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Does Natural Insulation Work?

Natural insulation can certainly work and is often used in construction for its environmental benefits. These natural materials include sheep’s wool, cellulose (made from recycled paper), wood fibre, cork, hemp, flax, and even straw. The effectiveness of insulation is typically measured in terms of U-values or R-values.

R-value measures how well a material resists the conductive flow of heat: the higher the R-value, the greater the insulation performance. U-value measures the rate of heat transfer through a material: the lower the U-value, the better the material is at insulating.

The R-value or U-value of a natural insulation material will depend on a variety of factors including its thickness, density, and the specific material in question. Here’s a brief overview of the approximate R-values for some natural insulation materials:

  1. Sheep’s wool: R-3.5 to R-3.8 per inch
  2. Cellulose: R-3.2 to R-3.8 per inch
  3. Wood fiber: R-3.6 to R-3.8 per inch
  4. Cork: R-3 to R-4 per inch
  5. Hemp: R-3.5 per inch
  6. Flax: R-3.5 per inch

Keep in mind that these are just estimates and can vary depending on specific conditions and installation methods. Other factors like moisture resistance, fire resistance, and lifespan should also be considered when choosing insulation materials.

In many cases, natural insulation can offer comparable performance to synthetic materials, while also offering benefits in terms of sustainability and indoor air quality. However, they may have a higher upfront cost and might not always be the most practical choice depending on the specific circumstances of your project.

Calculation of required thickness

The U-value of a wall or roof is determined not only by the insulation used but also by the other materials in the structure. It’s a measure of the rate of heat transfer through a structure divided by the difference in temperature across that structure. The lower the U-value, the better the material is at insulating.

To achieve a specific U-value using different insulation materials, you would have to adjust the thickness of the insulation according to its thermal conductivity. You’d also need to account for the thermal properties of the other components of the wall or roof.

The thermal conductivity of an insulating material is given as the lambda (λ) value in W/mK (Watts per meter per degree Kelvin). The lower the lambda value, the better the material is at insulating.

To calculate the required thickness of insulation (d) to achieve a specific U-value (U), you can use the following formula:

d = λ / (U – U_other)


  • d is the thickness of the insulation
  • λ is the thermal conductivity of the insulation
  • U is the desired overall U-value
  • U_other is the U-value contribution of the other materials

Let’s take cellulose insulation as an example. The lambda value of cellulose is typically around 0.038 W/mK. If the U-value contribution of the other materials in the structure is 0.1 W/m2K, and you want to achieve a total U-value of 0.3 W/m2K, you would need:

d = 0.038 / (0.3 – 0.1) = 0.19 m = 190 mm

So, you’d need about 190 mm of cellulose insulation.

The U-value of other materials in the structure can be quite complex to calculate. It depends on the specific properties of each material layer, including its thermal conductivity, thickness, and surface properties. If you’re not sure how to calculate it, you may want to consult with a professional or use a U-value calculation software.

Comparing with the EWI Pro insulation range

  1. Mineral wool (Rockwool): R-value is approximately R-3.0 to R-3.3 per inch. The thermal conductivity is around 0.034 to 0.040 W/mK.
  2. Expanded Polystyrene (EPS): R-value is approximately R-3.6 to R-4.2 per inch. The thermal conductivity is around 0.030 to 0.038 W/mK.
  3. Phenolic foam: R-value is approximately R-4.8 to R-8.2 per inch, but it often averages around R-6 per inch. The thermal conductivity is very low, around 0.018 to 0.024 W/mK.

Given these numbers, to achieve the building regulation U-value of 0.3 W/m2K (assuming U_other = 0.1), the approximate thickness of insulation required for each material would be approximately 110mm for Mineral Wool, 90mm for EPS, and 60mm for phenolic foam like Kingspan. We recommend these numbers as a general guideline as the substrate will have inherent U-values that affect the thickness required. The range can be purchased with our sister company EWI Store.

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