Loft Insulation

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

Reply to
Dave
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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.

Sand them down (belt sander) and paint them.

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

Reply to
harry

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.

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.

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.

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.

Reply to
Tim Watts

What depth are the rafters?

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.

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

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.

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.

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.

Reply to
John Rumm

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!

Reply to
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

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.

Sand them down (belt sander) and paint them.

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.

Reply to
Dave

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

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

Reply to
Alex_b_ltd

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