Let’s say you’re building a house with plywood or OSB sheathing. You plan to install 2 or 4 inches of rigid foam on the exterior of the wall sheathing, followed by vertical rainscreen strapping and siding. Where does the housewrap go?
Depending on who you talk to, you get two different answers:
- It goes between the rigid foam and the vertical strapping, or
- It goes between the sheathing and the rigid foam.
I’ve heard a variety of arguments in favor of each position. For example, I’ve heard that the housewrap belongs between the foam and the strapping, “because that way it’s easier to integrate with the flashing at the logical drainage plane.”
On the other hand, I’ve heard that the housewrap belongs between the sheathing and the foam, “because that way it protects the sheathing,” or “to prevent the housewrap from flapping in the wind,” or even “to protect the housewrap from extreme temperatures which might degrade the plastic.”
Both sides of this argument have merit. If you have a strong opinion favoring either position, it’s safe to say that either approach can work well, as long as the housewrap is properly integrated with all of the window flashing, door flashing, and the flashings protecting other penetrations.
Simplifying the decision
If this quandary has you discombobulated, though, here’s an easy way through the thicket:
If you’re unfamiliar with the innie/outie terminology, an innie window’s flanges are in the same plane as the OSB or plywood wall sheathing, while an outie window’s flanges are in the same plane as the back of the siding. (For more on this topic, see ‘Innie’ Windows or ‘Outie’ Windows?)
Integrating housewrap with “innie” windows
Innie windows are installed just like they are on a house without exterior foam. The housewrap covers the plywood or OSB sheathing, and the window is installed with your favorite flexible-flashing details. The foam is installed later, and there’s no reason to integrate the free-draining rainscreen gap with any of the other flashing details on the wall.
All you have to do is come up with exterior “jamb extensions” — including a sloped secondary sill tucked under the sill that comes with the window — to cover the edges of the foam at the window and door openings. It’s perfectly reasonable to argue that these exterior jamb extension details don’t really have to be watertight — just resistant to weathering, durable, and attractive.
Integrating housewrap with “outie” windows
Outie windows are usually installed with the window flanges in the same plane as the back of the siding. This usually requires the installation of a plywood box in each window rough opening; the box extends x inches beyond the plywood or OSB sheathing, with x = the thickness of the foam + the thickness of rainscreen strapping.
After the foam is installed but before the housewrap goes up, you need to install a “picture frame” of strapping lumber (installed flat to the foam) around each rough opening. The outer face of the strapping should be flush with the outer edge of the plywood frame. Then you install your window.
The housewrap goes up next, on top of the foam. At each window head, the housewrap is creased and extended out over the “picture frame,” and then down over the window flange or the flexible flashing at the window head. At the window sill, the flexible flashing protecting the rough sill extends over the housewrap. Then the rainscreen strapping is installed over the housewrap.
The method I’ve described is just one way to install housewrap over exterior foam. Of course, there are many variations to these installation and flashing details.
Can I just skip the housewrap?
Some builders argue that rigid foam is a perfectly good water-resistive barrier (WRB), so foam-sheathed walls don’t need any housewrap at all.
If you decided to go this route, be sure to do your research before proceeding. It’s important to note:
- Not all brands of rigid foam have been approved for use as a WRB.
- Rigid foam can only be used as a WRB if you follow the fastening and seam-sealing details listed in the ICC-ES report used to obtain acceptance for your brand of foam to be used as a WRB.
- Some building experts note that over a period of years, rigid foam may shrink, raising the question of whether an installation of taped foam will remain waterproof over the long term.
For more information on this option, see Using Rigid Foam As a Water-Resistive Barrier.
Advice from the Building Science Corporation
To read more information on this question, see the Building Science Corporation’s Guide to Insulating Sheathing.
Last week’s blog: “Superinsulated House Specs.”