Green Building Forum - Insulating plaster - what about the internal walls?

Insulating plaster - what about the internal walls?

Nick Parsons — Sat, 28 Sep 2019 09:05:41 +0100

'I will take look at Sempatap also. The U value looks pretty good for 10mm?

I am not hearing anyone encouraging me in the direction of the Bauwer plaster :-) ''

It's horses for courses.

You are quite right that for older homes, for 'straight onto the wall' installations (installations with a properly insulated cavity are different) breathability (and in my view the ability to 'buffer' moisture well) is highly desirable at least. The other thing you are looking for is a good insulation value, and an easy way, if possible, to make the installation air-tight.

A few lambda (thermal conductivity) values:

Bauwer light: 0.068W/mK (Designed as part of a breathable system)

Sempatap (Claimed thermal resistance 0.194m2K/W): 0.0515W/mK (Breathability n/k)

Rigid wood-fibre board: In a range from high 0.030s to 0.050W/mK (Designed as part of a breathable system)

PIR: c0.022W/mK - (Used in non-breathable systems)

These are thermal conductivities for a notional m3 of the materials - not U values.

To go from thermal conductivity to U value you do the following:

Divide thickness of insulation in m by the lambda value. That gives you the R value - thermal resistance

Find the 'base case' U value of the wall you are going to insulate. SAP now assumes a U value of 1.7W/m2K for a 225mm solid brick wall. It used to assume (a worse value of) 2.1W/m2K.

Convert that 'base case U' to a 'base case R' by getting the reciprocal. 1/U = R and vice versa.

Add the two R values, and the reciprocal is the U value.

Insulating plaster - what about the internal walls?

modernvictorian — Sat, 28 Sep 2019 20:53:14 +0100

Thanks for a pointer on the maths. Makes sense. I am ruling out PIR and Sempatap as not breathable/likely not breathable. I'm still keen on the plaster with its ability to fill all those nooks and crannies that will surely show up.

I reckon I will have about 50m2 of external wall to see to, or 80m2 if I include the party wall (which seems to transmit a lot of noise so might be worth doing). So the cost for 100mm is 3Kish. Less for 50mm. Expensive but not prohibitive. Assuming I can get a plaster to do it! The resulting U value from 50mm thickness would be 0.89 W/m2K (according to previous thread) for my solid wall. And for 100mm thickness it would be 0.54 W/m2K. I have no idea how meaningful this difference is in terms of bills/comfort?

Actually on the old thread they were assuming a U-value for solid walls of 2.75 W/m2K so if 100mm offers a '5 times improvement' that would bring the U-value down to 0.34 using a 'base case' U value of 1.7 W/m2K??

Insulating plaster - what about the internal walls?

Jeff B — Sat, 28 Sep 2019 22:01:29 +0100

I think you are right to assume that Sempatap is not "breathable". I have a sample and it is composed of a firm spongy foam with a fibreglass surface layer. OK for my application but not for yours.

Insulating plaster - what about the internal walls?

WillInAberdeen — Sat, 28 Sep 2019 23:12:09 +0100

Hi MV you need to check the numbers as Nick explained

Untreated wall U=1.7 so R=1/1.7=0.59

Add 50mm plaster which has R=0.05m/0.068=0.74 so overall U= 1/(0.59+0.74) = 0.75 - so only saving half the heat

Increase this to 100mm plaster then U=1/(0.59+0.74+0.74) = 0.48

If you used 100mm PIR then U=1/(0.59+0.1/0.022) = 0.19 which is more like what you are needing to make all that effort worthwhile.

You would need another 15mm thickness for plasterboard+skim to cover the PIR for a fire resistant and hard-wearing surface, and there might need to be some studs to support said plasterboard which would bridge some heat through the PIR.

We did this and it made a big difference to the room.

Jeff, have you considered removing existing plaster and replacing with Marmox Multiboard or similar?

Insulating plaster - what about the internal walls?

tony — Sun, 29 Sep 2019 06:30:53 +0100

Better without studs, use plastic or nylon insulation fixings.

Insulating plaster - what about the internal walls?

modernvictorian — Sun, 29 Sep 2019 13:05:17 +0100

Understood thanks. Better to actually do the maths than to rely on the manufacturers calculation!

Insulating plaster - what about the internal walls?

Jeff B — Sun, 29 Sep 2019 17:37:41 +0100

Posted By: WillInAberdeen

Jeff, have you considered removing existing plaster and replacing with Marmox Multiboard or similar?

