Green Building Forum - external insulation Thu, 16 Apr 2026 18:41:38 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295090#Comment_295090 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295090#Comment_295090 Sat, 05 Mar 2022 20:47:49 +0000 chrisinbrighton
The walls are all 700mm thick stone and one of these walls is covered in slate with a 50mm gap. I was thinking this would be a good place to add insulation.Is there a particular type which would be best and does a vertical slate wall need a breathable membrane?
Thanks.]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295094#Comment_295094 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295094#Comment_295094 Sat, 05 Mar 2022 22:35:58 +0000 tony external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295097#Comment_295097 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295097#Comment_295097 Sun, 06 Mar 2022 07:14:25 +0000 Peter_in_Hungary Posted By: chrisinbrightonIm just starting to plan the renovation of a Welsh chapel,i can imagine il`l have lots of questions on here!
What do you mean by 'renovation'
Or are we to assume renovation = conversion to a dwelling?
Are you governed by planning restrictions concerning the external appearance (or anything other aspect)?]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295098#Comment_295098 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295098#Comment_295098 Sun, 06 Mar 2022 10:05:47 +0000 chrisinbrighton
Will 50mm of cellotex be hardly any benefit?]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295100#Comment_295100 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295100#Comment_295100 Sun, 06 Mar 2022 10:49:47 +0000 Jonti external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295102#Comment_295102 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295102#Comment_295102 Sun, 06 Mar 2022 11:40:00 +0000 Nick Parsons
Also, while 50mm is too little, as others state, if you use exg battens it's 50mm insulation with full-depth timber interruptions every however many mm. If you *had* to stick with 50 could you perhaps do 25 ins against the wall, battens, then 25 between, then slate.

Agree with Tony re membrane. It wouldn't do any *harm* but you almost certainly don't need it.

Edit to add Conservation Officer]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295103#Comment_295103 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295103#Comment_295103 Sun, 06 Mar 2022 14:10:27 +0000 Peter_in_Hungary Posted By: chrisinbrightonYes conversion to dwelling but listed so slates have to stay(I would want to keep them anyway)

Will 50mm of cellotex be hardly any benefit?
Putting 50mm rigid insulation between the wall and slates will (approximately) change the u value from 2.02 to 0.45 so it would have a reasonable effect. (I assumed a sand stone wall, other types of stone will have different values)

However if that is the only wall you can do and especially if the slates are in a reasonable condition, I would leave that wall alone and concentrate in the internal surfaces.

If you externally insulate on that wall only and internally insulate the rest you will get problems at the juncture of the internal and external insulation (corners) that would make it advisable to internally insulate a good bit of the externally insulated wall to mitigate the cold bridge at the corners so best to just treat the internal insulation as a whole and forget about insulating the outside.

One approach that has been made by at least one other here is to build an insulated box inside the walls of the existing structure as opposed to internally insulating the existing walls. Each solution has its own problems and advantages.

If you want to play the 'what ifs' with various types of insulation and needs for VCLs against what you have to start with then look at
https://www.ubakus.de/u-wert-rechner/?
Use the demo version]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295107#Comment_295107 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295107#Comment_295107 Sun, 06 Mar 2022 17:03:38 +0000 chrisinbrighton The slates have got to come off anyway so it will be a good time to add insulation.I like the idea of doing a two layer unfoiled design(Nick Parsons) and will try and push for more than 50mm.
At present I cant see that the other stone (slate possibly) walls can easily be internally insulated.]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295111#Comment_295111 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295111#Comment_295111 Sun, 06 Mar 2022 18:30:38 +0000 chrisinbrighton external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295117#Comment_295117 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295117#Comment_295117 Sun, 06 Mar 2022 19:06:38 +0000 fostertom
There's a quite modest limit to how insulative IWI can safely be, before it leaves the wall outboard of it so cold all winter through, like it's never before been, that so much winter condensation builds up within it, that it never quite evaporates in summer, leading to gradual accumulation of water in the masonry. EWI has no such problem, thicker the better, as that keeps the wall warmer than it's ever been!

If wanting to IWI up beyond that danger level, that's where Peter's 'insulated box inside the walls' comes in, which must be coupled, strangely, with really free ventilation to outside air of the airspace between wall and box, so the wall becomes effectively a freestanding garden wall, at which it should have no problem. However I doubt you'll be trying to insulate by IWI up to that danger level.]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295179#Comment_295179 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295179#Comment_295179 Tue, 08 Mar 2022 18:06:02 +0000 chrisinbrighton ...and EPS or phenolic are not breathable I understand ?]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295182#Comment_295182 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295182#Comment_295182 Tue, 08 Mar 2022 20:15:31 +0000 djh Posted By: chrisinbrightonDoesn`t external wall insulation need to be breathable.Im assuming warm dampness will work its way through the thick stone walls.Not necessarily. As long as the EWI is thick enough that the temperature on its inner face is above the dewpoint for whatever humidity you have inside the house then there'll be no condensation on the inner surface so no problem. The problems can come with IWI, because it's cold outer face is against the stone (or other) wall surface, so condensation is much more likely.

