If you have any comments or corrections on this article, please email us and we can publish your comments. Skip to main content. Building Codes. Building Science. Energy Efficiency. Originally published by : Applied Building Technology Group, LLC — December 12, The following article was produced and published by the source linked to above, who is solely responsible for its content. The minimum thickness of foam sheathing for US Climate Zones is shown the table below.
You can use either foil-faced, EPS, or extruded foam. Of the three, EPS is the most permeable, allowing some drying the exterior if the wall cavity ever gets wet — for example, from a flashing leak.
EPS has a perm rating permeance of about 1 for a 1-inch-thick board, making it a Class II vapor retarder similar to the Kraft-paper backing on fiberglass batts. For that reason, I prefer unfaced foam on the exterior, although many people use foil-faced polyiso foam without problems.
EPS and polyiso board are the most environmentally friendly choice as they typically use pentane as its blowing agent, which is fairly benign to global warming. XPS is the worst as it uses HFCs hydrofluorocarbons which are very potent contributors to global warming. XPS manufacturers could solve that problem completely by switching to HFO hydrofluoroolefin , but have been reluctant to change due to the higher cost.
In answer to your other questions, the water-resistive barrier WRB , such as asphalt felt or housewrap Tyvek, Typar, etc. The important thing is to properly integrate the WRB with the window and door flashing and other wall flashings.
The placement of the windows in the wall will determine the best place for the housewrap. For manageable details, you want the housewrap in the same plane as your window flanges, usually on the outside of the foam sheathing. This provides some modest energy benefits in terms of wind penetration, but introduces a lot of detailing problems with wide exterior sills and jamb extensions.
Because window flashing is the number 1 place for exterior water leakage, I like to keep things simple here, so I prefer outies.
Both innies and outies are simplified by installing plywood boxes in the window openings. Unlike house wrap, which works to prevent infiltration air coming into the building but is poor at stopping exfiltration air moving out of the building , rigid foam is able to do both. Must be installed properly to limit air leaks and act as a weather-resistive barrier Rigid foam does not require specialized equipment to install it but you do need to follow strict seam-sealing procedures to meet code.
However, if you want to use rigid foam in place of wood sheathing it needs additional bracing to prevent racking. Slightly more expensive Adding a layer of rigid foam on top of plywood or OSB sheathing will increase the cost of the project.
However, this is just a short-term, fixed cost. Rigid foam often pays for itself with lower utility bills over the long term. And it may put off or prevent costly work to repair rot in walls or framing.
These decisions are determined somewhat by climate. The lowest cost, highest performing rainwater management strategy is rigid polymeric foam sheathing with sealed joints Lstiburek , There is an existing construction challenge of sealing the joints in rigid polymeric foam sheathing in a reliable and durable manner to prevent water ingress.
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