Underfloor Insulation: Small Gap Between Joist and Wall

25 Dec 2014
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United Kingdom
My bathroom is above an unheated storage area accessed from outside, presumably originally a coal shed. The bathroom has been rather cold and drafty during the winter, so I've pulled down the plasterboard ceiling in the shed and fitted 100mm Celotex between the floor joists.

Between the outermost joist and the external (cavity) wall is an annoyingly small gap, around 40mm wide (see attached photo). I've tried and failed to friction fit small blocks of Celotex due to the uneven wall, and can't accurately hold them in place to fit with a foam gun.

Should I persist with fitting Celotex somehow or would just using gun foam for the whole gap be appropriate? The gap runs vertically to the underside of the bathroom floorboards.


  • Wall Gap.jpg
    Wall Gap.jpg
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Sounds like your problem is air tightness not insulation, as 40mm missing wouldn't make the whole room cold. As such I'd go for your idea of foaming the whole gap to stop the air. Make sure the air isn't bypassing the insulation elsewhere. If you know where the air barrier is then make sure the are no holes.

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