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Housing - Renovation: Insulation option with extension roof

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    Hi,

    I am building an extension and have done the walls, and I have the rafters in place.

    I have phenolic boards that will go across the rafters underneath, to deal with the cold bridging issue.

    I have some YBS reflective breather foil which will go across the top of the rafters.

    These items were available pretty cheap, which is why I am basing my insulation choice around them.

    I want to completely fill between the rafters ( 200 mm) with glass/rockwool/cellulose - whatever. BCO has approved this plan.

    I am quite keen to not be filling between the rafters from underneath, and am planning to fix the phenolic boards under the rafters with glue/big headed-clout nails so that I can put the between the rafters stuff in from above, onto the boards, before rolling the breather foil over the top. I have a few days of dry weather ahead...

    I guess loose-fill stuff would be easiest to use, but I have read that settling undermines its performance.

    Recommendations for material to use and whether to use loose-fill or batts would be very welcome,

    Thanks,

    John
  2.  
    I forgot to mention that the roof has a 15 degree pitch, making the loose-fill method pretty easy.
    • CommentAuthortony
    • CommentTimeSep 10th 2007
     
    What will be the consequences of the loose fill blowing arround? Horrid so dont use it.

    Reflective so called breather foil is not a good idea either

    I dont think that 'they' will let you fill the rafters right up and you may well find that you need a ventilation gap somewhere.
  4.  
    If I add the loose fill when there is no wind and lay the breather membrane over it... or is the stuff so light that a gentle breeze would blow it about?

    'Reflective so called breather foil is not a good idea'

    Can you expand on that?

    The breather foil is like bubble wrap, and you lay it down with the bubbles downwards. Air can escape between the overlaps. Normally, the foil is laid over sarking board, but I have spoken to someone at YBS who told me that the purpose of the sarking board was to prevent the space between the bubbles being clogged with mineral wool. I have a spare roll of breather membrane ( Nilvent ), and they said I could use that to roll out over the mineral wool, then lay the foil over that. BCO reckons that is OK, and using BuildDesk, I see no risk of condensation.
    • CommentAuthortony
    • CommentTimeSep 10th 2007
     
    'Reflective so called breather foil is not a good idea'

    Can you expand on that?

    Read the threadabout multifoils!!!
  6.  
    I have read quite a bit of that thread - has anyone read all of it??

    I am not relying on the breather foil for insulation - just whatever the foil part of it helps, fine.

    I have 25 mm phenolic boards under the rafters, 200 mm of glass or rockwool between the rafters. I reckon the breather foil is just a bonus, for as long as the reflective surface lasts. I have gone this route because I was doubtful of the longevity of multifoil ( from that thread ), and didn't want to rely on it.

    Doesn't this seem to be enough insulation - I am in a sheltered position in South Devon, with a south facing extension.
    •  
      CommentAuthornigel
    • CommentTimeSep 10th 2007
     
    The reflective membrane will be effective at reducing thermal gain during the summer rather more than insulating.

    You could put the roof on first and then put most of the phenolic foam boards on and then blow in warmcell from the highest point on the inside.

    Although the foam boards will act as a vapour control layer you should also consider a continuous vapour control layer.
  8.  
    Nigel,

    'The reflective membrane will be effective at reducing thermal gain during the summer rather more than insulating.'

    That was what I had in mind - I guess it still counts as insulation whichever way the heat is travelling, and whether it is radiant heat or convection or whatever. I could have expressed my thoughts more clearly though...

    Nice idea with the warmcell - I will check the price. Thanks.

    I will be going for a continuous vapour control layer in spite of having the phenolic boards. I am trying to do the job cheaply, but I think I can stretch to a roll of polythene rather than risk condensation in my new roof. :)
    • CommentAuthortony
    • CommentTimeSep 10th 2007
     
    I would worry about condensation forming on the underside of the bubble wrap. With or without a vapour barrier.
  10.  
    'I would worry about condensation forming on the underside of the bubble wrap. With or without a vapour barrier.'

    Despite all I say above, what YBS ( the bubble foil manufacturers) have recommended to me on the phone ( just use another breather membrane to separate the mineral wool from the bubble foil ), what BuildDesk says about the condensation risk ( no risk ), and the approval of the BCO, I still do worry about the risk of condensation.

    Maybe I should try leaving something like a 25 mm gap between the mineral wool and the first breather membrane....

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