Insulating a roof is no small task. Depending on the type and thickness of your roofing materials, you will need to determine the R-value. This value determines the thermal resistance of your insulation. 

Although it may seem complex, a simple formula and a reference table of materials can be used to calculate R-value. 

Duro-Toit explains how to calculate the R-value of insulation and why it should be calculated. 

What is R-value?

R-value is a rating of the thermal resistance of a material. More precisely, it is a measure of the efficiency of insulation. The R-value is used to find the necessary thickness of insulation to be installed to achieve the desired level of insulation. To find the required thickness of insulation, we calculate the sum of the R-values of all the materials. 

The higher the R-value, the more it indicates that the material has a high level of insulation, protecting against heat and cold. 

Why calculate the R-value of insulation?

It is necessary to calculate the R-value of insulation in order to determine the thickness of the insulation. In short, determining the R-value is necessary to properly insulate a building. Moreover, it helps to choose the right insulation layer and maximize its performance. 

This prevents poor insulation if the thickness is too small and helps to limit costs and save money if the thickness is too large. In both cases, it is necessary to ensure that a building’s insulation performs at its best. 

How to calculate the R-value of insulation

There are two elements to consider when calculating R-value. The first is the lambda value (λ) of a material. Each material has its own insulating properties. The other element to consider is the desired thickness of the insulation

Calculating the R-value of the insulation is simple. Simply divide the thickness by the lambda of the material. 

R = thickness / lambda 


You want to insulate your attic with a 15cm thick layer of glass wool. The lambda of glass wool is 0.035

Here, R = 0.15/0.035 ≈ 4.3 

Note: There are many online calculators to simplify the calculation of this value for you! 

R-value and RSI-value

To accurately understand everything you need to know about the R-value, you need to know the RSI-value as well. 

R-value is an imperial value. At the international level, it is called RSI-value (R-value Systeme International). This value is different from the R-value. To convert an R-value to an RSI-value, simply multiply it by 0.1761


If R = 11, RSI = 11 x 0.1761 ≈ 1.9. 

In sum, 1 R ≈ 5.714 RSI. 

This conversion is important to know if you are only given the RSI value. Please note that the R-value is to be taken into account for both flat roof insulation and pitched roof insulation

How to interpret R-value 

Depending on the type of attic or roof, a minimum R-value is estimated to be necessary for sufficient insulation. 

In the case of a roof, here’s what you need to remember: 

Floors of an unused roof space: R ≥ 7.0 

Attic ceilings: R ≥ 6.0 

Terrace roofs: R ≥ 4.5 

Caution: The R-value is a theoretical indicator and you should not place total confidence in it. It does not determine with absolute certainty how effective insulation is. The R-value is applicable in a laboratory setting. On a building site, other elements can play a role in the insulation of a building. To find out how well your roof is insulated, it is strongly recommended that you contact a roofing specialist accustomed to insulating buildings. 

Let Duro-Toit handle your roof insulation! 

R-value is a valuable tool to consider if you want to insulate your roof properly. However, it is only a theoretical reference and is no substitute for the expertise and advice of a qualified roofer

Duro-Toit insulate roofs in the Montreal area on a daily basis. Our many certifications demonstrate our expertise and knowledge. We were recently elected Consumer’s Choice 2022 for the 3rd year in a row!  

Contact us now! Our team will be happy to hear from you and answer your questions about your roof insulation. 

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