I think we know know why...the real discussion issue is 'what is the
simplest way to fix it?

Yes, but at two levels.

The first is what can I do quickly (like in half a day) to dry out
the loft and allow the rafters to start drying. I won't be able to do
a long-term fix until later in the year.


Open the loft hatch mebbe and let some warmth in there, the ventilation
will do the rest.


I might try something with the dehumidifier, such as having it upstairs
and discharging into the loft, but I need to do some measurements to
make sure it extracts enough moisture so it doesn't make the situation
worse, and I didn't have a humidity meter on me when I was there last
time. (Unfortunately it won't work in the loft as it's too cold for it.)

Second is what's a proper long term fix (which isn't too much work of
course;-) This requires proper diagnosis, since fixing the wrong
problem won't do any good.
Somehow you need a vapour barrier in there if the insulation is as good
as it seems to be.

And it really is old lah and plaster.

A lot depends on the rooms below. A simple fix is to reline below over
existing with foil backed plaster board, and skim or use lining paper to
finish off. and repaint..

I am not sure about fixing from above, but lifting insulation and laying
plastic sheet over the lot and re laying insulation might work as well

Or even strips of plastic laid between the joists: its not to make it
watertight just to cut it down till the existing ventilation can cope.



It looks like moisture through the lath and plaster might be significant.
So, what's the best way to do this? I could probably roll back the
insulation to get to the laths (it runs between the joists, and then
another layer across the top of the joists).

Could I simply buy some of the thinner damp proof membrane and lay
this over the laths, or is there a better material? I have a large
roll of non-breathable sarking which might work for this.


Sounds ok to me..see what others say.

Can it go up and over each ceiling joist? Cutting it to fit between
the joists would leave long unsealed stretches along the joist edges,
defeating the object of trying to seal. However, if it goes up and
over each joist, the joist tops will be sealed cold spots (with only
one layer of insulation over).

As I said. major reduction is the game. not 'sealing' ..think of it as a
path with a leaky plug and a dripping tap..as long as you are getting
rid of it faster than its coming in, it wont end up in the rafters.

Anything will do. celotex is foil baked as is fooiled plasterboard.
Could cut strips of that and simply flop them down between joists.. betweem

Another option is replacing the ceilings, but taking down two large
lath and plaster ceilings is not something I like the idea of, and
overboarding always seems to me like a bit of a bodge.

It isn't really. I've got bits of doubled up plasterboard here..its
standard for extra sound insulation in many places.

[robgraham]

On Feb 28, 9:28*pm, (Andrew Gabriel)
wrote:
In article ,
* * * * The Natural Philosopher writes:



I think we know know why...the real discussion issue is 'what is the
simplest way to fix it?

Yes, but at two levels.

The first is what can I do quickly (like in half a day) to dry out
the loft and allow the rafters to start drying. I won't be able to do
a long-term fix until later in the year.

I might try something with the dehumidifier, such as having it upstairs
and discharging into the loft, but I need to do some measurements to
make sure it extracts enough moisture so it doesn't make the situation
worse, and I didn't have a humidity meter on me when I was there last
time. (Unfortunately it won't work in the loft as it's too cold for it.)

Second is what's a proper long term fix (which isn't too much work of
course;-) This requires proper diagnosis, since fixing the wrong
problem won't do any good.

It looks like moisture through the lath and plaster might be significant.
So, what's the best way to do this? I could probably roll back the
insulation to get to the laths (it runs between the joists, and then
another layer across the top of the joists).

Could I simply buy some of the thinner damp proof membrane and lay
this over the laths, or is there a better material? I have a large
roll of non-breathable sarking which might work for this.

Can it go up and over each ceiling joist? Cutting it to fit between
the joists would leave long unsealed stretches along the joist edges,
defeating the object of trying to seal. *However, if it goes up and
over each joist, the joist tops will be sealed cold spots (with only
one layer of insulation over).

Another option is replacing the ceilings, but taking down two large
lath and plaster ceilings is not something I like the idea of, and
overboarding always seems to me like a bit of a bodge.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

I know this is going slightly off topic, but why, Andrew, do you see
overboarding as a bodge ? I have an L & P ceiling that is cracked,
has been for the 40 years we've been here, and I'm about to strap it
with suitable timber framing and board it. What's wrong with that?
Rob

[Andrew Gabriel]

In article ,
robgraham writes:
I know this is going slightly off topic, but why, Andrew, do you see
overboarding as a bodge ? I have an L & P ceiling that is cracked,
has been for the 40 years we've been here, and I'm about to strap it
with suitable timber framing and board it. What's wrong with that?

