A common attribute among high-performance homes is a well-insulated wall. The code minimums for R-Value will depend on where you live but regardless of climate zone, a good wall will have enough insulation to mitigate thermal transfer. Historically a stick-framed house would only take as much insulation as would fit in the stud bays… 3.5 inches for 2×4 framing and 5.5 inches for 2×6. But as energy efficiency became more important, whether imposed by code or simply a desire to build better, alternative framing and building strategies evolved to allow for thicker, better-insulated walls. Two common approaches today are the Double Stud Wall and Continuous External Insulation.
A Double Stud Wall
The name describes it well. Two walls are framed (2×4 or 2×6) with a gap of several inches in between. The entire space can be filled with blown-in insulation to make a thick wall with a thermal break. Unlike traditional framing where studs create repeating thermal bridges, a double stud prevents this nicely as the insulation layer separates the inner wall and outer wall. As for R-Value, a double stud wall not only beats code but is a common choice among Passive House builders. If you fill your walls with a high-performing insulation, like Havelock Wool Blown-In, you can get R52 (12in x 4.3 R Value per inch).
Constructing a double stud wall is relatively straightforward as it’s simply framing 2 walls as you would normally. There isn’t much new technique to learn. There are options for which wall is load-bearing and dimension choices but generally speaking, putting the walls up is easy. Choosing your continuous air barrier is key and a common technique is to do that via a ZIP System with sheathing and tape. We were just on a build site where the external double stud walls had Zip sheathing and tape as the air barrier and there was no vapor barrier installed inboard of the insulation. The thinking here was that 12 inches of vapor open insulation would manage any moisture (ie dry out). If you do want to prevent vapor diffusion into the insulation, placing a smart vapor control layer inboard makes sense. This layer would be vapor open in the summer to facilitate inward drying and vapor retarding in the winter to prevent vapor accumulation into the insulation. There are also advocates for placing a layer of OSB or plywood on the outside of the inner wall. Similar to a smart vapor barrier, this layer’s permeance can change with relative humidity which gets you that very important drying action.
Continuous External Insulation
So a double stud wall gets you a high R Value, reduces thermal bridging, and allows for various air and vapor management strategies. Well, the same goes for external insulation. Common applications for external insulation are when solid walls don’t allow for cavity insulation, ie masonry walls. In this instance, insulation is placed outboard of the wall (framed or solid) and is finished either with a coat of render or with an alternative cladding material to protect it from the elements. You’ll often hear this whole setup referred to as EIFS – Exterior Insulation Finishing Systems.
Continuous External Insulation is a popular choice because it bolsters the R-Value of any existing wall, does an excellent job mitigating thermal bridging, especially at rim joists at ground and upper levels of the house, and it limits moisture accumulation, especially in cold climates, and moist coastal areas. This is because moisture can condense on the interior of the wall sheathing, provided the surface is cold enough. If you have enough insulation on the exterior of the sheathing to maintain a warm surface temperature then it won’t become the condensing surface. For more on dew point measurement and condensation, see this helpful article.
For residential applications, there are several choices for external insulation. Some have very high embodied carbon so we’d suggest avoiding those. Wood fiber is a sensible choice given performance and low GWP. It’s typically attached with staples but then bolstered with furring strips. A vented rainscreen provides that all important air gap.
Conclusion?
Well, we don’t really have a preferred choice between Double Stud Walls and Continuous Exterior Insulation. We would note that both applications allow for the use of low embodied carbon, high-performance materials like Havelock Wool and we’ve been fortunate to be used in lots of these wall assemblies. Maybe exterior insulation requires some more detail around installation but its thermal bridging mitigation is excellent. Double stud walls are perhaps faster to install but moisture management can require more thought. Regardless both methods are part of the growing movement for better-built walls. Here’s to it.
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