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Insulation for Warm/Cold Flat Roof

rhennessy · 2009-12-16 19:06:36

Hi, I am currently constructing a warm flat roof (insulation outside the hollow-core slabs) where the roof will serve as a patio area that can be accessed from the first floor living-room. Due to the height between the top of the hollow-core and the FFL of the first floor (as I also need to apply screed for falls, asphalt,…

16-12-2009 7:06pm

#1

rhennessy

Registered Users, Registered Users 2 Posts: 21 ✭

Join Date: January 2009

Posts: 11

Hi,
I am currently constructing a warm flat roof (insulation outside the hollow-core slabs) where the roof will serve as a patio area that can be accessed from the first floor living-room.

Due to the height between the top of the hollow-core and the FFL of the first floor (as I also need to apply screed for falls, asphalt, paving slabs, etc.), I will only manage approx 100mm insulation which will give me a U-value of 0.23, not even meeting the regulations of 0.22. To achieve a U-value of 0.16 I would require approx 150mm which I won't be able to accomodate. I realised too late that I did not have enough between flat roof and first floor FFL so I am stuck with this.

However, as I am constructing a warm deck roof where the hollow-core slabs are inside the insulation, the dew point is outside the structure and the risk of condensation (and damp) is greatly reduced.

Cold flat roof construction is frowned up due to the insulation being below the deck but what if I added insualtion below the hollow-core slabs as well as above it? Does anyone see this as causing a problem? If not, then this would allow me to achieve a far better U-value than the 0.23 I am currently looking at.

Any comments appreciated!

Thanks

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