Caulking (also called caulk) is a flexible, waterproof sealant used to fill and seal gaps between a window frame and the surrounding wall surface. Properly applied caulk is one of the cheapest and most effective weatherization measures available — a $5 tube of caulk applied around a leaky window can reduce drafts as effectively as adding insulation.
Types of Window Caulk
- Silicone — the gold standard for exterior window joints. Excellent adhesion to glass, metal, and vinyl; highly flexible; waterproof; 20–50 year lifespan. Cannot be painted. Best for: frame-to-glass joint and exterior frame-to-siding joint.
- Siliconized acrylic (latex) — paintable, easy cleanup with water, flexible. Lifespan 10–15 years. Best for interior joints and anywhere you need to paint over the caulk.
- Polyurethane — strongest adhesion, excellent for large gaps, paintable. More rigid than silicone; harder to tool. Best for gaps larger than ¼ inch.
- Butyl rubber — extremely waterproof; used in flashing and behind window flanges during installation.
Where to Caulk Around Windows
- Exterior: all four sides where the window frame meets the siding or trim
- Exterior: joint between window trim boards (at corners)
- Interior: joint between window casing and wall (for air sealing and appearance)
- Do NOT caulk the bottom exterior sill-to-siding joint — this is a weep area that must drain
Signs Caulk Needs Replacement
Inspect window caulk annually. Replace it when you see: cracks, gaps, shrinkage from the surface, peeling, or discoloration indicating mold. Failed exterior caulk can allow water to enter the wall cavity, causing insulation damage and structural rot far more expensive than a $5 tube of caulk.
Application Tips
For a clean job: cut the nozzle at 45° to a 3/16-inch opening, apply in a continuous bead, tool immediately with a wet finger or caulk tool, wipe excess with a damp cloth. On exterior joints, apply only when temperature is above 40°F and no rain is forecast for 24 hours.
