A basic knowledge of R-values will take you a long way towards understanding building insulation and knowing how to choose the right insulation product.
What R-value means
R-values are scientifically calculated measurements of the ability of materials to resist the transfer of heat. The higher the R-value, the better the insulating properties.
When you look at R-values of various insulation products, it’s important to compare apples with apples. They can differ two ways.
- Thickness. The thicker a product, the longer heat takes to move through it. For example, an 80 mm rigid insulation board has a higher R-value than a 50 mm board made of identical material.
- Physical structure. Even if they look similar, the composition of competing products means they can perform quite differently. For example, if one brand of loose fill insulation is less effective at resisting heat than another, you’ll need a thicker layer to achieve the same R-value.
Why one insulation product can have two R-values
In summer, when you want to keep the inside of your building cool, your insulation’s job is to prevent heat coming in. It’s the opposite in winter, when you want your insulation to trap the heat.
Because heat can move through insulation in both directions, there may be two R-values. Some products, for example those with a reflective layer of foil one surface, have different R-values for heat flow inwards or “down” and heat flow outwards or “up”.
Product R-value and Total R-value
The R-value of an insulation material measured in isolation is its Product R-value.
The materials in your walls and roof have R-values too. For example, a masonry wall has a relatively high R-value and a metal roof has a low R-value. When you add insulation, the combined performance of the product and building materials is the Total R-value, expressed as RT.
Calculating Total R-value is a bit more complex than simply adding the R-values of the insulation and the building materials. That’s partly because heat goes through gaps and spaces, window glass, metal window and doorframes and anywhere else there’s no insulation. Although you may have little control over those, two other factors in the equation are closely related to your choice of both insulation product and installer.
- Compression. When insulation material compresses and loses thickness, it loses some of its R-value. This can happen when soft insulation is fixed to wall and roof timbers. Where possible, choose a rigid insulation material that doesn’t compress.
- Installation. Incorrectly installed insulation may have gaps, damage such as rips and tears or excessive compression due to poor fixing technique. Any of these will lower the Total R-value.