Air infiltration and the air-barrier interface
Airtightness of the unit and of the installation: the window, the rough opening and the building's air barrier.
Two different leaks
Air leakage through a window assembly has two sources: leakage through the operable unit itself (sash to frame, hardware, gaskets), and leakage around the unit, at the interface between the frame and the building's air barrier. Product ratings only cover the first. Blower-door results on finished buildings are dominated by the second.
Unit air infiltration
Air leakage of the product is tested to ASTM E283 and reported in cfm per square foot of window at a 1.57 psf (25 mph) pressure difference. Codes typically allow 0.3 cfm/ft²; passive house targets are an order of magnitude tighter. GlasFëns F-Series units use European multi-point locking hardware that compresses continuous gaskets around the full perimeter of the sash, the approach that makes tilt-turn windows the standard for airtight construction.
The air-barrier interface
The window must be connected to the wall's air barrier, not just to the sheathing. Practically that means: an interior air seal (tape or sealant over backer rod) between the frame and the rough opening, continuous with the wall's interior air barrier; a sill pan and exterior flashing that manage water; and insulation in the shim space so the interior seal is not the coldest surface in the wall. The exterior seal is a water barrier; the interior seal is the air barrier. Getting them reversed is the most common installation error we see.
Why it matters for energy
Uncontrolled air leakage carries heat and moisture through the envelope and undermines the value of a low U-factor. In a passive house the window installation detail is drawn, modelled for thermal bridging (the psi-install value) and inspected. We provide installation details for the F-Series for common wall assemblies and are glad to review a project's details before the shop drawings are approved.