Insulation of exterior wall advice

[nostos156]

Considering a DIY job where I will knock down an old lath & plaster exterior wall in an attic bedroom. The house is an old Victorian one with exterior walls of (very thick) solid stone. 

 

The wall in question is the coldest one in the room. The other exterior walls (with sloped ceilings) that face the slates have all had rolls of glass wool put in between the studs years ago, as well as in the loft above the room. The wall thickness is about 5 cm based on shoving a screwdriver in until it hits stone. From what I can tell, the timber stud work is the old kind where they hammered in wooden pegs to the stone, that had the very thin studs attached. Not sure what they're called.

 

The basic idea is to knock that wall down, then build out a new 2×2 stud framing a short distance from the stone wall to create an air gap (25 mm is the minimum, right?). Using 2×2 as the room is not all that large and trying to minimise space lost.

 

In between the studs, the plan was to use Celotex PIR at 50 mm thickness, but pushed back from the face of the studs slightly to allow for a service void for cables and such; rather than adding additional battening for a service void. The edge of the boards to timber would have spray foam to seal any air gaps from the cutting.

 

Afterwards, I'd slap on standard plasterboard and paper over. 

 

How does this all sound? I've been informed that I should be using foil backed plasterboard for a vapour barrier, but don't those only really do something when the vapour barrier is continuous? Using it on one wall where it'd have seams due to the plasterboard seems like it wouldn't do much? Could I cut the foam down level to the PIR and then use foil tape at the edges and continue it around the side and face of the stud work instead?

 

The main reason for doing all this is that this is the only good wall a wardrobe can go on, and I'd preferably want it pushed up against the wall. Would that kind of insulation be enough to stop condensation issues?

 

If not, I am additionally planning to put a 3 mm layer of wallrock thermal liner, just to act as an additional cold bridge if necessary. That would hopefully be enough.

[jayc89]

Insulating should help, but damp behind wardrobes, or any other fixed furniture, is generally indicative of additional ventilation being needed too.

 

The VCL does indeed need to be continuous and in your case sealed around the perimeter of the insulated wall, so a plastic VCL over the top of the PIR might be better in your case, generally finishing behind the abutting PB and sealing with an acrylic sealant like this - https://www.ecologicalbuildingsystems.com/product/orcon-f

 

The use of impermeable materials, such as PIR, on solid wall structures is contentious. Some are firmly in the camp of only ever using breathable materials, such as woodfibre insulation, and a lime plaster finish, whereas other's don't consider internal breathability to be as big a concern.

 

The biggest imminent risk is interstitial condensation, where warm moist air reaches the cold side of the insulation and condenses, so foam and taping to create the continuous barrier is important. Some breathable materials are "safer" in this regard, wood fibre, for example is hygroscopic so will retain moisture during the cold season and release it during the warm. 

 

What's on the outer face of the wall? Render/paint, if so, what kind, cement?

 

I've taken a more pragmatic approach to my renovation, I'd love to use breathable materials, but they were cost prohibitive for me, and plasterers that will work with lime around me are like hen's teeth. There's certainly no one-size-fits-all-solution. What works for one house may not work for another. I'm of the opinion that so long as a wall can "breathe" to at least one surface, the risk is reduced (although not removed), so things like ensuring gutters are working, not leaking, ensuring good pointing, any paint/render on the external face is breathable, and that there are now raised ground levels, has been key to my renovation - eventually I plan to install a French drain round the perimeter of the house too. Internally, the walls are back to brick, junctions around joists etc have been pointed using lime (I didn't want to risk trapping moisture directly against the joists) and I'll be using the "Warm Batten" approach to gluing PIR directing to the wall using PU adhesive, and screwing battens through it which will act as a service void.