WillinAberdeen - no I wouldn't contemplate that, it would be too complicated. I've tried to attach a couple of photos to show why but for some reason they are inverted after attaching them to my reply!

Insulating plaster - what about the internal walls?

Jeff B — Sun, 29 Sep 2019 18:28:47 +0100

Tried inverting the photo using the software but still appears upside down!

Insulating plaster - what about the internal walls?

tony — Sun, 29 Sep 2019 20:02:17 +0100

There is a switch in the jpg code that makes it do that

Insulating plaster - what about the internal walls?

Jeff B — Sun, 29 Sep 2019 20:11:22 +0100

I've imported the two photos into a WORD document. This works ok. The first shows the ceiling above the shower and the second the area around the Velux window. I would like to cover the ceiling and the dwarf wall with Sempatap.

Insulating plaster - what about the internal walls?

modernvictorian — Fri, 31 Jan 2020 09:09:07 +0000

Reviving an old discussion here. After much deliberation I am now moving more in the direction of woodfibre IWI. As others have pointed out it is difficult to find anyone who has experience with the insulating plaster. I also feel like the woodfibre approach is more 'tried and tested' (and hopefully less expensive!).

One of the reasons I looked into the insulating plaster was because I liked the idea that the plaster itself would act as the airtight layer. As far as I understand the same can be achieved by using lime plaster over a woodfibre board. The other option is to use flexible batts and plasterboard over. I am trying to understand the pros and cons of both of these approaches. Does anyone have any insight on this?

Insulating plaster - what about the internal walls?

jfb — Fri, 31 Jan 2020 10:32:24 +0000

The airtight layer would be the lime render behind the woodfibre board. If it was the finishing layer it will be punctured by sockets and whatever else.

What is the state of your walls at the moment? Bare brick? Brick with lime render? Brick with cement render/ skim? If the latter I would take it off and render with lime.

If you have to redo the backing layer of lime render you can make sure you get details around windows/doors airtight with tape like this
https://www.ecomerchant.co.uk/pro-clima-contega-solido-sl.html

Also consider wall/floor junctions. Best to run the IWI all the way up the wall including around joists. Bit more invasive and easiest to do if you can do it all at once rather than room at a time.

Insulating plaster - what about the internal walls?

modernvictorian — Fri, 31 Jan 2020 11:28:15 +0000

Ah I see. The walls are currently brick with lime plaster and some sort of foam insulating wall paper in places. Plaster is blown in many places (according to the survey). I need to get all the wallpaper off to take a better look.

Insulating plaster - what about the internal walls?

Mike1 — Fri, 31 Jan 2020 19:09:02 +0000

Posted By: modernvictorianThe other option is to use flexible batts and plasterboard over. I am trying to understand the pros and cons of both of these approaches. Does anyone have any insight on this?

Just looking into this myself. I have a mansard roof so I'll be going with hemp fibre batts to infill the timbers on that; not yet sure if I'll use the same for the stone walls too.

One thing to think about is whether and where you want to hang anything on the wall. With studwork you do have some ready-made supports, though still not necessarily in the right places.

Insulating plaster - what about the internal walls?

djh — Fri, 31 Jan 2020 20:36:15 +0000

Posted By: Mike1With studwork you do have some ready-made supports, though still not necessarily in the right places.

Whilst true, that's a lot less useful than it might seem. Once they're covered over and plastered you'll have very little area where they actually are, unless you took extraordinary care in measuring up etc.

Insulating plaster - what about the internal walls?

blacksmith — Fri, 31 Jan 2020 22:17:09 +0000

modernvictorian - just wanted to drop my experience here on "Insulating plaster - what about the internal walls?" I picked up some Bauwer Light from another project that could not find skilled trades so I got it very cheap. I had seen it a few years ago but as I was already going the hemp/lime insulation route so had not taken it any further.

I experimented with it out of curiosity and found that it is an unuseual material to work with if your used to regular trowel applied materials, it is incredibly light weight and can be put on at incredible thicknesses which I think could be off putting first time round. My first attempt was insulating what will be my tank/airing cupbord - over stone walls approx 700mm thick. It requires a very comprehensive key to adhere to - I used an old tyrol sprayer with soaked hemp shiev in lime mixed to a slurry and sprayed liberally over all the suface - after four days (this was in mid November, mid Wales) I then mixed up a bag of thermal plaster and found that troweling it on was a bit of a chore - I then tried just loading up a bucket trowel and flicking it at the surface and it went on a treat - I went over the whole surface like this 40mm minimum in one go and then with a plastering trowel gently worked it up. It was not ideal but for a first attempt it stayed up and did not come out too bad - I am not looking for a perfectly flat level wall - im in an old barn and quite like the gentle undulations and im not a professional plasterer but found it workable.

