External wall finish

As long term forum members might recall, I have been refused external wall insulation because my house can be seen from a conservation area (with great difficulty). When said area was built there was an orchard here, but that’s another matter.

I need to renovate the outside of the house without insulating or rendering it. I have “solid” masonry walls which are not airtight and not particularly waterproof. Fortunately I have deep soffits and am not in an exposed area.

I would like to treat the walls to make them air and liquid water proof but still breathable. I would further like to use a paint for this. The council planning office have said that white would be considered favourably and as luck would have it that was my second choice (after sandstone, which would admittedly give the impression that the building was stone and not brick).

Blowerproof do a white paint that would seem to meet requirements but can only be in UV for three months.
Intelligent Membranes (aka Passive Purple) do weatherproof paint in black and dark grey (I know, who wants a black or dark grey house?!?).

Any suggestions?

Exposed/face sealed paints aren’t durable. They’re exposed to temperature cycling, UV, may blister due to salt build up and freeze thaw cycles. I’d consider it only for aesthetic purposes.

With respect to durability and performance, what I understand is that the bricks aren’t the main issue. The primary source of moisture absorption and air infiltration will be through the grout/mortar. The quality and mix of the grout, and how well it’s repointed, will have a large impact. This can include microporous additives. You need a good bricky.

The use of silane/brick creams are probably only advisable if large amounts of IWI (+100mm) are feasible.

Ultimately, planning laws will change to allow different finishes once the priories adapt to reflect the urgency of the climate crisis. Until then, we’ll have to put up with solid masonry wall needing more thermal energy to stay durable, or we use moisture tolerant building materials.

My thought entirely but the government record on rapid enhancements is abysmal and I was hoping for an enhancement that wouldn’t interfere with and hopefully enhance future retrofit.

I did think it was a tall order but there’s no harm in looking into it.

Hi Tim,

Did your borough commit to any CO2 reduction targets for 2030? If so, perhaps you can take your planning application to appeal and encourage some councillors to reassess the decision on that basis?

That happened locally to someone I know in our town, and designers of The Entopia Building retrofit in Cambridge did the same when specifying very different windows to replace the traditional, original multi-pane sashes. There must be other examples of that happening in London, especially given the ambitions of the London (local) Plan.

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I’m looking at a so-called insulation paint for internal and maybe external for my 1975 cavity wall house. The manufacturer will send me the results of the BRE test any day now. Initial findings are looking good. But, I don’t want to promote it yet until I get the test results and have tried it myself. I have a painting and decorator lined up to do the work in July/August this year. Currently the paint is only in white, but I’m told it can be mixed by their distributor into other colours. The paint, requiring at least two coats, is expensive around £600 for a 20 litre tin, but almost certainly cheaper than IWI or EWI.

Watch this space (to dry)

I have to say that insulating paint doesn’t sound realistic. If it’s been sent to BRE for testing I suppose at least the manufacturer has faith in the product.

This will only be changing the emissivity of the surface. I’d be cautious if there are any condensation risks.

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Brick slip any good??

As an insulator? No.

@Tim_Gilbert @Frank_Reif Here’s the link to insulation paint I mentioned.

What are your thoughts? What is missing to help your understanding of how good it is? Is there anyone in this Forum with building physics expertise?

Note in my case, I’m looking to put this paint internally on external cavity walls. The experiment appears to have been done using a solid-brick wall.

There are only two modes of heat flow that matter here, either conductive, or radiative. We can ignore mass exchange/air leakage for a sec.

Insulators designed to reduce conductive heat flow all do so diffusively. Essentially, it’s fibrous materials or polymer chains that slow the jiggling of heat through the solids, between which small pockets/pores of air are formed that reduce convection.

Beyond a certain point, you can only improve the performance of conductive insulators by increasing the depth. Even aerogels, the best performing substance outside a vacuum, at 1 mm thickness would not confer good conductive thermal resistance.

Therefore, the only significance of an “insulating” paint is to reduce radiative heat loss. Metals/foils are one such way of doing so. I suspect they use a coating which is has low emissivity. Much like a space blanket in survival situations, it’s good at reflecting heat from warmer objects. But the air and surfaces in the room will remain cold.

I personally think this is a deceptive product, who’s authors and funders should be treated with suspicion. I would not be surprised if this is a petrochemical product which has dubious influences on indoor air quality.

As a rule of thumb, there’s no new material that we’ll discover to substantially change what we do already.

The results seem very improbable and implausible. I certainly wouldn’t want to be one of the pilot customers, even if it was free to such people. I would want to have at least empirical evidence that the product worked in real live and didn’t generate unintended consequences. Think “Preston”.

If the paint is thermally reflective it shouldn’t qualify for a U value in its own right and I was pleased to see that one wasn’t given, only the before and after U values. Contrary to what the article states, it will not improve EPC grade until the BIM database is upgraded to include it, and quite possibly then only for the full EPC created at design stage, and not the rdSAP truncated database.

I await the peer reviewed research paper.

Thanks @Frank_Reif and @Tim_Gilbert Attached is the report produced with Innovate UK funding that they made available to me yesterday. I haven’t fully digested it yet.
Zenova IP LJMU BRE 1920S House Project Report.pdf (2.5 MB)

Any further thoughts very welcome

I have read through the report and didn’t really know what to make of it. It almost reads like a marketing document. To have any weight it needs to be peer reviewed and I am by no means a peer in this context.
Is there anyone on this forum with building physics knowledge and experience who would care to take a look?

I remain deeply sceptical. Initially because I have never seen this sort of approach in any project, unless the idea is being ridiculed. I’ve studied hundreds of case studies. Insulations paints is a running joke in the building science community.

It’s been a while since I did a practical experiment using these surface temp transducers. However, I’ve seen PHDs with dubious results, so it’s best just to stick with what we already know how to do.

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When you say ‘masonry’, do you mean stone, Tim? I’m wondering about EWI with some sort of cladding that replicates as close as possible the current appearance of your house.

Red brick, now somewhat porous. There are random gaps in the inner part of the wall (as seen when insulating window reveals) and hence the word solid in quotes.

Adding slips to the wall is a good way of approximating the original appearance, but not, to my mind, of enhancing it. If I lived in a detached house people would quickly forget the original appearance and accept the modified look, but in a terrace or semi the new look would stand out like a sore thumb.

Brick or stone slips are expensive, particularly if you are trying to match an existing look. Fitting them is also expensive, so the cost of the insulation project does go up a lot.


Here is the photo of the external wall insulation with a brick slip finish I took at Salford University.
Its in a public area alongside the demonstration house.

It certainly looks neat. Do you know how much it added to the overall depth?

On Zenova IP: the report is odd, but even if the performance is as claimed, 1mm of the paint is pretty much irrelevant in the context of a useful improvement to the wall. They say that the U-value was reduced from 3.37 to 2.27 W/(m2K), i.e. the thermal resistance was increased from 0.30 to 0.44 m2K/W. According to Table 4.3 of Building Regs Approved Doc L1(a), the minimum allowed U-value for an existing wall which is improved with internal or external wall insulation, is 0.3 W/(m2K), i.e. a thermal resistance of 3.33 m2K/W.

So of the required 3.03 m2K/W required increase in thermal resistance, 1mm of the paint has achieved 0.14 m2K/W, less than 5%. I wouldn’t waste my time with it, even if the test figures are valid. I certainly wouldn’t believe their untested assertion that increasing the thickness from 1mm to 6mm will yield six times the thermal benefit.

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