 

To stick the board, I applied a continuous bead of adhesive around the perimeter of it and a continuous zig-zag bead across it, the idea here is that any cold air that does make it through the wall gets is limited to one of these cavities thus reducing any thermal-bypass. Once a board was on the wall, I also applied a bead of the adhesive around the edges of it to be doubly-sure any cold air behind can't travel between boards. I used window packers to leave a 10mm space around all boards, including those at wall and floor junctions, which I later filled with expanding foam and then taped using aluminium tape - this could have been overkill, my thinking is in an older house, nothing is square, so there will always be slight gaps and deliberately leaving a larger gap to foam up later would make it easier to capture them all. Once one wall had been fully foamed and taped, including around floor and ceiling junctions, I started on the return wall so any abutments should be pretty snug. 

[nostos156]

2 hours ago, jayc89 said:

Insulating should help, but damp behind wardrobes, or any other fixed furniture, is generally indicative of additional ventilation being needed too.

 

The VCL does indeed need to be continuous and in your case sealed around the perimeter of the insulated wall, so a plastic VCL over the top of the PIR might be better in your case, generally finishing behind the abutting PB and sealing with an acrylic sealant like this - https://www.ecologicalbuildingsystems.com/product/orcon-f

 

The use of impermeable materials, such as PIR, on solid wall structures is contentious. Some are firmly in the camp of only ever using breathable materials, such as woodfibre insulation, and a lime plaster finish, whereas other's don't consider internal breathability to be as big a concern.

 

The biggest imminent risk is interstitial condensation, where warm moist air reaches the cold side of the insulation and condenses, so foam and taping to create the continuous barrier is important. Some breathable materials are "safer" in this regard, wood fibre, for example is hygroscopic so will retain moisture during the cold season and release it during the warm. 

 

What's on the outer face of the wall? Render/paint, if so, what kind, cement?

 

I've taken a more pragmatic approach to my renovation, I'd love to use breathable materials, but they were cost prohibitive for me, and plasterers that will work with lime around me are like hen's teeth. There's certainly no one-size-fits-all-solution. What works for one house may not work for another. I'm of the opinion that so long as a wall can "breathe" to at least one surface, the risk is reduced (although not removed), so things like ensuring gutters are working, not leaking, ensuring good pointing, any paint/render on the external face is breathable, and that there are now raised ground levels, has been key to my renovation - eventually I plan to install a French drain round the perimeter of the house too. Internally, the walls are back to brick, junctions around joists etc have been pointed using lime (I didn't want to risk trapping moisture directly against the joists) and I'll be using the "Warm Batten" approach to gluing PIR directing to the wall using PU adhesive, and screwing battens through it which will act as a service void.

 

To stick the board, I applied a continuous bead of adhesive around the perimeter of it and a continuous zig-zag bead across it, the idea here is that any cold air that does make it through the wall gets is limited to one of these cavities thus reducing any thermal-bypass. Once a board was on the wall, I also applied a bead of the adhesive around the edges of it to be doubly-sure any cold air behind can't travel between boards. I used window packers to leave a 10mm space around all boards, including those at wall and floor junctions, which I later filled with expanding foam and then taped using aluminium tape - this could have been overkill, my thinking is in an older house, nothing is square, so there will always be slight gaps and deliberately leaving a larger gap to foam up later would make it easier to capture them all. Once one wall had been fully foamed and taped, including around floor and ceiling junctions, I started on the return wall so any abutments should be pretty snug. 

 

Yeah, I had issues with minor mould growth (located near the floor of the wardrobe) with an Ikea PAX that had a couple of cm of spacing from the wall. However, I think in this case it was due to the fact that there was a gap between the bottom of the skirting and the floorboards, which the carpet covered up. In especially windy days, you could feel a cold breeze coming through there… probably cold enough to condense things up around there.

 

I just don't want to shove one up against the wall and have the cold bridge through enough that it would still cause issues due to thermal bridging. I don't know enough about R/U values to know if my plans would be enough to stop that or not. For example, the studs themselves could be a source of cold bridging. Insulated PB on top of the celotex would solve that, but I'd lose a lot more space going that route.

