UPDATED on April 3, 2016
Builders in northern states and Canada often specify exterior wall foam for new construction as well as for residing jobs on existing houses. Installing rigid foam on exterior walls reduces thermal bridging through studs and (as long as the foam is thick enough) greatly reduces the chances of condensation in wall cavities. Current trends favor thicker and thicker foam; many cold-climate builders now routinely install 4 or 6 inches of EPS, XPS, or polyiso on exterior walls.
Innies or outies?
Builders installing thick exterior wall foam can install windows two ways: with the window flanges in the same plane as the back of the siding — so called “outie” windows — or with the window flanges in the same plane as the OSB wall sheathing — so-called “innie” windows. Either way will work.
Let’s consider a wall with 2×4 studs, OSB sheathing, 4 inches of exterior foam, 3/4-in.-thick vertical strapping, and fiber-cement lap siding. How should the windows be installed?
Outies, step by step
A builder who prefers “outie” windows would do it this way:
- 1. Window rough openings are oversized by 1 1/2 in. in both dimensions.
- 2. Each rough opening is lined with a frame (a window buck) made of 3/4” x 8 3/4” plywood strips. On the interior, the frames are installed flush; on the exterior, the frames project out 4 3/4 in. beyond the outside face of the OSB sheathing — that is, 3/4 in. beyond the outside face of the foam. (Instead of using a site-built plywood window buck, it’s also possible to use manufactured foam bucks like the ThermalBuck.)
- 3. Once the exterior foam is installed, a frame of 3/4 in. strapping, fastened flat to the foam, is installed around each plywood window projection. The outer face of the 3/4 in. strapping is flush with the outer edge of the plywood frame.
- 4. The bottom of the plywood box is flashed with peel-and-stick flashing, just like a conventional rough opening.
- 5. Windows are installed in the plywood frames; they are attached to the studs (through the plywood) with metal masonry clips (masonry brackets).
- 6. Peel-and-stick flashing is installed on the sides and top of the window, covering the window flanges and extending back to the foam.
- 7. Plastic housewrap is installed over the foam. The housewrap laps over the peel-and-stick flashing. Under the windows, peel-and-stick flashing laps over the housewrap.
- 8. Vertical furring strips are installed on top of the foam and the housewrap. Siding is attached to the furring strips.
Innies, step by step
On the other hand, a builder who prefers “innie” windows would do it this way:
- 1. Plastic housewrap is installed over the OSB sheathing.
- 2. The rough opening is flashed with sill pan flashing and jamb flashing.
- 3. The windows are installed conventionally, with the flanges on top of the OSB sheathing. The plastic housewrap is integrated with the window flashing.
- 4. The exterior foam is installed on top of the housewrap.
- 5. Vertical furring strips are installed on top of the foam.
- 6. The exposed foam under the window sill is cut at a slope.
- 7. The exposed foam edges facing the window (including the foam at the sill area) are protected with peel-and-stick flashing.
- 8. The exterior of the window is trimmed out with water-resistant jamb extensions and a water-resistant sill made of cedar, cellular PVC, or copper flashing.
The innie vs. outie debate has been going on for decades. Back in 1984, here’s what builder John Hughes of Edmonton, Alberta, had to say about the debate: “It’s possible to locate windows and doors anywhere on these broad sills, but most people like to keep windows flush with the new exterior wall. This creates a wide sill inside the house, looks good on the outside, and eliminates the necessity of installing a broad, weather-resistant exterior sill.” (The quote is from Hughes’ article, “Retrofit Superinsulation,” published in the April/May 1984 issue of Fine Homebuilding.)
Both innie and outie windows have strong advocates. When Energy Design Update reported on the innie-versus-outie controversy in July 2002, building scientist Joe Lstiburek came out in favor of outie windows. “It looks cooler that way,” he explained. Since the vast majority of residential windows are installed as outies, there’s no doubt that outie windows look “normal” to most Americans.
