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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorPingy
    • CommentTimeMar 31st 2010
    Where the brickwork meets the roof there is gap which is allowing the wind to penetrate into the cavity. I'm at a stage where I can still access some areas of this junction from the inside as well as from the outside. What would be the best material to use to seal these gaps - mortar, expanding foam from a can, or something else?
    • CommentAuthorCWatters
    • CommentTimeMar 31st 2010 edited
    Caution, that gap might be deliberate. The cold side of roof/loft insulation is normally ventilated to prevent condensation forming. Typically a continuous gap of around 12mm is left or special vents are installed.

    Take a look at eaves trays if you haven't got them already. These form tunnels at the eaves so insulation can stuffed down in there without blocking the ventilation.


    Example installation..

    • CommentAuthortony
    • CommentTimeMar 31st 2010
    Sounds to like there is no cavity insulation? and it should join up with your roof or loft insulation. dont worry about the gap.
    • CommentAuthorPingy
    • CommentTimeMar 31st 2010
    Apologies for not describing the construction more clearly - that's what happens when you're in a rush. Let's start again..........

    I'm converting an agricultural building and at the same time adding some extensions. The external 'skin' consists of 9" solid brick where the original walls still stand and 100mm" single brick for the new bits. Behind these I've created a 250mm cavity, fully filled with mineral wool cavity batts, and used dense concrete block walls for the inner skin. The wall plate for the roof sits mostly on the inner skin but one area it sits on the original 9" wall (that's how I can access the cavity from the inside at the moment). The roof is constructed of 150mm rafters panelled over with Panelvent boards, breathable membrane, counterbattens, laths and slates. All ceilings will be vaulted.

    I'm currently constructing a second 'false' roof on the inside using 100mm rafters (similar to Keith's roof construction - see page 34 of Autumn 2007 issue of Green Building magazine). These sit on a wall plate attached to the vertical face of the inner blockwork to create a thermal bridge free cavity below the main load bearing rafters. This cavity will be filled with cellulose fibre insulation.

    Whilst working on the 'false' roof I've noticed how much wind is blowing through the gaps where the brickwork meets the Panelvent boards (the top layer of mineral wool will be fitted before boarding the underside of the false roof). Even though the cavity will be fully filled with mineral wool I imagine the wind will have a significant chilling effect on the insulation. I would have thought it would be more effective if the cavity was sealed.

    With regard to moisture all of the external brickwork has been built using NHL3.5 (no OPC) to enable any moisture to be wicked away to the outside through the mortar. Cellulose fibre is being used in the roof for the same reason - any moisture should escape through the Panelvent and beathable membrane.

    The air tightness layer for the inside of the building will be wet plaster on the walls and a vapour barrier on the underside of the false roof. As long as I get the air tightness right on the inside I won't experience the wind that I'm currently getting into the building. I can see, however, the wind will blow through the mineral wool around the top of the cavity thereby reducing its effectiveness.

    Can you see what I mean now?
    I would fit bevelled noggins between the rafters to the top of the outer leaf & then fill any remaining gap with polyurethane foam. You may be interested in the following thread:


    • CommentAuthorPingy
    • CommentTimeMar 31st 2010
    Thanks for the advice David. You've confirmed what I was thinking. Looks like it's going to be the polyurethane foam coming to the rescue! Interesting thread you've linked.
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