Green Building Forum - IWI transition risks/mitigation

IWI transition risks/mitigation

ChrisinYorkshire — Mon, 16 Mar 2026 11:29:31 +0000

Hi Everyone,

Hopefully this makes sense – if its hard to follow let me know and I’ll look at posting a diagram.

I would like any thoughts about my planned action for insulating a wall where my house transitions from being underground to above ground.

Context
The house is built against a valley side. It is mostly of traditional thick sandstone construction, although 30-40 years ago or so some areas were rebuilt with cavity walls.

To one side of the house two floors are effectively underground – the top floor pokes above ground approximately at floor level. The wall against the earth has been fitted with a cavity drainage system, metal studs, with PIR insulation and careful attention to airtightness/VCL.

My current area of concern – the transition point of said wall, as it pokes out from the ground. This involves 3 large wooden beams that are bedded into the wall (upon which sit the top floor floor-joists). I am switching to breathable insulation for the top floor room (a kitchen).

Further context and observations

It has been a very wet winter, the kitchen is unheated and has no mechanical ventilation. Where one of the beams enters the wall I am getting a 23% moisture reading ( this is the top face of the beam)

Floorboards have been cut away (approx. 70-80mm) from the wall during summer last year. They are reading 20% moisture at the edge closest to the wall, this is in the coldest corner of the room.

A floor joist running along side the wall, but separated with DPM and a gap, is reading 21% moisture.

Therefore I think there is a general humidity/condensation issue, though the beams bedded into the wall might also be wicking some moisture from the wall.
The coldest corner of the room, is a bit damp, also from condensation I believe. Note this corner is going to be behind kitchen cabinets.

Current course of action:
- I have dubbed out and parged the wall with NHL 3.5 and sand mix.
- Planning to meet the PIR insulation with wood fibre board which will then continue up the wall.
- I have considered but don’t want to use an eps/xps plinth layer at the transition from PIR to wood fibre boards, because I don’t want to trap condensation/moisture around the large beams that enter the wall.
- There is a leaky gutter externally that will be repaired/replaced.
- Kitchen will have mechanical ventilation and heating installed.

Any concerns over ‘wet process’ vs ‘dry process’ wood fibre boards? Wet process appear to be more capillary active than dry process boards. Frustratingly it's hard to find suppliers of wet process boards in tongue and groove.

IWI transition risks/mitigation

djh — Mon, 16 Mar 2026 15:17:06 +0000

Posted By: ChrisinYorkshireKitchen will have mechanical ventilation and heating installed.

I'm glad to see this because it will help resolve issues.

The coldest corner of the room, is a bit damp, also from condensation I believe. Note this corner is going to be behind kitchen cabinets.

That sounds like it may pay to put a little extra heat or ventilation or both in that corner if it can be arranged

What is the land surface like outside the buried wall? I would recommend digging a ditch, putting a drain at the bottom and filling the ditch with (sharp) gravel to minimise damp and rain bounce-back against the wall.

Sorry, I can't comment on wood fibre question.

IWI transition risks/mitigation

Nick Parsons — Mon, 16 Mar 2026 21:19:09 +0000

How do the ends of the beams *feel*?

Yes, <20% is nice, but if you have some local variations and the hardness of the timber suggests it can 'cope' it may not be such a major issue. If you felt it was, how about steel 'shoes? Major engineering operation, though.

I'm not certain re dry vs wet process. I think my supplier (and yours) was wet process, and said it was good. If you want to stick with wet process and can get it in square-edged, why not go for that? With less-than-straight walls T&G can be more of a pain than a blessing (think insulating over a concave area and following the 'shape' - the tongue gets in the way of your attempts to 'stuff' the joint.

IWI transition risks/mitigation

ChrisinYorkshire — Tue, 17 Mar 2026 09:25:24 +0000

Thanks djh and Nick,

djh - land surface directly outside is a stone-flag pavement of a public highway. It is a conservation area and I don't think I can put in a french drain or ACO, though I can tend to the pointing. The paving/road sweeps down and away from the house so water tends to run off.

Nick - When we did the bottom room I checked the beams and they were in good condition. One of them I can still check within a cavity wall section above and it remains in good condition - the wood is solid, you cant drive a screwdriver into it etc.

So I am possibly over worrying - having now replaced 4 rotten beams in the property I am cautious about getting this right for decades to come. Vapour open capillary active materials while lowering the humidity feels right.

Thanks for your points on wood fibre boards - i'll go for wet process square edge and see how I get on.

IWI transition risks/mitigation

tony — Fri, 27 Mar 2026 09:28:36 +0000

Have you measured any temperatures, eg of the surface of the plaster low down in the corner?

Will you be insulating between the kitchen floor and the ceiling below where the Hohn ground level is?

Have you thought about EWI?