Last month Jasper and his team from Border Lime Construction came to install Diasen Diathonite, a breathable, insulating lime plaster product, on the inside of our front wall. This was the only wall which hadn’t yet been insulated, so this makes up the penultimate part of our insulation strategy, with only the floor now remaining.
The front wall (Figure 1) presented a challenge on the insulation front. We didn’t want to change the appearance of the outside, so were constrained to internal wall insulation (IWI). The walls are solid sandstone (with a rubble core), and approximately 450mm thick, with an estimated U-value of 2.7 W/m²·K. The front wall is south facing, and along with the west gable, takes the brunt of the harsh weather, although it had no signs of damp issues. Nevertheless, we didn’t want to introduce a moisture-risky strategy. Solid stone walls would originally have existed with seasonal wetting-drying cycles, with moisture drying to the inside and outside. Moreover, inside dwellings, the open fires and range cookers would have provided heat and continuous draw of ventilation to the building, with much lower internal moisture loads (baths not showers, laundry dried outside etc). Disrupting this moisture cycle can result in moisture accumulating in the stone wall, either from internal humid air leaking to the outside and condensing within the fabric as the moisture-laden air is cooled (interstitial condensation), or from rain loads not being able to dry. Whilst some argue that stone is not adversely affected by being permanently wetted, the moisture load does significantly increase heat loss from the stone, as well as increasing the risk of freeze-thaw effects and spalling, and thus presents a long-term risk to the building fabric, which seems particularly relevant for heritage features.
Options for Consideration
In the early days of the project, and with rather less knowledge than we now have, we had considered installing studwork, and 300mm of Thermafleece sheep wool insulation to achieve the target U-value of 0.15 W/m²·K. An intricate dual-layer of offset studs would prevent thermal bridges through the studs themselves. My approximate costs for this worked out at approximately £60/m2, £3000 based on 50m2 (front wall and returns for gable walls and partition), to include timber studs, suitable membranes, and plasterboard finish. However, this approach presented challenges with several thermal bridges (pathways through the insulating layer which can result in significant heat loss, especially in highly insulated buildings), the most obvious of which were the major structural timber roof trusses which bear into the front wall, and also a brick partition wall which separates the west quartile of the building. On discussion with various people, and through learning on the CarbonLite Retrofit course, I eventually moved away from this approach, reconciling to higher (poorer) U-values. The highly insulating layer would lead to the front wall receiving almost no heat during the winter, and the membranes would limit moisture transmission for drying into the house, and so seasonal drying would be prohibited, and could lead to problems with the stone over the long term. Also, without careful and meticulous use of membranes and airtightness products, there would be a potentially high possibility of mould growth on the cool side of the insulation.
A case study presented by Bill Butcher at Green Building Store introduced me to the idea of a breathable lime plaster product, Diathonite Evolution by Diasen. It is a lime based plaster containing cork particles for insulation. This product was described as providing good insulating performance (thermal conductivity of 0.045 W/mK), simultaneously providing the airtightness layer by the wet parge coat method, as well as being breathable and thereby allowing the transit of moisture in either direction. Finally, the alkaline properties of lime mean that it inhibits mould growth. A brief look at this revealed it’s eye-watering cost however: A 150mm application would yield a U-value of 0.23 W/m²·K but had a ball-park price tag of >£10k, just for the front wall (50m2).
I sat on this thought for a long time. I returned to this front wall time and time again, probably over a 2y timescale, thinking and re-thinking the approach. £10k was simply beyond what we could afford or justify for one wall, and that was before we considered the need to address the wall-floor thermal bridge and the traces of damp which were evident at the back of the house. But every time I thought about it, or discussed it with someone with more experience, read case studies, or heard about solid wall insulation, I became more convinced that Diathonite, or an alternative lime-based product was what we needed.
