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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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    • CommentAuthorDen
    • CommentTimeSep 28th 2020
    Hi All. First post here and trying to get some opinions. We are about to purchase a Cat 3 listed house in Scotland and am trying to work out what materials will work best for different elements of the house. Listed consent issues aside for one part of the house there is a single story cottage, with rubble stone walls and a vaulted ceiling, currently un-insulated.

    I am trying to work out what approaches for insulation might work best and wondered whether the following 'hybrid' insulation strategy for the the roof might work:

    50mm+ ventilation gap to slates/sarking
    100mm flex woodfibre between rafters
    breathable membrane under rafters
    50mm celotex under rafters (apart from woodfibre perimeter to avoid impermeable insulation touching wall)

    All 4 walls have no internal insulation and are limewashed rubble.

    I would also plan to create a small curved coombe ceiling to maintain an air gap at the apex between both sides of the roof. Depending on the exact structure of the roof covering I could also add additonal conservation vent slates assuming LA happy with this but that will depend what's under the slates as to whether its necessary. Additonal ventilation provided in the warm space wil be via 2 new velux windows.

    Does this construction look ok?. I'm trying to get a good balance between cost and performance but happy for any comments!
    • CommentAuthorEd Davies
    • CommentTimeSep 29th 2020
    Hi, welcome Den.

    Posted By: Denbreathable membrane under rafters
    What's the purpose of this? Are you sure you mean ā€œbreathableā€?

    Breathable membrane is usually used in circumstances where it's desirable to resist liquid water getting in but still allow water vapour out. I'd think that if you're getting liquid water below the woodfibre then a) things have already gone badly wrong and b) you'd want it to flow away as quickly as possible even at the cost of it dripping into the room. Also, putting something vapour open directly on top of Celotex seems a bit odd.

    A vapour-open (ā€œbreathableā€) membrane above the rafters and woodfibre but below the ventilation gap would be a lot less surprising.

    Maybe meant a vapour barrier (e.g., polythene sheet) under the rafters?
    Welcome Den,

    Similar query to Ed's, plus I don't fully understand this:

    ''50mm celotex under rafters (apart from woodfibre perimeter to avoid impermeable insulation touching wall)

    What do you feel is the issue with the impermeable insulation touching the wall? As the walls are un-insulated and potentially subject to condensation I think I would be more worried about the wood-fibre touching the walls (it may be water-vapour permeable, but my feeling is that it won't be helped by liquid water, which it could then transfer, like a sponge, to the plasterboard). I could be wrong!

    I think if it were me I would be treating the face of the Celotex as a vapour control layer -VCL- and taping at all joints and perimeters. Some people feel it's OK to have the VCL a little bit into the 'sandwich', although I usually do it 25mm in when I have to, not more than that (just a 'gut feeling' on my part), but as Ed says, that would mean having a VCL under the wood-fibre, not a breathable membrane.

    And I echo Ed's feeling that maybe the breathable WF is odd in what has been made a non-breathable set-up by the Celotex. However I suppose one could argue that, if the VCL were less than perfect the WF could do its 'moisture-buffering' job prior to passing the WV into the ventilation gap to dissipate harmlessly.
    • CommentAuthorDen
    • CommentTimeSep 29th 2020
    Thanks both for your replies. My reason for the flex fibreboard between the rafters was like you say to act as a great vapour buffer for any water that might find its way in there. Between the rafters there's a relatively large surface area for each rafter in contact with the insulation, and from what I'd read on Historic England/Scotland, impermeable insulation between the rafters was not a good idea.

    Under the rafters I'd reasoned that the Celotex would be fine since the surface area was much smaller and everything above it was 'ventilated'. I agree now that you mention it, I should have written VCL layer. Maybe it's better to tape all the joints on the Celotex then put the VCL between the Celotex and the plasterboard?

    Does that sound ok?
    If you are putting a VCL between the Celotex and the plasterboard why go to the trouble of taping the Celotex joints?
    Any reason for using Celotex rather than one of the other insulation boards used for insulation e.g. EPS
    What is the rafter spacing? Anything more than 60cm will risk the plasterboard sagging over time. The solution would be to cross batten the rafters at the insulation joints (which I presume are 50cm wide slabs) then fix the PB to the battens.
    • CommentAuthorDen
    • CommentTimeSep 29th 2020
    Only reason for the Celotex was cost vs performance (50mm was ~Ā£5/m2 when I compared the costs), think the rafters are close ~40cm, but I can't say for certain until I take some of the boards off that are there now...can't do that until the sale goes through though :)
    I am planning a similar hybrid roof insulation system. Wood fibre batts between rafters and Celotex below. My reason was because a) concerns about future gaps around the celotex between rafters and also lack of breathability in an old roof, and b) I couldn't use a full wood fibre build up and still meet Building Regs because of the head height constraints in the loft. Actually the roof still doesn't meet Building Regs but Building Control approved it based on a whole house SAP calculation.

    I am also hoping the wood fibre will offer some improvement vs Celotex on buffering the summer heat.

    My vapour barrier will be under the celotex.
    We have a listed building built circa 1752. We have vaulted ceilings in the bedrooms and I took the view that while the solid stone walls needed breathable materials and lime plaster, the roof was the same construction as modern houses. I therefore installed TLX Gold breathable foil/membrane above the rafters (these are taped up to create an airtight layer), a 50mm air gap then celotex between and under the rafters and then plasterboard.

    If I were doing it now, I'd probably counterbatton the roof so that the TLX was pulled tight, rather than draped into the rafters. This would allow me to fill the rafters completely with celotex. I'd also take a week off work and stand over the roofers to make sure that the joints were all taped up properly (or just do it myself). As it was, ours weren't taped and we have issues in winter where meltwater is pushed under the membrane and leaks into the roof.
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