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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthordelprado
    • CommentTimeMar 9th 2018
    And by that I mean insulating at rafter level, after you have insulated at ceiling level. I have 300mm of wood fibre at ceiling level and am now thinking about using lcynene at rafter level to deal with my eaves, stop rubbish coming in from the outside, and making the wood fibre perform even better by reducing airflow up there. I can't see any downsides?
    i have 400mm at ceiling level and 200mm at rafters without any problems but i am sure otherswill disagree.
    • CommentTimeMar 9th 2018
    As ever, you need to consider condensation risk and also the impact of whatever ventilation or airflow is required to avoid that on the thermal performance. At a minimum, given the emphasis on avoiding voids in insulation, what do you think the effect of a void as large as an entire loft will be? In general you'll get better performance from putting all the insulation in one thicker layer.
    • CommentAuthordelprado
    • CommentTimeMar 9th 2018
    I suppose there is nothing stopping me using the lcynene between rafters then using my wood fibre on top of that, but would be a pain to fix.

    Its really hard to say what the effect would be, but I would assume diffusion and vapour pressure would get rid of condensation, on the outer line of lcynene as thats where the cold is
    • CommentAuthorrevor
    • CommentTimeMar 10th 2018
    I don't remember where I got this bit of information from but if you are stacking different insulation the insulation with the lower R value goes on the cold side.
    Do you have a vapour barrier between the ceiling and the insulation?
    Do you have condensation in the roof space at the moment or experienced it in the past? If you haven't then I cannot see you will get problems by insulating at rafter level because now the roof space will retain the heat that is coming through the existing insulation thereby keeping any moisture as vapour. A lot will depend on how much ventilation you are currently getting in the roof space. You could suck it and see and if condensation becomes problematic then introduce some ventilation. If then you were concerned about heat loss then have a fan linked to a humidistat and duct the moist air outside or if you have MVHR then back into the dwelling for the MHVR to deal with it. But if you have MVHR then there should not be a condensation problem in the first place

    I have insulated between rafters with rock wool then over the rafter with pu. I have created a vapour barrier and sealed for air leakage at this point so moist air won't leak into the space between the insulation and breathable roof membrane, the roof is slate so plenty of ventilation above the membrane. At ceiling level I have a further layer of rock wool but that is there for sound insulation although it will have some contribution toward heat retention below.
    • CommentAuthordelprado
    • CommentTimeMar 11th 2018
    Hi revor, I think that is what I am concluding. This will actually help keep condensation away from rafters by making everything warm.

    Ventilation wise, at the moment its not ventilation (in the roof ridge or eave way), but obviously its not air tight because all the soffits are old cracked lath and plaster.

    There is no VCL at ceiling level.
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