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You can do a better job with (1), and comply with the Regs, too.

Posted By: Nick ParsonsYou can do a better job with (1), and comply with the Regs, too.

Sorry too much mess - can't face that!

What I didn't say is that I think (2) AND (1) is the best way to go but I thought you REALLY didn't want to hear that!

Have you considered a future air tightness layer, and how you could build that in, even if gradually?

You also have to consider things like vapour control there.

Facing exactly (and I mean exactly) the same thing here and it looks I should mentally prepare for 1.... And we had the ceilings plastered only 5 years ago :-(

Is it really possible to get as good an insulation installation between the rafters from below as it is above? I mean: insulation touching all planes, no gaps once the p/board has gone back up, hopefully not pushing away any insulation.

Whichever way you approach it you have to consider the air barrier and in many cases access from above/outside will be required to keep this continuous.

I've got the same setup (except I have some (bituminous) underlay).

You can get decent insulation levels by adding some on the inside without removing the plasterboard, which as you say makes a huge mess. Glue a sheet of PIR on the inside as well as putting some insulation between the rafters. That way you can still leave an airgap but also get below U=0.15 on the skeiling section. You are still severely constrained at the wallplate where there really isn't room to do a very good job unless you extend the rafters.

I did your 3 when I moved in a decade ago (using EPS as I didn't know about PIR/PUR then). As you say the main issues are that you have to do it in 2 pieces to get past the joists, you have no idea how well you've fitted to the wallplate at the bottom, and it's a massive PITA to do, lying in the corner of the loft.

Just yesterday I finally got to take off the tiles and underlay and and have a look from outside, as I have a scaffold up (for EWI). The fitting is not terrible but there is a quite a lot of gappage too in places. Now I can finally foam it up.

I only left a 25mm gap with 150mm rafters (I used a 25mm sheet and a 100mm sheet), which of course is not to spec. However it has worked fine, even when we had a massive flood in the loft due to a plumbing disaster which soaked much of the fluff, and some plasterboard, and took a couple of months to properly dry out.

Does anyone know what the angled bricks covering the top of the cavity are for - is it to keep the rats out? For some reason they are cemented onto the outer leaf but just butted up against the inner. So I've removed them and foamed a 50mm XPS piece into the top of the cavity to stop warm air getting out the top, so the EWI works.

The hardest bit is insulating between the top of the EWI/soffit board to the wallplate to join the loft insulation. It important to do this bit because as-built there is just 1 brick between inside and outside, and even after adding the internal PUR there is a great big thermal bridge. It's an awkward shape. You could cut and foam (or gapotape) rigid board, or stuff some fluff in and put a windcover of some sort over. Ideally you want PUR over the really narrow bit where there is only room for maybe 25mm over the wallplate corner.

I worry a bit about covering it entirely and sealing it in with foam/PUR. Now the wallplate can't easily dry if it gets damp for any reason. Fluff plus a breathable windcover would be better in this regard but significantly worse thermally.

What do we reckon?

Hi Wookey,
A month ago I had to treat a small section of the skeiling. Insulation was fitted when the roof was retiled to comply with building regs. Incredibly shoddy job (see the thread/rant about building inspectors) with loosely fitting bits of PIR board thrown in, blocking the required air gap and still leaving holes/gaps all over the place. There is proof of condensation being an issue as there are mould spores on the breathable membrane fitted over the rafters.
"Unfortunately" I have solar panels fitted over the roof, and the pieces of badly fitting insulation in there cannot be removed from above, which means I have to attack it from the inside - OP option 2.
After cutting away a small section of the lathe and plaster (huge mess) I stuffed the cavity formed by the soffit & fascia with mineral wool and covered it by a shaped piece of PIR board that can be wedged into the soffit/fascia corner and fixed in place. Luckily there is no wooden wall plate anywhere to be found. While I was at it I fitted additional PIR on the inside to get to the required U value. I hope to continue this along the full length of the skeilings, but as I have to take it room by room it might take years to complete.
The top of the outer leaf remains the weakest point in the insulation envelope with only 50mm PIR between the brick and the outside world, but there is not much I can do about it.