What are R values and U values?
R-values express a material’s thermal resistance (how well it blocks heat) while U-values express thermal transmittance (how well it passes heat along). A material with a high R-value will therefore have a low U-value and vice versa. R-values are used in the building and construction industry while other measures of thermal resistance are used in other industries - textiles, for example.
How are R values established or measured?
R-values are obtained using a formula that measures the ratio of temperature difference between two different sides of a material, and the amount of heat that can pass through it.
When an R-value is recorded for an insulation type, it is specifically measured against that material alone, without any additional layers or installation specifics being taken into account.
R-values used in Australia conform to the International System of Units (SI) while those in the US (based on measurements in Fahrenheit as opposed to Celsius) are approximately 5.71 times those measured using SI. It can be difficult to determine the difference between US and SI measurements as R-values are typically expressed without acknowledging which system it’s based on.
- If you're looking at R values for a particular product, in most cases the figure the manufacturer provides will represent the total R value for the entire product (e.g. R-3.0 for a single glasswool batt), rather than the SI standard R value for that particular type of material (likely to be 0.6 for glasswool).This makes it much easier to calculate the total R values of your walls, floor and roof. Things like cellulose and spray foam, which don't come in pre-prepared sizes like batts do, will be measured using the standard 'kelvins per metre squared' R values. The total R-values for these sorts of insulation will depend on their density and thickness.
What's considered high, low or average for general insulation?
This will largely depend on the type of climate you live in. The Building Code of Australia (BCA) makes provision for minimum values according to climate types. Because the priority for insulation will change depending on the climate (i.e. for cold climates the priority is heat retention whilst in tropical climates reducing heat gain is the main focus), minimum values need to be checked for your area. Different states and territories have particular provisions that need to be addressed too.
What are 'up' and 'down' R values?
In short, 'up' values measure the flow of heat rising up through the insulation while 'down' values measure the amount of heat passing down through the insulation. These are sometimes known as winter and summer values, respectively. These measurements are especially important when dealing with reflective insulation, as they are included in the regulations set out in the Building Code of Australia (BCA).
Shortcomings of R values
While R-values are the best indicator of thermal performance for a material, it is worth noting that many other factors can change the end result, for better or worse. Improvements can be made by adding layers such as a reflective foil vapour barrier, adding additional thicknesses of insulation and ensuring all gaps are sealed. Things that can reduce R-values include thermal bridges and heat loss through windows and wall studs.