I guess I'm just too much of an anal perfectionist.

The existing L&P ceiling seems to be a bit heavy for the joist size/span.
From memory, I think they're 3x2's and probably about 4m long.
It sags now, but not noticably when you're in the room. I would be a bit
concerned how much sag would be added.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

[robgraham]

On Feb 28, 11:34*pm, (Andrew Gabriel)
wrote:
In article ,
* * * * robgraham writes:

I know this is going slightly off topic, but why, Andrew, do you see
overboarding as a bodge ? *I have an L & P ceiling that is cracked,
has been for the 40 years we've been here, and I'm about to strap it
with suitable timber framing and board *it. *What's wrong with that?

I guess I'm just too much of an anal perfectionist.

The existing L&P ceiling seems to be a bit heavy for the joist size/span.
From memory, I think they're 3x2's and probably about 4m long.
It sags now, but not noticably when you're in the room. I would be a bit
concerned how much sag would be added.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

OK, that's reasonable. My joists are 15 ft 8x3's. Don't think I've a
problem. Sorry to go OT :/

Rob

On 28 Feb,
(Andrew Gabriel) wrote:

Another option is replacing the ceilings, but taking down two large
lath and plaster ceilings is not something I like the idea of, and
overboarding always seems to me like a bit of a bodge.

How about painting the ceilings with an oil based impermiable paint. When I
did my extension the drylined surfaces were painted with gyproc drywall
topcoat as a vapour check coat. Don't know how effective it is, haven't had
any damp in the loft other than when the whole roof was iced up externally
and then thawed, leaking through nail holes in the sarking. Sometime soon I'm
going to have to strip off one quadrant of the roof to find the leak. Not
this weather though.

--
B Thumbs
Change lycos to yahoo to reply

[Andrew Gabriel]

In article ,
(Andrew Gabriel) writes:
In article ,
Roger Chapman writes:

Does this sound plausible?

Yes. (But see below).

Any bright ideas on fixing it, apart from rolling up the loft insulation
and blocking off the new air vents? It seems to me such a roof is
dependent on heat leaking from the house.

Are you sure that there is no leakage of air from the main part of the
house into the loft. Your increased ventilation may have the affect of
providing a pressure difference between house and loft and an influx of
warm humid air into a cold space will certainly cause condensation.

There are no holes punched through the ceiling. The loft hatch
and ceiling isn't going to be completely air-tight of course, but
it's no worse than it was for previous 25 years. Nothing has changed
in the house either regards producing moisture.

There will be some slight transfer of heat to the loft however deep you
make the insulation so I have my doubts about the roof managing to
radiate down to below ambient. Grass frosts typically occur below 3C but
the grass is almost isolated from the ground and even with the limited
heat coming up from the earth air temperature above bare ground is much
closer to zero before that attracts a frost coating.

With a similar problem in the past I have used EasyLog data loggers to
establish the extent of the problem but that was transient rather than
continuous through the winter. (EL-USB-lite IIRC).

The house has temperature logging, but not in the loft. So, I just
added a sensor in the loft and I'll compare it with the outside sensor.
Having come out of the loft and closed the hatch, the loft temperature
is rapidly dropping towards outside temperature, but too early to know
where it will settle yet.

It looks like we might get a couple of nights at 0C this week, but not
with clear skys, but I'll see what readings I get.

To follow up...

I did two things - more ventilation in the house, and some heating in
the loft to keep it 2C warmer than outdoors. (It was dropping below
outside temperature on some nights).

Within 10 days, the resistance of the rafters had increased to above
what my test meter could read, so they dried out quite quickly.
Some ~20 days later, I bought a damp meter, and that's showing the
timber moisture content at 17-18%, which seems to be a pretty normal
reading ffrom what I read. The other thing that's happened and
surprised me a bit, is that the fine surface mold which had appeared
has also started disappearing; I imagined that would stay even if it
didn't grow anymore, but seems not. No damp smell anymore either.
Timber is all still rock-solid.

Not yet done anything to change loft ventilation or provide a moisture
barrier at ceiling level, but at least one or other of the short-term
measures seem to have had an instant impact.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]