warm flat roof question

[seka1980]

Hi all

can anyone tell me if its OK to use insulation between and under the joists on a warm flat roof as well as the usual one's on top ( apart from extra cost is there any down side?)


im building a garden office, so there's no building regs involved and I tend to do things over kill


Thanks

Jon

 

[sweeney90]

The idea of a warm roof is that the insulation placed on top of the joists, therefore the roof requires no ventilation. This is because there is very little risk of condensation build up. A properly installed warm roof includes a vcl above the osb also.

Once you start adding insulation between joists, you will then need atleast a 25mm air gap above the insulation, which will need to be fully ventilated in every bay. The idea of adding insulation under the joist is to stop cold bridges where the joists are. Although timber is a fairly good insulator, it does not compare to modern requirements, as products like PIR board.

There is no reason you cannot do both with the right ventilation, however I'm sure 150mm of PIR will be fine between alone.