Wood Rot

Wood rot in windows is caused by wood-decay fungi — organisms that digest the cellulose and lignin in wood when moisture content is sustained above 19%. It is the single most common reason wood window frames and sills need professional repair or replacement. Because windows are at the intersection of interior and exterior environments, they are particularly vulnerable: failed paint, caulk, or glazing compound allows water to soak into the wood, creating ideal conditions for fungal growth.

Where Rot Appears First

In roughly decreasing order of frequency:

1. Exterior sill — lowest point; catches water; most exposed. First to show rot in virtually every wood window.
2. Bottom rail of sash — sits on the sill; absorbs water pooling in the sill track
3. Jamb bottom — where the jamb meets the sill; often traps water
4. Head casing — where failed flashing or caulk lets water in above the window
5. Muntins — where glazing compound has failed at multiple pane intersections

Identifying Rot

Probe suspect areas with a screwdriver or awl. Sound wood resists penetration. Rotted wood accepts the point easily or crumbles. Surface discoloration (gray, black, or brown staining) and soft, spongy texture are visual indicators. Early rot may be under intact paint — look for paint bubbling or bulging as moisture pushes out from below.

Repair Options

• Epoxy consolidant + filler — for rot that has not penetrated more than 1/3 of the wood depth. Remove all soft wood; apply liquid epoxy consolidant to harden remaining fibers; fill void with two-part epoxy filler; sand, prime, paint. Durable when done correctly; $100–$250 professional repair per window.
• Partial replacement — for deeper or larger rot: cut out the rotted section and splice in new wood, bonded with waterproof glue and primed on all surfaces before installation.
• Full sash or frame replacement — when rot is extensive, replacing the entire sash or window is more economical than repair.

Prevention

Paint all wood surfaces (including the bottom of sills and underside of sashes) before installation. Re-caulk exterior joints every 5–7 years. Keep glazing compound intact. Ensure positive drainage at the sill. These steps dramatically extend the life of wood windows.