flat roof insulation

[moonlightandjj]

Hi
Can anyone tell me if you can use that polystryrene (sp) type insulation as roof insulation in a flat roof. And is so will it pass building regs? I have seen jablite 100mm thick and thought about using 150mm + in my new extension roof.

I have already been quoted for celotex and so far the price has come in over £990, which seems so expensive for roof insulation. I have been told that I need to have 150 mm of celotex and for that I need to double up a 75mm board @ £33 + vat - which means I need 30 boards - what a rip off if you ask me! My bricks, 3 windows,door, poured cement only cost £200 more for my extension.

I was wondering if I could use the jablite, even if I used 200m it would still only cost me £200 saving me £700.

So any help would be gratefully recieved.

 

[noseall]

are you talking full fill or warm deck?

b.c will allow a single layer of ply backed insulation sheet to be used straight over the top of the roof joists. you can then apply the roof covering i.e felt, rubberized sheet etc. to this ply layer.

i think the overall thickness is 100mm ish.

the u value for the last job i did was 0.25.

 

[jeds]

You will need closer to 200mm of expanded polystyrene to achieve a U-Value of 0.25

PIR foam (i.e. Celotex) is approximately twice as good thermally - and therefore you would only need half as much. In fact 90mm gives you about 0.24 so on the right side. (don't know where your source got 150mm from?)

Do you have the space for 200mm?

 

[moonlightandjj]

Hi
thanks for your replys. I am putting the insulation between the joist. It was Celotex tech advisor who told me I needed 150 mm thickness. I gave them a ring ( got the number off their website and they took details of my joist sizes etc and they came up with: 50mm air gap, 150mm plus vapour barrier then 12.5 mm plaster board which will give me a u value of 0.2.

I thought it would it would be something simple like you do in the loft...lol. just get some insulation and stick it up - but oh no- how wrong can you be.

So now I have no idea. Do I need 150mm or just 90mm? I have 75mm celotex under my concrete subfloor.

Different manufactures gave me different quotes most mean adding battens to make my 8"x2" rafters even deeper, even doing that it would still be cheaper then buying 150mm of celotex.

I am so confused.

What do I need?? Do I really need 150mm of celotex or just 90mm, in that case I would buy 90mm celotex, if not go for cheaper option save £££££'s

Help

 

[oddballodd]

I recently converted my garage which had a flat roof portion. BC were happy with 50mm cross ventilated air gap plus 75mm Xtratherm (like celotex but much cheaper)...

 

[noseall]

if you fit between the joists then full fill with partial air gap. if you go across the top or the joists then 90mm ply bonded celotex or kingspan should do the trick.

 

[noseall]

i'm not entirely sure all b.c's enforce it, but the last flat roof full fill i did we also had to underdraw the joists with 25mm celotex too. this was done prior to plasterboarding.

they said it was to prevent cold bridging through the joists.