Flat roof insulation

[breako]

Hi,

We are in the middle of building a flat roof extension. Builder is doing 150mm rigid insulation warm roof insulation 

 

In the actual room there are 300mm joists and engineer has advised to make this even better to add 100mm wool or fibre glass.

 

A friend is saying this could cause condensation but I am not sure how 

 

Could anyone advise?

 

Thsnks

[atlantication]

There is an increased chance of condensation in the internal wooden structure with a so-called hybrid roof, as moisture can travel through the mineral wool and the wood is now cold (as it's in the middle of the insulation temperature gradient). This contrasts to a warm roof where the wood is warm so there cannot be condensation, or a cold roof where there the moisture barrier should stop condensation, and it's ventilated. There would be no way for the moisture to escape your roof except the same way it got in - back to the inside in summer when the thermal gradient works the other way around.

 

However, looking at your particular insulation thicknesses, it doesn't look like condensation is actually going to happen - see attached screenshot.

[Gus Potter]

3 hours ago, breako said:

Hi,

We are in the middle of building a flat roof extension. Builder is doing 150mm rigid insulation warm roof insulation 

 

In the actual room there are 300mm joists and engineer has advised to make this even better to add 100mm wool or fibre glass.

 

A friend is saying this could cause condensation but I am not sure how 

 

Could anyone advise?

 

Thsnks

It's a fair point until you look at the extra cost particularly labour wise of installing the glass wool under the warm roof. For the glass wool layer to work properly it needs to be really carefully fitted and that take time = cost.

 

Can you just make your PIR 200 mm thick and it will have probaly the same effect as you do away with the repeating bridges of the roof joists.

 

I have done this on my own house where the warm roof is fixed together with glue not mechanical fixings. But lots of folk want a mechanical fix.. finding the fixings it a bit tricky for 200 mm PIR.. but it can be done.

 

 

 

 

 

 

[Dave Jones]

just a heads up 150mm celotex and 100mm wool would fail new build requirements for a warm roof as its right on the limit of 0.12 against a limit of 0.11. Obviously you can do a worse job on refurb on extension although don't know why.

 

If you don't have the height available as the architect didn't plan for it drop the PIR boards down into the rafters as well, use gappo tape on the cut edges to get a tight fit and don't forget to alutrix or other vapour barrier under the whole lot lapped down onto the internal walls.

[breako]

Thanks for feedback.  The warm roof u-value guideline is now 0.18, I think  https://www.enviroplyroofing.co.uk/u-value-changes-for-flat-roofing/#:~:text=This type of insulation is,need to increase to 150mm.

 

So if you have150 rigid on top, which is thermal conductivity 0.022 / 0.150 = 0.146 which is well below 0.18.

 

So wondering, is it worth the risk of adding more fibre glass underneath? I don't think it will make much difference and just incurs a risk. 

 

Thoughts appreciated.