Hi everyone

About 10 years ago I insulated the two smallest rooms in my house internally which are both solid brick wall.

I used Insulated backed plasterboard with 25mm PIR at the time.

Now I'm considering external wall insulation. Probably will end up using XPS on the outside, but want to look into PIR as an option.

In an ideal world I won't rip off the internal wall insulation and all that would entail. But if I keep it, presumably there is potential problems in the brickwork itself if I insulated externally. XPS breathes, but PIR doesn't, but maybe the render wouldn't breathe anyway.

Or should I just not worry about it? I did a pretty good job of taping and sealing each board, but have of course drilled holes for fixings.

Any thoughts. Thanks

+1

Thanks guys.

Front wall no limit on thickness. The side wall goes into the alleyway, and a good chance trucks that go down there may bash it if too thick. So whilst I will probably go EPS, let me just make sure my understanding is correct. If I did go for non breathable insulation and it was insulated internally and externally, that effectively seals the wall on 2 sides, but if I haven't sealed all anchor holes properly or around the edges of the insulation, and moisture gets in it, it could get trapped and cause problems?

From a condensation POV adding EWI only improves matters over the current IWI-only situation. From a moisture-movement POV, it's as Tony said.

Unless the wall is soggy with an obviously-failed DPC (or the building predates DPC) or water leaking in from above somewhere I wouldn't worry much about it collecting too much water, so long as the IWI is reasonably well-sealed to the wall and there is not an airtightness problem. Water vapour diffusing in is not going to be a problem, it's air movement moving lots of vapour in _and_ _then_ _it_ _condensing_ which can move enough water to cause trouble, but if the EWI is decent (i.e significantly better than the 25mm PIR IWI) then the wall won't get cold enough to condense water anyway (typically needs to be <13C), except maybe very occaisionally. And if the airtightness is decent then there won't be massive air-movement through the wall anyway.

If you are worried about bashing the wall consider woodfibre as well as EPS. It's significantly more solid. Those are the two main EWI forms. Woodfibre is a lot more breathable, better decrement delay and lower carbon, but more expensive, and much less robust to water ingress/bad detailing.

I see Kore have just started making low-carbon platinum EPS: https://www.kore-system.com/products/kore-low-carbon-insulation/ (In Ireland, not sure if there is a UK supplier)

Thank you guys for all your comments. This is a great help.