UPDATED on December 18, 2017 with a corrected energy savings table.
If you live in the world of 2×4 walls, as I do, you may have wondered about the savings you’d get by going to a more robust wall assembly. The typical house in southern climes has 2×4 walls with R-13 insulation in the cavities. The two ways to beef that up would be to add continuous exterior insulation or to go to a thicker wall. But which saves more energy? And how do they compare to the plain old 2×4 wall?
This question comes up fairly often in our HVAC design work. Clients want to know not only about how much energy they’ll save but also if they’ll be able to downsize the HVAC system. The former saves on operating cost, the latter on first cost.
A wall assembly is a combination of materials that allows heat to flow in both series and parallel. For this article, I’m going to show you the results I got for four different wall assemblies. First, I’ve set up a spreadsheet that calculates the total R-value of an assembly and then used that tool to calculate the R-values for these four walls. Here’s a summary of them:
The first one is your standard 2×4 wall with plywood or OSB sheathing. The second one is the base wall plus half-inch, R-3 continuous exterior insulation. The third one is the base wall plus two-inch, R-10 continuous exterior insulation. The last one is a 2×6 wall with R-19 cavity insulation and no continuous exterior insulation.
The best R-value is the one with 2 inches of exterior insulation. The 2×6 wall is roughly equivalent to a 2×4 wall with a half-inch of exterior insulation.
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To see what effect these different R-values have on the size of…