Celotex type insulation.

[Dave]

Hi

I'm planning to insulate my loft with insulating board for occasional use as an office.

Need some help on use of foil; backed insulation products. Looking at Celotex website they recommend a 2" gap between old fashioned non permeable roofing felt and the Celotex fixed in between rafters, to allow ventilation. In my case with only 3" rafters means that I can only use the 25 mm insulation board. Does anyone have experience of using a smaller gap say 1" to allow thicker insulation between rafters and does this lead to condensation problems.

Second I understand the foil on Celotex acts as both an impermeable membrane avoiding condensation, and a thermal barrier by reducing radiation loss. Does this radiation loss reduction work when for example 2 celotex boards are mounted on top of each other or is an air gap required for the foil to work effectively.This to my mind also would give extra conductive insulation if the air gap is sealed.

Does anyone have experience on using multi layer foil insulation, it seems to give only a small thickness but expensive, but main thing does it work. the manufacturers seem to claim better insulation performance than board products but only give very overall figures in combination with celotex boards and not for foil used alone.

Regards Dave

Please note only respond by newsgroup as email not working.

[The Natural Philosopher]

That is really almost irrelevant.

Yui can line with foil backed pboard if you really care.

Forget the radaiation. Its am almost irrelevant issue.

If you put celotex between and then celotex over it will be fabulous.

However even an inch is WAY better than nothing. and the key is toi totally get rid of draughts from e.g. eave and ridge vents and let that lot circulate above the 'tex.

so use decorators caulk, extending foam and the foil tape to really eliminate all the draughts and the result will be well good enough for occasional use even with an inch.

Id p'board over fill and sand and then use lining paper and cheap emulsion for a basic 'room' finish

[tabbypurr]

Foil does not insulate. Multifoils have poor insulation, the manufacturers game the specs. You need foil on the warm side to avoid interstitial condensation.

NT

[Tim Watts]

2" is the minimum *unless* you have breathable membrane, then it can be reduced to 1". Straight from my building inspector's mouth.

If you want more, you'll have to go under the rafters.

In that case it makes no difference. Just layer them.

The celotex is a better insulator than air (it cannot convect for one). Don't leave air gaps between sheets.

Con...

[David]

Slightly OT - the head office is in Hadleigh, not that far from us, so you would think that you could nip over for some seconds.

Looks like they don't have a factory seconds shop, though :-(

Cheers

Dave R

[David]

Reading stuff about insulating the rafters, there seems to be an implication that there will be ventilation at the ridge as well as the eaves.

If I insulate, there will be a ventilation gap beneath the tiles and membrane from eaves to ridge and back to eaves, but no ridge vents (unless I pay someone to go up on the roof).

How necessary are ridge vents?

I can see that warm moist air (for example from a sunny day in winter) would rise up through the 50mm gap between the insulation and the roofing but potentially have nowhere to go, unless there is an air flow across the roof down to the opposite eaves.

A requirement for ridge vents seems to change this from a simple (possibly in slow stages) DIY project to one requiring the services of a roofer.

Cheers

Dave R

[Tim Watts]

Necessary unless you have breathable membrane. Otherwise the air cannot circulate and is trapped (with breathable membrane it is sufficient that moisture diffuses through the membrane and out between tile joints.

I faced that dilemma too. Solved as the roof was more knackered than I thought, so I got it re tiled, adding breathable membrane at the same time and additional ventilation on the flat roof section.

[The Natural Philosopher]

Not really necessary at all

[The Natural Philosopher]

The point being that with celotex in, there wont BE any warm moist air. Any rain will run down towards the eaves vents.

[Martin Bonner]

Well, not unless you want to prevent condensation on the timber rafters. (Or, to put it slightly less elliptically, "Actually quite necessary").

[Tim Watts]

And the cold moist air?

Your advice seems to be at odds with my discussions with the BCO.

[The Natural Philosopher]

Where does the water come from then?

(Or, to put it slightly less elliptically, "Actually quite unnecessary").

[The Natural Philosopher]

Cold air isn't moist enough to condense of rafters that are no colder than it is.

What about 'condensation requires warm moist are to come into contact with *colder* surfaces' do you fail to get?

If the insulation is done properly, the rof beyond and the air beyond are all at the same temperature, and you already have an air gap from eave to eave (or if you don't, don't take the insulation right up to the ridge top - leave a cross roof space.

the space beyond the insulation is simply 'outside' and will no more rot than an open barn with a roof will.

BCOs don't have degrees in engineering by and large.

Worthy souls though they are.

[Tim Watts]

My shed had no humans living in it. Nor does it have a source of water vapour other than the air.

However, if I leave it all winter, during the worst months it will have condensation running down the windows, mould growing on odd bits of ply and tools will go rusty.

That was last year - before I starting running a dehumidifier in it.

Warm air gets in, cold makes it condense, without adequate ventilation the water remains liquid and makes things damp.

I believe the same thing is likely to happen in a roof unless you have an adequate air path.

The BCO advised me to ensure I had a through air path under my flat roof decking as I was sealing the inside up with celotex. He even advised mushroom vents either side of teh chimney as that was blocking the natural air path from the north soffit vents to the south soffit vents (being continuous airpath inside).

[Tim Watts]

See my other post re: shed.

Now let me see. Listen to the BCO with his knowledge of the regs and years of experience in this field - or someone on the internet?

[The Natural Philosopher]

well it cant be ventilated then

My garage is the same, but has none of that

[The Natural Philosopher]

It is obviously badly ventilated.

Or more likely is not even waterproof.

[John Rumm]

Ideally you would need ridge and soffit vents.

You will need *some* ventilation at the ridge. This could be swapping in some vent tiles toward the top between each pair of rafters, or a continuous vent bedded in under the ridge itself.

If you can't manage that, then you could cut away a 6" strip of the underfelt at the top of the roof just before the ridge to allow some air to pass through the tiles - but it may not be adequate.

You might just be able to install a vent tile from the inside...

[Tim Watts]

Reversing your lines to answer:

It's *very* waterproof thank you. It has tyvek under the cladding and a felt roof in excellent order plus moderate roof overhang. The door fits well.

Thank you!

That is the whole point I have been making. And the same will happen to the OP's roof if he doesn't have an air path right through (ie ridge or top tile vents in his case).

By the way - I just noticed that my BCO does hold a BSc(Hons). Not sure what in, but likely a relevant subject as he has it in his email sig. I'll ask him next time.

[Andrew Gabriel]

Trouble is, a well insulated roof is often colder than the outside air (actually, always colder with a clear night sky), and does get condensation on it, both inside and outside. The stuff inside needs to be able to get out later.

That's the way it works.

Unfortunately, that's not true. A roof has a much higher emissivity than the air, and so radiates its heat away into space much faster. That's why dew forms on (and under) roofs which are well insulated from the house heat, particularly in winter when the the air is 100% humidity much of the time, so any temperature difference generates condensation.