We have a new insulated concrete slab, almost flat (but not quite). Standing on it are new timber/insulation/plasterboard wall leafs, which are almost straight (but not quite), these walls are the airtight layer.

The joiners put some nice straight skirting boards on. So there are wavy gaps, both behind and below the skirting, up to 5mm in places, the wind coming through.

First step was to remove skirting boards. There is a 12x25ish mm void where the bottom of each plasterboard is lifted above the floor, I filled this void with squirty foam. But when I trimmed the excess foam off, found that it had not bonded well to the concrete, big bubbles make voids between concrete and foam, not very airtight.

Next step was to refix skirting boards with stupidly large beads of caulk behind and below. This stopped the gap but looks a bit of a bodge.

How should I have done this right? How does everyone else seal junction between stud walls and concrete slab?

Sorry Nick, some more background. Existing walls are 1800s granite (so not very airtight at all). New concrete floor slab extends to inner face of existing granite walls. New timber/insulation/plasterboard wall layer stands on top of slab, approx 50-100mm inboard of granite walls (replacing previous lath/plaster lining). Cavity inbetween granite and timber leafs is open to ventilated roof space.

Existing granite is to keep rain out and support the roof. New timber layer is to provide insulation, VCL and airtightness functions. But needs to seal onto floor slab.

Ah OK thanks Ed, the insulation is 100mm Kingspan-equivalent with foil faces both sides, plus poly sheet VCL on inboard side.

How to seal the gap between to the floor at the skirting board junction??

Ah that's a good idea. Thanks David! Sadly the Vcl is already trimmed back, but maybe I could stick a strip of it to the floor and turn it up behind skirting. What glue do you like? Would a wide adhesive tape do the same?

Sure, but in the case of a pre-existing structure that wasn't built to be air tight that's not really an option Tony.

In the case of a pre existing room, I would remove the skirtings and seal the air tightness layers together and then seal gaps and voids behind the skirting area then refit them.

Almost certainly, unless there was a wind-tight layer fitted before the insulation.

Posted By: davidfreeborough...but probably wouldn't be good value as the cavity is open to the roof.

Good point - I'd missed that bit.

Kind of a bit late now, but if a new slab was laid, I imagine there was a DPM? If so, that should have been left long (up the external walls). It could then be folded flat onto the slab (with a bead of mastic), then the timber frame walls ontop. The excess still protruding into the room could then have been lapped up the inner face of the timber frame walls and lapped and sealed to the VCL. Sorted.

As for a fix now, it will be a matter of sealing the vcl to the slab in some way. How is a more difficult thing to answer and will likely involve some membrane (like DPC), and some adhesive to bond it to the slab, and then lapped with the timber frame walls and seald.

Thanks all for excellent info!
The bottom of the granite walls has been lined with torched bitumen felt and the new dpm lapped onto this for damp proofing, we had to dig below gnd level to make depth for the floor insulation.
I was not keen to parge the granite internally as it absorbs a certain amount of rain, AIUI the ventilated cavity helps penetrating rain to evaporate or drain.
The insulation is foil faced kingspan so wind penetrating the insulation not a problem I hope.
It would have been neat to seal the vcl onto the slab, wish we had thought ahead and not trimmed it, looking into tapes instead!
Will