It all depends on what you are trying to achieve.

If you want full coverage of a wall with no gaps then go external - unless you have a rendered house it will look very odd.

If you want to make things better, without going all PassiveHauss go internal.

Be realistic - it is really had to get PH levels of air tightness in old buildings and probably not worth the bother.

As with all modern insulation even quite thin insulation makes a big difference. If you don’t have much space you can use a really high performance board.

NB not all internal insulation is equal so you can pay more for the same effect from a thinner board.

In a lot of more minor rooms don’t worry about the decorative plaster - if it is plain just leave it - cut the insulation butt up to it and then get a length of matched ornate plaster made and go over the top if it.

Windows will need to have timber reveals or plasterboard returned to the windows and a new deeper sill made.

TBH adding 50mm of internal mid grade wall insulation to a rear return makes an unimaginable difference to the heatability and coolability of those rooms.

You do need to consider how fire proof the insulation will be.

Foam insulation needs to be Class O which is self extinguishing. In practice this is achieved by the layer of plasterboard stopping v oxygen getting to the material behind. This does mean that detailing is vital.

Specifically it is absolutely necessary to close the top of boards so it isn’t open to the ceiling void. Otherwise fire can easily spread behind the boards.

This is easily achieved by pre bonding slips of red (fire proof) plaster board to the ceiling where the tops of the boards will butt up against.

Happy to chat about this if you want to give us a call.

A friend who is an architect told us that Wandsworth wants a planning application even for external wall insulation at the back of a house, not visible from the street.

Not all councils want it, so it is not clear to us if Wandsworth is overstepping its authority or not.
This is such a shame. At a time when councils and government should be doing anything in their power to facilitate these interventions, they make them harder with unnecessary red tape.

In the end, we did not go ahead, because adding the EWI would have required changing most windows, which otherwise would have clashed against the added protrusion of the EWI. In our case, it would have cost us ca. £20k between new windows and EWI, so we decided not to do it.

Last summer we bought those silver sun covers you put on the outside of a car windscreen, and placed them outside our south-facing windows. It looked awful, and it was certainly not as effective as full EWI would have been, but it did the job.

An alternative in our case could be an internal insulation, basically a layer added to the inside of the room.
We didn't do it because it would have reduced the available space, although not by much, and because it wasn't quite clear to us how effective or not it would have been.

If you learn anything new on the topic, please do post back.
 

Shame you didn't get any replies here. Did you take your project further?

We are looking at similar.
I would agree EWI to rear ought to be tolerated as PD would permit one to say paint your whole house white and rendered EWI would not look *that* different.
Also generally council allow more change at rear of

Council have some vague sustainable goals now in their new local plan.
I can find a few examples of Planning permission on the Wandsworth site.
E.g. 241 Nimrod Road.

I'm also surprised this isn't considered more. Yes it is more expensive but if you are planning building work to rear addition any way e.g. side return /mansard loft extension then the added cost becomes more manageable especially Vs the space loss from doing internal.

Would love to hear more, if you found out more.

Many thanks

Does any one have any experience with adding external wall insulation in Wandsworth?
Specifically, I'd like to understand what, if anything, is needed from the council, and I'd love to hear from someone who has direct experience of this - or at least indirect, if you know of someone who has done it, or of firms which have installed EWI in Wandsworth.

The property is a mid-terrace house, no conservation area, not listed, and we own the freehold.

Some other councils mention that adding EWI is permitted development as long as the appearance remains similar - so I believe it all comes down to how this is interpreted.
https://www.hastings.gov.uk/planning/need/externalwall/

We'd be adding it only to the back, which is not visible from the road, and is currently red bricks.

We will need to contact the council at some point, but it would be useful to contact them once we already have a clearer idea - eg so we can point them to specific cases or details.