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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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Products: Mixing and matching insulation good or bad?

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    Hi,

    I have been debating this idea with a colleague and would like some other opinions:

    Is it a bad idea to mix insulation such as mineral wool and PUR in a warm roof applicatIon. Specifically mineral wool between the rafters and t&g PUR over the top? I have always avoided this but I am being told that there is no problem, any thoughts?
    •  
      CommentAuthorMartian
    • CommentTimeApr 30th 2007
     
    Hi Mike,
    I think that putting mineral wool between roof joints and then solid insulation boards over the top, followed by boarding out the loft is about the most sensible uses of the different materials I can think of.
    My own is done like this and it works very well.
    You should use a vapour barrier on the warm side of the insulation either as foil-backed plasterboard on the first floor, or on retro-fits, just lay thin plastic sheeting over the joists and p/boards prior to fitting the mineral wool. Do not tape the joints on the insulation slabs so that they can allow out any small amounts of water vapour which may get in from below.
    You should probably also upgrade the wiring on the first floor lighting circuit to allow for the possible heat gain.
  3.  
    I am a great believer in layering different materials, each material type works best in a certain position (and size), like mineral wool between timber members and sheet material inside and out to reduce any thermal bridging. By layering you also have surface resistance of each material and pockets of trapped air which contributes to a higher effective thermal resistive area. Also can be a little more forgiving with average installation standards.
    • CommentAuthorGuest
    • CommentTimeMay 7th 2007
     
    After having my bungalow roof re-felted I began to experience a lot of condensation in cold weather. It seemed that the 'breathable' membrane trapped the moist air in the roof space. I initially tried to insulate the roof with 'Kingspan' but this seemed to make the condensation worse, so bad in fact the rafters began to soak up the moisture. It was obvious that the air gap between the insulation and the membrane was to blame so I put a layer of rockwool/fibreglass between the Kingspan and the membrane. I then covered the area with cardboard as a temporary measure to see if it worked. It seems to work very well and I intend to plasterboard now. The house has certainly seemed much warmer this Winter and savings on heating are noticable.
    • CommentAuthorGuest
    • CommentTimeMay 7th 2007
     
    I would tend to go for using the rigid board insulation to the inner face of the timbers, mainly because this is the correct ordering of materials with regards to vapour permeability. Having some thickness of mineral wool (which has almost no water vapour resistance) then following this with any rigid board material (XPS,PUR,PIR etc) which all have much higher water vapour resistance could lead to condensation forming on the inside face of the rigid board. May only be a theoretical risk, chances of this occuring are somewhat reduced by using a vapour barrier on the warm side of all insulations. Using a laminated plasterboard will typically offer the quickest thermal response time, also the rigid boards should be more air tight again offering improved thermal performance.

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