Deciding between Internal and External Wall Insulation


Appreciate this has already been discussed on other topics, which I’ve read and tried to understand but I’m fairly new with this and was hoping to get some advice for our particular situation.

We are having a builder do some retrofit work as part of an extension project, including swapping boiler for ASHP, MVHR and UFH downstairs. The property is a 1930s semi that has already had an extension previously. Both the previous extension and new extension has CWI. The original property has I believe blown in CWI, but the builder has said that this is with rockwool and is patchy.

The options suggested to us are:

  1. Internal wall insulation either using insulated plasterboard (builder), or insulated plaster (architect) on the gable wall. This would cover all the gable wall downstairs, but not between floors and not in the bathroom/bedroom parts of the upstairs (so maybe only 1/3rd coverage). We’d lose maybe an inch on the inside of the gable wall which is mildly inconvenient at worst.

  2. Fix the cavity wall insulation - potentially with bonder beads, but would have to clear the cavity first. Would probably need a survey to determine what the situation is.

  3. External wall insulation, after making sure the cavity is closed with air pressure tests.

Our questions are:

  1. Should we be looking to do one, two, or maybe all 3 of the options? Or are there diminishing returns? If doing both EWI and IWI are there extra considerations to make them compatible (think I saw something about an overlap…?)

  2. Having read some of the discussions on IWI it seems like there are risks with it in terms of moisture/thermal bridging - we’d be relying on the builder to make sure that these risks are mitigated, but is it worth the hassle?

  3. Is it silly to only improve the gable wall insulation - will it all just leak out the front/back instead?

  4. Is it worth improving the CWI if it’s patchy or is this not a huge problem? Is EWI less effective if patchy but not air flowing through?

  5. For EWI if it’s just the gable wall are there extra considerations on the corner edges to ensure it’s water tight?

Are there other questions/info we should be looking at? We were hoping to be guided by the architect/builder on this, but there seems to not be a consensus so it comes back to us to make a decision on what we want (which is a nebulous ‘good’ amount of insulation for not too much money and that can be achieved as part of the original building project)

Thanks, sorry for rambling!

From one Tim to another, welcome to the forum.

Unfortunately your last point is spot on. There is no definitive answer and the final choice is down to you. Members of this forum have done any combination of your choices or done years of research and still not done any. What they mostly have in common is that their research and training sessions they have attended have all been focused on their case and their homes. You need to take that into account when reading their sometimes conflicting advice.

You are correct that an overlap is recommended between EWI and IWI. This is to avoid thermal bridging at the (non-)join. IWI can be implemented with more or less any condition of cavity other than wet. EWI needs as a minimum a sealed cavity and at best a fully insulated one. My first cavity walled property had two airbricks roughly opposite each other but not aligned, so sealing the cavity would have been quite an undertaking. I put foam CWI into that one and ended up with an uninsulated patch between the airbricks. Incidentally, foam insulation has a bad press but is perfectly good in its niche.

My preference is for EWI, which I don’t have yet as I need to wait for my council’s planning policy to become more energy focused. I could have just gone for IWI instead but decided to wait it out and get on with other measures in the mean time. The house needs major work to its rendered walls and some of the EWI costs can come from the maintenance budget.

Extensive use of both internal and external insulation on the same wall is not recommended without specialist help and surveys as you could easy inhibit “breathability” of the wall fabric, however I had no qualms about insulating inside fitted wardrobes to prevent cold walls and air in the confined area leading to damp and potentially mouldy clothes. The insulation will remain in place after EWI is fitted.

Please provide yet more detail about your house.
Semi or detatched?
Exposure to wind and rain?
State of brickwork and pointing or render?
Frequency of subzero temperatures?
Is the MVHR in place yet?
Permeability of current wall coverings, in and out?

Thanks Tim, that’s helpful.

To answer your questions (sorry should have put some of this in the first post but got lost!)

Semi, gable wall facing NNW
Not very exposed, subzero a couple of times a year (close to manchester city centre)
Good brickwork (I think…)
MVHR not in place yet
Not sure on permeability - how is this determined?


There is a scale of measured permeability that requires specialist tests. I should have worded myself better.

Is the wall surface/covering permeable? A yes/no answer is all that’s needed, at this stage anyway. Impermeable is wall sealing paint or vinyl wall paper. Any plastic paint can be considered impermeable. Clay or lime based paints are not.

Ah ok - currently it’s stripped back to brick (builder thought we were definitely doing IWI), so could be anything

Ok. So currently you have a permeable wall. You also have a good, accidental, opportunity to check for moisture and integrity of the inner leaf. If anything needs fixing get it done now.

As you have reached this point it does swing the balance towards IWI, as you need to resurface the inside of the wall anyway. You mentioned in your original thread that IWI would not go below the floor or between stories. Why not? You could trim back the ceiling plaster and floor boards and take the insulation from DPM to abutting the floor above, to be picked up when that room is done.

Assuming that a 1980s house had dot and dab it was as well to strip it off before IWI. Wet applied plaster in good condition does not need removing first and gives a good base for insulation, provided all organic matter is first stripped off, such as paper, paste and wooden dowels used for fixings. You need to remove all “mould food”.

A bare brick or block wall needs parging first to smooth it and remove potential air pockets that would form behind the insulation. A good parge will also improve airtightness.

Currently we’re not having work done on the two rooms that also on the gable wall upstairs - one is used as storage during the work too, so makes it difficult to include in the project. The bathroom is above the kitchen which is currently gutted so could access the space between floors there, but the bedroom is above the hallway which currently has the ceiling intact.

