Internal vs external wall insulation

Join Retrofit Coordinator Dr Sarah Price and learn how to choose between external or internal wall insulation.
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[enemyofthestatewearein]

Great video, this is all good advice but I think it's worth noting too the following :

1) The maximum thickness of external wall insulation may be limited by the depth of existing roof overhangs (i.e. soffits and/or gable verge). Also meter boxes, conservatories, bay windows and external soil pipes are difficult to move or adapt, so normally these just get insulated around, leaving a significant thermal bridge.
2) External wall insulation material must be water resistant (e.g. EPS/mineral wool), these have significantly higher lambda value than PIR board (e.g. Kingspan/Celotex) which can only be used internally. Thus by insulating walls internally, it's possible to get about 1.5x better insulation for the same thickness. So in truth, Internal insulation can be significantly better value vs External because it's possible to get 1.5x more insulation for similar (or even lower) cost - but note that this does of course depend on the spec and materials used in each case!
3) With external wall insulation, it's difficult/impossible to achieve continuity between the wall & roof insulation, leaving a thermal bridge in the gap. This is an important junction where both heat loss and condensation risk (from internal, airborne moisture) is high, arguably this area is higher risk than ceiling cavity & joist ends between floors, where heat and moisture can escape into the room above, instead of into the building fabric.
4) In the UK windows are normally replaced from the outside. External wall insulation (EWI) will therefore make later window replacement more difficult/expensive as the insulation & render will need to be cut back and repaired after. So if you are installing EWI then it's worth considering if windows will need replacing as it's better to do this job first. On the plus side, EWI may allow to reduce the size of large, drafty full height windows often found in 1960s - 1970s houses, as the EWI & render can be used to cover and restore the external facade below new, smaller windows. With internal wall insulation, windows can be replaced as-is, anytime, without any additional work or cost.
5) It is perfectly possible to make internal insulation continuous, into window reveals, between floors, and into the loft, to effectively eliminate thermal bridging and improve air tightness. However this is both disruptive and labour intensive as it requires removal of part of the ceiling/floor, careful vapor sealing around joists, and correct fire-stopping details. Unfortunately, there are currently very few contractors who know how (or care!) to do this properly and poor installation may result in condensation inside the wall, with consequent risks due to frost or rotted timbers.

I would therefore agree that overall, for a basic simple install, external insulation is the lower risk option (but do the windows first, if needed!). However if you are looking for maximum energy savings/best value, then my view is that internal insulation, where it's possible, will usually be the better option, particularly if it's done alongside a wider refurbishment e.g . re-wiring, new heating, new kitchen or bathroom etc. However internal insulation does need a proper assessment, design, and careful installation to mitigate potential risks from interstitial condensation, and it is currently difficult to find both designers and installers with sufficient expertise.

[orange]

With all insulation pay special attention to ventilation and dehumidification but even more so when you seal the outside of the building .

[andrewfitzsimons]

You need to watch skill builder’s take on EWI as he would argue against it with facts .

[ckafam]

Hi, Great video. Short and to the point. Thank you Dr.
I am considering External wall insulation for my brick vaneer single story house. The house is on a concrete slab on ground therefore I am unable to insulate under the slab. I have seen examples where a trench is dug up to 2 feet deep around the slab perimeter and the EWI is set into the trench to minimise the thermal bridge at the edge of the slab. How much of a difference does this make as opposed to not trenching? I am in a mild climate where it rarely gets below zero in winter. Thanks again.

[tedoakes]

Please keep uploading content. Fantastic work.

[Marenqo]

the only thing I don't like is that I would lose the brick look of the house

[brianlee]

Thank you. I'm planning on building a home in Florida where it's very hot. I plan to do exterior insulation with vertical harty board on the outside. In your opinion what is the best construction for a hot climate other than ICF? I was seriously considering ICF but it's out of my budget. But I know in Florida they build a lot of cinder block and they say they work well in the heat. What is your opinion on this? Should I go traditional wood or cinder block or some other option? Thank you for any info you have 👍

[scottwebber]

I put 1/2” sheet foam on the interior underneath the drywall -do u have any feedback on this as I’m under the impression that insulation holds the heat in.

[thomasneely]

Can you have EWI and Kingspan k18 internally

[erickalcala]

how about if only the front is possible for external insulation and the other walls are in the land limit... ? do I go full internal?

[jeremyratcliff]

Sarah. I've sent an enquiry to you regarding a design I have for constructing a passivhaus building. Look forward to hearing from you in due course.

[thewanderer]

I've got a 1920s house with render on brick on the front wall. A renovator has advised not to use external cladding, because of the lime mortar being used for the brickwork. Three builders have said it will be fine. Who is right, how do I make the call :-) ???