Flanged window as in-betweenie in a double-stud wall — sill detail

Double-stud walls create plenty of room for thick insulation

Double-stud walls use common materials and familiar assemblies to create a low-tech, energy-efficient wall with lots of room for thick insulation. This framing method virtually eliminates thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the studs and greatly reduces sound transmission through walls.

The basic strategy is simple: The exterior walls are built from two parallel stud walls with a gap between the rows for extra insulation. Many builders use two parallel 2x4 walls with a 5-inch gap between them to create a 12-inch-thick wall. Of course, the wall can be thicker or thinner as circumstances dictate.

The most commonly used insulation for this method of construction is dense-packed cellulose, although other types of insulation (including blown-in fiberglass, mineral wool batts, or open-cell spray polyurethane foam) can certainly be used.

For more information, see GBA Encyclopedia: Double-Stud Walls.

Great windows are only great if installed well

A good window installation converts a hole in the wall into an integrated part of all three important barriers that make up a wall assembly: the air, thermal, and water barriers.

Because all windows leak at some point, rough openings need to be designed to handle water entry. The rough sill should be flashed with either a pre-formed manufactured sill pan or a site-built pan.

This detail shows a site-built sill pan installed in a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation.. The water-resistant barrier (WRB) can be asphalt felt or plastic housewrap.

For more on windows:
GBA Encyclopedia topic page
Video Series: Flashing a Window in a Foam-Sheathed Wall

Four Rules for Window Installation

Protect the head of the window from rain by tucking the window in from the face of the wall. Overhangs don’t help much on the coast — water blows sideways, up and down — and on taller buildings, an 18-inch overhang doesn’t help the ground-floor windows at all. On a building with 12-inch thick well-insulated walls, we’ll slide the windows back 3 1/2 inches so that every window has an individual built-in overhang right above the window head. That means we install most windows as “in-betweenies” instead of “innies” or “outies.”

Treat the window flange as an installation aid only, not as a water or air seal. Expect that water will be able to enter behind the flange during extreme conditions, and always provide a pathway out. Follow the manufacturer’s instructions; your warranty depends on it.

Since water will probably get behind the flange at some point, make sure it can’t pool on bare wood and rot your wall. Wrap rough openings properly, with tape or flashing that is lapped top over bottom, and make sure to install a sloped sill pan below the window. It’s now part of most window manufacturers’ instructions to place a clapboard or sloped shim at the rough sill, but we still rarely see this recommendation followed until we ask for it. The critical and often overlooked air-sealing area is at the interior of the sill. If the window is not shimmed at its sill — the usual situation we see — there needs to be a bed of caulk or a gasket seal at the inside face of the window to prevent water from blowing up under the open exterior window flange, right under the window frame, and into the building.

Establish a long-lasting air and water seal at the inside face of the window unit, and connect that seal to the rough opening. The best material for this seal is a flexible and long-lasting construction tape like Dow Weathermate, 3M 8067 Flashing Tape, or a European tape like Siga Rissan or Pro Clima Tescon. The tape will stay nice and warm on the interior side of the window; it won’t go through extreme freeze/thaw cycles like exterior tape. It should also remain flexible longer than spray foam. However, we realize this is a tedious and non-standard technique for most U.S. builders and will likely lead to an upcharge, so we usually have to accept spray foam as our interior air seal. (It should go without saying that fiberglass scraps do not belong in a rough opening.) We insist on a blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. to ensure the air seal around the window was installed properly, and we often make builders come back with caulk to close up the holes they are surprised to find in the spray foam installation. Our typical non-Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. airtightness standard for residential work is 1.5 ach50, but we always give the builder the cfm50 number that would allow them to beat the 0.4 ach50 achieved by one of their competitors. It never hurts to let them know where the bar really sits.


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