Thermal Insulation

Choosing the right thermal insulation

A range of criteria needs to be considered when picking energy-efficient insulation
Friday, November 8, 2013
By Rockford Boyer

Many property owners are now demanding that their buildings be as energy efficient as possible. Though there are many factors to ensure high-performance energy results, a successful strategy will need to include the right type of thermal insulation.  

While the main question being asked today with regard to thermal performance is “What is the R-value?”, this is only one aspect of insulation’s properties and performance. Thermally speaking, designers should consider a range of criteria in order to determine which thermal insulation material to use, as there are many properties that can affect a building’s performance.

The main criteria to consider are:

Stable R-value (aging)

Thermal insulation obtains its R-value by trapping still air or gas within a manufactured system or product. Foam insulation use low thermal conductive blowing agents; but over time, gasses trapped within the boundaries of foam products are replaced with air.

This doesn’t imply that foam insulation loses all of its R-value. Rather, it means that it could lose a certain percentage of R-value throughout its service life.  In their warranties, some major foam manufacturers acknowledge the potential for a 5–20 per cent reduction in R-value after the first year of service. This does not occur in mineral wool insulation due to the properties of air, unless convection occurs within the material.

Dynamic R-value

Thermal insulation materials in North America are required to state their R-values at 24 degrees Celsius, which allow all manufacturers to report at the same temperature. However, since this is essentially room temperature, there is limited heat flow to resist due to the lack of temperature difference between the exterior and interior environment. Therefore, this practice does not provide a representation of the insulation’s performance within its regional climatic zone.

Dimensional stability

Temperature impacts R-value performance as well as the dimensional stability of materials. Having a material that can expand and contract due to thermal cycling can potentially have a negative impact on the building envelope’s performance. To maintain the building envelope’s thermal integrity, materials that have low coefficients of linear expansion, such as mineral wool, can be utilized to minimize the potential for open gaps at lower temperatures.

Product durability

It is imperative to design building envelopes using materials that are durable and last for as long as possible. To function and perform suitably, insulation materials must resist several damaging variables such as fire, ultraviolet rays, moisture, heat and wind damage. To maintain thermal efficiency, the material must be resilient to warping and should maintain its shape over time. Stone wool, XPS, and polyiso are all durable options, but their performance in each of the aforementioned categories can vary.


Moisture damage is the main cause of building envelope failure. Thus, designing an envelope that can dry in both directions is ideal for enhancing the assembly’s durability. Using permeable materials throughout the building envelope will ensure that vapor drive or drying potential is not restricted in any direction. Several tools are available for designers that can assist in determining which materials will minimize the potential for moisture-related issues.

Climates in North America can vary significantly with regard to heat and moisture conditions. When choosing insulation material for the building envelope, it is important that the climate the building is being constructed in is taken into consideration. And given the energy performance expectations of the system, how different insulation are likely to perform given the performance-based testing should also be considered.

Keeping these factors in mind can mean a more educated decision about insulation materials given their physical properties and performance under varying environmental conditions. This can help meet the property owner’s expected performance levels, which is just a good building practice to follow.

Rockford Boyer, B. Arch., is the North American manager of the Energy Design Centre. He is also a master of building science candidate at Ryerson University.

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