Insulating lath and plaster walls from the loft -- bad idea?

[alexgb]

Hi forum,

I just had a contractor double their quote when they arrived on site, so I'm having to consider doing this myself. It's an old Scottish tenement with sandstone walls, lath and plaster interior -- perhaps a 5cm gap between them, though it varies. I would insulate by making holes, but I can't find a blowing machine here up in Scotland.

So, since it's a top floor flat, I thought I might drop some cellulose or EPS beading in there. The wiring is new, and the boiler flue is isolated, if anyone is wondering.

Thoughts?

 

[claraclattermole]

I think you could potentially cause issues with damp but hopefully someone else with more experience will be along to confirm or advise. Plus it could impact your neighbours so you might need permission from the building owner and management people. It will be very experlnaove and destructive if you need to remove it so I would definitely be cautious.

 

[alexgb]

I think you could potentially cause issues with damp but hopefully someone else with more experience will be along to confirm or advise. Plus it could impact your neighbours so you might need permission from the building owner and management people. It will be very experlnaove and destructive if you need to remove it so I would definitely be cautious.

Thanks for the reply. Damp always gets brought up but we don't have to speculate, Historic Scotland has done a number of studies. Quoting from case study 4:

In addition to measuring the improvement in thermal performance, the trials at Sword
Street also measured changes in moisture levels within the fabric of the masonry
walls that may have resulted from the application of the insulation. This was
measured by the use of probes at the junction of the inner masonry and the
insulation, and at a depth of 50 mm into the thickness of the wall (Fig. 8). The results
of this monitoring are summarised in Table 2. It is considered that none of the
insulation measures being trialled is leading to a damaging increase in moisture over
time, at either the interface of the insulation and the wall or within the fabric of the
stone. Whilst the relative humidity was, in three cases (aerogel board, hemp board
and wood fibreboard), higher than that in the room, only in the case of the wood
fibreboard was there a considerable difference and all of these were still within
acceptable limits. The humidity for the blown cellulose and bonded bead insulation
was lower at both the interface and within the fabric of the wall. It is also important to
note that whilst the results are averages, in no cases did the humidity rise
significantly over the 18-month monitoring period. These results are significant as
10
Historic Scotland Refurbishment Case Study 4
they go some way to proving that insulation can be safely applied directly to mass
walls without using a vapour barrier. Further monitoring will continue to examine this
issue over a longer period of time, and this paper will be updated in due course

The property is freehold, so there is no management company (we call them factors in Scotland).

I am more worried about the logistics of getting the insulation into the loft. I also hear the insulation might go into the subfloor void (solum)?

 

[Robola]

So you ask for thoughts then dismiss them straight away.

The case study you quoted states:

"All internal wall linings had been stripped out and replaced with dry lining in a previous extensive refurbishment. This meant that there were no limitations regarding the retention of original wall linings or decorative cornices, which would be the case in many similar properties."

So not the same as lath and plaster in situ.

and

"At Sword Street this lining was plasterboard;further trials are on-going as to the application of this type of insulation behind lathand plaster walls (see Refurbishment Case Studies 2 and 5)."

If you look at case study 5 they conclude with

"The technical reservations regarding the filling of previously open cavities beneath roofs are still open to question. Longer term monitoring and assessment of the relative humidity within the walls and the filled cavity will be required to resolve the technical issues more confidently. This is particularly true for the area affected by the water leak. The true success of this type of intervention will therefore not be known for some time."

 

[Robola]

My thoughts are that this will be highly building dependent.

I wouldn't do it in my house as:

• It is very exposed and I know driven moisture evaporates from the internal stonework behind the lath and plaster.
• There are gaps behind the skirting that go straight through to the next floor/flat below us.
• The timber framing behind the lath has dwangs that mean that each void isn't vertically continuous, it will be patchy.

 

[montyjohn]

Hi forum,

I just had a contractor double their quote when they arrived on site, so I'm having to consider doing this myself. It's an old Scottish tenement with sandstone walls, lath and plaster interior -- perhaps a 5cm gap between them, though it varies. I would insulate by making holes, but I can't find a blowing machine here up in Scotland.

