Drafty Windows? It Might Not Be the Glass (Check Your Caulking)
Too Long; Didn't Read
- The Diagnosis: Use a candle. Flame flickers at the edge = Caulking failure. Flame flickers at center = Convection (Cold Glass).
- The Fix: Remove old caulk, insert "Backer Rod", and apply 100% Silicone.
- The Warning: NEVER caulk the "Weep Holes" at the bottom. You will rot your wall.
- The Cost: DIY = $50. Pro = $5-$10 per linear foot.
Answer First: Is your window drafty, or is it just cold? Do the Candle Test. On a windy day, hold a lit candle near the edge of the window frame (interior). If the flame dances or blows out, you have an air leak—your caulking has failed. If the flame is steady, but you still feel a chill, that is Convection—the glass itself is cold and cooling the air near it. Caulking won't fix convection (but Triple Pane Glass will).
The $10 Tube That Saves Hundreds
In Toronto, we blame the glass for everything. "My windows are old, I need to replace them." Often, the windows are fine. The seal around them is dead. Builders in the 90s often used cheap "Acrylic Latex" caulking. After 10 years of Canadian freeze-thaw cycles, that acrylic turns hard, cracks, and falls out, leaving a 1/8th inch gap around your entire house. That gap is equivalent to leaving a window wide open all winter.
1. The Caulking Truth Table: Silicone vs. The Rest
Go to the aisle at Home Depot, and you will see 50 options. Here is the only one you should buy.
| Type | Acrylic Latex (Painter's Caulk) | Polyurethane (Construction) | 100% Silicone (The Winner) |
|---|---|---|---|
| Use Case | Interior Trim / Baseboards. | Concrete / Brick expansion joints. | Exterior Window Seals. |
| Flexibility | Low. Cracks when frozen. | High. | Extreme (-40°C to +200°C). |
| UV Resistance | Poor. Chalks and fades. | Fair (Needs paint). | Excellent. |
| Paintable? | Yes. | Yes. | NO. (Choose color wisely). |
| Life Span | 5 Years. | 10-15 Years. | 20+ Years. |
Pro Tip: Do not buy "Siliconized Acrylic". It is just acrylic with a marketing budget. Buy 100% Silicone (like Mulco Supra Expert or Tremco Tremsil).
2. The Danger Zone: Weep Holes
Before you go crazy with the caulking gun, you need to identify the Weep Holes. These are small slots (or holes with plastic covers) at the bottom exterior edge of the window frame. Function: Windows are designed to take on some water during heavy rain. The internal channels drain that water out through the weep holes.
The Mistake: Homeowners see these "holes" and think, "Aha! A draft!" and caulk them shut. The Result: Water gets trapped inside the frame, freezes, expands, and shatters the vinyl welds. Then it rots out your wood framing. Rule: Caulk the Top and Sides. NEVER caulk the weep holes on the bottom.
3. How to Recaulk Guidelines (The "Backer Rod" Secret)
You cannot just squirt new goo over old goo.
Step 1: Removal
Use a "5-in-1 tool" or a sharp utility knife to slice out the old caulking. You need a clean surface. Cleaning: Wipe the brick and vinyl with rubbing alcohol to remove dust. Silicone won't stick to dust.
Step 2: Mind the Gap (Backer Rod)
If the gap between the window and the brick is wider than 1/4 inch, you cannot just fill it with caulk. It will sag and tear. Insert "Backer Rod": A grey foam snake. Stuff it into the gap.
- Why? It prevents "Three-Sided Adhesion". You want the caulk to stick to the Brick (1) and the Window (2). If it sticks to the back of the gap (3), it cannot stretch. The backer rod acts as a bond breaker.
Step 3: The Bead
Cut the nozzle at a 45-degree angle. Pull the gun towards you (don't push). Apply a consistent bead ensuring it touches both sides.
Step 4: Tooling (The Finger)
Dip your finger in soapy water (or use a popsicle stick). Run it along the bead to smooth it out and push it into the pores of the brick. Note: Silicone is messy. Have paper towels ready.
4. When is Caulking Not Enough?
Sometimes, the rot goes deeper.
- Rotten Wood: If you push on the window frame and it feels "spongy" or moves, the wood studs underneath are rotted. Caulking is a lipstick on a pig.
- Foggy Glass: If the draft is coming from between the panes, the thermal seal has failed. See our guide on Glass Replacement.
5. Professional Costs
Don't want to hang off a ladder?
- Cost: Pros charge $4 to $8 per linear foot.
- Average House: $1,500 - $2,500 for a full exterior re-caulk.
- ROI: Massive. It is the cheapest energy retrofit with the highest payback.
6. Tools of the Trade: Don't Buy the $5 Gun
If you are doing your whole house, professional tools make the difference between a clean bead and a sticky mess.
The Caulking Gun
- The Ratchet Grid (Bad): The cheap orange guns ($4). Every time you squeeze, it ratchets. When you let go, it keeps pushing caulk out.
- The Dripless (Good): A smooth rod mechanism ($15+). When you release the trigger, it instantly stops the flow. Look for a 12:1 thrust ratio or higher. Thick silicone requires high thrust.
The Finishing Agent
- Silicone is hydrophobic. It repels water. But it sticks to dry skin instantly.
- The Trick: Fill a spray bottle with water and a few drops of dish soap. Mist the bead before you run your finger along it. The soap prevents the silicone from sticking to your finger, allowing you to create a perfect, smooth concave fillet.
7. Timing is Everything: Winter vs. Summer
"Can I caulk in January?" Short Answer: No. Long Answer: You need a surface temperature of at least +5°C (40°F) for proper adhesion.
- Frozen Moisture: Even if the air is above freezing, the brick might be frozen. If there is a microscopic layer of ice on the brick, the caulk will adhere to the ice, not the brick. When the ice melts in spring, the caulk will fall off.
- Curing Time: Silicone cures by reacting with humidity in the air. In dry, cold winter air, it takes days to cure.
- The Window: The best time to caulk in Toronto is May to October.
8. Safety First: Ladders and Reach
Most drafts come from the second floor because wind speeds are higher up there.
- Extension Ladders: Ensure the "4-to-1 Rule" (For every 4 feet up, the base is 1 foot out).
- Stabilizers: Use a "stand-off" bar to rest the ladder on the brick, not the window frame or the eavestrough.
- The Reach: Never overreach. If your belt buckle passes the side rail of the ladder, you are dead. Move the ladder.
9. Frequently Asked Questions
Q: How much caulking do I need?
A: Measure loosely. A standard 300ml tube gives you about 25 linear feet of a 1/4 inch bead. An average window is 4x4 (16 feet perimeter). So, assume 1 tube per 1.5 windows. Buy extra; you can return unopened tubes.
Q: Why did my new caulk crack after 1 week?
A: Acrylic shrinkage. You probably used cheap acrylic caulk. As it dried, the water evaporated, and it shrank by 30%. Because it was "taut" across the gap, it tore itself apart. This is why we use 100% Silicone—it doesn't shrink.
Q: Can I caulk over old caulk?
A: No. New silicone does not bond well to cured silicone. It will peel off like a rubber band. You must remove the old stuff.
Summary
Draftproofing is not sexy, but it pays for itself in one winter. Grab a candle, find the leaks, and spend the afternoon sealing your envelope. Just remember: Silicone Only and Save the Weep Holes.
Need a full assessment? If your windows are leaking water (not just air), read our article on Why Your Window is Leaking to diagnose if it's a caulking issue or a structural failure.