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Dormer bungalow - block airflow between ground and first floors

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  • 05-10-2022 11:22am
    #1
    ac1977
    Registered Users Posts: 30 ✭


    Hi,

    I bought a 90's dormer/mansard house. I was replacing floorboards in an upstairs bedroom, and when I lifted some of the floorboards I noticed that I could see directly out at the soffit vents. This means that cold air that gets through the soffit vents can go directly between the ceiling joists , which means that in winter there would always be cold air flowing through the house, over the downstairs ceiling and under the upstairs rooms' floorboards.

    On windy days, any open doors upstairs start hitting off the architraves, which probably shows that airflow is coming directly into these rooms via the floorboards. I can also feel draughts coming up through these floorboards.

    I took down some of the soffit outside to take a look, and I could see straight through the house between all the joists.

    I'm planning on blocking this entry point between the joists with 100mm foam board, and sealing around the board with expanding foam to try to block all this airflow.

    Is this a good idea?

    Has anyone done this, and seen any benefit?

    Thanks



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    #2
    10-10-20
    Registered Users Posts: 7,159 ✭✭✭10-10-20


    Is the ceiling of the upper bedroom insulated and if so, where does this terminate? At the bottom of the knee-wall? Photos might help.



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    #3
    ac1977
    Registered Users Posts: 30 ✭ac1977


    house.png

    I don't have photos right now, but here's a crude drawing.

    It's a mansard roof, so the knee wall is basically the entire upstairs bedroom wall(light blue). But that's kinda unrelated here, as the airflow occurs below these walls and under the floor.

    The soffit are the orange, which I poked my head through, and had clear visibility along the route of the blue arrows, which also show how the cold air gets access to flow through the house.



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    #4
    10-10-20
    Registered Users Posts: 7,159 ✭✭✭10-10-20


    Ok, but is there insulation in situ at all?



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    #5
    ac1977
    Registered Users Posts: 30 ✭ac1977


    joist1.jpg
    joist.jpg

    Found some photos. This is what I can see when I poked my head up through the soffit. I can see straight between the joists



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    #6
    ac1977
    Registered Users Posts: 30 ✭ac1977


    We have attic insulation, and we have the downstairs cavity walls pumped.



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    #7
    mickdw
    Registered Users Posts: 23,368 ✭✭✭✭mickdw


    What you need to do is block the hole at end of joists under the line of the knee wall.

    This will involve doing each joist gap one by one. You could use a flexible fire stop type material.

    This will allow small side roof space between roof and knee wall to remain ventilated. Insulation should be to back of knee wall and to small section of ground floor ceiling outside knee wall.



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    #8
    10-10-20
    Registered Users Posts: 7,159 ✭✭✭10-10-20


    That's crazy, a massive void left open-vented. The benefits of getting this closed off and the insulation envelope closed is pretty big.

    Question though, on the second photo we can see what looks like a vertical plywood panel, is that the knee-wall (light-blue in drawing) and if so, where is the insulation there?



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    #9
    ac1977
    Registered Users Posts: 30 ✭ac1977


    https://www.boards.ie/discussion/comment/119703857#Comment_119703857

    Yeah, that basically what I'm doing. I'm cutting sections of 100mm rigid foam, and plugging each gap. Big job for me, so I'll have to insulate the back of the knee wall later.



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    #10
    ac1977
    Registered Users Posts: 30 ✭ac1977


    https://www.boards.ie/discussion/comment/119703864#Comment_119703864

    Yeah, that's what I thought, crazy to have this open to the elements. And the plywood is the external bedroom wall, no insulation!



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    #11
    ac1977
    Registered Users Posts: 30 ✭ac1977


    While I'm plugging the gaps with the board, should I also stick whatever rockwool I can in there first?



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    #12
    10-10-20
    Registered Users Posts: 7,159 ✭✭✭10-10-20


    While you're at it, investigate this gap too:

    image.png

    That's either the top beam of the external leaf wall-plate and the internal block further in or it's the soffit-board under the joist against the external leaf.

    If it's the brick cavity, it should be closed off and you should have pumped insulation up to the closure. Just see if you can identify what it is.



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    #13
    ac1977
    Registered Users Posts: 30 ✭ac1977


    On the 4 or 5 joist gaps I've already plugged, I checked that gap, cos I was worried about airflow getting in the cavity wall, but it appears to be filled with concrete thankfully. But I expect at some point in the job I'll find gaps into the cavity wall. If so, what's best to plug that gap?

    Also is the kneewall insulation something a diy-er like me could do, with spray foam or something?



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    #14
    10-10-20
    Registered Users Posts: 7,159 ✭✭✭10-10-20


    https://www.boards.ie/discussion/comment/119703909#Comment_119703909

    Here's the catch - the whole knee-wall should be insulated - insulating just the bottom section of what you can reach is going to make an incomplete solution. Plus consider that perhaps the ventilation for the upper section of roof is carried along the rafters from the soffit, so what effect is insulating this area going to have on the ventilation of the upper roof? I'm not sure what the best advise is here.



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    #15
    ac1977
    Registered Users Posts: 30 ✭ac1977


    https://www.boards.ie/discussion/comment/119703952#Comment_119703952

    I think you're right. Any kneewall insulation would also require getting attic roof vents. The gap where the two roof angles meet to allow air into the attic is pretty small as it is, so spraying foam all the way up might block it completely.

    Anyway, I'll do the joists gaps for now which I think is the most urgent work, and come back to the kneewalls later.



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    #16
    10-10-20
    Registered Users Posts: 7,159 ✭✭✭10-10-20


    Also is the kneewall insulation something a diy-er like me could do, with spray foam or something?

    I'd be thinking along the lines of taking down the plywood within the walls and applying rock/glass-wool into the gap (retaining any required ventilation gaps against the sarking) then using insulated plasterboard to replace the walls. The question of VCL would probably require that it be installed over the wool with the insulated board over it.

    You weren't too partial to the decor of the upstairs bedroom then.... I hope?

    For the question of what insulation to fit now, I'd be doing as mickdw said and insulating from the external side of the wall-plate (wooden beam) to the bottom of the knee-wall with 200mm wool. That should improve the insulation of the upper parts of the block wall, preventing condensation/mold in the top 6".

    Any issue with stuffing that small cavity with wool, @mickdw ? I think it would stop any convection, etc.



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