I did my research. We evaluated a few other similar options, including hemp-lime plaster from Eden Hot Lime with thermal conductivity of 0.113 W/mK, and another insulating plaster with a conductivity of 0.068 W/mK by a company called Bauwer. I also learned in 2018 about a new addition to the Diathonite range called Thermactive, with a conductivity of 0.037 W/mK – the best I had seen of any of these type of products. In September 2018 during the Cumbria Green Build Festival I visited a case study which used Diathonite Evolution, and saw first hand the finish achievable, and met the contractors. And still I went home pondering whether we should revert to wool-filled studwork. However I simply couldn’t settle on an approach which was inherently risky. We revised down our expectations, and I worked up the numbers, obtained new quotations and got in touch with Border Lime Construction for a discussion.
Of the three products mentioned above, we ruled out Eden Hot Lime because we felt that it didn’t offer high enough performance in reference to our approach on the rest of the house. We then compared the Bauwer and Diathonite products more carefully. On the face of it, Bauwer was the cheaper option at £10 per 25kg bag (ex VAT), but when I explored this in more detail it was not the case:
- Diathonite Thermactive, £46 per bag ex VAT, with an applied thickness of 75mm, k = 0.037 W/mK,
- R = 75/1000/0.037=2.027 (m2K/W)
- U = 1/R = 1/2.027=0.4933 W/m²·K
- Using Bauwer Lite, £10 per bag ex VAT, k = 0.068 W/mK, thickness required to achieve the same U-value:
- 2.027*0.068*1000=137.836 mm
- Needs 140mm of product
- Coverage = 25mm thickness per bag, therefore 140/25=5.6, round up to 6 bags per m2, = £60
For the front wall area in question, I would require 65 bags of Thermactive, at £2,990, but 300 bags of Bauwer Lite at £3000. In fact, the cost was effectively the same, but the labour costs of installing the thicker coating of Bauwer Lite would be higher. On this basis, we settled upon Diathonite.
For the simple reason of wanting to reduce the cost further, we evaluated thinner applications – on cost and performance. I expected the Thermactive to be more costly than the Evolution, which it is per area at the same thickness, however this seemed not to be the case when considered based on thermal performance. I’ve included my cost comparison below.
|Thermal conductivity (W/mK)
|Thickness applied (mm)
|Thermal resistance, R (m2K/W)
|Quantity per bag
|Yield per bag
|Coverage per bag (m2)
|£ / bag (2018)
|£/ unit R-value
|U-value, with 450mm sandstone wall W/m²·K
Based on material cost alone, Diathonite Thermactive offered a lower cost per unit R-value than Evolution. It should also offer lower labour costs because there is less material to transport and install, and so for me was the clear leader on cost where insulation was the priority.
I wanted to make a final check of the product thickness. 50mm seemed to be a reasonable compromise of cost and performance, but this was not based on quantitative data. Some homeowners have installed up to 180mm, so I wanted to do an analysis.
Using Therm, a free software program for modelling heat transfer through 2-dimensional constructions, I modelled different thicknesses of Diathonite Thermactive based on an external air temperature of 0°C and internal temperatures of 17°C and 20°C respectively. The model allowed me to see the effect on the surface temperatures in these different conditions.
As shown in Figure 2, with 17°C internal air temperature, the surface temperature of an untreated (simplified) sandstone wall would be 10.6°C. The isotherms (lines on the diagram representing layers at a specific temperature) are evenly distributed through the wall from outside to inside.
As shown in Figure 3 however, the addition of 50mm of Thermactive on the inside surface increases the surface temperature to 15.7°C, which is more than 5°C higher than the untreated wall. This is an important consideration for the formation of condensation on internal surfaces. Condensation depends on a number of factors, such as humidity air temperature and air movement, and whilst this was not a condensation analysis, I felt more comfortable with <2°C temperature differential between the air and wall surface, than >6°C for the untreated wall.
The graph in Figure 4 illustrates the results for Thermactive thicknesses ranging from 0mm to 100mm for internal temperatures of 17°C and 20°C. In both cases, the first 10mm applied makes the biggest difference, with subsequent increases in thickness yielding increasingly small improvements. This graph allowed me to draw the conclusion that a 100mm application increased the surface temperature by only 0.6°C compared to a 50mm application.
We finally decided on Diathonite Thermactive at 50mm. This was based on the cost per unit R-value, the labour cost for installation, the limited further improvement by installing even thicker layers, and what we felt like we were prepared to spend on the job.