Foil Insulation for underside of flat roof

[jonnybegood]

Hi Guys, looking for some advice

I have a cold roof constructed flat roofed dormer that I need to insulate after ripping down the existing ceiling and finding no insulation present.
There is so much choice of insulation available and then there is concern around vapour control.

I have measured the depth of the rafters and they vary between 200mm and 170mm, but the biggest problem is that they are not the same width along, 400mm at majority of places but narrowing down between 300 - 330mm at certain points were whoever constructed the roof used additional support, plus there are nails are all sorts of different pieces protruding along the length of the rafter.

So inserting the celotex type insulation could be quite difficult as it would be difficult to tight fit it along the full length of the rafter and would involve alot of cutting around these narrowing points.

This weekend in a moment of madness I have ordered superquilt foil insulation with the intention of stapling to underneath the rafters and not bothering between the rafters.

There seems to be plenty of ventilation below the ply deck and there will be up to 200mm of ventilation space above the foil and below the deck.
So after stapling the foil to the rafters and taping the joins I will then be screwing 2x2 timbers to the rafters to level the ceiling and to allow a 50mm gap below the foil as per man instructions.
Then finally finishing off with foil backed plasterboard and skim finish.


Concerns:
Will it work? Have I done enough to stop moisture getting to the ply deck and rafters.

Foil insulation seems to be recommended for pitch roofs but not flat roofs, why??

Will I be better off sticking with the celotex or some other form of insulation rockwool etc.

Any experience on here of using the foil, is it effective??

Cheers

 

[Deluks]

As long as there are actual soffit/fascia vents letting air into the space above the insulation then you'll be ok, you can add more insulation and reduce the airgap if you like, stuff some loft roll or mineral wool slabs up there before you add the superquilt, leave a good 100mm space between insulation and deck. 50mm is considered a minimum airgap, but a bit more won't hurt.

 

[jonnybegood]

Thanks for that...is it worth the effort of buying/ fitting 50mm of mineral wool for beteen the rafters..my rafters are max 200mm deep with a 50mm air gap above the foil (man guidelines)and 50mm of mineral and then 100mm gap above mineral to ply it gives a total of 200mm, so 50mm is max i can use, i know 50mm is better than nothing but is it worth the effort, if i put 50mm celtex boards then costs will rise considerably.

 

[mointainwalker]

Any experience on here of using the foil, is it effective??

Foil is very controversial.

It does not pass standard European tests ( including BRI )

These foils originated in France but a report from the French CTSB ( BRI equivalent) has said that they are enormously difficult to install sufficiently well to insulate "reasonably" and even then only gave approx 40% of the claimed insuklation value. This refered to Tri-Iso.

I was surprised to see that they insulated as well as 40% of claimed value.