Suggestion on IWI of an old 1850ca cottage

[frzac]

I am approaching the start of the refurbishment of my old 1850 listed cottage and I would like to hear your experience regarding the insulation of solid stone walls.

Mine are made of 500mm of solid Welsh stone covered with about 20mm lime plaster, entirely substituted or patched in some rooms with gypsum plaster(pink one). Everything is covered by glued paper that has been painted over the years (clearly the walls are not so breathable).

On the external, 3 sides are exposed to external weather, one is sheltered by a neighbouring building. The only exposed gable wall is covered by 20mm of cement plaster (not perfectly adherent to the wall) which should offer a degree of protection from rain. This layer will be removed in the refurbishment to expose the rubble stones.

 

Our architect suggests repointing all walls (wise) and using an insulated plasterboard with integrated VCL directly attached to the wall, with 30 mm insulation + 12.5mm plasterboard. I am not sure about this method since it could lead to potential future issues especially when we remove the external plaster on the gable end.

 

Which insulation type would you suggest for those external walls to avoid or mitigate any potential condesation/infiltration risks?

Would you suggest removing the plaster before applying any insulation?

[Mr Punter]

Does the building suffer from damp?

 

The render was probably added to reduce moisture getting through the wall and I would expect it to get damp when this is removed.

 

For IWI, have a look at CharlieDIYTE.  I like this method.

 

https://charliediyte.co.uk/blog/blog/the-best-wall-insulation-for-old-properties

[Iceverge]

I would be slow to expose the external stone. 

 

These walls were often not designed to have exposed stone work. 

 

 

Hit these points and you'll be ok. 

 

1. Stop bulk water. Good pointing/external render. Good gutters and chutes. Take care of external water table (French drain) . 

 

2. Excellent internal airtightness via a parge coat, variable membrane, tapes and airtight paint where needed. This really is massively important. 

 

3. Continuous mechanical ventilation in the house to remove damp something like dMEV, MVHR, PIV. 

 

4. Internal insulation build up from only materials of higher vapour permability. Something like wood fiber, mineral wool, plasterboard, cheap trade contract paint are all fine. Nothing to trap the moisture in the wall. That rules out PIR, plastic membranes, vinyl paint. Foil back PIR plasterboard is certainly not a good idea. 

 

 

 

For maximum bang for your buck, excellent airtightness with 50mm of mineral wool everywhere will massively outperform 100mm PIR here and there with no attention to air sealing. 

 

 

 

 

[Iceverge]

I made a very boring video detailing this.

 

https://youtu.be/sI98d1R3ntQ?is=z8mp6B7uVoDurT7o

[scottishjohn]

On 19/08/2026 at 12:04, Mr Punter said:

Does the building suffer from damp?

 

The render was probably added to reduce moisture getting through the wall and I would expect it to get damp when this is removed.

 

For IWI, have a look at CharlieDIYTE.  I like this method.

 

https://charliediyte.co.uk/blog/blog/the-best-wall-insulation-for-old-properties

treat exterior walls once old render is removed with permagaurdmicro shield ultra   or other similar product

 

 

 as long as pointing is ok then it will definatly work and stop an ingress through the stone work 

 check it out will seal up to 2mm gaps 

worked a treat on my old house --

 do not even bother with thompsons or similar cheaper type products 

 

 do it once and do it right

 

goes on like wallpaper paste and gets sucked in and drys clear  and does not change colour noticably of the stone 

 

 in about 2-3 hours 

 

job done for next 20uyears +

[scottishjohn]

On 18/08/2026 at 22:07, frzac said:

I am approaching the start of the refurbishment of my old 1850 listed cottage and I would like to hear your experience regarding the insulation of solid stone walls.

Mine are made of 500mm of solid Welsh stone covered with about 20mm lime plaster, entirely substituted or patched in some rooms with gypsum plaster(pink one). Everything is covered by glued paper that has been painted over the years (clearly the walls are not so breathable).

On the external, 3 sides are exposed to external weather, one is sheltered by a neighbouring building. The only exposed gable wall is covered by 20mm of cement plaster (not perfectly adherent to the wall) which should offer a degree of protection from rain. This layer will be removed in the refurbishment to expose the rubble stones.

 

Our architect suggests repointing all walls (wise) and using an insulated plasterboard with integrated VCL directly attached to the wall, with 30 mm insulation + 12.5mm plasterboard. I am not sure about this method since it could lead to potential future issues especially when we remove the external plaster on the gable end.

 

Which insulation type would you suggest for those external walls to avoid or mitigate any potential condesation/infiltration risks?

Would you suggest removing the plaster before applying any insulation?

if you can afford to use 100mm +on inside of each exterior wall then build a  4"stud system --leave all the old plaster etc where it is  and have a 50mm gap from studs to walls

 

 just make sure you have ventilation from below floor into your roof space -- job done 

 

 If not then  you will need to make sure outside of walls are lower than floor lv

 

 house will have no DPC --so you will always  have some rising damp from ground -- 

If you can dig down round the house and make a gravel channel ideally top to be  6" below floor level  and about a foot deep   fit a french drain in bottom under the gravel --that should stop any rising damp as well

 

then do what you like with choice of insulation and new PB  -

 

this air gap will also give you space for rewiring and plumbing etc 

[scottishjohn]

If you cannot afford to loose a bit of space inside 

the i would sort the exterior walls first and let it dry out on inside 

 

the permagaurd is breathable  to a degree--how much I cannot quantify 

 

 but the moisture can evaporate from the inside 

it would have originally been done with lathe and plaster --which does leave small gap between stone and plaster 

 

 you may have seen older houses with wooden panelling "wainscotting" from floor up to about 3 ft --that was to stop the rising damp from wetting the plaster --you can buy them  but easy to make with t+g floor boarding --and a handy shelf on the top 

 

 old house with suspended floors did not need it as much if outside ground level was low enough outside 

Edited 1 hour ago by scottishjohn