Flat roof insulation

[Paul_B]

Hi Guys,
Looking at replacing my flat roof, and need some insulation to put between the joists. Can I use polystyrene sheets to do this? As below

http://www.wickes.co.uk/invt/210801&bklist=

Many thanks

Paul

 

[Deluks]

That's the cheapest and less effective option. What is the roof covering, an outbuilding, garage, kitchen? If it's part of the house then you will need a minimum depth to meet regs (and save you money!) Plus it depends if you want a cold or warm roof. A cold roof means there is a void between underside of roof deck and insulation. This requires ventilation and gets a bit fiddly.

Anyway, back to your Q, a 'proper' job would use kingspan/celotex/xtratherm, not cheap but very effective, but you'll need thicker than 25mm for it to be effective.

Lots of good info here