So, since it's a top floor flat, I thought I might drop some cellulose or EPS beading in there. The wiring is new, and the boiler flue is isolated, if anyone is wondering.

Thoughts?

My biggest concern would be all the beads disappearing down a hole somewhere. Filling a neighbours cupboard etc.
I had a 1950's cavity wall semi detached done a while ago, and beads got into every orifice.

Then use a thermal imaging camera to look for voids with no insulation.

Ty mawr with wood fiber boards only sell up to I think 60mm. Their opinion that beyond this it;s too much insulation meaning the walls are too cold and don't dry out. Can't see over insulation being a problem for you, but worth noting if you have any wide voids.

As long as the insulation value isn't too high, and it's breathable insulation, and you don't expose too many cold-bridge location in the process then I can't see it being an issue.

 

[montyjohn]

So you ask for thoughts then dismiss them straight away.

That's a bit harsh. He gave a reasonable counter argument. He's under no obligation to accept all suggestions and I suspect he's more interested in getting to the truth so it's good to bounce info back and forward. It's collaborative.

 

[alexgb]

Thanks forum. Less concerned about the moisture but as @Robola has mentioned, the fact that there could be gaps going down to the neighbour's property below is a concern (and I guess there would be no way to be sure without ripping all the skirting board out). Also the patchiness of the insulation due to the stud construction etc.

Has anyone in this forum had experience with thermal wallpaper like Erfurt or Wallrock?

 

[Robola]

That's a bit harsh. He gave a reasonable counter argument.

Sorry if it came across as harsh but I found this to be quite dismissive

Damp always gets brought up but we don't have to speculate,

A certain amount of speculation is inevitable as the case studies are inconclusive.

 

[paulc]

Has anyone in this forum had experience with thermal wallpaper like Erfurt or Wallrock?

I know of one or two people that have used that type of stuff. It doesn't have sufficient thickness to provide any worthwhile level of insulation. However, it can be enough to raise the wall temperature sufficiently to reduce mould. But it can trap moisture behind and lead to bigger damp problems.

 

[fernicarry]

Its something I'd like to do myself, particularly around window reveals where removing the old wood panelling is too risky. I think you'll find it challenging to get a good result.

The inner face of the stonework won't be particularly smooth. Neither are the plaster nibs on the back of the laths. The blown insulation will catch and back up. You may end up with many more holes than you were planning. I also wonder about how the holes are made good. If you have ever fitted an electrical outlet you will know that once the laths are cut mid span tney end up flapping around providing no support to the plaster. Dealing with dozens of those would not be much fun.

As you say the void behind the plaster is open at the bottom. Generally the floorboards run out below the wall but should stop just short of the stonework. They might act as a back stop to hold the insulation but lengths will vary. Preferably the insulation should carry on below floor level (and above ceiling height) to maintain its effectiveness. If you stop short at floor level then the last few inches above the floor are less effective and you create a colder spot right in the corner where it is least desirable. Ideally you would lift the floorboards and insulate between the joists then bring the wall insulation down to form a continuous barrier.

You say the wiring is new. I don't believe even new wiring is immune to plasticiser migration when in contact with polysytrene. I've certainly witnessed it with cables bought in the last 15 years. You may also need to derate circuits if they end up buried for any distance in the insulation. And as others have said those polystyrene beads leak out all over the place. Cellulose would be the better choice in terms of embodied energy but it'll be harder to coax it into position being a bit fluffy.

Damp bridging could be an issue. It is why the cavity behind the plasterwork was put there in the first place to act as a capillary break. I've experimented with solid IWI in one room where it had already been redone with plasterboard. The build up is as breathable as I can make it, insulating lime plaster, hemp insulation, wood wool boards, lime plaster finish, breathable paint. The hemp being hygroscopic will act as a buffer if damp does penetrate. Separately I am in the process of hacking off the cement pointing outside but that's a multi-generational task.

Why did the contractor double their estimate? The nature of the construction or something specific to your case? Myself and a friend did once look into obtaining a machine and used ones are available so if you are committed to this approach you could buy a machine, use it and then flog it on.

