Q: I’m tearing the house down to bear studs inside and outside. I’m putting 1-inch rigid foam board on the outside. Do you suggest foil-faced or unfaced foam board? Where and how should I use moisture barriers in the construction of the interior walls? I live in zone 5, should I use rain screen? What do you think of cement lap board for siding? – Ron B.
A: Foam sheathing can help protect walls from moisture problems, but only if it is thick enough and installed properly. There are three key issues with foam sheathing:
1) Use a thick enough layer of foam to minimize condensation on its interior face. Interior moisture can enter the wall from air leaks or diffusion through the wall finish. The goal is to keep the inside face of the foam sheathing above the dew point of the interior air.
2) Don’t use a vapor barrier, such as poly, on the interior. If the wall cavity get wet for any reason, then the wall will need to dry primarily to the interior. It’s best to use only a Class III vapor retarder such as painted drywall. If you want a continuous vapor retarder, consider one of the “smart” (and expensive) vapor retarders such as MemBrain or Intello. These have low permeance when humidity is low (in the winter) and high permeance when humidity is high (during the summer or if the wall cavity is wet).
3) Use an effective interior air barrier to keep warm, moist interior air out of the wall cavities. This could be sealed drywall or sealed sheathing such as the Zip system.
For Zone 5, the IRC and IECC recommend minimum R-5 foam sheathing for 2×4 walls and R-7.5 for 2×6 walls. Remember that these are minimums – the greater the R-value of the foam the better, as this creates a warmer wall cavity with less condensation. These numbers assume that the wall cavity is filled with fiberglass or cellulose insulation. 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. Extruded polystyrene (XPS) is about alf as permeable as EPS, and foil-faced foam is completely impermeable.
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. The drying potential through EPS or XPS is low, but better than nothing. 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.), can go on either side of exterior foam sheathing. 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.
However, some people like to recess the windows so they sit against the plywood sheathing (“innies” vs. “outies”). 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. With outies, this solves the problem of nailing your window flanges through thick layers of mushy foam.
Rains screens are always a good idea, but not a necessity with fiber-cement siding, which is highly resistant to moisture and holsd paint very well. If you were installing wood siding over foam sheathing, I’d recommend some type of rain screen. If you want to longest lasting siding and exterior paint, then go ahead with the rain screen. But it does add cost and complexity to your wall system.
See also Exterior Foam vs. Dense-Pack