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??

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?

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

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

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.

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.

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.

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.

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.

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.