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Insulating Converted Attic Roof

  • 07-09-2010 11:55pm
    #1
    Southern Comfort
    Registered Users, Registered Users 2 Posts: 188 ✭✭


    Hi all. First post here. I'm currently going about insulating the roof of my converted attic. My two main problems are that my rafters are only about 100mm (4 inches) deep and I can't afford to lose any more head height internally because of a low pitch.

    Basically, I'm wondering how best to go about it, giving me the best U value possible. The guy who's going to do it has called a few times. I find him a good guy to deal with. The main option we've discussed is removing the tiles and felt, putting in 100 mm of Rockwool Flexi above the existing plasterboard ceiling and covering it with a breathable felt before replacing the tiles. That would give me a U value of around 0.38. The recommended one is 2.0.

    Another interesting option he came up with was to use a 9 mm sheet of Aerogel insulation, covered by a sheet of thin plasterboard on the inside. Even losing this much head height is a problem, however. Also, neither of us is sure what U value that would give me. I think it would be about the same as 25 mm of rockwool and, if that is correct, it wouldn't be great.

    The other options would be to use 100 mm of Rafterloc or Aerodorm, both of which are slitted and can be compressed. My rafters vary from about 22.5 inches to 23.25 inches apart. 30 year old house that could have been better built!!! They would probably have to be cut and doing it accurately would be difficult and time consuming.

    I'm presuming (rightly or not) that the breathable felt cuts out the need to allow a space above the insulation for air circulation...............

    What do you all think my best option is? I'm wondering if putting the 9 mm layer of Aerogel and compressing 100 mm of Rockwool Flexi on top of it would be a good option. Is it ok to compress Rockwool? I know it shouldn't be done to fibreglass insulation. A problem with doing it this way is the cost. Aerogel alone is around €30 + VAT per square metre but I'd consider it if I thought it would provide a good long term solution. It would give me the equivalent of about 125 mm of Rockwool (I think).

    I want to do the best job possible now and leave it at that, rather than doing something else again in a few years from now.

    I'd appreciate your thoughts or opinions or the experiences of any of you who had a similar problem.

    Thanks in advance.


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