If you could do internal wall insulation on a damp wall any way you wanted, what would you choose?

[DampedOut]

TL;DR if you had to insulate a damp facade that will always see some water ingress, how would you do it?

So. This summer we had the cement facade removed from the house. The house now appears to be a 17th century Cumbrian farmhouse, then pub, then BnB, then private home. Removing the facade was a huge success—lots of beautiful character features revealed on the outside.

There's always a downside though: it faces south/south-west in Cumbria and driving rain has been pushing water in, so all the (inappropriate) internal plasterboard and gypsum has water stains now. Great.

The facade is a likely 60cm-75cm stone wall with rubble in the middle. Once the plaster in the inside comes off in the future, decisions have to be made about picking a good insulation system for a wall that will experience penetrating rain every year in autumn and probably in winter too. Water will definitely find its way in. As the climate changes, this might even get worse with more rain and less snow.

External insulation is a no-go. Conservation area + lots of beautiful revealed features on the facade, including original wooden lintels above some windows with rose head nails intact, means the facade has to stay revealed and pointed with lime.

Another limiting factor: The home owner, a relative, doesn't want to use an insulation system where the insulation has to be screwed into the solid masonry external wall and can damage the stone.

Things I've thought about but can't decide between:
1. lime plaster and calcium silicate boards sandwich—can be stuck to the wall with adhesive
2. lime plaster and wood fibre boards sandwich—probably has to be screwed in place, right?
3. hempcrete (here be dragons, right?)

Things I think are probably a bad choice:
4. solid insulating lime plaster since it's expensive to build up a layer thick enough to do significant insulating
5. battens with sheepswool in between because of condensation forming in the wall and potential for rodents

What am I missing here? How would you do it?

p.s. although I haven't posted in years, I often read on this forum and thank you to everyone here for always providing such great insights, information and advice.

 

[lngn2]

Internal studs with wood fibre between and over? You'd lose room space but wouldn't have to drill the walls.

 

[malcolm]

I've done something a little similar insulating a wall with a leaky but breathable timber frame on the outside. I used a stud wall with celotex panels with a gap of about 50mm between the stud wall and the exterior wall. The gap is ventilated to the attic to help dry out the cavity between the insulation and the wall.

 

[Drayton]

The amount of water getting into a wall subject to frequent gale-driven lashings of rain is considerable - think repeated pressure-washing. Though I'm coastal so maybe it's worse here - pressure washing and sand blasting all in one. And definitely getting worse in recent years.

No solution from me, I'm afraid, my external stone and rubble walls remain uninsulated.

Sounds like a lovely house with the owner it deserves, I hope you find a good way to insulate it while letting it dry through whenever the weather allows.

 

[DampedOut]

Yeah, I'm wondering if wood fibre boards could really hack it with a lot of water ingress or if they'd just rot. In which case, I guess it's options 1 or 3.

 

[Lacan07]

I would have thought hemp would be the most obvious solution. The u value wouldn't be great but it would give you something.

Some folk aren't too keen on wood fibre board on a wall that's liable to get wet repeatedly

 

[DampedOut]

Hemp plaster would be something, that's true! And something is better than nothing.

I just don't know if it's as effective as the other options. It might be really competitive price-wise, but I haven't even started to figure that out.

Here's my notes on the u-values** of different kinds of breathable insulation. Hope this comparison list and the links will help someone else out and save them some effort, but if anyone sees an error, please let me know and I will amend it.

I also think some of these would perform better against a damp wall than others, but that's kind of what I want to make sure of with this post and question. If they're just about equal in terms of working against a very wet wall, then it just comes down to which ones perform better for the least money.

