Insulating a loft conversion

[Francis]

Hi,

We are having the loft converted to provide more living accomodation. Box style with flat roof at the back and pitched roof at the front. The sides are being done with celotex but parts of the pitched roof are being done withwool and covered with plasterboard. There seems to be a max of 3 to 4ins thickness of the wool. Should we be looking to getting a greater thickness or asking for celotex instead? What sort of vapour barier would be needed?

With regard to the flat roof - there are 6in joists with an angled piece of woo atop each to provide a slight runoff angle. What sort of gap or barrier do wee need here and would a double thickness of celeotex be used here? (The stuff they have at the moment is 3ins thick.

Sorry if this is basic but it is new to me and I want to get the best job done now - the pitched roof in particular is north facing.

TIA

[Stuart Noble]

If the roof has a breathable membrane under the tiles, apparently you don't need ventilation between celotex and tiles, so in theory you could use double thickness. Wool is cheaper and easier, especially if the joist spacings aren't uniform. Whether this complies with building regs and/or your contract with the builders is another matter. Query it and see what they say.

[John Rumm]

What you do depends a bit on whether you have a breathable sarking under the tiles. If you do then you can fully fill the gap between rafters. Otherwise you need to leave a 50mm gap and arrange for soffit and ridge ventilation.

If use just wool type insulation then a foil backed plasterboard would be appropriate.

When I did mine I used 50mm foiled foam insulation between the rafters (with 50mm gap behind) and 35mm under them, followed by the plasterboard. That thickness was slightly in excess of what was required to meet building regs at the time.

Firrings being the posh term for the tapered bits.

Is it on top of the firrings, or just in the gaps? The former is what they call a "warm deck" construction - 75mm is about the minimum in this case IIRC.

Might be of use:

[Francis]

I am told it is "breathable membrane"

No insulation has been put in here yet and the roof is on. So, IIUC, a gap between the roof and the insulation is required, so enough room for about 100mm of insulation directly above the ceiling ?

Thanks for the avice to you and Stuart.

Regards,

Francis

[John Rumm]

That is a cold deck roof then. No requirement for a gap that I am aware of since the space is probably not ventilated anyway. With these the gap between the rafters is usually just stuffed with insulation.

Many of the insulation manufacturers have quite handy "how to" guides that may make interesting reading. For example:

[Hugo Nebula]

On Sun, 25 Nov 2007 12:13:45 +0000, a particular chimpanzee, Francis randomly hit the keyboard and produced:

This should have a Building Regulations application, and if it does, insulation and ventilation of the roof are two of the things covered.

Mineral fibre is not a good insulant for the roof. You would need at least 200mm between rafters, with all the attendant problems of how to maintain the 50mm gap over. PIR (Celotex, Kingspan, Xtratherm, etc) would be needed between and under the rafters (IIRC, 50mm & 70mm respectively).

If it's a habitable room, then taping the joints of the PIR insulation is usually enough. If it's a bathroom/showerroom, then a separate polythene vapour barrier is usually recommended.

With the flat roof, you would probably need 140mm PIR between the joists, with a minimum 50mm gap over at the lowest point (ignoring the firrings or sloping piece of wood).

Get in touch with Building Control now before it's covered over and you have to undo it all.