Flat Roof Insulation

24 Dec 2012
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United Kingdom
I have a couple of questions regarding flat roof insulation.

I have a house with dormer flat roofs either side of a pitched roof, you can see the void between the ceiling and roof, when in the loft and there is no insulation in there at all.

I have done lots of reading on this site and many others regarding insulation, air ventilation and condensation.

Ideally removing the existing roof and install a warm roof, but that is not an option.

The void in 130mm in total, so im planning to install some rigid insulation between the rafters at a depth of 70mm leaving an air gap of 50mm, the main pitched roof has ventilation. but as far as i can see there is no vents in the flat roof but they lead unobstructed into the pitched roof.

my only questions are concerning condensation:

1. At the moment the roof does not have any condensation, would adding insulation effect this?

2. would it be better to have the 50mm air gap between the plasterboard and insulation, or insulation and the deck of the roof?

Any help greatly appreciated.

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There is no point in having an air gap (cold roof) unless you can adequately ventilate the gap.

This gap would go on the cold side of the insulation i.e. directly below the deck.

You may be best off filling the roof void solid and fill any small gaps with foam or silicone and use foil tape in abundance as well as fitting foil backed plasterboard beneath.
Thanks for your reply.

Am i right in thinking as there is no condensation now, why this would change if i add the insulation, as presumably warm moist air is already going u through the ceiling and hitting the cold roof? This wouldn’t change with insulation?
If you are not going to take the ceiling down or the roof off, and you are just going to slide the insulation in the best you can, then leave a 50mm gap above the insulation.
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You dont have to remove the existing roof to create a warm deck.

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