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Insulating Lime Plaster
Insulating solid walls can be complex, costly, and, if not done correctly, problematic.
Why Insulating Lime Plaster?
Insulating lime plaster is a popular choice due to its thermal performance, ease of installation and ability to preserve a buildings character.
Sourcing Options
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Pre-mixed products – Reliable and tested, but often expensive.
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Custom mixes – Many skilled plasterers create their own insulating mixes, offering significant savings, though traditionally these mixes haven’t been tested—until now.
Our Research
We believe lime work should be both accessible and affordable. For many homeowners, asking their plasterer to prepare a custom mix is the most practical option.
Over the last 12 months, we’ve independently tested a range of insulating mixes widely used by tradespeople.
The results? Proof that affordable, custom-made options can perform just as well—giving homeowners peace of mind and tradespeople the confidence to recommend them.
Below you can download the test certificates and compare prices and performance so you can make an informed decision.
If you are a tradesperson, feel free to use these test results to show your clients.
What was tested
An insulating lime plaster is traditionally made by replacing the aggregate in a mix (sand) with an insulating material. In these tests, we have used the insulators Perlite, Hemp Shiv and Aerogel.
Perlite
Perlite is derived from a naturally occurring volcanic glass known as obsidian.
Find out more about perlite HERE
Hemp Shiv
Aerogel
Aerogel is known as the lightest solid on earth, composed of up to 99% air. It is created by replacing the liquid in a gel with gas.
Find out more about aerogel HERE
Supplier: Hobens International
Product: P45
Supplier: Yorkshire Hemp Company
Product: HempBuild Fine
Supplier: StortGroup
Product: Hydrophobic Aerogel
A standard lime mix typically uses a 1:3 ratio of lime to aggregate. When perlite replaces sand, a 1:2.5 ratio is preferred due to perlite’s coarser grading, which requires more lime for workable plaster. Aerogel was then added as a percentage of total dry materials to assess performance benefits versus cost.
We deliberately did not test a lime and hemp shiv mix, as achieving a workable plaster with shiv alone requires a high lime to shiv ratio, which would not provide as low a K-value as a combination of perlite and hemp.
The Mix Ratio Tested
Sample panels of various lime plaster mixes were tested to determine their thermal conductivity. The Heat Flow Meter was calibrated using NIST 1453 (thickness = 13 mm) to ensure accuracy. Each sample was tested at least three times, and the average value was recorded. Testing was conducted by Stork Chemicals, an aerogel manufacturer exploring new applications for their product.
Methodology for Testing
The Results
Best viewed on desktop
Insulating Lime Plaster | Tested | Plaster Mix | K-Value (W/m-K) | Cost per bag (ex vat) | U-Value at 50mm on a brick wall | U-Value % Decrease |
|---|---|---|---|---|---|---|
Standard Lime Plaster | 1 lime: 3 aggregate | 0.6 | 2.04 | 11% | ||
Woodfibre Insulation Board 50mm | 0.036 | 0.55 | 76% | |||
Celotex Insulation 50mm | 0.022 | 0.37 | 84% | |||
Warmcote by Best of Lime | Airlime and Perlite | 0.14 | £46 for 25kg | 1.26 | 45% | |
Cornerstone by Cornish Lime | NHL2 & foamed glass | 0.12 | £16.48 for 9kg | 1.17 | 49% | |
Diasen Diathonite Thermactive | NHL5, airlime & cork | 0.037 | £55 for 15kg | 0.74 | 76% | |
carringtonLIME Perlite Mix | 1 lime: 2.5 perlite | 0.099 | from £4 for 25kg | 1.06 | 54% | |
carringtonLIME Perlite/Hemp Mix | 1 lime: 1.25 perlite: 1.25 hemp shiv | 0.102 | from £4 for 25kg | 1.08 | 53% | |
carringtonLIME Perlite/Hemp/5% Aerogel Mix | 1 lime: 1.25 perlite; 1.25 hemp shiv plus 5% aerogel | 0.086 | from £67 for 25kg | 0.98 | 57% |
Frequently Asked Questions
Below you will find a list of frequently asked questions. If your question isn’t covered in the list, feel free to reach out
What is methyl cellulose?
Methyl cellulose, derived from cellulose—a natural plant fiber—is commonly used as a thickener, stabilizer, or emulsifier. In lime plaster, it enables hydrophobic aerogel to disperse uniformly, preventing it from repelling water and floating. Cellulose has been used in lime plasters for thousands of years to aid stickiness, strength, durability and workability.
If I add more aerogel will it result in a better k-value?
In theory yes. As part of this initial testing, we have carried out tests with 8%, 10% and 15% aerogel which all received positive results. However, the results weren't consistent. We believe that results may be effected by the volume of water. This theory is currently being tested and results will be released once complete.
Why did you use a paddle and not a standard mixer?
Aerogel can be easily damaged and so we used a paddle to create a gentle mix action. Once it has been coated in the methyl cellulose, we used our standard mixing equipment.
How long can the aerogel/methyl cellulose mixture be stored?
Mixing the aerogel and methyl cellulose is a time consuming process so we recommend doing this in advance. We have successfully stored the mixture in an airtight container for up to 3 weeks. We intend on testing longer term storage.
Can I achieve building regulations requested U-Value of 0.18 using natural materials?
To reach a U-value of 0.18 on a solid brick wall, you’d need around 500 mm of insulating plaster (lime:perlite), which is impractical due to space loss. A more compact alternative is 180 mm of wood fibre.
However, U-value isn’t the only factor to consider.