Tbh I don't think there is any silver bullet for retro-fitting insulation. Your choices boil down to a) take down externally facing wall linings and rebuild as I did with my wall, you could lose historic lath and plaster and you risk damaging joinery and cornices in the process but you should be able to get back to the same finished level, b) new stud work set inside the existing walls deep enough for your proposed insulation, cover up period details and lose floor space, c) do nothing, spend a fortune failing to heat the building to a comfortable level and face the prospect of losing value as the govt continuously ratchets up the requirements on building performance and limits choice of heating systems.

If you can think of a plan d) please do let me know...

 

[alexgb]

Its something I'd like to do myself, particularly around window reveals where removing the old wood panelling is too risky. I think you'll find it challenging to get a good result.

The inner face of the stonework won't be particularly smooth. Neither are the plaster nibs on the back of the laths. The blown insulation will catch and back up. You may end up with many more holes than you were planning. I also wonder about how the holes are made good. If you have ever fitted an electrical outlet you will know that once the laths are cut mid span tney end up flapping around providing no support to the plaster. Dealing with dozens of those would not be much fun.

As you say the void behind the plaster is open at the bottom. Generally the floorboards run out below the wall but should stop just short of the stonework. They might act as a back stop to hold the insulation but lengths will vary. Preferably the insulation should carry on below floor level (and above ceiling height) to maintain its effectiveness. If you stop short at floor level then the last few inches above the floor are less effective and you create a colder spot right in the corner where it is least desirable. Ideally you would lift the floorboards and insulate between the joists then bring the wall insulation down to form a continuous barrier.

You say the wiring is new. I don't believe even new wiring is immune to plasticiser migration when in contact with polysytrene. I've certainly witnessed it with cables bought in the last 15 years. You may also need to derate circuits if they end up buried for any distance in the insulation. And as others have said those polystyrene beads leak out all over the place. Cellulose would be the better choice in terms of embodied energy but it'll be harder to coax it into position being a bit fluffy.

Damp bridging could be an issue. It is why the cavity behind the plasterwork was put there in the first place to act as a capillary break. I've experimented with solid IWI in one room where it had already been redone with plasterboard. The build up is as breathable as I can make it, insulating lime plaster, hemp insulation, wood wool boards, lime plaster finish, breathable paint. The hemp being hygroscopic will act as a buffer if damp does penetrate. Separately I am in the process of hacking off the cement pointing outside but that's a multi-generational task.

Why did the contractor double their estimate? The nature of the construction or something specific to your case? Myself and a friend did once look into obtaining a machine and used ones are available so if you are committed to this approach you could buy a machine, use it and then flog it on.

Tbh I don't think there is any silver bullet for retro-fitting insulation. Your choices boil down to a) take down externally facing wall linings and rebuild as I did with my wall, you could lose historic lath and plaster and you risk damaging joinery and cornices in the process but you should be able to get back to the same finished level, b) new stud work set inside the existing walls deep enough for your proposed insulation, cover up period details and lose floor space, c) do nothing, spend a fortune failing to heat the building to a comfortable level and face the prospect of losing value as the govt continuously ratchets up the requirements on building performance and limits choice of heating systems.

If you can think of a plan d) please do let me know...

Yeah you raise a good point re: the wiring. Best solution I can think of is cellulose for the walls with sockets (there are only two) and EPS beads for the others.

The contractor doubled their estimate as a bait-and-switch, I don't believe they ever intended to do the work for the quoted price (they were sent various photos and the floorplan/square footage, they could see how big the flat was).

I am not keen on rebuilding the walls due to the original cornices and window reveals being in good condition. But it also sounds like insulating behind the lath would be quite challenging as a DIY job because of floorboards, skirting board removal, and all the other litany of issues mentioned here.

I think I will just stick to wallpaper. Between 300mm of loft insulation, various draughtproofing measures, and double glazing the property already achieves a satisfactory EPC measure.

 

[Gothichome]

My thoughts on insulation between external brick and internal plaster is not to insulate. At the very least the insulation will not allow the very little airflow inherent in cavity construction to equalize humidity in the outer brick work and plaster to changes in day to day ambient humidity levels.

