To insulate or not to insulate solid stone wall...

[Suzi Martin]

Hi,

Just joined this forum and hoping for some advice.

We recently bought and are living in an early to mid 1800s sandstone and whinstone cottage in Fife, Scotland. We are renovating room by room and are considering whether or not we should install internal wall insulation on the original stone walls.

We recently removed the red brick fireplace in the kitchen to reveal the original fireplace, which we will be keeping as a feature (with the addition of a log burner). This revealed that the wall consists of a stud frame overboarded with plasterboard and then skim plastered. There is a gap between the stud frame and the solid wall; so the stud frame is not fixed directly to the stonework.

So, the question is, should we bother insulating? We haven't spent a winter in the cottage yet, but so far it isn't particularly cold (and we haven't had the heating on as we've not ordered any heating oil yet!). We're a bit wary because we don't want to create damp/mould problems and we don't have the cash to go down the route of traditional materials.

Any advice much appreciated!

Thanks,
Suzi

 

[fernicarry]

Another sandstone and whinstone building here.

Traditional construction in Scotland, west coast definitely, maybe less so east coast as its a bit less wet, is for the lath and plaster to be carried on studs dooked(*) into the stone wall. This leaves a void space between the inner face of the solid stone wall and the inner decorative finish. If driven rain penetrates through the stonework it won't wick through to the inner surface. Same principle as cavity wall construction where the inner leaf should stay dry even if the outer leaf is saturated. This relies on the void space being ventilated to the solum around the ends of the floor boards and into the roof space so that the wall can dry out. Often the void gets blocked by debris, especially if builders have been up to their usual tricks of shoveling rubbish under the floor. See my other recent thread on how that turns out a few decades down the line...

The original studs are gone from your room but whoever renovated it followed the same approach by building studwork stood off the wall. I suspect thats more for the convenience of getting a flat surface to attach boards to than any great understanding of how the building is intended to work. Our kitchen was exactly the same.

If you you are going to reinstate with modern materials then you need to keep the venitlated void and ensure its working properly. Downside is that air movement through the void will reduce the thermal performance of the building. You'll get drafts coming in around the joinery and electrical accessories unless you are really obsessive about detailing. Hence a better approach, from a thermal pov, if you have the opportunity is to line the wall with breathable IWI and use breathable boards and plasters. I'm doing one wall of one room like this at this exact moment. How it will behave when the wall gets saturated this winter is a bit of an experiment. My thought process is that it can't possibly be any worse than what was previously there...

Breathable materials aren't necessarily more expensive than modern materials. If you were redoing that studwork you would be well advised to get a bit of celotex between the studs and create an insulated box inside the room which will involve some amount of expenditure as well. My motto is to improve air tightness and insulation whenever you get the access to do so, especially if the original fabric of the building has already been mucked about with. You also have to consider the direction of travel for energy policy in this country and the obsession with net-nothing to the detriment of all else. 12 years ago when I bought this place I could honestly say that heating costs whilst high were still third after mortgage and council tax. Its a very different landscape today....

(*) dook -- wooden wedges hammered into gaps between the courses then cut down to give a regular fixing surface.

 

[mh12]

Hence a better approach, from a thermal pov, if you have the opportunity is to line the wall with breathable IWI and use breathable boards and plasters. I'm doing one wall of one room like this at this exact moment. How it will behave when the wall gets saturated this winter is a bit of an experiment. My thought process is that it can't possibly be any worse than what was previously there...

Breathable materials aren't necessarily more expensive than modern materials. If you were redoing that studwork you would be well advised to get a bit of celotex between the studs and create an insulated box inside the room which will involve some amount of expenditure as well.

Novice here. Can you please expand on what method you would use for IWI and approximate cost say for 20sq metre wall? Also how doe sone ago about minimsing cold bridging at the site of electrical switches?
Thanks

 

[fernicarry]

A good starting point is the IndiNature install guide. You can work out the cost yourself based on the materials you need.

https://www.celticsustainables.co.uk/content/Indinature/IndiNature%20IndiThermInstallGuide.pdf

minimsing cold bridging at the site of electrical switches

Electricity and insulation aren't great bed fellows because of the possibility of having to downrate the circuit capacity. Best approach I can think of is to run the cables in conduit where they have to pass through insulation.

 

[Kearn]

How thick is the stone? Chances are, when dry, its thermal properties won’t be much enhanced by adding further insulation. When wet, it’s a great conductor though.

Boarding and adding plasterboard and gypsum isn’t ideal.

Standard practice is to keep the thermal mass solid and consistent = no air gaps which are a magnate for dew point condensation to form.

So, a nice thick coat of insulated lime plaster straight onto the stone. Avoid hemp if the wall will be damp though, as it can rot and discolour through and decorative finish.

 

[mrJames77]

Hallo,

People recommend using breathable materials like wood fibre or lime to avoid trapping moisture, especially with stone walls. It might be worth focusing on reducing drafts and improving air-tightness first if you're not ready for a full insulation project. Since you haven't lived through a winter in the cottage yet, testing a small section before fully committing could be a smart move.

James

 

[Glenn586]

16th century cottage here, walls half a metre thick. We lime plastered all the walls. One particular room with 3 external walls was very cold and exposed to the weather. Also a metre underground in places. I retro fitted 40mm wood fibre on the 3 external walls and the difference is night and day. The room is so much warmer, too warm with the wood burner going. The underground sections are now bone dry almost as if the wood fibre is disbursing the moisture better.