Some electrical issues were discovered recently so work will have to be done in both rooms eventually - just might have to wait a while till we can move the stored things out into the other rooms upstairs first…

Do you know of any risks in having the IWI only partially covering the whole wall? Would it be that we have to do the remaining rooms with some urgency to avoid compromising the work already done, or is it just a case of it only being 50/60% (made up numbers) effective until complete?

I will get the builder to inspect the inner leaf with those questions in mind, if they haven’t already


One of the beauties of IWI is that it can be done piecemeal as you do up each room, however the effect of what is already done is much less than the area covered, due to the heat that would have been lost through the treated bit of wall migrating to untreated areas. You also need to be extra careful about heating and ventilation near the edge of the insulation so as not to get condensation forming at the nearest points to the now colder section of outside wall. This is why an overlap is required with EWI, as the two insulation layers do not abut.

It is good that you are considering electrical work as that enables you to move all electrical fittings to internal walls.

I think we’re still leaning towards EWI despite the wall being partially stripped already. That opens up a couple of other questions:

  1. Should we just put normal plaster back on the wall where it was stripped, or does it still make sense to parge it and maybe put some better plaster on (not sure at what point it becomes IWI…)

  2. Should we do IWI on the front/back of the house as it will be difficult to do EWI there - does that then raise overlap issues at the corners…?

  3. My partner likes the idea of diathonite - is this better than insulated plasterboard? Or just aesthetically different? Sounds like it would be thicker? (builder offered 25mm or 50mm thickness on the insulated plasterboard when talking about IWI)

Diathonite is a great product. It has been successfully used for EnerPHit projects and I think retrofit Passivhaus. Diathonite can be bought and applied exactly as plaster or you can get their installers to spray it on. The spray can be applied between wall and first parallel joist where manual application can be a problem. Paying for a crew to spray apply Diathonite probably isn’t worth while if only doing one room. It’s more a whole house approach.

You won’t need to parge if using Diathonite. The minimum depth is 2.5cm but it can be layered up to almost anything you want to reach a target U value. 2.5cm should be airtight. Being vapour open you are free to do virtually what you want (if anything) to the outside surface, by way of insulation and trim.

Insulated plasterboard is not so vapour open and needs careful application to be airtight at the joins. It is, however, a lot cheaper than Diathonite.

If using plasterboard I would be very cautious about coating the external surface of the wall.

You are correct about overlapping IWI & EWI. The recommended overlap is a minimum of 1m. This leads to awkward aesthetic choices, unless you happen to have a suitable feature one meter from the corner. I’m thinking of an alcove or chimney breast that you are retaining. The distance has been calculated previously based on the U value of a metre of brick wall.

Thanks for answering all these questions, it’s been really useful.

Being vapour open you are free to do virtually what you want (if anything) to the outside surface, by way of insulation and trim

I might be misunderstanding this, does this mean we could use it as IWI alongside EWI without needing overlap?

Separately is this the sort of overlap you’d need with combining IWI/EWI (green EWI, blue IWI):

So there’d be some funky lip where the IWI ends? Guessing wrapping the EWI round the corners is probably more hassle than it’s worth if we need to modify the roof…

You can use the IWI as overlap to the EWI but you can’t go from EWI to no insulation. You could maybe do so short term but you have to act fairly quickly to correct the situation.

The diagram is correct. In your example, and most cases, the join is at a corner. Either or both layers can wrap the corner but as you say, there are usually restrictions preventing use of the EWI.

Note to fellow members:

I am not happy being the only source of responses to this topic when I know that there are some among you that have extensively researched the topic and others that have undertaken similar work.

I would very much appreciate counter points, views and advice.

Edit: my own advice is based on the AECB CarbonLite Retrofit course and subsequent research into what was right for my home, which turns out to be very different from what I had in mind when I started.

Thanks again Tim, I feel like we have maybe asked too many questions, but we really appreciate the advice you’ve given - and if anyone else wants to chime in then we would also appreciate that!

I think we’re close to deciding on EWI for the gable wall, just need to make sure it’s sealed, then we’re going to work out what to do with the front/back as that’ll probably be some form of IWI as we can’t do EWI without affecting drains from the other semi. Diathonite sounds good, but not sure if it’ll eat into the room space too much!

Make sure the whole house has a sealed cavity, not just the wall you are insulating. Otherwise the air warmed in the cavity will migrate elsewhere to escape.

If you go for the minimum Diathonite simply as a plaster rather than insulation you can get away with 2.5cm, not much different from ordinary plaster but fully breathable and slightly insulating. You will get a faster heat up time for the room, which might be beneficial, bearing in mind that you need to replaster anyway.


Edit: see also

Don’t be put off, the page is in English!

You can never ask too many questions but you can get too few answers.

My answers naturally express my point of view. Others will differ and it is the resultant discussion that helps knowledge to develop, not just yours but mine too.

Do you (or anyone else reading!) know of any companies that do surveys to check that air isn’t circulating inside the cavity? We’ve got CWI but the builder thinks it’s patchy, and I’m not sure how far around the front/rear it goes. My googling just brings up companies doing air tightness checks for new build interiors…

They may not advertise as such but the age of the property is irrelevant to the rest process. I had a test of our house a few years ago. Built in 1937, better than 2018 regulations. In other words, better than rubbish! 7 ach against 10 ach.

Anyway, to the point in hand, a CWI company could advise. You also ought to find out whether the cavity is shared with your semidetached neighbour although I don’t think that practice continued into the eighties.

If you can lie on the loft floor and see the cavity, then clearly it is open. Photos of such have been posted on this forum and my first cavity walled property was like that too.

Thermal images can also help, taken at dawn on a cold winter’s day with a well heated house.