Exterior Insulation on 2×4 Walls Versus 2×6 Walls With Cavity Insulation Only

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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?

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[source: http://www.greenbuildingadvisor.com/articles/dept/building-science/exterior-insulation-2×4-walls-versus-2×6-walls-cavity-insulation-only]