 

[mh12]

16th century cottage here, walls half a metre thick. We lime plastered all the walls. One particular room with 3 external walls was very cold and exposed to the weather. Also a metre underground in places. I retro fitted 40mm wood fibre on the 3 external walls and the difference is night and day. The room is so much warmer, too warm with the wood burner going. The underground sections are now bone dry almost as if the wood fibre is disbursing the moisture better.

Thank you. Were the wood fibre boards mechanically fixed to the wall or applied with lime plaster acting as 'glue'? What were the rest of the dimensions of the wood fibre boards- length and width? Finally did you lime plaster over the wood fibre boards?
Apologies for so many question, I'm in the process of doing refurb and want to follow the breathable insulation route which I'm not 100% conversant with.

 

[Glenn586]

The wood fibre boards were glued to the existing lime plastered walls using Secil Isovit. I know most people mechanically fix them but because my boards were only 40mm thick they had enough bend and flex in them to cope with the natural undulations on the walls. You then trap 4mm reinforcing mesh on top of them using lime plaster. Then top coat with lime plaster. An incredibly helpful girl at Mike Wye told me what to do. I did all the work myself, I’d call myself a competent DIY’r. Took 3 weekends. With lime plastering you don’t want it looking like glass so you get away with a lot

 

[Glenn586]

STEICO Therm

STEICO therm is a multifunctional, rigid wood fibre insulation board. For internal wall and roof applications and sub-screed application.

www.mikewye.co.uk

 

[a twig]

Handy guide to various breathable insulation options and how to apply them here by various forum members in this thread

 

[mh12]

The wood fibre boards were glued to the existing lime plastered walls using Secil Isovit. I know most people mechanically fix them but because my boards were only 40mm thick they had enough bend and flex in them to cope with the natural undulations on the walls. You then trap 4mm reinforcing mesh on top of them using lime plaster. Then top coat with lime plaster. An incredibly helpful girl at Mike Wye told me what to do. I did all the work myself, I’d call myself a competent DIY’r. Took 3 weekends. With lime plastering you don’t want it looking like glass so you get away with a lot

Bravo. You DIY'd it, Im so jealous. Honestly since being on this forum I feel inept. Everyone seems so handy, clever, skilled etc etc.. Did you have any previous experience with plastering? Although Im tempted I know my wife will not allow me to go down the DIY route for the final finish plaster but very tempted to apply the the insulation board my self.
Hoping there are some you tube videos on how to do this sort of task

 

[mh12]

STEICO Therm

STEICO therm is a multifunctional, rigid wood fibre insulation board. For internal wall and roof applications and sub-screed application.

www.mikewye.co.uk

Sorry, one more question. Did you opt for the Wet or Dry boards? Looking at the spec there appears to be a minor difference in U value. I suspect the application for both will be the same?

 

[Rosemouse]

The wood fibre boards were glued to the existing lime plastered walls using Secil Isovit. I know most people mechanically fix them but because my boards were only 40mm thick they had enough bend and flex in them to cope with the natural undulations on the walls. You then trap 4mm reinforcing mesh on top of them using lime plaster. Then top coat with lime plaster. An incredibly helpful girl at Mike Wye told me what to do. I did all the work myself, I’d call myself a competent DIY’r. Took 3 weekends. With lime plastering you don’t want it looking like glass so you get away with a lot

Hello, your replies are incredibly helpful - I am in France with an early 19th century farmers cottage, nothing special but trying to strip out all the concrete cover and acrylic paint. Did you do the lime plastering yourself as well? Can a novice learn to do these things? I completely live in France and the French government is offering me quite a large subsidy for insulation but I am not convinced that the methods they will employ are the best and I would rather make sure that the house remains healthy. Have you any reading material you could guide me to? Sorry for all the questions, this is the first house I have ever owned and I love her.

 

[Lacan07]

Bravo. You DIY'd it, Im so jealous. Honestly since being on this forum I feel inept. Everyone seems so handy, clever, skilled etc etc.. Did you have any previous experience with plastering? Although Im tempted I know my wife will not allow me to go down the DIY route for the final finish plaster but very tempted to apply the the insulation board my self.
Hoping there are some you tube videos on how to do this sort of task

You could easily apply the board to the wall yourself, there's plenty of guides online.

I found wood fibre board the easiest background to lime plaster, there's zero suction, the plaster just hangs around forever. Too long in fact, you really need to be careful with your timings, especially if doing the 2 coats in one day. Personally I wouldn't be doing it if I were coming to plastering for the first time though, not if your looking for a decent finish. I found learning to lime plaster a long slow curve, but then again i never got the chance to do a course. Nothing beats experience and practice.

 

[Glenn586]

Sorry for the delayed response. Yes I did the plastering myself. Just had a look on youtube for a few tips. All I would say is try it. If you don't like the finish then pay a plasterer to skim a few bags over the top. I'm doing some insulating this weekend so will upload a few photo's for reference.

 

[Glenn586]

Just an update for reference after my second foray into plastering with Lime on top of wood fibre insulation. It went really well and I found it much easier than the first time round. I gave it two coats on top of the mesh with Cal AC Reabilita. I wanted to finish it wlth the trowel but I couldn't control the suction well enough to do this so I finished it with a sponge. For anyone wondering whether to have a bash I'd say go for it. I honestly can't see what you have to lose.