However, Chris Makepeace, a certified engineering technologist at Alberta Infrastructure in Edmonton, Alberta, favors innie windows. “The window should be totally supported by the structure of the wall,” Makepeace told EDU. “If the window is toward the inside, then the bulk of the window frame is able to ‘see’ the interior heat, and the window is at a more constant temperature year round. If we extend the window farther to the exterior, we increase the water leakage potential.”
Jack Hebert, president of the Cold Climate Research Center in Fairbanks, Alaska, also prefers innie windows. “It creates a pocket where you don’t get as much wind across the face of the window,” Hebert told EDU.
Thorsten Chlupp, a builder in Fairbanks, Alaska, agrees with Hebert that innie windows are best. In an article published in the Journal of Light Construction, Chlupp explained his preference for innies: “Windows can be installed either at the face of the sheathing — in a recess — or out at the face of the wall. From a performance standpoint, a recess is better, because the window is somewhat protected from wind-washing and the interior glass is more easily warmed by the heat in the room. By contrast, windows installed at the face of the wall are in an interior recess, separating them from the warm air inside (especially if a curtain is drawn) and exposing the outer layer of glass to cold wind. I’ve observed that in extremely cold weather — when it’s 25°F below zero, for example — frost tends to form inside windows installed at the face of the wall, whereas frost rarely occurs on inset windows. I’ve installed windows both ways, but because of the frost problem I now do only recessed installations. A recessed installation is more complicated because the sides of the recess must be covered with exterior jamb extensions. On vinyl-sided homes, we make the extensions from 20-gauge metal coil stock. The bottom is sloped to shed water, and there are flanges on both edges — an inner flange that gets fastened to the sheathing and an outer flange that laps over the 1×4 strapping that we install on top of the EPS around the window. We’ve also made extensions from wood and cellular PVC. These solid extension jambs are glued and screwed at the corners and fastened to the wall over a thick bead of sealant. We either toe-screw them to the framing or fasten them from the inside with metal clips.”
At a workshop at the recent Affordable Comfort conference in Kansas City, Massachusetts architect Betsy Pettit showed slides of deep energy retrofit jobs with both innie and outie windows. “With innie windows, you have to return the exterior trim and extend the window sills,” said Pettit. “By the time you do that work, innie windows aren’t the most economical approach. However, innie windows should last longer, because they’ll receive less rain deposition. The innie approach puts the windows in the right location.”
On a builders’ Web forum, a Portland, Oregon, architect recently posted the following comment on the topic: “Outie windows make for interior ledges for cats. Innie windows make for exterior ledges for pigeons. I suppose the perfect answer is a ‘halfie’ so the cat can sit and watch the pigeon.”
Builders deciding between innie and outie windows should remember:
- The WRB (for example, housewrap) can be under the foam or over the foam. A third option is to omit the housewrap and use taped foam as the WRB. In most cases, innie windows require a WRB under the foam, while outie window require a WRB over the foam. (For more information on the question of where to locate the WRB, see Where Does the Housewrap Go?)
- No matter where the windows are located, flashing details need to tie the perimeter of the window into the WRB.
What about retrofit jobs?
For a deep-energy retrofit job that includes new windows and new exterior foam sheathing, it’s worth considering the use of “Dudley boxes.” For more on the Dudley box approach, see Window Installation Tips for a Deep Energy Retrofit.
For more information on the relative advantages of innie and outie windows, including installation details, see “REMOTE: A Manual,” a useful document published by the Cold Climate Housing Research Center in Fairbanks, Alaska.
Our GBA detail library includes details for both innie windows and outie windows. Links to these details can be found here: Building Plans for a Deep-Energy Retrofit.
The Building Science Corporation has published a useful article with step-by-step instructions and details for flashing both innie and outie windows. Here is the link: Windows Can Be a Pain.
Last week’s blog: “Raising the Bar for Energy Star Homes.”