Clients often ask us for information about the R-value (a measure of thermal resistance of a material) of the walls in our homes, hoping to make comparisons to help them to decide who to build with. Unfortunately the R-value is not the best tool for the job in this case – what you really want to know is the overall thermal performance of the home!
Can’t I just I use the R-Value as a comparison?
The R-value is really only useful when directly comparing the thermal resistivity of two materials (ie, two types of batts), which is of little use when trying to gauge the relative “warmth” of two homes built to different designs. This is because many more factors contribute to how warm a home feels, such as the size of the spaces, the floor and ceiling insulation, the amount and location of the glazing, the type of heating (radiant or convection), use and control of solar gain, etc.
Even when comparing different materials in the same overall design, sometimes the comparison breaks down. The R-Value doesn’t take into account all the properties of a material that can contribute to the thermal efficiency of a home. One property is thermal mass, which is of particular relevance in a solid timber home (we discuss this in Part Three of this series!).
R-value versus thermal efficiency
In a real world situation, what is important is the thermal efficiency of the entire building (essentially a measure of the effort needed to heat and cool). Calculating the overall thermal efficiency is part of the process that we go through when detailing the performance of a building for consent, and considers all the contributing factors mentioned above. The R-value of the materials used is only one part of the equation, and is not particularly useful on its own.
In terms of the cost to heat, the type of heating in your home is as important it’s thermal efficiency. Any time you are using a heat pump driven system (as in a standard heat pump, or hydronic installation), for every dollar you put in, you’re getting $3-4 back in generated heat. If you are using electrical resistance heating, the best you can hope for is 1:1.
Click here to read Part 2 of this 3 part blog series, where we talk about what to look out for in a home design if the thermal efficiency of your new home is important to you (it should be!).