Internal Wall Insulation - Solid Masonry Walls

[James1234]

Hi all. I am doing a full refurb to my house and extension and have spent far too long debating how to manage the IWI. Everyone I have discussed this with seems to have very different theories/approaches.

 

The house is 100 years old, 450mm thick Cotswold stone walls with lime mortar and rubble fill. Externally it has been re-pointed with cement mortar. There is a bitumen DPC which seems to be doing the job well because the walls are dry and where we have opened new doorways we have seen it is in good condition. We are having an MVHR system installed.

 

Key principles I have taken from research is not to over insulate to prevent to masonry getting too cold in the winter and to allow the wall to breath into the house. Ideally I would re-point externally to improve the ability of the wall to breath outward but that will have to be a future job. Pointing is in reasonable condition.

 

Options considered

1) Originally I was looking at using the Steico wood fibre system (lime skim, 60mm steico, lime skim) but the price tag is significant.

2) An architect suggested 50 x 50 battens straight onto the bare stone with rockwool batts between, AVCL, plasterboard and skim. I questioned the breathability of plasterboard and skim but he made the good point that we install a VCL in other situations because the plasterboard doesn't prevent moisture passing through it. I also found this system https://www.swipiwi.co.uk/ which is essentially what I am proposing but reducing the thermal bridging of the battens (which for me feels like a small benefit for the additional cost). If their system uses normal plaster board then I assume they also feel that plasterboard is breathable.

 

Options ruled out

3) a surveyor suggested tanking the whole 2 storeys with plastic egg crate sheeting and then insulated plaster board on battens. He was of the view that this approach keeps the moisture out of the house. Given we have timber lintels and floor joists built into the walls I felt that preventing the walls from drying into the building was too much of a risk

4) dot and dab pir (either insulated plasterboard or separate PIR and plasterboard with AVCL between). Again I ruled this out due to the risk to the embedded timbers becoming damp)

 

I had settled on option 2 based on the fact that it seemed to tick most boxes. My builder discussed this option with the BCO today who said that he would not approve that arrangement and he would want to have a cavity between the wall and the insulation/battens. This seems to fly in the face of the advise of keeping the insulation tight to the wall to avoid an air gap where condensation can form (appreciate with my bare masonry walls that there will be some gaps regardless. This also feels like a draughty option.

 

Option 5 could just be to do away with the insulation and just replaster in lime plaster, but while we have it all stripped back it feels like we should be doing something.

 

Appreciate your thoughts and if you think option 2 is reasonable any reference material to argue my case with building control

 

 

 

[Mike]

40 minutes ago, James1234 said:

2) An architect suggested 50 x 50 battens straight onto the bare stone with rockwool batts between, AVCL, plasterboard and skim.

I'm currently doing similar, but using metal studwork and either 100mm or 145mm (depending on elevation & exposure) of hemp insulation batts, though in France, not the UK, and with render not pointed stone.

 

40 minutes ago, James1234 said:

Key principles I have taken from research is not to over insulate

A good rule of thumb, and wise to stick to it in the absence of a WUFI evaluation. From playing with the trial version of WUFI, in some circumstances more insulation actually reduces the risk (at least when using hemp insulation, which has superior vapour-handling properties to non-natural insulation materials).

 

40 minutes ago, James1234 said:

I questioned the breathability of plasterboard and skim but he made the good point that we install a VCL in other situations because the plasterboard doesn't prevent moisture passing through it.

Plasterboard and skim is indeed vapour-permiable. Make sure that the paint that you use is too though, or you'll negate that.

 

40 minutes ago, James1234 said:

AVCL

Chances are it's not needed, especially with MVHR, but but 'intelligent' VCLs are commonly specified.

 

40 minutes ago, James1234 said:

Pointing is in reasonable condition.

Good pointing is crucial. Especially on an exposed wall, moisture ingress from outside may be a much bigger factor that vapour ingress from the inside.

 

Airtightness from the outside is also important. Junctions of materials (wall to window etc.) are the weak points. Plenty of other threads on that here.

 

40 minutes ago, James1234 said:

My builder discussed this option with the BCO today who said that he would not approve that arrangement and he would want to have a cavity between the wall and the insulation/battens. This seems to fly in the face of the advise of keeping the insulation tight to the wall to avoid an air gap

Agreed. And contrary to official UK Government advice too - https://www.gov.uk/government/publications/retrofit-internal-wall-insulation-best-practice

 

40 minutes ago, James1234 said:

Option 5 could just be to do away with the insulation and just replaster in lime plaster, but while we have it all stripped back it feels like we should be doing something.

It would be crazy not to!

 

Edited July 19 by Mike

[Iceverge]

Good luck with falling down this rabbit hole, it's a minefield!

 

The one big principal to remember that over the course of a year "DRYING" must exceed "WETTING"

 

Unless you have lots of driving rain or poor gutters and pointing then almost all of the damaging moisture comes in the form of airborne water vapour from inside. 

 

Damaging amounts get into the walls via air leaks, not via diffusion through materials. EG It doesn't get through plasterboard and skim, but rather through poor sealing around skirting and sockets.  Total Airtighess (a function of planning and workmanship) is your friend here. 

 

As you noted, internal insulation limits "DRYING" but excellent workmanship (ZERO air leaks!!) and MVHR you will massively limit the amount of "WETTING" of the internals of your wall.  You could then get away with quite a chunk of internal insulation. Add some vapour permeable materials like mineral wool and lime and you could get a chunk more internal insulation without damage.

 

Calculators can give an idea of the lightly hood of success but in reality no two families generate the same amount of vapour, and no two builders do the same diligent job on airtighess so rules of thumb and even sophisticated simulations are little more than educated guesses. 

 

Your BC has to assume poor workmanship and an MVHR that won't be used so therefore specifies a ventilated cavity in the wall to make sure Drying exceeds Wetting in the worst case scenario. 

 

 

 

[Iceverge]

Airtighess is the most important thing you will have to tackle in this renovation. Not only for overall energy saving and comfort but for the survival of the building itself. You need to keep the damp air out of the internals of the wall. 

 

The largest issue you have is around joist ends and internal walls.

 

I would parge their inside of all walls with a lime/sand/cement slurry. I would use airtight paint or tapes (returned to the parge) to seal around all penetrations, joist ends, doors and windows. Seal to the floor slab and the ceiling membrane. Then DIY blowertest it for leaks. 

 

That would leave you free to internally insulate. Something like 50mm (or more) mineral wool batts between battens or studwork. Plasterboard and skim.

 

 You wouldn't need an AVCL as your airtighess would be taken care of by the parge, itself of moderate vapour permanbility and some flexibility. Also putting the AVCL where your architect suggests leaves it full of holes from wires and makes it very tricky to join to other ACVLs in adjoining rooms.

 

 

Edited July 19 by Iceverge

[James1234]

Thanks all, some useful points and reassurance. I will go and have a conversation with BCO!

 

Which ever solution I go with I will be paying close attention to the airtightness. It is a semi detached  and we are extending so the back wall of the existing house will become internal... so it won't be perfect... but will be taping around joists, running membrance or parge around the back of internal stud work etc and connecting to membrane in the roof and floor.

Edited July 20 by James1234