Even the best window details can be hard to follow because so much depends on the sequence of installation. Use the GreenBuildingAdvisor window installation series along with these window details:
Flanged Window at Head. Exterior Foam Sheathing (Padded Opening); Fiber-Cement Siding.
Rigid insulation as a WRB
Tape all joints of rigid insulation to make an effective air barrier
The primary reason for using rigid insulation on the exterior of a wall assembly is to keep the wall assembly warm to reduce the chance of condensation or moisture accumulation. If desired, rigid insulation can be taped — according to some, but not all, experts proper taping allows the rigid insulation to act as an exterior air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. — and can also serve as the wall's weather-resistive barrier (WRB).
To be either of these, the tape must bond well to the surface of the rigid insulation. That means the rigid insulation must be clean and dry. It also means that you should match tapes to insulation; use tape specifically designed for the rigid insulation you are using.
Not all types of rigid foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. can be used as a WRB. In order to meet code requirements for WRBs, the foam manufacturer must submit its product for testing by the ICC-ESThis is the International Code Council Evaluation Service. ICC-ES is a non-profit public benefit corporation that evaluates building products, issuing final reports on code compliance of building products and materials. These reports on then made available at no charge to the building community at large.. If the ICC-ES has issued an evaluation report that qualifies the foam as a WRB, it can be so used — but only if the builder uses the same tape and flashing details shown in the ICC-ES report. For more information, see Using Rigid Foam As a Water-Resistive Barrier.
If conditions — weather, mud splash, careless workers — make a good clean bond between the tape and the rigid insulation unlikely, then use housewrap, asphalt felt, or building paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt. as a WRB.
Metal cap flashing for doors and windows - installation tips
Slope the middle leg of a window's metal cap flashing
To shed water, a window cap must be installed so that the "horizontal" leg is not horizontal but tipped forward to let the water drain off the cap. The cap must be flexed and bent away from the 90° of the bend that comes with it. Tucking the window cap down tight to the window unit creates a "gutter" that collects water rather than a flashing that drains it.
If possible, use cap flashing with integral end dams to prevent water from dribbling down the jamb casing.
Window head flashing for walls with exterior rigid foam
Different ways to flash the window head, such as self-adhered flashing
GBAGreenBuildingAdvisor.com's window head details for foam-sheathed walls show three ways to flash rough openings for windows and doors: double coverage of self-adhesive flashing membrane; regletting (inserting) the flashing into a kerf (groove or notch) in the rigid insulation; and Z-flashing the membrane into a break in the rigid insulation about 4 inches above the window.
All three options work equally well, depending on the type of claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. or thickness of the exterior rigid foam insulation, but of course there are other techniques. If builders are using foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. as the wall's WRB, it's important to verify window flashing details with the foam manufacturer; these details must match the method shown on the ICC-ESThis is the International Code Council Evaluation Service. ICC-ES is a non-profit public benefit corporation that evaluates building products, issuing final reports on code compliance of building products and materials. These reports on then made available at no charge to the building community at large. report that granted approval to use the rigid foam as a WRB. If no such ICC-ES report exists, another type of WRB — usually housewrap — is required. For more information, see Using Rigid Foam As a Water-Resistive Barrier.
Canned spray foam around window and door frames
Air sealing with expanding foam
The best materials to air-seal the gap between a window frame (or door frame) and the rough opening are canned spray foam, specialized European window tapes, or specialized gaskets sold for this purpose. If you choose canned spray foam -- by far the most common option -- choose minimally expanding spray foam, a type that is less likely to cause the window frame to be distorted.
Don't be tempted to fill these gaps with scraps of fiberglass insulation. Fiberglass is air-permeable, and therefore it can't be used as an air-sealing product.
For more product information:
Lakesideca Advisor: Gaskets and sprayable caulk
Rigid insulation - choosing between EPS - XPS - polyiso
Rigid foam is a versatile and effective type of insulation that can be used in walls, roofs and foundations. Even the least thermally efficient variety of foam has a higher R-value than fiberglass or cotton batts and blown-in fiberglass or cellulose.
There are three types of foam board—expanded polystyrene, extruded polystyrene and polyisocyanurate. Key differences are R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. per inch, water resistance, compressive strength, how permeable they are to water vapor, available facings and, of course, their cost.
Board insulation is manufactured with the help of a “blowing agent” that creates tiny bubbles in the foam which slow down the flow of heat. Manufacturers of expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) and polyiso board use hydrocarbon blowing agents that don’t damage the earth’s ozone layer. A switch away from the ozone-damaging hydrofluorochlorocarbons (HFCFs) used to make extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.)is not expected to take place until 2010, giving this type an environmental disadvantage for the time being.
Expanded polystyrene. The least expensive of the three, EPS can be manufactured in different densities to increase its compressive strength but it’s still not as strong as extruded board and it’s more susceptible to crumbling at the edges and to other job-site damage.
Extruded polystyrene. Because of it shigher strength and water resistance, XPS is often used below grade to in-sulate slabs and foundation walls.
- R-value: About 5 per in.
- Permeance: 1.1 per in. up to 2 in. then 0.55 per in.
Polyiso. With higher insulating values and no ozone-depleting blowing agents, polyiso board has some advantages over XPS for above-grade use (it’s not recommended for below-grade applications because it can absorb water). Polyiso often comes with a foil facing, which gives the material a very low vapor permeance.
- R-value: up to 6.5 per in.
- Permeance: 0.03
Further Resources
GBAGreenBuildingAdvisor.com Encyclopedia: Rigid Foam Insulation
Green Product Guide: Board Insulation.
Building Science Corp:
Vented air space behind siding - furring strips or mesh?
Furring or drainage mat in vented air space?
This vented air space can be achieved with vertical furring strips of wood or a sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. drainage mat (also called spacer mesh), but which is better? There are enough advantages to each; use the one that you prefer.
Most building scientists agree that a vented air space of about 3/8-inch (or 10 mm, per the Canadian building code requirement) is a minimum depth for the air space for wood or wood-based claddings. Commercially available drainage mats result in a 1/4-inch to 3/8-inch vented air space. But the use of 1x3s or 1x4s (3/4-inch thickness) is common. (Wider furring strips are less likely than narrower strips to split — a common problem when strips are screwed into studs through foam sheathing.)
Both systems drain freely and, with openings top and bottom, create plenty of airflow behind the cladding for excellent drying. Many sources recommend that the wood furring strips be pressure-treated, but GBAGreenBuildingAdvisor.com could not find any research that proves that the pressure-treated wood furring strips perform better than non-pressure-treated wood furring.
For more information on this topic, see Fastening Furring Strips to a Foam-Sheathed Wall.
Tags: Hot climate
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