Exterior rigid insulation improves the performance and durability of walls by keeping the sheathing warmer, protecting the weather resistive barrier, and reducing the penalties of thermal bridging. Keeping the wall warmer means, it stays drier and lasts longer. The energy benefits are best illustrated with an example.
In a 2 x 6 wall, a standard section is a cavity surrounded by two studs, a bottom plate and usually a double top plate. Then, there are the irregular sections such as where a window is framed that might have some additional king, jack, and cripple studs, as well as a header. The insulative value of wood is about 1 per inch, so a stud provides an R-value of 5.5. A cavity with fluffy insulation can achieve R-21.
When taking the whole assembly R-value, with an assumed framing factor, that R-21 wall is really about R-16 due to the thermal bridging. Let’s say we want to increase the R-value of the wall and add R-6 to the cavity, providing R-27 total. In such a case, the whole wall R-value will increase to R-18 or by about 13%.
If, instead, we add that R-6 as continuous exterior rigid insulation to disrupt the thermal bridging, the whole wall R-value will increase to R-23, or by about 47%. This shows that it is substantially more effective to add insulation to the exterior of wall then to the interior.
There are a variety of rigid insulation products and with majority biobased materials leading the pack. They have low environmental impact and can even have a negative carbon impact over their lifecycle. The two prime examples are cork and rigid, wood fiber boards. However, both these products come with a significant cost premium. We are anxiously awaiting TimberBoard release in Fall 2023 by TimberHp, which is a US produced wood fiber board. Hopefully this will reduce wood fiber insulation incremental cost as most of it is imported.
Rigid mineral is next on the preferred list as it has a relatively low environmental impact. Foam plastics follow with polyisocyanurate, expanded polystyrene (EPS), and extruded polystyrene (XPS [with an emphasis on the low global warming potential options]).
Some cladding can be installed directly over some types of foam that are up to 2” thick. For example, there are some approaches with stucco and some types of lap siding. However, it is important the cladding is considered drainable (vinyl siding, some lap siding, corrugated metal) or a rainscreen mesh should be added over the rigid insulation. For thicknesses over 2” or beyond manufacturers limitations, furring strips should be installed. Additionally, as insulation thicknesses increase over 1”, nail length could be 3-3.5” to ensure proper embedment, and it might be worth using furring strips.
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