Here's what I know of so far:
1. calcium silicate panels = a u-value of 0.068 W/mK [https://www.ecomerchant.co.uk/skamowall-board-1-000mm-x-600mm-x-25mm-or-50mm.html]
2. wood fibre boards = 0.038 W/mK [https://www.mikewye.co.uk/product/steico-therm/]
3. hempcrete = 0.07 W/mK [https://www.ukhempcrete.com/hempcrete-buildings-thermal-performance-and-costs/]
4a. insulating lime plaster = 0.12 W/mK [https://cornishlime.co.uk/products/lime-mortars/cornerstone-insulating-render/]
4b. Cork plaster = .10 W/mK [https://www.lime.org.uk/ecocork-insulating-plaster.html]
5. sheepswool insulation = 0.0385 W/mK [https://www.sheepwoolinsulation.com/product/optimal-rolls-2/]
6. hemp plaster = .07–.09w/mK [https://www.bre.co.uk/filelibrary/pdf/projects/low_impact_materials/IP14_11.pdf]

**for future readers who start where I started on this whole insulation-question: a u-value measures how much heat is lost through a material, so the lower the number is, the lower amount of heat lost. The number closest to 0 wins. In which case, the most insulating material in this list is wood fibre boards, then sheepswool, calcium silicate panels, hempcrete, hemp plaster and insulating lime plaster.

Tbh though I still haven't worked out how to get the u value or r value for different thicknesses of material. If anyone can explain, I'd be grateful.

 

[paulc]

Add to that list, another insulating lime plaster.
Secil EcoCork - 0.10 W/mK

 

[Sam W]

I was advised against hemp plaster for a damp wall by Ty Mawr, my understanding was the hemp fibres may rot.
Insulating plaster does get expensive if you're building up a decent thickness, just over £1/mm/m2 for Cornerstone, EcoCork 73p/mm/m2. I recently did some calcs and got to 40mm of Cornerstone being roughly equivalent to 10mm of standard Kingspan PIR insulation.
I couldn't use boards because of a combination of very uneven walls and awkwardly placed doorways, but they definitely seemed an easier way to stop heat loss than building up very thick insulating plaster.

 

[Zebra]

Polystyrene on the lower part and woodfibre higher up.

 

[Zebra]

Yeah, I'm wondering if wood fibre boards could really hack it with a lot of water ingress or if they'd just rot. In which case, I guess it's options 1 or 3.

I left a woodfibre board outside for ten years just to see what would happen. Apart from being used as a cat scratching post, it was fine where it could dry again after rain. Where it was in contact with the ground it rotted, much like timber would have.

 

[a twig]

Ditto - personally used 20mm wood wool boards with an NHL skim on damp walls with no issue

 

[DampedOut]

So, I wanted to come back and share the calculations I've done in a spreadsheet on the different types of insulation above and how they perform. I was hoping to create a reference that someone else might find helpful and to give back to the community.

I've done a few sets of calculations. The first ones, Tables 1 and 2, show the R-value and the U-value of these insulating materials at different thicknesses.

Here's some quick definitions:

• The R-value is the resistance to heat loss. A higher number is better. It looks like this: 9.99 m2K/W.
• The U-value is the amount of heat lost through a material at a given thickness. A lower number is better. It's also worth pointing out that thermal conductivity looks like the u-value but is different. The U-value looks like this: 9.99 W/m2K. Note the 2—this is how you spot the difference between U-value and thermal conductivity value. To really do a u-value calculation properly, you should include other things about the surfaces and convection and radiation heat loss. But I'm not willing to go that far with it.
• Thermal conductivity is how much heat is lost through the material. It's not calculated to a thickness, so I've had to do that myself for all these materials. It looks like this: 9.99 W/mK. Every single insulating product you encounter should list the thermal conductivity. Lower numbers are always better.

Thermal conductivity is the basis of all the maths. We use it to find the R-value first (the resistance to heat loss). You take the thickness in metres, divided by thermal conductivity. If we had 30cm of insulating lime plaster, our math would be .3/.12 (because insulating lime plaster has a thermal conductivity of .12). That gives us an R-value of 2.5 m2K/W.

And we can take the R-value now to get the U-value for insulating lime plaster at that given thickness as well. We divide 1 by the R-value. 1/2.5 = .4 W/m2k. Now we know that, by itself, 30cm of insulating plaster will not meet Building Regs requirement for a U-value of .3 W/m2K or less.

