Insulating between joists that are below DPC ?

12 Dec 2021
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United Kingdom
On replacing some of the joists on the ground floor of my old terraced house I noticed that some were sat on a bit of slate or DPC whilst others were just on the brick plinth. On closer inspection, the original tar/bitumen DPC for this room is located two bricks above the brick plinth the joists sit on (see image) meaning the exposed joist ends were just wicking this moisture up.


Originally I think the joists (since replaced) were bedded onto bitumen/tar to prevent rot with the surrounding below DPC bricks free to air. My question is about insulating this sub-floor as I'm concerned about this moisture wicking from these bricks into any insulation, or being trapped, instead of evaporating and exiting through air brick ventilation.

Could/should I just cover this whole plinth (and side walls) with DPC/DPM of some kind to isolate any insulation (likely mineral wool) from the wet bricks below DPC ?

My other thought was to not insulate it but just lay a DPM over the joists lapping up behind the skirtings to draught-proof and help isolate the flooring from the walls - the chipboard I pulled up was curling at the ends where it had been laid touching the walls. Is this a good idea though without any insulation (condensation on underside of DPM?) ?

The final floor covering will be chipboard, underlay, and then laminate/wood flooring, btw.
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If possible I'd try to lap up a DPM across the ledge and up to above the height of the original DPM. Whatever else, any insulation you put in will need to be protected from moisture - wet insulation doesn't work all that well
If the floor is still open then why not post pics showing the joist tails seated on the brick ledge?
And a pic showing soil conditions?
You have air bricks - above or below the bitumen DPC in the wall?
Is your FFL above the outside ground level?
Are any joist ends wet / damp / rotted now?

As for insulation, keep it back about from the wall, should be OK there, provided there is a good under floor air flow.

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Unfortunately, won't that leave a cold margin?

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