On 14/03/16 23:51, Dave wrote:
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.

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.

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.

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

This to my mind also would give extra conductive insulation
if the air gap is sealed.

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

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.

Con...


Regards Dave

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