TL/DR I've actually done all this math with a spreadsheet and now after that preamble, here it is. I highlighted what I consider superior performance in bold:

Table 1: R-values of Insulation at Different Thicknesses (higher numbers are better)

	1	2	3	4a	4b	5	6
	Calcium Silicate Panels	Wood Fibre Boards	Hempcrete	Insulating Lime Plaster	Cork Plaster	Sheepswool Insulation	Hemp Plaster
Themal Conductivity (W/mK)	0.068	0.038	0.070	0.120	0.100	0.039	0.070
R-Value at 2cm	0.294	0.526	0.286	0.167	0.200	0.519	0.286
R-Value at 4cm	0.588	1.053	0.571	0.333	0.400	1.039	0.571
R-Value at 6cm	0.882	1.579	0.857	0.500	0.600	1.558	0.857
R-Value at 8cm	1.176	2.105	1.143	0.667	0.800	2.078	1.143
R-Value at 10cm	1.471	2.632	1.429	0.833	1.000	2.597	1.429
R-Value at 12cm	1.765	3.158	1.714	1.000	1.200	3.117	1.714
R-Value at 14cm	2.059	3.684	2.000	1.167	1.400	3.636	2.000
R-Value at 16cm	2.353	4.211	2.286	1.333	1.600	4.156	2.286
R-Value at 18cm	2.647	4.737	2.571	1.500	1.800	4.675	2.571
R-Value at 20cm	2.941	5.263	2.857	1.667	2.000	5.195	2.857
R-Value at 25cm	3.676	6.579	3.571	2.083	2.500	6.494	3.571
R-Value at 30cm	4.412	7.895	4.286	2.500	3.000	7.792	4.286
*higher values are better

Table 2: U-values of Insulation at Different Thicknesses (lower numbers are better)

	1	2	3	4a	4b	5	6
	Calcium Silicate Panels	Wood Fibre Boards	Hempcrete	Insulating Lime Plaster	Cork Plaster	Sheepswool Insulation	Hemp Plaster
Themal Conductivity (W/mK)	0.068	0.038	0.070	0.120	0.100	0.039	0.070
U-Value at 2cm (W/m2K)	3.400	1.900	3.500	6.000	5.000	1.925	3.500
U-Value at 4cm	1.700	0.950	1.750	3.000	2.500	0.963	1.750
U-Value at 6cm	1.133	0.633	1.167	2.000	1.667	0.642	1.167
U-Value at 8cm	0.850	0.475	0.875	1.500	1.250	0.481	0.875
U-Value at 10cm	0.680	0.380	0.700	1.200	1.000	0.385	0.700
U-Value at 12cm	0.567	0.317	0.583	1.000	0.833	0.321	0.583
U-Value at 14cm	0.486	0.271	0.500	0.857	0.714	0.275	0.500
U-Value at 16cm	0.425	0.238	0.438	0.750	0.625	0.241	0.438
U-Value at 18cm	0.378	0.211	0.389	0.667	0.556	0.214	0.389
U-Value at 20cm	0.340	0.190	0.350	0.600	0.500	0.193	0.350
U-Value at 25cm	0.272	0.152	0.280	0.480	0.400	0.154	0.280
U-Value at 30cm	0.227	0.127	0.233	0.400	0.333	0.128	0.233
*lower values are better

 

[DampedOut]

p.s. I did more calculations to account for the external wall that I'm working with. It's an 80cm solid sandstone wall. With all our period homes, the walls represent a huge variable in calculating the efficiency of insulation.

I might share those calculations in the future so people can see the effect in terms of the maths of having a better or worse performing solid wall. Spoiler alert: once you get up to higher thicknesses of insulation, the efficiency of the solid wall matters much, much less. The difference to the U-value is half a hundredth! At lower insulation thicknesses, it makes a much bigger difference.

Still, calculating for your solid wall is a good idea in my opinion because after crunching all the numbers, it can be the difference between one thickness of insulation and another.