I like fibreglass batts perfectly built in - rare in the real world

Don’t let them use dot and dab whatever you do.

Or stud frame incl EWI. Thinnest walls, for given standard (v gd = easy) of insulation and airtightness, especially if EWI-ing the rest of the house (by far the best way).

Thanks all, for your input. The house is brick on the ground floor and pebble dash render on the first floor. I'd love to get EWI all over the whole house, but I haven't got the budget for it and also it wouldn't fit in with next door's side (it's a semi-detached house), so I am thinking a cavity wall probably makes the most sense.

So I guess it's either mineral wool batts (fully filling the cavity) or getting them to fill with EPS beads as the wall is built? Is there a reason why (correctly installed) mineral wool batts are better than EPS beads?

I will try to be at home as much as possible, but I thought it better to specify a technique that less prone to cutting corners (I probably shouldn't but I worry a bit about getting on builders' nerves if they think I'm constantly breathing down their necks!)

I already planned not to use dot and dab - I've got a plasterer I've already used so I was going to see if it's possible for him to finish the walls.

Posted By: tonyor when services are retrofitted especially pipes?

Pipes and services in cavities ??

Or timber frame as Tom suggested ...
With brick slips, or rendered as desired.

Posted By: mattpI met the architect yesterday, and his preferred specification is for a cavity wall with 80 mm PIR board and a 20 mm cavity as this minimises the thickness of the wall. I said that I was concerned about gaps in the insulation reducing its efficacy, but apparently if it is installed right (taping all joints) then it is a good method. Is this right?

The architect is correct in that his specification is OK - except that it needs to be installed correctly and with care. Taping the joints will help a lot but airtightness tape is expensive (north of 20 quid for a 25m roll) and unfortunately standard gaffer tape or duct tape will not do as the adhesive dries out in a few years which then renders it useless and you can't go bach and make good the failed tape. What are the chances of your builder doing a 100% job ? Also if the wall adjacent to the conservatory needs to be built from the inside then putting in the cavity insulation correctly fitted and taped on the outside of the inner skin will be v. difficult bordering on impossible. If a cavity wall is a must then for this wall at least I would go for full fill as it is built.

Posted By: mattpEWI with brick slips sounds like it could be a possibility assuming a reasonable match to the existing bricks can be found.

Brick slips are expensive to buy and labour intensive to install. Putting EWI on a cavity wall requires that the cavity has no air flow through it (otherwise the air flow will nullify the EWI). The usual way to ensure the cavity is suitable for EWI is to full fill - but if you are doing full fill cavity the additional advantage of EWI is v. marginal.

Posted By: mattpOne of the walls is going to be built up against the party boundary against my neighbour's conservatory, so any insulation / covering will have to be installed as the wall goes up.

How much space is there between the new wall and the boundary, or if on the boundary then between the wall and the conservatory ?

If the conservatory will cover all or most of your new wall then there would be a possibility for this wall to be single skin with EWI. The render finish would have to go on as the wall is built, a bit of a pain but doable.

If the upstairs is pebble dash and one side wall is (more or less) covered by your neighbours conservatory then would single skin with EWI be so out of place as the back wall would match the upstairs (rendered) leaving only the side wall as a render instead of brick. If the return to the boundary on that side was also rendered then the whole of the rear of the house is rendered up to the fence line of your neighbour without the conservatory.

Posted By: Peter_in_Hungary
The architect is correct in that his specification is OK - except that it needs to be installed correctly and with care. Taping the joints will help a lot but airtightness tape is expensive (north of 20 quid for a 25m roll) and unfortunately standard gaffer tape or duct tape will not do as the adhesive dries out in a few years which then renders it useless and you can't go bach and make good the failed tape. What are the chances of your builder doing a 100% job ? Also if the wall adjacent to the conservatory needs to be built from the inside then putting in the cavity insulation correctly fitted and taped on the outside of the inner skin will be v. difficult bordering on impossible. If a cavity wall is a must then for this wall at least I would go for full fill as it is built.

This is useful, thank you. Especially your point about building from the inside.

Posted By: Peter_in_Hungary
How much space is there between the new wall and the boundary, or if on the boundary then between the wall and the conservatory ?

I have attached a photo so you can see what I am talking about. My neighbour and I currently have a conservatory that straddles the party line. It is a timber construction with a timber wall dividing them in the middle. The lean-to will be rebuilt on the same footprint as now, but extending across to the boundary with the neighbour. It will be about half a metre higher than currently built (similar to the neighbour's extension to the left of the image). To avoid having to rebuild the neighbour's conservatory, the plan is to build the new wall close to the boundary, but not across it. The neighbour's conservatory roof will be joined to the new wall with flashing or similar to prevent the elements getting to the now exposed timber wall.

Posted By: Peter_in_Hungary
If the conservatory will cover all or most of your new wall then there would be a possibility for this wall to be single skin with EWI. The render finish would have to go on as the wall is built, a bit of a pain but doable.

If the upstairs is pebble dash and one side wall is (more or less) covered by your neighbours conservatory then would single skin with EWI be so out of place as the back wall would match the upstairs (rendered) leaving only the side wall as a render instead of brick. If the return to the boundary on that side was also rendered then the whole of the rear of the house is rendered up to the fence line of your neighbour without the conservatory.

