Help sought: insulating an internal wall that is partially also an external wall

[low_and_there]

I've attached a diagram to accompany this and will post a couple of photos of the inside of the rear reception room that this concerns.

 

The problem statement is: We have a split level victorian mid-terrace, where the rear addition (aka outrigger) is 1.4m lower than the reception rooms. Doing heat loss calcs I have just realised that as a result of this, we've got some walls that whilst appearing internal, they partly face the ventilated void space under the reception rooms, which means that unless we insulate them, we're going to have some 14sqm of solid brick wall facing a cold ventilated void (made colder by the fact we're putting insulation above it). We're using breathable insulation throughout - sheepswool, woodfibre or cork.

 

If we don't insulate, I think we'll  may end up with cold bridges and higher heat loss, so I am looking for suggestions on ways to approach the insulation detail.

 

It's not possible to place the insulation on the inside face on all of it, because it would eat into the Staircase, which is already narrow, but I could do some on some sections.

 

In my sketch attached, the black lines indicate the areas where we've got internal walls facing the ventilated void upto a height of about 1.4m. For stretches 1 and 3, I could insulate on the internal face, losing space in the kitchen and WC respectively if there's no suitable alternative; for stretch 2, I can't do that - it has to be done on the opposite side of the wall. I've got the floor boards up, so have access to do that right now.

 

Options for stretch 2:

 

a) Apply a lime parge coat to the bare masonry under the original DPC, then mount 100mm cork board - not applying any render the cork board as it's not visible and not facing any rain

 

b) Construct a timber cavity in the underfloor void, line it with the Vapour Breathable membrane I'm using for the UFI, and stuff Sheepswool in there (assume this is a bad idea but floating it anyway!)

 

c) Construct timber cavity and fill with loose fill cork insulation

 

 

Some concerns I have are avoiding breaching the original DPC and also not creating paths for moisture to rise from the chalk ground.

 

Suggestions extremely welcome.

 

 

 

 

* the joist hangers and battons underneath are to host deeper underfloor insulation, nothing to do with this particular problem statement (I'm doing that because the joists are only 100mm deep).

 

Edited June 15, 2022 by low_and_there
Added photos

[Redbeard]

Hi

 

Well thought-out re the sub-joists for the UFI. Is the cross-ventilation really *excellent*?

 

'EWI' on wall 2 would work. Remember that any insulation on those walls wants to 'oversail' as much as possible to cloak 'diagonal thermal bridges', same as you'd do for IWI where the ext wall meets an internal or party wall.

 

Haven't got time to look in further detail yet (particularly re junctions, weak-points etc.) but hope to comment later.

[low_and_there]

1 hour ago, Redbeard said:

Remember that any insulation on those walls wants to 'oversail' as much as possible to cloak 'diagonal thermal bridges', same as you'd do for IWI where the ext wall meets an internal or party wall.

 

thanks @Redbeard, that's reassuring to hear you think it would work; in terms of 'oversailing', I hope I have understood what you mean - I will for example, be returning the IWI 90cm in from external walls to mitigate a thermal bridge in the corner. I am less clear on how to achieve here though...

 

1 hour ago, Redbeard said:

Is the cross-ventilation really *excellent*?

 

I think so, yes; some of the sleeper walls are honeycombed and for the rest, I have knocked through several bricks lower down below the future insulation level, throughout the sleepers and through the external wall at the front the house and the rear. The front of the house I dug down about a foot into the soil there to create those new airbricks.

[Redbeard]

Take wall 1, for example. Draw a diagonal line from the words 'dining room' to outside, running between the EWI and the IWI. Although you have shown a (very) slight overlap between IWI and EWI what happens below 'main house floor level'? The EWI and IWI do little, as there is a path through from warm dining room to cold sub-floor void. Maybe take the IWI across the window so that you lengthen the path the heat has to take to escape. You very rarely 'get rid' of thermal bridges, but you can mitigate them.

[low_and_there]

Thanks @Redbeard for the guidance!