...and EPS or phenolic are not breathable I understand ?

EPS is breathable to some extent or at least not impermeable to moisture. µ is 20-40 for EPS 70. Phenolic is a lot more vapour resistant, as is XPS]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295297#Comment_295297 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295297#Comment_295297 Sat, 12 Mar 2022 07:29:36 +0000 Beau
In hindsight, it would have been nice to stop the thermal bridge of the uprights to the wall but it's way better than it was. The house used to get condensation on the inside of walls but this now never happens]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295324#Comment_295324 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=295324#Comment_295324 Sun, 13 Mar 2022 22:36:47 +0000 chrisinbrighton external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310283#Comment_310283 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310283#Comment_310283 Mon, 23 Feb 2026 21:20:04 +0000 chrisinbrighton One thing im not sure about is the 700mm thick granite stone wall is very uneven so will there be a `large` air gap(mabe averarage 15mm) between rigid insulation and stone...will this air gap blow away the warmth from the building?
Could it be better to use Rockwool that can squash up against the rough stone?]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310289#Comment_310289 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310289#Comment_310289 Tue, 24 Feb 2026 15:35:37 +0000 Peter_in_Hungary Posted By: chrisinbrightonOne thing im not sure about is the 700mm thick granite stone wall is very uneven so will there be a `large` air gap(mabe averarage 15mm) between rigid insulation and stone...will this air gap blow away the warmth from the building?
IMO yes.If there is any air flow in the gap between the insulation and the wall then heat will be removed which will affect the effectiveness of the EWI.

What I do when installing EWI is to use standard EPS 80 grade batts at whatever thickness meets your target U value which I attach to the wall with a band of adhesive on all 4 sides (at the edge) and then 5 dollops in the centre. This I press firmly on to the wall until the adhesive squeezes out at the visible sides and to match the height of the adjacent batts, any adhesive squeezed beyond the visible edges I scrape off to mane the next batt easier to butt up. In this way there is no path for air to wick away heat.

I don't bother to try to get the walls to be straight and plumb but rather I follow any contours the wall may have. No one expects a 200 year old stone/rubble walled building to have straight and square walls

If you want something a bit more breathable than standard EPS Baumit do an EPS with perforations that allow better vapour diffusion than standard EPS (there may be others equally as good)
See https://baumit.co.uk/products/external-wall-insulation/opensystem-breathable-ewi/baumit-opentherm
They also do and internal render designed for such walls.
I have no connection with Baumit, just (so far) a happy user.

If you are going to put slates hung on the EWI then under the horizontal battens either vertical battens or spacers will be needed to hold the horizontal battens off the EWI to allow and water to drain away

I would put the EPS fully across the wall (rather than between studs) and then fix battens through the EPS to the wall.]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310297#Comment_310297 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310297#Comment_310297 Wed, 25 Feb 2026 13:10:46 +0000 Nick Parsons external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310304#Comment_310304 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310304#Comment_310304 Wed, 25 Feb 2026 17:56:31 +0000 chrisinbrighton Also the vertical studs need to stay i think as they are holding alot of weight.]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310306#Comment_310306 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310306#Comment_310306 Thu, 26 Feb 2026 08:50:54 +0000 GreenPaddy
not 100% sure it's the same product, but I used the tissue faced kooltherm boards before, and it turned out they have to be kept completely dry in storage and during installation, or the tissue face peels of, making them useless. Discovered this after the supplier dropped them off in the rain at site on a Friday, so by Monday they were a soggy mess. Please double check the fine print before getting the tissue faced product, as storing pallets of boards under cover before installation might be a challenge.]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310322#Comment_310322 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310322#Comment_310322 Sat, 28 Feb 2026 13:08:10 +0000 Beau Should stop any air movement and hold it in place]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310354#Comment_310354 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310354#Comment_310354 Sat, 07 Mar 2026 09:30:48 +0000 chrisinbrighton
And yes a few cans of squirty foam seems obvious to glue the boards on!]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310434#Comment_310434 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310434#Comment_310434 Tue, 31 Mar 2026 15:38:17 +0100 Doubting_Thomas
https://www.passivhaustrust.org.uk/UserFiles/File/Technical%20Papers/Thermal%20bypass%20risks%20v1.0%20222909.pdf

Some good guidance on how to deal with wind washing through gaps in insulation.]]> external insulation https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310437#Comment_310437 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17406&Focus=310437#Comment_310437 Wed, 01 Apr 2026 17:37:28 +0100 chrisinbrighton