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Insulating a flat roof


I have a flat roof extension built in the 1970s. Due to a rewire I've had to remove parts of the plasterboard ceiling and have found that there is no insulation in the roof - I can see the roof deck and joists, there is no vapour layer and the plasterboard is not foil-backed. I am after advice as to what the best options are to insulate the roof - joists are approx 400-500mm apart and the ceiling will be overboarded and skimmed in the nest few weeks.

3 Answers from MyBuilder Insulation Installers

Best Answer

Hi Dave

The correct method for a flat roof is under the current regulations to have a warm roof. You would only do this however if replacing the roof covering on top and you have the height above the roof to do this.

In a nutshell you would sandwich 130mm of solid insulation between two decks allowing to place a vapour barrier on top of the first deck. The roof needs to be unventilated to comply with the British Satndards which also states that there should be no insulation within the void (dotting the i's and crossing the t's).

Anything hybrid and outside these parameters may give rise to interstitial condensation within the roof void based on the room being a habitable zone during the coldest months of the year (normally Jan/Feb).

It all depends on the use of the room, your concerns regarding energy efficiency, premature failure and budget.


Drew @ Fibretechs


Answered 28th Apr 2016

you could put foil backed insulation between the joists then cover with insulated plasterboard
good luck Alex


Answered 26th Apr 2016


I've had a similar issue in the past and I used Kingspan.

The current building regulation for a roof is 0.18. Therefore for a flat roof you would need 100mm of Thermpitch TP10 and 52.5mm Kooltherm K18. Note that the Kooltherm is 40mm insulation and 12.5mm plasterboard attached together.

AVP Property Maintenance


Answered 28th Apr 2016

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