Double-up celotex boards?

[light-boy]

Does anyone know if there are any reasons why you can't double-up celotex boards as a means to getting to the regulatory U-value?

(or anthing that has less thermal conductivity for the same or less thickness?)

Thanks

lightboy

 

[Mr Winston]

If you mean e.g if you need 100mm celotex floor insulation you could use 2 x 50mm celotex boards to = 100mm

ive never encountered a reason why not - has someon said something??

 

[light-boy]

No, no-one has said anything... It just seemed like an obvious opportunity to do what I need to do and then I thought.... is it too obvious???!

Based on my calculation I need 140mm celotex board to get to achieve the required U-value of 0.25 W/m²K, but they don't do them so could I use 2x70mm's or 1x100 and 1x40 to achieve the same value?

 

[Mr Winston]

sounds like you have a new flat roof to insulate, am i right? - i think either of those options would do the trick - you must also allow another 50mm over the insulation for ventilation to flow through your soffit vents so your joists should be 190mm deep minimum.
a better detail would be to underdraw 25mm of celotex below the joists and have 90mm between the joists = less insulation for better performance (i.e no cold bridges)

 

[chappers]

in fact 70 between and 25 under should be sufficient, just check with BC first though.

 

[Mr Winston]

celotex produce a 'handy guide' a5 booklet and ive quoted 90+25 out of therem - if you call them they will send you one

For some reason their website only quotes for warm deck roof. A warm deckroof is good if you have difficulty getting cross ventilation - its a special TD3080Z celotex board with ply backing 80mm thick and goes entirely over the top of the joists.