Symptoms
- Window worked fine before a rainy or humid stretch
- Sash feels tight or won't move in the frame
- Wood looks slightly raised or textured near the joints
- Problem improves in dry weather
Likely causes
- CommonHumidity or rain absorption into untreated woodBare or poorly sealed wood soaks up moisture and expands.
- CommonMissing or failed paint/sealant on the exterior woodA worn protective coating lets water into the wood grain.
- PossibleWater intrusion from a failed sill or flashingLeaks around the window let water sit against the frame repeatedly.
- PossibleCondensation sitting on the sill over timeChronic indoor condensation can slowly swell interior wood.
What to do
- Wait a few dry days if this just started after wet weather; wood often shrinks back on its own.
- Gently work the sash rather than forcing it, using a putty knife along the seam to release any sticking points.
- Once movable, sand down the swollen area lightly so it doesn't bind against the frame.
- Check the exterior paint or sealant for cracks or bare wood and touch it up to block future moisture.
- Inspect the sill and flashing above the window for gaps that let rain behind the trim, and re-caulk as needed.
- If indoor condensation is contributing, address room humidity as described for condensation issues.
- For frames that swell every season, consider having a carpenter plane down the sash slightly for a more permanent fit.
Tools and parts
- Putty knife
- Sandpaper
- Paintbrush
- Exterior caulk
Exterior wood sealantSearch: “exterior wood window sealant paint”
When to call a pro
Call a carpenter or window repair pro if swelling happens every year, since the wood may need to be planed or the water source professionally sealed.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.