Snohomish County averages 38 inches of rain per year, 140+ overcast days, and winter temperatures that routinely dip into the mid-20s°F. That combination of persistent moisture, limited solar gain, and cold snaps puts more stress on residential windows than any other climate in the continental U.S. When windows fail here, they don’t just leak heat. They allow moisture infiltration, trigger mold growth behind drywall, and drive heating bills well above regional averages. Choosing the right replacement window for a Pacific Northwest home isn’t about picking the prettiest frame. It’s about matching glass, frame, and seal specifications to what this climate actually throws at a building envelope.
Why Pacific Northwest homes lose more heat through windows than you think
Windows account for 25-30% of residential heating and cooling energy loss in a typical U.S. home, according to the U.S. Department of Energy, and in Pacific Northwest homes built before 1990, that figure runs higher. Older construction in Snohomish County frequently used single-pane aluminum-frame windows with no thermal break, and even early double-pane units installed in the 1990s have degraded seals that no longer meet their original performance ratings.
Older homes in Snohomish County face a specific problem
Pre-1990 homes in the Puget Sound region lose an estimated 35% of their heating energy through windows and doors combined, based on DOE building stock analysis. The core issue isn’t just outdated glazing. It’s aluminum frames with no thermal break. Aluminum conducts heat roughly 1,000 times faster than wood or uPVC. That turns the frame itself into a cold bridge, radiating heat out of the room regardless of what the glass unit does. In a climate where indoor-outdoor temperature differentials regularly hit 30-40°F from November through March, that conductivity difference translates directly into higher gas and electric bills every single month.
Homes built between 1990 and 2005 face a different but equally common problem: failed IGU (insulating glass unit) seals. When the seal between panes breaks down, argon or krypton gas fill escapes and is replaced by humid air. The result is visible fogging between panes, along with a U-Factor that has climbed back toward single-pane performance levels while the homeowner assumes the window is still working as installed.
What U-Factor and air infiltration ratings actually mean for a wet climate
Windows rated U-Factor ≤0.28 and air leakage ≤0.30 cfm/ft² are the minimum performance thresholds for ENERGY STAR qualification in Snohomish County, and in a climate defined by wind-driven rain and persistent humidity, the air leakage number matters just as much as the thermal rating. Most homeowners know to ask about U-Factor. Almost none ask about AL.
The AL rating: the spec most homeowners miss
Air leakage below 0.30 cfm/ft² is the NFRC-certified threshold that separates a weather-tight window from one that will admit moisture-la 

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