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© Copyright - Work-4 Projects Ltd.

REPORT

Window Installation Details for Effective Sealing

By M.A. Lacasse and M.M. Armstrong

The effects of inadvertent water entry at windows are well known: water can damage interior finishes and in the case of wood-frame construction, may lead to wood rot or the formation of mould in the wall assembly.
Rainwater may enter because of deficiencies in the window components, either inherent or after the window has “aged.” When water is blown to the window (Figure 1a), the risk of entry at any deficiency increases. Water entering a defective window or along the perimeter interface between the window and the cladding may find its way into the wall assembly and eventually cause damage. Water reaching the sill must be drained (Figure 1b) because if left stagnant it may lead to the formation of mould.



Wind blowing on the exterior of a building brings rainwater to the windows and to the wall-window interface; it also gives rise to pressure differences across the assembly. The magnitude of the pressure difference is a function of the wind speed and interior pressure conditions; higher wind speeds yield greater pressure differences.
Openings along the wall-window interface (e.g., gaps in the sealant behind the window flange, gaps in the seal between the glazing and the window sash, or defects in the window frame) may permit the passage of air and water. If water is present at an opening through which air may pass, and there is a pressure difference, water will be driven through it.
Pressure equalization is the key to preventing water entry and promoting effective drainage. Ideally the pressure in the wall-window interface (the gap between the window frame and rough opening) and the rough sill must be equal to the pressure at the exterior of the wall (caused by wind). This will eliminate driving pressures that force water to enter across the window installation, and also eliminate any pressure that may impede drainage from the sill area. To achieve pressure equalization, both the exterior and interior airtightness details must be considered.

Managing Rainwater Entry
On the basis of the results derived from NRC laboratory testing and observations of water entry at the window perimeter, recommendations can be made to mitigate the effects of a defective or poorly installed window.



When a jointing product is applied to the back of the window flange (Figure 2a), the largest pressure drop in the assembly can occur across the bead of caulking (the plane of airtightness). Any imperfections in this seal may, in the presence of water, allow water to enter through these imperfections (cladding not shown). Moving the plane of airtightness away from the location of potential wetting (Figure 2b) reduces the pressure difference across the flange and diminishes the risk of water entry behind it or through defects in the window frame. This can be accomplished by installing a backer rod and spray polyurethane foam (SPF) or sealant between the window frame and rough opening.
Apart from locating the plane of airtightness at the back of the sill, it is important to ensure that flanged windows have a gap behind the flange at the sill such that pressure can equalize between the sill cavity and the exterior. This also allows any water that has reached the sill area to easily drain out, since there is little or no pressure to counter water drainage from the sill. The gap between the bottom window flange and the sill can be created using cap nails. Locating the plane of airtightness toward the interior of the window and creating a space behind the bottom window flange allows any water that gets into the opening behind the flange to drain to the base of the window and out of the assembly at the sill.
Research also highlights the importance of a continuous interior air barrier in reducing the driving force for water entry. A continuous air barrier can be readily achieved with a jointing product and backer rod or by installing a backer rod and SPF.

Implications
While a new functional window installation that is well sealed behind the flange was shown to initially prevent water from entering, it was shown to be inadequate for mitigating inadvertent water entry through defects that develop over time.
Relocating the plane of airtightness away from the window flange and toward the interior results in a robust window installation that will effectively manage water throughout its lifetime. When combined with other important window-interface elements—including a sloped sill, back dam, sill flashing membranes wrapping up the jambs and over the sheathing membrane at the sill, and insulation to the interior side of the sill (leaving the drainage path unobstructed)—the window installation details described here are adequate for managing even the most significant rainfall events occurring in North America.

This article has been excerpted from Construction Technology Update #80, with permission from the National Research Council of Canada.

Dr. M. A. Lacasse is a Senior Research Officer in the Building Envelope and Material group at NRC Construction. Ms. M.M. Armstrong is a Research Council Officer with the same group.

 


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