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General: Insulating the eves of a hipped pyramid roof

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  1.  
    • CommentAuthorHuwblut
    • CommentTimeJul 2nd 2024
     
    Hi again everyone

    I’m still plugging away with my little bungalow project. Following on from my post about insulation I’m now trying to figure out how to get it into the eves but still leave a 50mm gap for ventilation.

    I’ve manage to get a photo downloaded so I hope that it helps. There’s lots more photos which I’m sure would be really useful but I can’t seem to get more that one in…

    I’m told that it’s a hipped roof in the shape of a pyramid. It’s got two little bays at the front.

    The corners are of particular concern as it’s a real junction of timber! There was some insulation in there previously but it was patchy which left cold spots and which resulted in some condensation / mould.

    The ceilings are down and there’s access from above and below. I’ll be insulating the main body of the ceiling / floor from underneath.

    I’ve been looking Manthorpe Refurb Eaves Panels:

    https://www.manthorpebp.co.uk/roofing/roofspace-ventilation/refurb-eaves-panel-0

    but the irregular spacing of the joists and rafters make it difficult.

    I’ve also been looking at their eves ventilation roll:

    https://www.manthorpebp.co.uk/roofing/roof-ventilation-%E2%80%93-solutions-modern-day-problem/roll-panel-vents-1

    which may do a job as well.

    I’m wondering if all the roof needs a 50mm gap or could I compromise and do as much as I can do and hope that there’ll still be enough air movement?

    All the soffits will be replaced as well so I could include some extra ventilation there.

    Apologies if I’ve not explained this too well.

    I’d be lost without the help of the forum!

    Cheers

    Huw
  2.  
    The danger would be to create closed spaces at the base of the hip rafter between that and the jack rafter. However depending upon the depth of insulation, the slope of the roof and the spacing of the rafters you may find that the hip rafter and the jack rafter create a closed space even if 50 mm vent gap is maintained

    Closed spaces are a potential condensation point.

    You could drill (lots of) holes at the top of the jack rafters to allow air movement between the jack rafters and do so to adjacent jack rafters until clear roof space is reached. It is never a good idea to drill holes in structural timbers (holes are supposed to be in the middle 1/3) but given the shortness of the jack rafters they are structurally very over specified but are the same size as common rafters for ease of construction and tiling..

    You could also put vent tiles at the points of closed spaces, although cutting tiles at the bottom of a hip is always a bit fraught without the complication of vents.

    To me it looks like the 2 products you cite provide through ventilation but do not provide ventilation for what is below - and it is below that it will be needed. I can imagine condensation forming on the underside of either product either if the insulation touches it or they are put in a closed space (closed by the junction of the hip rafter and the jack rafter)
    • CommentAuthortony
    • CommentTimeJul 2nd 2024
     
    I think 25mm ventilation for short lengths.

    How much insulation will you be putting in the loft - I reckon 400mm while you are at it

    It should be possible to place the eaves ventilation trays, push fibreglass roll down over the wall plate and into the top of the cavities, do the corners first.

    Then the first meter of the ceiling, then I add a vapour/air tightness barrier then plasterboard

    Happy insulating !
    • CommentAuthorHuwblut
    • CommentTimeJul 3rd 2024
     
    Thanks very much for the responses, it really is a bit of a dilemma.

    For context there's a couple of small rooms in the top of the pyramid which are insulated with Celotex and some low walls which I'll probably insulate with Celotex or similar. Therese's a crawl space around the perimeter which gives reasonable access to the roof, I'll insulate this as well hopefully to 400mm ish.

    The idea of a vapour barrier underneath the plasterboard and insulated joists has been mentioned before but discounted due to the above space being a "warm" space rather than a typical attic.

    I'm happy to do this but is it really necessary? There'll be adequate extraction in the kitchen and probably 2 small; dmev constantly running fans - one in the kitchen and one in the bathroom with a 10mm undercut on the doors.

    Apologies for so many questions but I'm learning a lot here...
    • CommentAuthortony
    • CommentTimeJul 4th 2024
     
    Ventilation in or of the house doesn’t negate or reduce the need to ventilate the loft void(s), I like vapour control layers but these don’t negate the need for loft ventilation.

    I always worry about wind or outdoor air getting under the floors of loft rooms, that is where you don’t want ventilation

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