 

[IHateDamp]

Here is a very interesting cautionary thread on LinkedIn about how problematic single wall interior insulation can be and how basically hardly anybody has a clue about insulation consequences
https://www.linkedin.com/posts/coli...-the-failed-activity-7384308307823493121-MXmC

 

[fernicarry]

Good points in that report (read the linked PDF not the AI summary). Much of which matches the received wisdom on this forum such as importance of breathability, good detailing, ventilation etc.

The report was originally written in 2016. I presume its been resurrected on LI because of recent stories related to the almost universally poor outcomes of government funded insulation schemes and this chap wants to say "I told you so".

Like I said above I don't think we have a lot of choices. Pick your poison.

 

[alexgb]

An interesting point about thermal mass decoupling leading to overheating. I am going to put solar control blinds in all the south-facing windows to combat this.

Internal moisture IWI There is concern about
too much internal
insulation preventing
heat flow into walls
which may be needed to
drive out latent moisture
and thus prevent
external surface or
interstitial condensation
Lack of breathability,
moisture trapped within the
structure of the wall,
interstitial condensation,
frost damage, movement,
premature decay. Surface
condensation.
The presence of an
un sealed Vapour
Control layer (VCL)
could lead to the
movement of
moisture from the
exterior to the
interior, and vice
versa.
 Testing of materials
 Integration of a moisture
professional into the
design, survey, install,
sign off process.
 Standards researched on
ventilation requirements in
post insulation work

Reading through the report, most of it seems only to apply to synthetic materials, for example the bit about VOCs doesn't apply to cellulose.

. UK Building Regulations
requirements increase
likelihood of thermal
bridging problems
Achieving upgrades to U-values of
0.3 makes walls more sensitive to
thermal bridgin

Realistically 0.3 is not going to be achieved anyway, Historic Scotland stated that the lowest U value they had was 0.5 when insulating behind lath and plaster.

I plan to use an endoscope camera to make sure the insulation gets where it's supposed to.

 

[fernicarry]

Yeah, the thermal mass thing is unavoidable if you can only do IWI. And EWI a non-starter for me. If I was happy to live in a white rendered box then my life would be so much simpler

I don't expect overheating to be that much of a problem here although we did hit 25' for a few days this summer. Not on the same level as experienced by some new builds in the SE. Unintended consequences... The good thing about traditional sliding sashes is that you can get a really big open area that allows a lot of air movement. The upstairs top sashes were wide open for days on end. Modern hinged replacements don't really allow this.

The thermal mass here is quite noticeable at this time of year. We are still coasting with the heat absorbed over summer although I feel that's just about running out. The room I did with IWI may have cooled down a bit faster than adjacent rooms but that could be any number of factors. It does have quite a large glazed area. Not enough data to draw a conclusion.

Are you still considering filling the cavity?

I wouldn't get too hung up on absolute U-values except possibly when selecting materials. If you can get to the point where you can comfortably heat the building with a flow temperature of 40' from your CH (for some given minimum outside temperature) then that gets you into the territory where a HP becomes viable, at least in terms of running costs. A very distant dream for this place sadly.

 

[alexgb]

Th
Yeah, the thermal mass thing is unavoidable if you can only do IWI. And EWI a non-starter for me. If I was happy to live in a white rendered box then my life would be so much simpler

I don't expect overheating to be that much of a problem here although we did hit 25' for a few days this summer. Not on the same level as experienced by some new builds in the SE. Unintended consequences... The good thing about traditional sliding sashes is that you can get a really big open area that allows a lot of air movement. The upstairs top sashes were wide open for days on end. Modern hinged replacements don't really allow this.

The thermal mass here is quite noticeable at this time of year. We are still coasting with the heat absorbed over summer although I feel that's just about running out. The room I did with IWI may have cooled down a bit faster than adjacent rooms but that could be any number of factors. It does have quite a large glazed area. Not enough data to draw a conclusion.

Are you still considering filling the cavity?

I wouldn't get too hung up on absolute U-values except possibly when selecting materials. If you can get to the point where you can comfortably heat the building with a flow temperature of 40' from your CH (for some given minimum outside temperature) then that gets you into the territory where a HP becomes viable, at least in terms of running costs. A very distant dream for this place sadly.

Thanks for the data point, appreciate it. I'm in Scotland so heating demand predominates.

Heat pump is my dream too. I think it will take years for prices to come down though!