Breathable plaster boarding solution -moisture condensing on stone wall.

[dobson156]

I have a house with rubble filled stone wall, when I bought it these had cement based strap pointing and had cement render and gypsum plaster on the inside. Predictably the walls were damp and cold.

I have had it repointed with hot mixed lime and have stripped off the cement render back to the stonework on the inside.

I am in two minds about how to plaster the interior. Having done some research and read these threads:

• https://www.periodproperty.co.uk/forum/threads/plasterboard-with-lime-plaster.18345/
• https://www.periodproperty.co.uk/forum/threads/plastering-question.16738

My options as I see it are:

1. multiple coats, full lime render and plaster it
2. batten out the wall, fill with soft breath insulation, apply wood fibre boards, lime skim. (let me know if that doesn't make sense).

I think I'd rather do no.2 as I am more comfortable with the skills required and I will gain a little bit of much needed insulation. But what I'd like to know is this, as the wall will be breathable, there will be no vapour barrier. In modern construction the vapour barrier stops moisture from migrating from the hot side of the wall to the cold side and condensing and causing mould and rot.

Other than the fact the wall is hopefully breathable now it's been repointed, I don't really seeing what stops this from happing with the system described in no.2. Water can migrate through the plaster, wood fibre board, insulation and then condense don the stone wall... no?

Thanks for any guidance in advance.

 

[WoodMont]

I’ve just responded to another thread with a parallel answer when it comes to IWI for a solid brick wall situation, I’ve opted for Steico wood fibre downstairs with a lime plaster for maximum breathability and thermal mass to back. Upstairs we decided to go for a product called ‘SWIP’, I enrolled onto a SWIP Installer course and have started installing their products myself which has the option of a vapour-open or vapour-closed system depending where you live. SWIP IWI is a performing but more importantly breathable option for solid walls, especially if you chose the vapour-open Pro Clima Intello Membrane.

 

[a twig]

What prompted you to use SWIP IWI?

Looks like it doesn’t offer much more performance wise than wood fibre does and costs nearly double per m2? Plus wood fibre doesn’t require a membrane.

 

[dobson156]

Upstairs we decided to go for a product called ‘SWIP’, I enrolled onto a SWIP Installer course and have started installing their products myself which has the option of a vapour-open or vapour-closed system depending where you live

I'm kinda aware of that system, and fundementally it seems to be the same sort of thing I was way going to make peicemeal but with insulating battens (which is a good idea!).

But it still raises the same question, that if I use the vapour open version, what stops moisture tracking through the plaster and condensing? I.e. the whole reason a modern wall has a vapour barrier?

 

[Kearn]

Option 1. And depending on the thickness of your walls, you frankly don’t really need insulation, just dry fabric and a low slow approach to heating said fabric as a thermal store.

Option 2 creates the risk of keeping the stone cold, and shifting the dew point somewhere into the middle of you're mix of materials, creating invisible interstitial condensation.

How thick are you walls?
Why do you feel you need to add insulation?

 

[WoodMont]

What prompted you to use SWIP IWI?

Looks like it doesn’t offer much more performance wise than wood fibre does and costs nearly double per m2? Plus wood fibre doesn’t require a membrane.

In all honesty it was ‘Charlie DIYte’ on YouTube that inspired me https://charliediyte.co.uk/blog/blog/the-best-wall-insulation-for-old-properties Charlie delivers a good explanation into the system. My walls are not plumb so the OSB backed with XPS allows fettling and eliminates cold bridging, the cost of timber is on par with SWIP’s studs per linear meter and requires fewer. Alternatively the costs involved in levelling walls, fixing wood fibre boards and lime plaster is out of my budget for the whole house.

 

[aockenden]

I.e. the whole reason a modern wall has a vapour barrier?

Uhhh, modern walls in the UK don’t have a “vapour barrier”… they have a DPM beneath, and two skins of block work with a ventilated cavity between, and insulation boards held up against the internal block skin.

 

[a twig]

My walls are not plumb so the OSB backed with XPS allows fettling and eliminates cold bridging, the cost of timber is on par with SWIP’s studs per linear meter and requires fewer. Alternatively the costs involved in levelling walls, fixing wood fibre boards and lime plaster is out of my budget for the whole house.

Mr DIYte’s exploits have come up a few times on this forum, rarely favourably but that’s not to say he is necessarily wrong on this occasion.

As evidenced by many threads on this forum woodfibre or woodwool installation is emininently DIYable even on the wonkiest of rubble stone walls, so I’m still finding it difficult to understand how it works out more expensive than SWIP.

Both wood wool and wood fibre boards are cheaper than SWIP. No timber or battening is required, and levelling the wall is literally a case of slapping on the mortar then attaching the boards with the mechanical fixings pulls everything level. Especially given the flexibility of wood wool.
The final finishing skim of lime plaster is usually only required to be a few mm thick

 

[a twig]

Lots of useful information in this thread covering both wood wool and wood fibre insulation options and installation methods with photos etc

 

[dobson156]

Mr DIYte’s exploits have come up a few times on this forum, rarely favourably but that’s not to say he is necessarily wrong on this occasion.

