External or Internal Wall Insulation (EWI or IWI)?

We’re considering external-wall insulation, on the gable-end only of our red-brick semi-d; Nick Parsons seemed to suggest this can often be worth doing, even if not doing the other two external walls (for various reasons.) Interested in any opinions on this, and any possible Gtr Mcr-area recommendations to do the work. PS cavity-wall insulation going in soon (at last.) Thanks!

Due to the relative overhangs of gable and eaves it is often easier to insulate the one wall. It does make significantly less impact than insulating all the walls and needs very careful finishing off to prevent penetrating water at the edges.

Given the costs involved it is unlikely to be worthwhile unless you are putting IWI in the rest of the house and/or the gable end is a) in bad need of rendering b) more exposed than the rest of the house. Where there is a discontinuity between EWI and IWI you need to address thermal bridging.

EWI does not make a wall airtight, so that needs doing too, on the inside (this bit doesn’t only apply to gables). Airtightness measures are ALWAYS on the warm side of the insulation. Putting airtightness membrane inside and insulation & render on the outside will trap any humidity in the wall, so if the wall isn’t already airtight you need to address release of trapped moisture.

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If you have at look at superhomes there are several examples of EWI on single walls and there should be examples over a number of years. I am sure I saw one in Chorlton.

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Thanks for those points Tim. 1) Our gable-end is a large proportion of our three external walls - and the other two external’s are primarily windows/doors…awkward for EWI, but might replace glazing. 2) I’m unclear re airtightness requirement, prior to EWI; is it the case that (internal-wall) airtightness is necessary for successful EWI? Or, is it ‘just’ good practice/desirable? I didn’t see that mentioned in ‘Sustainable Renovation’, Chris Morgan/Pebble Trust, that Carbon Coop pointed me to.

There are several factors involved with airtightness. Uncontrolled air movement through the structure cools a house in winter and potentially warms it in summer (e.g. summer 2022). Also air carries humidity far more rapidly than it distributes itself. As air passes through the structure from the warm side to the cool side it may reach the dew point, at which point the moisture condenses within the structure, this is known as interstitial condensation. If it is in contact with timber or organic matter that leads to rotting. If it is near the outside surface frost could lead to spalling or loss of efficiency of the EWI.

I’m surprised that an authoritative work doesn’t mention it.

The traditional way to build a house was far from airtight but then fuels were burnt in the house and needed an air supply and any resulting damp was at least partly dried out by the rapid airflow.

Until last year a house leaking an entire air change every 10 minutes would still pass building regulations, so there are a lot of new homes in need of a totally new airtightness plan.

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Thanks, think I’ve got that. So: if I airtight the internal surface, then next there’s the EPS-filled cavity, then the external leaf, and lastly EWI layer, where is it likely any water-vapour would encounter its dew-point? - assuming modern/high-standard components (and no previous history of damp walls/condensation.)

In the completed structure, in the outer layers of the insulation or the covering render.

Spalling isn’t an issue with EWI but you could get the render “blowing” from the insulation. It is very important that the render is breathable (vapour open) to allow vapour to escape before it builds up.

Ok, thanks. And what if I didn’t airtight the internal side prior to EWI? Would that be likely to produce bad effects, esp re vapour condensation - or would it ‘just’ lead to a less-effective thermal-insulation result?

This depends on your circumstances, or rather those of your house. Certainly the insulation won’t be as effective if there is uncontrolled air movement. As that probably means uncontrolled movement of humidity (in the air) that could cause various issues. The less humid the place you live the smaller the risk of interstitial condensation.

Thanks Tim, answers v much appreciated! I would probably consider EWI on the (large) gable-end wall, plus airtightness measures on the inside of the same wall, eg strip off existing plaster, tape joints/seams as appropriate, then pile on suitable new internal plaster, as a deep layer…But I’ll get someone to have a look and advise on the details of that.

If you put one to two times the thermal resistance of the CWI as EWI, you should be fine. The ratio determines where the original brick cladding will fall on the thermal gradient - we want it on the warm side most of the time.

Air tightness measures on the interior is always advisable. But as long as the original brick cladding is warm enough, it won’t be significant enough to cause failure in this climate.

Being warm most of the time, it’ll rarely be a condensing surface, and when it’s not, it’ll have a large moisture storage, and redistributive, capacity to make it through adverse conditions.

This also holds for solar driven vapour from the render passing through the vapour open insulation required for EWI - as not to create a vapour “sandwich” with the EPS. I think the risks would be too high with vapour closed EWI - but I’d like to be proven wrong if anyone knows better.

