Insulating Floor Joists that are Embedded in Wall

How do people insulate the ends of (ground-floor) joists that are embedded in the internal leaf of a (cavity) wall? Is it a case of ‘excavating’ the embedded end, installing an insulated-concrete block underneath (eg Marmox Thermoblock), then using tape to insulate the upper surface and the two sides of the joist, plus the exposed end? (Does the tape, or similar, still insulate on the uppermost side when compressed, after the load from the wall is reinstated?)

I presume that what you are looking for here is continuity of insulation. I can’t see any point in insulating the seat of the joist if it is exposed to cold air underneath. If, however you are fitting insulation under the joists then you are in a position to look at reducing thermal bridges, but why only under the joist ends?

The thing that joist ends unfortunately do, much more than loose heat, is leak air. Unless your cavity is insulated with foam at joist height you need to make the wall/timber interface airtight using suitable airtightness tape. If you are determined to have continuity of insulation, which is the ideal, you would need to replace not just the bricks upon which the joists rest but the whole course (or 2) with Thermoblock or similar. A very much cheaper, but less effective, alternative would be to continue the underfloor insulation down as far as possible, with a very small gap at the dpc. This should overlap with your CWI, insulating both sides of the leaf. Below dpc I suggest damp proof insulation.

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Ok, thanks Tim. I read (in a case study) that tongue-n-groove suspended flooring in good condition has a good level of airtightness; if the aforementioned joists are beneath such a floor, are they not therefore outside of the airtightness-envelope, or is it still good practice to pull up a couple of floorboards along the wall and use airtightness tape on the joist/wall interfaces? PS The cavity has just been filled with EPS beads, which presumably must now be surrounding/extending below the joists. PPS no immediate plans for insulating the underside of the floor, v big job!

Brand new and undisturbed tongue and groove does have a fairly good airtightness, but not perfect. Any joins without the tongue and/or groove will not be airtight and on course at wall margins and penetrations. As the timber matures and dries out further the timber will shrink and the joins will become less airtight. My own floor boards are flat edged and with about 3mm between boards. When the house was new I expect the gaps were much smaller, if any. Above that I have a 3mm dense foam underlay with foil faces on both sides. I am treating that as airtight for now, however the edges are a weakness. At some point soon(ish) I need to redesign the floor but I can imagine the house will be unsuitable for the children while work is in progress.

In your case you are presented with a quandary. If you take up any boards to undertake airtightness work you will damage the tongue and groove connection with the adjacent boards, resulting in a loss of airtightness there instead. Without insulation under the floor it is essential in our climate that the floor remains vapour open.

What floor finish do you have? How much gap is there between boards and wall? (There has to be some to allow for expansion and contraction with temperature and humidity changes.)

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You need something to make a flexible seal between the edge of the floor boards and wall. The foam below is primarily promoted for sealing between window frames and walls but works equally well in this situation.

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Thanks. Haven’t actually removed any skirting boards as yet, but a quick check of a couple of locations without skirting, eg meter cupboards, and a check of the apparent 80%-ish proportion of floorboard that’s visible alongside the skirting board (ie, parallel to the external wall) indicates that the plastering/rendering was done in a way that overlaps the floor by about 10mm; the plastering is about 25- 30mm thick, judging from a bit of exposed brickwork near the mains elec inlet cabling/ducting. The floors are (currently) varnished floorboards (which we quite like); I regularly hunt down and fill any gaps with PVA+sawdust mix.

If the plaster overlaps the floor boards by 10mm and is 25-30mm thick does that mean that the floorboards end 15-20mm from the masonry? Do you have access to that gap?

Something in that ball park… ie, last floorboard running parallel to wall isn’t tight up against the inside leaf/bricks, but is overlapped by the plaster. Could access that gap by chiseling off a channel of plaster - although the gap may not be consistent, of course.

It is a bit unconventional but I suppose you could seal the gap between plaster and board instead. I doubt that the results would be quite as good but access would be easier.

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But in any case, does such airtightness-taping not mean I couldn’t subsequently run the IWI down past the floorboards, as I believe is usually good practice, because the final floorboard would be airtight-taped to the wall (or, to the plaster)?

It is indeed good practice to run IWI down to the damp course (or even lower) and have it contiguous with any future underfloor insulation. If that is a plan for the near future then buy cheap tape, otherwise get good stuff. Airtightness tape is not easily removed but can be cut if necessary, and future IWI is a case where it will be necessary. Your airtightness measures are only a temporary fix until you do the whole job.

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Aha…ok, got that, thanks.