Insulating a flat roof from inside

[yottie]

We have an extension with a flat felt roof, the difference between the extension and other rooms is about 8 deg F. The walls have foam lining. Is it possible to fit some sort of insulation to the ceiling without disturbing the boards.

 

[jeds]

Yes, as long as the flat roof void is vented. Just fix thermal plasterboard to the ceiling. Kingspan K18 or similar.

 

[yottie]

I seem to remember that the spaces between the joists had insulation blanket in them.

 

[jeds]

In that case you might run into trouble. Warm, moist air rises through the roof layers cooling as it goes. The moment it cools to x temperature it condenses into water. No problem if the roof is ventilated because the vapour will be diluted and will vent off. In your case the new insulation layers will alter the temperature through the layers and you run the risk that the vapour will condense below or within the mineral wool layer. If it does the mineral wool will become saturated with water.

 

[yottie]

Thanks (I think), is there no way I can do any-thing then.