CWI that has not been inspected during construction will likely be poorly installed, so I suspect it’ll have plenty of airflow pathways to cause thermal bypass and moisture transport. Another reason to go with extra EWI.

How that impacts the rest of the geometry by the roof is another story.

I’d like to know what best practices are for the exposed edges on the front an rear elevations, it’s something I’ve been meaning to study.

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You are overthinking this. 2cm or more of wet applied plaster is airtight. If you have plasterboard, replace it with a good layer of wet plaster. Remember to trim back any plasterboard on the front and back walls to allow full coverage. Also, you need to plaster top to bottom without gaps for ceilings, floors and intermediate walls.

In line with the “no gaps”, no sockets, switches, screw holes etc.

I’m assuming your referring to my post.

There will be a discontinuity in air tightness at the corners of building.

The floor joist system may be embedded into the gable wall, which is hard to access and air seal.

This is an issue with phased developments, you have to think harder about the unintended consequence.

Fortunately EWI is the safest option, and the materials can be selected to be all water tolerant.

I’d choose multiple layers of mineral wool up to 150mm.

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@Frank_Reif, I was responding to @mike_killian.

I got distracted (for several hours) and in between you posted your excellent answer.

You are correct about embedded timber. A true bane of any retrofitter. Even plastering between storeys won’t fully airtighten them and additional airtightness tape will probably be required.


Thanks Franks_R and Tim_G for your v useful comments. (I had assumed Tim_G’s comments were referring to my own earlier post!) Now I’m leaning towards IWI inside on gable-end wall, 2cm++ suitable wet plaster, in place of current v old (albeit still fairly sound) layer, ie straight onto the bricks, with taping as required…How does that sound?

As Frank, @Frank_Reif, said, EWI is always preferable. The only exception is where there is a proportionally small area of EWI that is somehow meant to meet up with a larger area of IWI. This is because of the thermal bridging at the “joins”.

Frank suggested 150mm insulation, for which you need at least 170mm weather protected clearance, to allow for the render. Ideally plus an overhang to stop driven rain getting behind the render. How far does your Gable roof overhang? You might need to reduce the insulation unless you are willing to extend the roof.

Personally I’d love to double that insulation but for my, and most properties, it is impossible. It’s also overkill unless you expect to heat your home with body heat only.

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With IWI you’re limited by a combination of the following factors:

  • value of the lost space, which per mm is more valuable than EWI.
  • the risk of interstitial moisture which increases with more thermal resistance inboard of the structure.
  • the diminishing returns of additional thermal resistance of the whole assembly (its closer to an exponential decay than a linear increase in value).

In which case I’d suggest a smoothing parge coat (gypsum, lime or cement - all of which have similar properties despite a disproportionate fuss made over them) on the condition it dries thoroughly and there are no vapour closed paints from previous redecoration.

All you hope to achieve is a flat enough surface for the insulation to press up against in order to limit small voids and gaps, for unintended air flow to occur.

In your case, due to the CWI, it’s likely safe to go up to 100mm of a vapour open semi rigid insulation board - it will probably be limited by the total depth of the build up and how it impacts your living spaces. But, having said that, the only way to be sure is with heat flux survey in the highest risk areas.

No vapour barriers. I would not bother with smart membranes in this case. However, airtightness is worth ensuring with a membrane.

A service cavity is worth it in my opinion. This 25mm cavity can house conduit for electrical cables and data. Put extra conduit in for the next electrician. I would include data to window headers, as automated blinds are likely to become standard.

I would use higher density plaster board for sound attenuation and reactive passive thermal mass. It’s not clear if it’s worth insulating the service cavity.

All of the difficulty will occur around floor systems and window openings.

I would not worry too much about insulating along internal solid walls to reduce thermal bridging. The additional complexity and cost is almost certainly not worth it.

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That’s service space between the insulation and living space, not behind the insulation.

If you have an alcove or some other existing wall length limiter I would insulate a party wall that far but no further, otherwise, as @Frank_Reif says, it isn’t worth it. On the other hand if you know that the next door neighbours live like Eskimos, maybe you should insulate the whole party wall. (That bit also applies for those fitting EWI.)

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Thanks Frank. So, you’re suggesting a possible way forward would be *leave existing plaster in place (approx 30-40mm), seeing as it’s in good nick *parge it *add airtight membrane, but vapour-open *affix high-density plasterboard, up to 100mm *add 25mm service cavity, for future cabling etc.

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Between “leave it” and “parge” I would put “strip wallpaper/plastic based paint”.