Hi,

We’re extending our 1953-ish semi and the majority of the external walls will be new.
The existing walls are 260mm thick and supposedly cavity wall, however, the cavity must be tiny.
We’ll check that when the build commences.

In terms of the finished building, 3/4 of the front of the building will be the original walls, but the rest of the external walls in this be house will be new.

We’re building new cavity walls and all the external walls (new and old), will also have internal wall insulation panels fitted (37mm).
The panels will include a damp proof membrane and will be taped, and we plan to fit an MVHR system, to take care of moisture in the air inside the house.

Our concern is what will happen to the external brickwork during the winter?
If it gets particularly water logged and then we have a spell of subzero temperatures, the brickwork may get damaged.

Is there anything we can do to prevent this or should we not be concerned?

We’re in the south of England.
Thanks in advance!

We’re not in a particularly exposed location.

As for planning constraints, I’m pretty certain we would need to obtain permission to use EWI, since all other houses in our road have brick facades, so we’d be changing the character.
Plus our attached neighbour is brick, so it would look odd.
Also, it’s more expensive than EWI.
I did consider it initially and I can see how it would be a good option in the right circumstances.

No particular reason to use a cavity build other than the existing house is cavity (?!) wall and nobody has suggested anything else up to now - and our building regs stuff has all been done and submitted already.

We are hoping to start building in September, so to change now would mean going back to the builder for a requote, going back to the structural engineer and architect, planning, et al.
So we’d be looking at a considerable delay.

I’ll look into your comments about her porosity of the bricks, since that makes sense and I should ensure our builder doesn’t ‘cheap out’.

Where is “here” for you?

37mm lining boards only have 25mm insulation in them, so won't make much difference either way. Probably needs a bit more insulation thickness than that to make it worth the cost/disruption of retrofitting all the existing house - maybe 50-100mm?

The new walls will need new linings anyway, so no harm in using this stuff rather than regular plasterboard, though the main thickness of insulation will need to go into the new cavities and there needs to be positively no air infiltration between the walls and the insulated plasterboard.

IWI and EWI each have pros and cons. For retrofitting unfilled cavity walls while avoiding cold bridges in a semi, IWI is possibly a good choice, if thick enough.

Thanks WillInAberdeen - the majority of the external walls will be new, so fortunately not too much disruption.
I mean, we have to rewire and replace the heating system as well, so we'll have to move out at a certain point anyway.

I'll discuss the IWI thickness with the architect - thank you.

Retrofitting seems like a big experiment.

Maybe I'm worrying unnecessarily over the brickwork.

As W in A points out 25mm is not going to excite anyone, not least Building Control. Who has drawn up the specification, and what reference is made to the Regs? 25mm PIR on a 225mm solid wall might give you around 0.85W/m2K, where the U value target is 0.3W/m2K.

DJH is quite right that building a new cavity wall with IWI on the inside is barmy. It's more expensive, removes thermal mass (and thus reduces comfort) and it's likely to reduce airtightness (sounds like you are using combined insulation+plasterboard board which are impossible to tape properly).

The right thing to do is build the cavity wide enough to achieve the U-value you want. I built my extension (in 2014) with a 200mm cavity containing dritherm 32 (rockwool). Other insulations will work fine too. Also use basalt wall-ties to reduce thermal bridging. That gives a U-value of 0.15 which is generally considered 'good' and what I would recommend for any new wall construction (unless you have done more detailed sums).

Another thing to do is make sure that you remove thermal bridges where new extension walls meet existing house walls. i.e connect the insulation. If you just butt up the new wall against the old one then the outside brick layer will be a massive thermal bridge to the interior at the corner.

If you are currently planning 100mm cavity plus 25mm of internal insulation then you want at least a 125mm cavity. It essentially costs nothing to go wider so I'd do at least 150. 200 is good.

(the best thing to do is EWI as has also been pointed out - I have a brick building and it was no problem getting permission to turn it into an insulated and rendered one on the front facade, or you could have use brick slips to keep it looking like bricks, but it's too late for this info to be useful to you).

The other things to think about are making sure the thermal bridge at the foot of the wall is treated (i.e the inner leaf is not just sat on concrete footings in the ground without a quinn-lite block or foamglass block as a thermal break. And similarly the eaves detail to minimise thermal bridging there.

I see no reason to worry about the external bricks. External bricks outside insulated cavities are totally normal and I've not heard any instances of them blowing, even in construction with wide and well-insulated cavities.

IWI on the inside of a cavity wall is fine if you can't/won't do EWI. I have this too on part of my house. The thing you really need to worry most about is airtightness. The standard bonded plasterboard+insulation panels are impossible to air-seal properly, because you can't tape the membranes together. It is cheaper and better to put up separate insulation and plasterboard, taping the insulation before adding the plasterboard.

If your airtightness layer is somewhere other than the internal wall then this doesn't matter. So you _could_ parge or airtightness-paint the existing internal surface, and get your airtightness test done before adding the IWI and then the bonded stuff is fine. If your walls are currently plastered and it's in good nick (well attached, not too cracked) then that's a good airtightness layer. Doing this behind the IWI has the advantage that when you screw things to the walls later it may not make holes in the airtightness layer.

I have 100mm of PUR on my cavity walls, to add to the 65mm of fibreglass in the cavity, which gets a U-value of 0.15. You could go less than that but I really would do at least 60mm PUR which would get you U=0.2.

You will also need to pay attention to where the 1st-floor joists go into the wall and ensure they are properly sealed (with tape) . You do not want a lot of soggy internal air going out of holes there and condensing on the ends of the joists inside the wall.

Hope that's useful. There are a lot more details it sounds like your architect may not have advised you very well on.

Most fundamentally, what quality of building are you aiming for? i.e what annual space-heating load? 50kWh/m2.yr, 25? 100? No-one should be building new things or doing retrofits that don't achieve 50 or so these days, because it's just dumb. Both LETI and the AECB agree on this target. Just doing 'building regs minimum' is not sufficient, as they were set more than a decade ago, before there was a climate emergency (well, before most people noticed).