Caulking

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.