Do sill pans pose risks to condensation on windows?

Evaluating the potential for condensation to form on the glazing, or at the window frame perimeter, was the focus of a window research project completed by the NRC Institute for Research in Construction (NRC-IRC) together with Canadian Mortgage and Housing Corporation (CMHC). It concentrated on windows installed with a sill pan and related building details.

Even a well-installed window can develop deficiencies with time and allow water to enter. If not properly managed, this water can enter wall components below the window and lead to material degradation. Sill pans are placed beneath the window at the sill to protect the rough opening by collecting and draining water that enters at the window and preventing it from reaching the interior of the wall assembly. Proper installation of a sill pan requires leaving an area open and free of insulation to encourage drainage out of the sill area.

Standard window condensation tests were performed on non-operable PVC windows with and without integral mounting flanges, and having a sealed double-glazed insulated glass unit. These windows were chosen because they are widely available and typical of those used in Canadian construction practice. The testing subjected the windows to a constant cold temperature of -30°C on the exterior, and to 20°C on the interior.

The tests simulated conditions that represented a mild wind buffeting the exterior of the assembly. Deficiencies were purposely created in the wall-window interfaces – one at the exterior to promote air leakage into the sill area, and another similar-sized deficiency atop the window towards the interior of the assembly.

Additionally, scenarios evaluated for potential window condensation included both a non-insulated cavity between the window frame and rough opening and a cavity containing either glass fibre insulation or spray-in-place polyurethane foam.

Results of the sill pan research reveal that insulating the cavity between the window rough opening and the frame with glass fibre insulation or polyurethane foam slightly increased window surface temperatures and slightly reduced the risk of condensation on the window. Thus sill pans can be installed without undue concern for risk to the formation of condensation on window components.

The most important aspect in regards to the likely formation of condensation on the window is the window itself, not the installation of a sill pan. Window installation details should, nonetheless, include insulation in the stud cavity, to ensure continuity in thermal resistance at window penetrations and minimizing heat loss at these critical points of the assembly. Insulation should be installed with care so as not to obstruct water drainage out of the assembly, allowing the moisture management system to work as intended.

This research, along with a previous study that assessed the robustness of specified window installations (see Construction Innovation, June 2006 and June 2010) clearly demonstrate that the installation of sill pans and similar components used to protect the sill beneath windows should be part of a moisture management and protection strategy that minimizes the risk of water entry into walls.


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