I'm in the process of loft insulating, not to meet building regs on heat
loss, but trying to get an occasional play space that can be heated. I am
going to put 75mm celotex under the rafters which are topped by a non
breathable membrane and tile roof..I will probably finish with plasterboard
one day. I have bought the celotex so I am not buying plasterboard topped
foam in case anyone advises me to.

My questions:

Is there a nice easily applied cheap gooey adhesive for sealing the joints
between the insulation boards?

There are 4 large purlins across my loft space: Do I insulate over them, I
do not really want to add 150 mm to their dimensions. Would it be reasonable
to seal them with a plastic membrane making them part of the cold space and
plasterboard over them., I'm worried about warm damp air getting into the
timber and condensing part way through. If I did the latter, they would act
as a cold bridge but they shouldn't become damp with rising warm air and the
heat loss through them should be small.

I see a similar problem with the floor joists below . Insulation will come
down the inside of the roof to the inside leaf of the cavity wall but will
seal in the floor joists so that the joist will be part in cold space and
part in warm with the potential for warm air to cause condensation in the
cold timber. There are also the occasional vertical timbers with top in cold
and bottom in hot space.What do the experts do with both these situations.

Dave

On Monday, 17 October 2016 00:31:36 UTC+1, Dave wrote:
I'm in the process of loft insulating, not to meet building regs on heat
loss, but trying to get an occasional play space that can be heated. I am
going to put 75mm celotex under the rafters which are topped by a non
breathable membrane and tile roof..I will probably finish with plasterboard
one day. I have bought the celotex so I am not buying plasterboard topped
foam in case anyone advises me to.

My questions:

Is there a nice easily applied cheap gooey adhesive for sealing the joints
between the insulation boards?

Cans of fixing foam.
Cheapest I know is"No no nonsense" from screwfix, around£3/can.
Cut foam undersize,support temporarily on nails & fill the gap.


There are 4 large purlins across my loft space: Do I insulate over them, I
do not really want to add 150 mm to their dimensions. Would it be reasonable


Sand them down (belt sander) and paint them.


to seal them with a plastic membrane making them part of the cold space and
plasterboard over them., I'm worried about warm damp air getting into the
timber and condensing part way through. If I did the latter, they would act
as a cold bridge but they shouldn't become damp with rising warm air and the
heat loss through them should be small.

I see a similar problem with the floor joists below . Insulation will come
down the inside of the roof to the inside leaf of the cavity wall but will
seal in the floor joists so that the joist will be part in cold space and
part in warm with the potential for warm air to cause condensation in the
cold timber. There are also the occasional vertical timbers with top in cold
and bottom in hot space.What do the experts do with both these situations..

Dave

Roof space conversions normally involve installing "hangers" from a purlin to a timber nailed to the ceiling rafters at right angles to them and immediately below the purlin.
This strenthens the new "floor" in the roof space.
This leaves a triangular space that can be used for storage.

On 17/10/16 00:31, Dave wrote:
I'm in the process of loft insulating, not to meet building regs on heat
loss, but trying to get an occasional play space that can be heated. I am
going to put 75mm celotex under the rafters which are topped by a non
breathable membrane and tile roof..

Then you *must* ensure a 2" air gap over the top of the celotex and
ventilation at the eaves and up on or near the ridge.

You said "under" so I assume you'll have a 3-4" air gap. You still need
soffit and ridge/tile vents.

These can be dispensed with if you have breathable membrane.

I will probably finish with plasterboard
one day. I have bought the celotex so I am not buying plasterboard topped
foam in case anyone advises me to.

My questions:

Is there a nice easily applied cheap gooey adhesive for sealing the joints
between the insulation boards?

Yes. Expanding foam. I recommend fire rated Screwfix foam. Ordinary
canned foam is quite flammable and I wish I'd used the fire retardant
stuff throughout. I switched to it after I did some tests.

There are 4 large purlins across my loft space: Do I insulate over them, I
do not really want to add 150 mm to their dimensions. Would it be reasonable
to seal them with a plastic membrane making them part of the cold space and
plasterboard over them., I'm worried about warm damp air getting into the
timber and condensing part way through. If I did the latter, they would act
as a cold bridge but they shouldn't become damp with rising warm air and the
heat loss through them should be small.

I would not worry about the trivial amount of moisture going through the
purlins. But you can always varnish them to reduce this. I have a number
of timbers that bridge my celotex unavoidably.

I see a similar problem with the floor joists below . Insulation will come
down the inside of the roof to the inside leaf of the cavity wall but will
seal in the floor joists so that the joist will be part in cold space and
part in warm with the potential for warm air to cause condensation in the
cold timber. There are also the occasional vertical timbers with top in cold
and bottom in hot space.What do the experts do with both these situations.

With ventilation on the cold side, it is unlikely to be a problem. My
floor joists do exactly the same.

My work has been under the direction of Building Control, which is where
all the information re ventilation came from.

Dave

On 17/10/2016 00:31, Dave wrote:

I'm in the process of loft insulating, not to meet building regs on heat
loss, but trying to get an occasional play space that can be heated. I am
going to put 75mm celotex under the rafters which are topped by a non
breathable membrane and tile roof..I will probably finish with plasterboard
one day. I have bought the celotex so I am not buying plasterboard topped
foam in case anyone advises me to.

What depth are the rafters?


My questions:

Is there a nice easily applied cheap gooey adhesive for sealing the joints
between the insulation boards?

