Solid brick walls & insulation (with a limited air gap).

[Db104]

Apologies in advance, I imagine this is a recurring query, though much of the advice I seem to be finding is wildly conflicting and wanted to check my thinking before making an expensive mistake.

My property is a Victorian era, solid sandstone, end of terrace property.

Following damp issues due to chimney ingress along with lath and plaster bridging I have removed the plasterwork from the majority of external facing walls within the property.

I had intended to insulate these whilst the walls are off though have met a few hurdles.

Initially I’d intended to use insulated plasterboard, though the suggestion is that as this is not breathable it would result in long term damp issues. Though would have had the benefit of leaving an air-gap.

I’d then looked at mineral wool/lambs wool with a lime plaster coat. However, due to the available gap being limited, i.e. I can’t expand this or it would impact the cornices, I would be unable to leave the required air-gap. The suggestion being that without the air gap this will bridge and again cause long term issues.

So, from what I understand neither are viable long term solutions and that I would be best just re-plastering without any insulation? Colder, but with reduced likelihood of damp.

 

[fernicarry]

Just to understand the situation, is it solid stone sandstone or rubble construction? The lath and plaster that was removed was that fixed to studs stood off the masonry leaving an air gap behind? How had the plaster become bridged to the masonry? Was it a build up of debris falling down the gap?

Provided you use breathable material you should be able to insulate directly onto the masonry without leaving a gap. I've done this in one room here which had similar issues to what I assume you have, water ingress through the fire flue and debris bridging the gap, not helped by plasterboard, gypsum plaster etc.. Manufacturer's guides (not that we should necessarily take those as gospel) and I believe even Historic Scotland are recommending this approach but the breathability of the materials is the key.

My build up was a parge coat of insulated plaster to level off the surface of the stone and fill gaps, hemp based insulation (because its quite squashy so confirms to the still not flat surface), wood wool boards onto the original studs which were left in place, lime plaster, lining paper, breathable paint. Because the whole system is breathable then any penetrating water will be able to evaporate off the inner face of the wall, although obviously we'd prefer that it leaves the way it came in. I've just started addressing the cement pointing on the outside but that's going to be a multi-year task.

I've only done a relatively small part of the external wall area so there is still lots of scope for air circulation behind the walls in adjacent rooms. I think there would be a tipping point where you need to commit to doing the whole external area rather than leave areas with compromised air circulation. This will be the first full winter its been done so we'll see what problems if any emerge.

 

[montyjohn]

I've been looking into wood fibreboard insulation with a skim of lime.

It tends to be promoted by English heritage. Although I'm not at a point where I am ready to commit, more opinions and research needed, but it's the most promising approach I am aware of so far.

It's full breathable. The only issue I can see is the due point will likely be at the interface between the board and the wall. Whether this still matters with breathable materials I'm not sure yet.

 

[Db104]

Thanks both for your replies.

@ fernicarry
Solid sandstone exterior, internally it’s a mixture of large sandstone and redbrick (I’m assuming that constitutes rubble as there is little notable pattern to this). Yes, you’re correct the lath and plaster sat on studs leaving and air-gap, the blockages were a build up of the dust and falling debris both accumulated around the fireplace and where previous pipes and cables had not been removed (now removed).

I had the gable end and chimney repointed prior to the interior wall removal so I am as confident as I can be that the ingress should be stopped. I am intending to rip off (potentially insulate) and replaster the walls in one go on that entire side of the building, was your suggestion that completing all of these walls with insulation directly in contact with stonework could potentially cause issues due to the drastic change in ventilation?

Finally, I’ve read a little about parge coats, though don’t really grasp the benefit if the wall below is sound. There is some suggestion that it helps seal, though with the onus on breathability this feels counter intuitive, albeit the suggestion is that it remains breathable.

@montyjohn, thanks I’ll investigate that as a potential compromise.

 

[fernicarry]

was your suggestion that completing all of these walls with insulation directly in contact with stonework could potentially cause issues due to the drastic change in ventilation?

If you can do the whole of the external facing wall then I don't think you will have a problem. I'd either make sure the air gap is clear to allow circulation from top to bottom of the building or I'd go to the other extreme and do IWI straight onto the whole wall and let the breathable materials manage any moisture than penetrates. If the materials are also hygroscopic that will buffer moisture reducing what reaches the internal surface. Mineral wool for example isn't, but sheeps wool and hemp are.

The parge coat is to get the wall relatively smooth so that the insulation sits in good contact with it. You don't want isolated voids inside the build up that warm air from the inside could leak into, cool down and condense. Its also said to help with air tightness but with thick stone walls that shouldn't really be a problem to begin with.

End of the day your choices are either a) do nothing, building remains difficult and expensive to heat, b) sacrifice space and original features to build new studwork for insulation whilst maintaining air gap, c) fill air gap with insulation, with potential risk of penetrating damp. If the rest of the building fabric (pointing, chimneys, rainwater goods) is in good order then I don't think c) will be a problem in practice.