Any problem with mismatching U-values for walls

Hi, hopefully a quick question:

Is there any potential consequence to having mismatching U-values for external walls in the same room?

One wall will just about make it to 0.3 but another could be taken much lower and I’m just sanity-checking whether there are any potential issues with doing that?

I don’t imagine that the difference would be enough to cause condensation on one wall over the other (neither should be able to condense at the internal surface anyway!)

As far as I understand basic box thermal physics, having one wall better insulated shouldn’t drive more heat loss through the less insulated wall (compared to if both walls were the same). Obviuosly it wouldn’t improve the lesser wall’s performance either…

The wall which has room for more insulation (EWI) is more exposed and so I imagine it would be worth the little extra to use the space…

It is more important to make sure that there are no thermal bridges at the joins than to worry about that. Personally I would insulate each wall to its maximum potential.
Remember to remove the soffits and insulate up to the top of the loft insulation/to the breather membrane.
If possible also go down to the footings. Special consideration is needed below DPC.

What insulation and cladding are you planning to use?

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Thanks @Tim_Gilbert,

The walls are solid brick, and I’m planning for wood fibre out and in, the rear wall will be external only, so as far as I’m concerned the insulation can be as tick as I want it to be there. The other wall is a ginnel/alley/passage, so can’t have much extra external thickness, so I’m planning 60mm external with a thin silicate render, and then 60mm internal to get to about 0.29U give or take.

Fortunately the ginnel wall is only 1 storey, and the insulation will be going up above the inner ceiling and wrapping over the ginnel, and then a layer and cladding suspended on the ginnel ceiling as well.

So the ginnel wall would be ~0.29, and rear wall likely 0.2 or bellow if practical

(I’ve discussed one idea for the EWI already on Feasibility: EWI with larsen truss + flexible wood fibre batts but may well got for a more standard 150mm solid block system or some other system depending on what my eventual contractor recommends / is comfortable with).

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Unless you can find a specialist fitter it is unlikely that they will suggest natural fibre. That isn’t any reason to reject it though.

It’s good to see that you have identified the suspended ceiling of the ginnel for insulating. This should improve comfort in the rooms above.

Is the ginnel entirely yours? If you try to insulate just your half, you are bound to get thermal bridging. Then fitting half a new ceiling is going to be an issue too.
I suggest that you try to share costs with your neighbour or offer to do the job for them (either all the ceiling or if you are generous the wall too). The ceiling will cost negligibly more and give a far better result. A nice smooth new and light coloured render in the ginnel will make the passage far more pleasant.

Oh it’s a prerequisite for me even considering a contractor. I meant the method of attachment to the wall, anchors vs trusses.

Yep, the extra metre and a bit makes a big different to the upper floors.

I’ve worked out the widths so that both sides could have the same amount of EWI and still leave 900mm at the narrowest point for access.

I know I’ll have to notify them under the party wall act either-way, and I do plan to offer to get their wall done at the same time if they’ll cover the extra costs, especially seeing as the path will have to come up anyway for a plinths, but it’s a rented property and I doubt the landlord would care about things like comfort and energy efficiency when they’re not the ones living there…

Some people dream of super-yats if they win the lottery, I dream of buying up this whole victorian terrace from the landlords and banks and the (800 year) leaseholds, gifting it all to the people who actually live here and paying to renovate the lot to EnerPhit in phases and putting people up in hotels whilst the work’s done… it’s good to have dreams…

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