When you're purchasing new roof insulation material and fittings, there are a range of factors you should be looking out for, such as price, ease of installation, flammability and environmental concerns. In the midst of all these different variables, one important property of insulation can be easily overlooked -- the insulation's R-value.
What are R-values?
Put simply, the R-value of a type of insulation tells you how effective it is at trapping and insulating heat. This is worked out in a test laboratory by measuring how quickly hot air passes through a piece of insulating material, then an R-value rating is assigned to that type of insulation. As such, a higher R-value generally means better insulation.
Is it really that simple?
Usually yes, but there can be a couple of complications:
- Thickness - Different thicknesses of a given material naturally have different R-values, so the R-value that is given for a certain insulation type at the point of sale only covers that thickness of the specific material. Do not be tempted to just eyeball insulation based on thickness, as some modern varieties of insulation give high performance with relatively thin applications.
- Up and down R-values - The up and down R-values of a piece of insulation tell you how effective the insulation is at trapping heat rising from below (the 'up' value) and coming down from above (the 'down' value). Most standard insulation types have identical up and down values, so their R-value is given as a single rating. However, if insulation comes with an adhesive vapour barrier on one side, or is designed with a reflective overlay, than these values may differ significantly. Homes that get hot in summer should be fitted with insulation with high down values, while homes that get cool in winter need high up values.
- Imperial and metric measurements - While Australian insulation has its R-value measured with metric means, insulation imported from some other countries, such as the United States, use imperial measurements. These imperial measurements give a higher value than metric measurements for the same amount of heat insulation, and the units used are generally not stated at the point of sale. Be cautious and make sure you know where insulation hails from before you buy it.
- Liquid insulation - Insulation that is applied in liquid form and sprayed into your home with a hose, such as cellulose or sprayable foam insulation, is difficult to give an accurate R-value to, since its insulation properties will depend largely on how it is applied.