Caulk is the window's raincoat. When it cracks, water gets behind the trim and rot starts. Pick the right sealant, measure once, and plan an afternoon for a few windows.
Time: about 30–45 min per windowTools: caulk gun, utility knife, scraper, ragWeather: dry, within the tube's temperature range
Quick quiz
Would you get this right?
Six questions, before you read the steps. Most people miss at least two.
What to buy
The right caulk for the job
1. Check the window maker first
Look for the brand on a label in a corner of the glass or inside the frame.
Their installation guide may name the sealant types they approve for your frame (vinyl, wood, aluminum, fiberglass).
Their rule beats any general advice, including this page.
2. Exterior grade, ASTM C920
Look for "ASTM C920" on the tube: the standard for exterior sealant joints that move.
Polyurethane: tough, paintable.
Hybrid / polymer: paintable, wide temperature range.
Silicone: very durable, usually not paintable.
3. Extras
Foam backer rod for gaps wider than about ¼ in.
Painter's tape for clean lines (optional).
Primer only if the tube's label calls for it on your surface.
Skip interior acrylic "painter's caulk" outside.
How much to buy
Caulk calculator
You need
3 tubes
Standard 10.1 oz tube ≈ 18 cubic inches. Includes 15% for waste. Gaps over ¼ in assume backer rod with the caulk half as deep as it is wide.
Step by step
Do it right the first time
1Find the brand and read its rules
Note the window brand from the label. Check its installation guide for the approved sealant and any drainage gaps to leave open.
2Cut out the old caulk
Run a utility knife along both edges of the old bead, then pull and scrape it out. Get it all: new caulk doesn't bond to old caulk, especially silicone.
3Clean and dry the joint
Brush out dust and loose paint, then wipe with a rag. Wait for a dry day: caulk over damp wood fails early.
4Fill deep gaps with backer rod
For gaps wider than about ¼ in, press in foam backer rod a little wider than the gap. Aim for caulk about half as deep as it is wide: it stretches better that way.
5Lay one steady bead
Cut the nozzle at an angle, a little smaller than the gap. Push the bead ahead of the nozzle at an even pace, one side at a time, without stopping mid-run.
Tape both sides first if you want razor-straight lines.
6Tool it smooth
Within a few minutes, smooth the bead with a damp finger or a tooling stick so it presses into both sides with a slight inward curve. Pull any tape right away.
7Leave the drain holes open
Many windows have small weep slots at the bottom that let water out. Never caulk over them, or over any gap the window maker says to leave open.
8Let it cure, then paint
Check the tube for rain-ready and paint-ready times; they vary a lot between products. Paint polyurethane or hybrid caulk once it's ready, if you like.
Practice
Try the bead before you touch the house
Hold the mouse button (or your finger) to squeeze, and run along the joint. The nozzle is cut at an angle pointing right. Try pushing to the right, then dragging to the left, and see the difference.
Covered0%
Packed in–
Even–
Smears0
Common mistakes
Caulking over old caulk instead of removing it.
Sealing the weep holes at the bottom of the frame.
Working on damp wood or outside the tube's temperature range.