 

If I went with a plastic VCL behind the PB, wouldn't I immediately break the barrier if I ever fish cables through that wall with my planned service void? At least if I just did some awkward foil taping around studs, I wouldn't be at risk of breaking it.

 

As for permeability, the wallpaper in this room before the renovations started was vinyl stuff. The permeability of the walls was actually thrown out a long time ago, and it doesn't seem to have caused any issues from what I could tell.

 

On the outer face of the wall is all solid stone, there is no render or exterior insulation. My understanding is that moisture can move through this.

 

 

Edited January 12, 2023 by nostos156

[nostos156]

So, getting to the point of knocking the wall down. Discovered a boarded up fireplace (just had plasterboard over it, not bricked up). Kind of suspected this, but this was one of the reasons I was doing a stud wall and not attempting anything near the actual brick, as there's the classic brown staining above it.

 

Was thinking of bricking it up, but I keep hearing about the need for a vent, even in an old stone and lime mortar house which is supposed to be breathable as hell. Since I'm leaving a gap between the wall and stud frame (as it isn't a cavity wall), could I just drill a few holes into the bricks I put down and call it done? There's a fair amount of air movement in the attic because the slates never had felt put down, so I'm figuring this should be enough ventilation without having to open a vent into the room itself and render the PIR pointless.

 

My relative who is a brickie kind of shrugged and said the vent likely wasn't needed, but I'm not sure. FWIW, the fireplace was full of old dust that needed clearing out, but was perfectly dry, even after a rainy day (chimney is not capped, just had mesh put over it). Seems there was no recurring damp issues, even in the middle of the winter.

 

 

[nostos156]

Well, went ahead and bricked it up, but decided to drill six 12mm holes in one of the concrete blocks I cemented down.

 

Is that adequete airflow?

[markc]

6 hours ago, nostos156 said:

Well, went ahead and bricked it up, but decided to drill six 12mm holes in one of the concrete blocks I cemented down.

 

Is that adequete airflow?

It will be fine, if chimney isn’t capped it will be venting anyway. Capped chimneys “suck in”/moisture and this then sinks to the bottom. If your old fireplace is dry and dusty in this weather you won’t have any problems 

[nostos156]

4 hours ago, markc said:

It will be fine, if chimney isn’t capped it will be venting anyway. Capped chimneys “suck in”/moisture and this then sinks to the bottom. If your old fireplace is dry and dusty in this weather you won’t have any problems 

 

That is fair. It hasn't been capped, just got a mesh covering on it. It definitely had prior evidence of mortar erosion as I could see it from the stone when I looked up, and there was a ton of dust that needed to be shovelled and bagged up before laying down the blocks, even if it was bone dry (and still dry during the periods of rain while doing all this we had).

 

Will leave it at the 6 holes drilled then.

[nostos156]

So I'm building the actual stud wall by this point and after putting the framing in and leaving a roughly 50mm cavity (as the wall is uneven it goes from 30mm at shallow parts to 50mm at deepest) from the stone to the framing. I am noticing an issue I never saw mentioned, so did not account for.

 

Basically, the 2x2's seem pretty weak to being bent. Even with noggins on, pushing or pulling them shows a fair bit of movement, which worries me. The obvious solution would be to drill holes in the stones and insert a bit of wood behind the stud and screw it down (with the wood wrapped in DPC), so it is also attached to the wall (although this would likely involve dismantling what I've already done in order to be able to drill the stone behind the stud. Could avoid that by wedging in a longer block of wood and inserting two screws at either side, but then I'm doubling the amount of thermal bridging through the metal.)

 

The last sentence comes to my next concern—with blocks of wood directly against the wall, doesn't that fairly increase the thermal bridging, especially with the fixings being metal? How do I prevent condensation risk on the metal fixings?

 

Is there another way entirely?

Edited February 19, 2023 by nostos156