Insulating plaster - what about the internal walls?

blacksmith — Fri, 31 Jan 2020 22:18:54 +0000

It has its own proprietary finish with I think a fibreglass mesh(?) embedded - I did not have that so in the spirit of experimentation, mixed up my own lime plaster and put that on and it worked a treat â€“ took it no problem and it has bonded well and has not cracked nor shrunk.

I then decided that it would be interesting to see if I could use it a base coat to fill all the gaps in the raked out pointing/stonework after I had repaired it â€“ again using a liberal sprayed coat of hemp/lime shiv to cover the wall I then applied a 35mm minimum coat of thermal plaster again using the flicking technique. I then used the point of a trowel to create a textured surface and left it to dry for 7 days. I then mixed up a plaster made from hemp/lime with the intention of using that over the deeply indented surface â€“ again I discovered it bonded well to the surface and was able to put on 40mm layers with ease.

Its not an intuitive material to work with but confess I am a convert â€“ now I have worked with it for a while I would be confident in using it as it was designed to be used â€“ this video gives a very good idea of how to use it and I adapted my method from it. If you are a keen DIY person then it is certainly within the scope of someone having a go. And if you follow the technique I see no reason not to be successful.

https://www.youtube.com/watch?v=oxogp2uRgaw&feature=youtu.be

Insulating plaster - what about the internal walls?

modernvictorian — Mon, 03 Feb 2020 16:18:11 +0000

Thanks for posting this. Looks you had great success. There are not so many examples of insulating plasters so it is great to see the layer buildup in progress. Whilst I would love to DIY the wall myself I have to be realistic about how much time I have.

Another reason for moving away from the insulating plaster is the thickness of plaster I would require due to the lower k-value compared to WF. I did also look at Diathonite Evolution plaster which apparently has a k-value of 0.045. Its formulation includes both cork (k-value 0.04 - 0.05 depending where you look) and lime (k-value much higher assumedly) so I was a little confused about why the k-value did not reflect the average of the components. In other words how can cork mixed with lime have a k-value the same as cork alone?

I think there was a similar discussion on the Bauwer thread a while back.

Insulating plaster - what about the internal walls?

Kenny_M — Wed, 05 Feb 2020 09:16:36 +0000

I did a cost comparison of Bauwer and Diathonite a while back, I think its on the forum somewhere.

In summary, Bauwer was slightly cheaper for the same r value, but needed to go on thicker and the additional weight for shipping would have evened up the cost. On that basis Diathonite seemed like the better option as it would achieve the same u-value for cost, with a thinner application.

However, wood fibre board, which seemed to be what most of building conservation groups were recommending, was less than half the price for the same r value.

In the end I just used EPS, as it was an upstairs bathroom that was going to be covered with impermeable paint anyway.

My impression of diathonite was probably best used in a commercial spray application, cutting down on labour costs, especially for uneven surfaces, curves etc. There was a suggestion in their literature that putting it on by hand could compress and reduce the effectiveness as insulation.

Insulating plaster - what about the internal walls?

modernvictorian — Wed, 05 Feb 2020 13:28:34 +0000

I remember your comparison and it was a useful reference thanks. The labour costs for installing the woodfibre would up the quite cost quite a lot for me so I have to take that into account but I'm not ruling anything out at this stage.

Insulating plaster - what about the internal walls?

modernvictorian — Tue, 14 Jul 2020 11:17:43 +0100

rosecottage, sorry I've not checked back here for a while. I have been having fun with Building Control approvals for a troublesome chimney demolition.

In the end I specified 40 mm woodfibre and lime plaster for all external walls and party wall. This got me through Building Control approval. Some areas will need more and some less (e.g. by front door there just isn't the space for 40 mm). But I am still thinking about the insulating plaster, particularly the diathonite thermactive product with has the same lambda value as the woodfibre. I am going to see what state the walls are in when the renovation starts before a final decision. Certainly I can see the benefit of the plaster for the gable end wall in the loft which is currently very rough brickwork.