Expanding foam - get the gun type rather than the cans with plastic tube
on top, and its much easier to apply and also not waste part used cans.

There are 4 large purlins across my loft space: Do I insulate over them, I
do not really want to add 150 mm to their dimensions. Would it be reasonable
to seal them with a plastic membrane making them part of the cold space and
plasterboard over them.,

You could - or just apply a (relatively impermeable) finish of some kind
to them.

I'm worried about warm damp air getting into the
timber and condensing part way through.

You are not going to get much moisture passing through the timber. Its
normally closed off (aka interstitial) spaces that you need to worry
most about.

If I did the latter, they would act
as a cold bridge but they shouldn't become damp with rising warm air and the
heat loss through them should be small.

Also at 150mm thick, you are going to have a u value for the purlin of
about 0.9 W/m^2K which while not "super insulated" is not actually that
bad.

I see a similar problem with the floor joists below . Insulation will come
down the inside of the roof to the inside leaf of the cavity wall but will
seal in the floor joists so that the joist will be part in cold space and
part in warm with the potential for warm air to cause condensation in the
cold timber. There are also the occasional vertical timbers with top in cold
and bottom in hot space.What do the experts do with both these situations.

Ventilation on the cold side is the key. Normally when you have a non
breathable sarking, soffit and ridge vents provide this. Depending on
the fit of the tiles, you may be able to get away with just cutting away
a strip of the sarking near the ridge (you get a fair amount of airflow
through traditional pan tiles - but less through modern closer fitting
concrete ones). For soffit vents you may find the round plastic push in
ones are easy enough to install - just put a small hole saw through the
soffit, and then fit the vent.



--
Cheers,

John.

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In message , Tim Watts
writes
On 17/10/16 00:31, Dave wrote:
I'm in the process of loft insulating, not to meet building regs on heat
loss, but trying to get an occasional play space that can be heated. I am
going to put 75mm celotex under the rafters which are topped by a non
breathable membrane and tile roof..

Then you *must* ensure a 2" air gap over the top of the celotex and
ventilation at the eaves and up on or near the ridge.

You said "under" so I assume you'll have a 3-4" air gap. You still need
soffit and ridge/tile vents.

These can be dispensed with if you have breathable membrane.

Slight aside and no help to the OP. Modern breathable membrane comes in
different weights related to batten spacing and expected wind speed.

I have just had a row with my builder because the only version supplied
by his builders merchant is only suited for small tiles or taped seams!

--
Tim Lamb

Thanks for the contributions I now feel I'm going in the right direction.
With respect to ridge vents, anyone got any links or supplier suggestions. I
had thought of putting vents in the side walls as there will be a pretty
large cold side space at the roof apex on the other hand this might force
the major air flow to be at the building walls and not well distributed. so
I'll see.

Regards

Dave
"harry" wrote in message
...
On Monday, 17 October 2016 00:31:36 UTC+1, Dave wrote:
I'm in the process of loft insulating, not to meet building regs on heat
loss, but trying to get an occasional play space that can be heated. I am
going to put 75mm celotex under the rafters which are topped by a non
breathable membrane and tile roof..I will probably finish with
plasterboard
one day. I have bought the celotex so I am not buying plasterboard topped
foam in case anyone advises me to.

My questions:

Is there a nice easily applied cheap gooey adhesive for sealing the joints
between the insulation boards?

Cans of fixing foam.
Cheapest I know is"No no nonsense" from screwfix, around£3/can.
Cut foam undersize,support temporarily on nails & fill the gap.


There are 4 large purlins across my loft space: Do I insulate over them, I
do not really want to add 150 mm to their dimensions. Would it be
reasonable


Sand them down (belt sander) and paint them.


to seal them with a plastic membrane making them part of the cold space
and
plasterboard over them., I'm worried about warm damp air getting into the
timber and condensing part way through. If I did the latter, they would
act
as a cold bridge but they shouldn't become damp with rising warm air and
the
heat loss through them should be small.

I see a similar problem with the floor joists below . Insulation will come
down the inside of the roof to the inside leaf of the cavity wall but will
seal in the floor joists so that the joist will be part in cold space and
part in warm with the potential for warm air to cause condensation in the
cold timber. There are also the occasional vertical timbers with top in
cold
and bottom in hot space.What do the experts do with both these situations.

Dave

Roof space conversions normally involve installing "hangers" from a purlin
to a timber nailed to the ceiling rafters at right angles to them and
immediately below the purlin.
This strenthens the new "floor" in the roof space.
This leaves a triangular space that can be used for storage.

replying to Dave, Alex_b_ltd wrote:
Hi Dave,

sounds like a good project you have here, I think you could probably avoid
both cost and hassle on this though. I noted you said you were putting Celotex
under the rafters and then a membrane and plasterboard at a later date?

You can always put up a thermal laminated insulation board - this comes backed
onto plasterboard. From here all you'll need is to include a breather
membranes under the rafters. Celotex insulation actually comes with a foil
facing so this prevents vapour or moisture leakage. If you seal up all joints
with a good quality insulating foil tape then you avoid any thermal bridging.

From experience I've used the Celotex thermal laminate, it's called PL4000.
Best price i've seen is here
http://www.insulationgiant.co.uk/spo...tex-insulation

it'll probably be the Celotex PL4060 that you'll need. I hope this helps mate

--
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