When you’re looking at the performance of different insulation products, the number that matters is Total R-value, sometimes called System R-value. In theory, it tells you how well a particular product will work once it becomes part of the structure of your building.
At this point, a quick summary of R-value basics may be helpful.
- R-value is a measure of resistance to the transfer of heat. Insulation products, building materials and enclosed air spaces all have R-values.
- Product R-value is the R-value of an insulation product.
- Total R-value is the combined R-value of an insulation product and the building materials and air spaces around it. Often referred to as RT.
- For both products and systems, it is possible to measure two R-values. This is because heat moves from the external environment to the inside of your building in summer, and goes the opposite way in winter.
For a more thorough overview, see What is an R-value?
Product R-value comparison
Product R-value is good for comparing insulation products of the same type, and that’s about all. Being the result of laboratory tests under controlled conditions, it can never provide the whole picture about how a product will help keep your building cool in summer or warm in winter.
Often, Total R-Value is used in a complete system
Some products, for example reflective foil insulation, have limited Product R-value. But when combined as a complete insulation system in a building, the Total R-Value will often meet the 6 Star Energy Efficiency Standards.
What affects Total R-value?
In principle, Total R-value is the sum of the insulation’s Product R-value and the R-values of everything on either side of it such as a wall cavity, cladding, bricks, timber studs or plasterboard. That’s basically how manufacturers determine the Total R-values you see on websites and in brochures.
In practice, the calculations can be much more complex because a range of factors may affect the Total R-value for a particular space inside a building. Here are some typical examples.
- Heat radiation through non-insulated areas such as windows.
- Heat conduction where insulation is in contact with timber or metal frames.
- Air movement through ventilation holes.
- Accumulated dust on reflective insulation. This is common with ceiling insulation.
- Compressed insulation that’s lost Product R-value because it’s thinner and heat goes through it more easily. This may be the result of poor installation.
- Damaged insulation or gaps that let air through, again possibly due to due to sub-standard installatio
What you can do
Given all the variables with competing brands, different types of insulation and construction methods, you should always seek expert advice. Use published R-values as a guide, but ask about a customised Total R-value calculation for your project.
It’s also essential to use a reputable installer who guarantees to handle the product carefully and install it according to the manufacturer’s specifications.