As evidenced by many threads on this forum woodfibre or woodwool installation is emininently DIYable even on the wonkiest of rubble stone walls, so I’m still finding it difficult to understand how it works out more expensive than SWIP.

Both wood wool and wood fibre boards are cheaper than SWIP. No timber or battening is required, and levelling the wall is literally a case of slapping on the mortar then attaching the boards with the mechanical fixings pulls everything level. Especially given the flexibility of wood wool.
The final finishing skim of lime plaster is usually only required to be a few mm thick

The reason I like using battens is know I can do a job and have confidence in the finish I can achieve. I also like that I can easily stick 25mm of insulation between the board and the wall. I've also done my fair share of removing any kind of mortar/adhesive type system and know I don't want to add any of that to my house.

My question is still what stops moisture condensing on the stone wall and causing mould, maybe it just doesn't matter?

 

[dobson156]

Uhhh, modern walls in the UK don’t have a “vapour barrier”… they have a DPM beneath, and two skins of block work with a ventilated cavity between, and insulation boards held up against the internal block skin.

Okay, so nothing to worry about?

Wood fibre board, onto battens, with breathable insulation, lime over top? All good?

 

[a twig]

You’re more likely to get interstitial condensation if there is an unventilated gap between the insulation and the wall, hence the recommendation to use lime plaster to level the wall when fixing insulation directly to the wall. I will admit that in many cases where my walls were straight-ish I skipped the line levelling and relied on the mechanical fixings to pull the insulation in tight and eliminate any gaps.

If using woodfibre then that is your breathable insulation and your finishing board, attach to the wall and then lime plaster straight onto it.

If you can use 40mm or thicker you’ll see a marked difference in performance, that said I still saw benefits in places where I could only do 15mm

 

[aockenden]

Okay, so nothing to worry about?

Wood fibre board, onto battens, with breathable insulation, lime over top? All good?

See twig’s response above ^^

 

[dobson156]

I will admit that in many cases where my walls were straight-ish I skipped the line levelling and relied on the mechanical fixings to pull the insulation in tight and eliminate any gaps.

If using woodfibre then that is your breathable insulation and your finishing board, attach to the wall and then lime plaster straight onto it.

I understanding what you are suggesting better now. My walls are very uneven, and I know I could get them dead straight with battens and packers. To be honest not having to put muck directly on a wall is as I see it a massive benefit to using the system I suggest. At the end of the day, everything we apply will one day have to be removed. I am sure the guy who rendered this wall in cement thought it'd be up for ever but he'd of been wrong.

I'd like to make the job as easier for that person as possible, removing boards attached to packed out battens is very easy.

If you can use 40mm or thicker you’ll see a marked difference in performance, that said I still saw benefits in places where I could only do 15mm

The system I am proposing would be 40mm. 15m of board + 25mm for the batts infilled with breathable soft insulation. I understand that the batts will technically be cold-bridging, but generally its still going to be an improvement over whats there.

You’re more likely to get interstitial condensation if there is an unventilated gap between the insulation and the wall, hence the recommendation to use lime plaster to level the wall when fixing insulation directly to the wall

I guess this another way asking the question I need answering. Does a void formed by battens, then filled with soft breathable insulation like Knauf Mineral Wool count as a "unventilated gap".

 

[Kearn]

Really simply put, no voids, no attempts to seal, no modern materials.

Any insulation has to be considered as an extension of your existing fabric. It needs to be one continuous mass, ideally with similar thermal properties.
boards are good, but need to be flush with the wall, so as said by a few above, a rough brick or stone wall needs a parking coat of lime to level.
Frankly IWI is a bit of an unknown and can cause more issues than it solves. Trying too hard to insulate moves you away from a traditional building in function, and you risk the dew point being in your make up where hidden condensation will form.

Wet fabric conducts well, dry insulates.

Which is why often no more than a direct 3 coat system of lime is adequate, ensuring the external wall is breathable and dry.

If you must, then there are insulating limes, but you need to be pretty thick before they really move the dial.

As you have stone, of undisclosed thickness, chances are there is zero need for internal insulation attempts. The stone wall, when dry, will act as adequate insulation.

Money would be better spent on secondary glazing or other weak points than trying to insulate a stone wall.

 

[mardinor]

As you have stone, of undisclosed thickness, chances are there is zero need for internal insulation attempts. The stone wall, when dry, will act as adequate insulation.

Money would be better spent on secondary glazing or other weak points than trying to insulate a stone wall.

Absolutely. A thermal camera will allow you to check and quantify this easily. They cost £200 but you could resell for half that on ebay, so relatively cheap. I could not condone buying one from a major online retailer and returning for free after use, of course.