Maybe you are right. I will discuss this with the architect and see what he says.

That's an unusual construction. Just to be clear, the photo is of the neighbour's house? Otherwise I'm confused about your mention of their extension.

As far as insulation materials go, I far prefer inert, i.e. not foamed, materials such as mineral wool or, even more preferable, wood-fibre insulation, though this would be not suitable for a cavity wall.

The main reason against foamed materials, such as PIR, EPS, XPS etc. is that I expect some off-gassing over time that I would not want to have in my house or even the wall cavity. As far as PIR is concerned, it is generally known to shrink a few percent over time, opening up gaps where there weren't any on installation.
Using high quality architectural tape over joints may accommodate this but the thermal bypass is unavoidable.

You won't get any of these problems using compliant ("squashy") insulation materials such as mineral wool batts or beads (known as "blowing wool", used to be a common solution for retrofit cavity wall insulation) that fill the available space and (in the case of batts) can be compressed a little against each other to ensure a durable continuous insulation layer. No off-gassing, too.

Where possible (not in a wall cavity though), I would always go for natural insulation materials, such as wood-fibre. They store carbon and, in the event of a fire, do not produce extremely toxic smoke that kills far more quickly than any fire. In so far, PIR is the last material I would install.

See more details in this scientific research paper:
"Assessment of the fire toxicity of building insulation materials", Anna A. Stec, T. Richard Hull, University of Central Lancashire, 2011
https://www.sciencedirect.com/science/article/abs/pii/S0378778810003671
If you can't get access to the full text then get in touch and I can send you a copy.

A final thought on full-fill cavity wall insulation (if a cavity wall is what you go for eventually): if adding EWI, there should be no danger of moisture passing through the insulation through the cavity, as is often a concern with full-fill cavities, as the EWI and its finish/rain screen should keep everything dry and, importantly, warm and thus also much less susceptible to interstitial condensation should your internal vapour control layer be poorly installed or compromised over time.

My first thought is 'Wow!' 220mm internally. Really? It's not technically impossible, and with a *fullly ventilated* cavity, could be 'safe' but most might think 220mm IWI 'brave'. Is (s?)he proposing flexi WF in a 'spaced' frame, or rigid WF on battens? If the latter have you seen the price of 260mm stainless screws? No, nor have I, but they won't be cheap!

The other issue is that the requirement for a fully ventilated cavity means that if you have single vertical battens you will need a minimum of 2 c150 x 225 air-bricks *between each pair of battens*. I cannot remember the source for this but it's self-explanatory unless you were to have a further cross-batten in which case say 4 air bricks at diagonal corners could suffice (or might not, because of course we have no idea of the size of the wall).

I may of course have misunderstood the proposal so yes, please, to the specification (and any dwgs).

Here is some of the text copied from the drawings below, since the images are hard to read.


Wall construction to comprise:
- 102.5mm Facing brickwork to match the existing house
- 50mm cavity space
- STEICO multi UDB vapour permeable membrane sealed at overlaps with airtight tape
- 80mm STEICO Therm Dry within the cavity
- 100x50 treated softwood timber studs at 400 centres with 100mm STEICO FLEX 036 between the studs
- STEICO Multi Membra 5 vapour control layer sealed at overlaps with airtight tape
- 40mm thick STEICO Internal boards lined with plasterboard
- 2.5mm thick plaster topcoat, hand trowelled smooth
- Include for Gyproc metal angle beads as required
U-Value: 0.177


Internal studwork leaf tied with stainless steel masonry ties, 600mm horizontal centres and 450mm vertical centres, staggered wall ties at openings to be no less than 300mm apart vertically and no more than 225mm from opening


Roof construction to comprise:
- Tiles to match existing on 25x38mm softwood timber battens and 25x38mm softwood counter battens to allow for effective ventilation
- STEICO multi UDB vapour permeable membrane sealed at overlaps with airtight tape
- 80mm STEICO Therm Dry above the rafters
- 150x50 treated C24 timber rafters at 22° at 400 centres with 150mm STEICO FLEX 036 between the studs
- STEICO Multi Membra 5 vapour control layer sealed at overlaps with airtight tape
- 40mm thick STEICO Internal boards lined with plasterboard
- 2.5mm thick plaster topcoat, hand trowelled smooth
- Include for Gyproc metal angle beads as required
U-Value: 0.149


over fascia ventilator to provide low level ventilation

Floor construction to comprise:
- Floor finish to clients requirements and flush with the existing on 18mm exterior grade plywood deck.
- STEICO Multi Membra 5 vapour control layer sealed at overlaps with airtight tape and to be continous with wall membrane.
- 200x50mm treated C24 timber floor joists at 400 centres with 200mm thick STEICO Flex between
- Min 150mm ventilated airspace
- 100mm concrete slab on damp proof membrane on 150mm compacted hardcore
U-Value: 0.146

Sorry about that. I have uploaded them on google, where they seem to be more legible:

https://photos.app.goo.gl/n6wUkinNdSNZywfu9

https://photos.app.goo.gl/RAP7rRGx32Lp5UdL6