With the colder weather has arrived so it makes sense to talk about insulation values.
We are all aware that higher R-values will generally provide for a more comfortable house, year round. Many customers surf the web and subsequently we often hear the question: “Why are construction R-values in the USA so much higher than in New Zealand?”
Now, it’s important to remember that the United States of America does not use the metric system, so when you see R-30; What does that mean to us? The conversion factor between SI (metric) and US units of R-value is 1 h·ft²·°F/Btu = 0.176110 K·m²/W, or 1 K·m²/W = 5.678263 h·ft²·°F/Btu. So, US R-30 = NZ R-5.28.
The USA have eight different climate zones for their Model Energy Code. Their Climate Zones 3 and 4 align to our three Climate Zones, as their Climate Zone 5 is certainly colder than our coldest zone, and their Zone 2 is warmer than our warmest zone.
Applying the conversion factors, and looking at their Climate Zones 3-4, we can therefore see that they advocate the following minimum construction R-Values, when we convert their values to the metric system:
· Sub-floor: R-2.2 in their Zone 3 and R-2.8 in their Zone 4. All three NZ Zones set compliance at R-1.3.
· Timber frame walls: (90x45mm framing) R-3.2 in both their Zones 3 and 4. We vary from R-1.9 for Zones 1 & 2 to R-2.0 for Zone 3.
· Ceiling: R-6.6 in their Zone 3 and R-8.6 in their Zone 4. We vary from R-2.9 for Zones 1 & 2 to R-3.3 for Zone 3.
Just to check, lets compare with their Zone 2 (Florida, Southern Texas, etc)
· Sub-floor: No requirement in their Zone 2. All three NZ Zones set compliance at R-1.3.
· Timber frame walls: R-2.3 in their Zone 2. We vary from R-1.9 for Zones 1 & 2 to R-2.0 for Zone 3.
· Ceiling: R-6.6 in their Zone 2. We vary from R-2.9 for Zones 1 & 2 to R-3.3 for Zone 3.
When I design for my clients, I set the R-values higher than Minimum Code Requirements because I believe it makes sense to do so. Some builders look at these figures and think they’re “too high”, but the evidence stacks up and these higher R-ratings makes sense:
· Sub-Floor or Floor: For on-ground concrete slabs, we usually use R-1.5 and for Timber sub-floors, R1.9 minimum.
· Timber frame walls: Minimum R-2.8 and up to R-4.8.
· Ceilings: Minimum R5.2, commonly up to R-7.2.
An interior photo of a very well insulated Martinborough home. This home maintains comfortable temperatures year round with only limited heating and cooling requirements.