The window seal is the hermetic closure that keeps the insulating cavity of a double- or triple-pane window intact. It is typically a two-part system: a primary seal (PIB butyl rubber applied to the spacer bar) and a secondary seal (silicone or polysulfide that bonds the assembly to the frame).
Why Window Seals Fail
Seals degrade over time due to repeated thermal expansion and contraction—glass and frames expand and contract with temperature changes, stressing the seal material. Other causes include:
- Pressure washing windows directly
- Painting over the seal, blocking its flexibility
- Low-quality original installation
- Normal aging (most seals last 15–20 years)
Signs of a Failed Seal
The most obvious sign is condensation or fogging between the panes—moisture that cannot be wiped away because it is trapped inside. You may also notice a hazy or dirty appearance inside the glass, or distorted reflections caused by the panes bowing inward under pressure change.
What Happens After Seal Failure
Once the seal fails, the argon or krypton gas escapes and is replaced by regular humid air. The desiccant in the spacer bar absorbs the initial moisture, but once saturated, condensation becomes permanent. The window’s U-value worsens, increasing your heating and cooling costs.
