Too Long; Didn't Read
- Silicone for glass and vinyl: Use 100% silicone anywhere the caulk will be exposed to UV light, touch glass, or need to last 20+ years. It does not shrink, crack, or break down in freeze-thaw cycles.
- Polyurethane for concrete and masonry: When caulking between a window frame and a concrete lintel, brick, or block wall, polyurethane grips porous surfaces better — but it must be painted or shaded to survive UV exposure.
- Never mix them in the same joint: New silicone will not bond to cured polyurethane (and vice versa). Mixing creates a delamination point that fails within one season.
- The Toronto climate makes the wrong choice obvious fast: One freeze-thaw cycle will expose a bad caulk choice. -20°C in February and +35°C humidity in August test every joint in your home.
Answer First: Silicone belongs on glass-to-glass and glass-to-vinyl joints. Polyurethane belongs on the joint between a window frame and concrete, brick, or masonry. Using the wrong one in the wrong place — or trying to layer them on top of each other — produces a joint that looks fine in October and fails by March. The difference comes down to adhesion chemistry, not brand preference.
Why This Question Actually Matters
Go into any Home Depot or Rona in the GTA and you will find an entire wall of caulking products. Silicone, polyurethane, acrylic, "siliconized acrylic," MS polymer, butyl rubber. Each one is marketed as "the best for windows."
Most of them are not.
The caulking conversation matters because windows in Toronto are not static objects. A vinyl window frame expands and contracts about 3 mm over the temperature swing between a humid July and a February cold snap. The joint between your window and the surrounding wall moves with it. A sealant that cannot flex with that movement cracks — and a cracked seal lets in water, air, and eventually rot.
Choosing the right product is not complicated. But it requires understanding one thing: caulk adhesion is substrate-specific. What grips glass will not grip concrete. What grips concrete may not last in direct UV exposure. Once you know that, the entire aisle simplifies.
[Image Idea: Labelled diagram showing window cross-section — glass pane, vinyl frame, rough opening, brick/concrete lintel, with arrows indicating which caulk type applies where]
What These Two Sealants Actually Are
Before getting into application, it helps to understand what you are working with chemically. The differences are not academic — they explain everything about where each product succeeds and fails.
Silicone Sealant — An inorganic polymer (polysiloxane) that cures by reacting with atmospheric moisture. Because it is inorganic, UV light cannot break it down the way it breaks down organic compounds. It stays elastic across a temperature range of approximately -40°C to +200°C, which is more range than any window will ever experience. It does not shrink as it cures, so it does not crack or pull away from the edges. The downside: it does not penetrate porous materials. On concrete or brick, it sits on top of the surface rather than bonding into it, which means freeze-thaw cycling can eventually pop it loose.
Polyurethane Sealant — An organic polymer that cures by reacting with moisture in a similar way, but the chemical backbone is carbon-based. That means UV light degrades it over time: it becomes brittle, chalks, and cracks if left unprotected in sunlight. The major advantage is adhesion to porous substrates. Polyurethane chemically bonds into the microscopic pores of concrete, brick, and mortar, creating a mechanical grip that silicone cannot match. It also bonds well to painted wood. Lifespan in direct sun without paint: 5 to 7 years. Lifespan when protected: 10 to 15 years.
Pro Tip: "Siliconized acrylic" is acrylic caulk with a marketing label. It is not 100% silicone. It shrinks as it dries (up to 30%), cracks in the cold, and has a realistic outdoor lifespan of 3 to 5 years. It costs less because it performs less. Avoid it for any exterior window joint.
Where Each One Belongs: The Joint-by-Joint Breakdown
The right way to think about caulking a window is not "what product should I buy?" It is "what surfaces are touching at this joint?"
Glass-to-Glass Joints
Any joint where two glass surfaces meet — the perimeter glazing bead in a storefront system, a shower door to a fixed panel, a glass-to-glass corner — needs silicone. Full stop.
Silicone is the only common sealant with sufficient elongation (typically 25 to 50% movement capability) to handle glass thermal expansion without tearing. It does not off-gas anything that degrades insulated glass unit spacer bars the way some polyurethane formulas can. For glazing on commercial storefront glass installations, most specs call for a neutral-cure silicone specifically because acetoxy-cure silicone releases acetic acid during curing and can corrode metal spacer bars.
Recommended products in Canada: Mulco Supra Expert (widely available at Rona and building supply stores across the GTA), Tremco Spectrem 1 for commercial glazing, GE SilPruf for high-movement structural joints.
Glass-to-Vinyl and Vinyl-to-Vinyl Joints
The joint between a window glass pane and its vinyl frame, or between a vinyl window frame and a vinyl extension jamb, belongs to silicone. Both surfaces are non-porous. Silicone bonds well to both. The thermal expansion rates of glass and vinyl are different enough that you need a sealant with genuine elasticity, not one that hardens and cracks.
This is the most common re-caulking job on older Toronto homes — the 1990s builder-grade windows in North York and Scarborough post-war semis were often set with cheap acrylic that has long since cracked and pulled away. Replacing it with 100% silicone is one of the better weekend investments a homeowner can make.
Frame-to-Concrete and Frame-to-Masonry Joints
This is polyurethane territory.
When a window frame sits inside a rough opening surrounded by a poured concrete lintel, concrete block, or brick mortar — as is common in Etobicoke bungalows and older Toronto brick homes — the joint between the vinyl or aluminum frame and the masonry needs a sealant that can grip a porous surface. Silicone will adhere initially but is prone to adhesion failure on masonry under repeated freeze-thaw cycling.
Polyurethane penetrates the surface texture of concrete and brick, cures into the pores, and holds. Products like Tremco Vulkem 116 and Sikaflex-1a are the standard for this application in the GTA. Both require a coat of paint or a UV-protective topcoat over the cured bead if they will be exposed to direct sunlight.
One important note: polyurethane requires a primer on some masonry surfaces for maximum adhesion. Read the product TDS (technical data sheet). On glossy or sealed concrete, a primer like Tremco Primer 953 or Sika Primer-3 N makes the difference between a joint that lasts 12 years and one that fails in 3.
Frame-to-Aluminum Storefront Joints
Aluminum framing systems on commercial storefronts in downtown Toronto, Mississauga office parks, or Markham retail plazas typically call for silicone. Aluminum is non-porous, moves significantly with temperature, and the sealant is visible. Silicone's UV resistance keeps it from yellowing or chalking in exposed locations. For commercial glass repair and storefront sealing, neutral-cure silicone is the near-universal choice in Canadian commercial glazing specifications.
The Mixing Problem: Why You Cannot Layer Them
This is the mistake that causes the most callbacks in the window and glazing trade.
A homeowner recaulks their windows with silicone. Three years later, they notice a spot that needs touching up. They grab a tube of polyurethane because it is what was on sale. They apply it over the silicone. It looks fine.
By March, it peels off in a sheet.
New silicone does not bond to cured silicone. New polyurethane does not bond to cured silicone either. And cured polyurethane will not accept a silicone topcoat. These are chemically incompatible cured surfaces. When you apply one on top of the other, adhesion depends entirely on mechanical grip, which is essentially zero on a smooth, non-porous cured sealant surface.
The only correct repair sequence:
- Remove the old caulk entirely — utility knife, oscillating tool, or a specialized caulk removal tool
- Clean the substrate with isopropyl alcohol (removes silicone residue, oils, and dust)
- Let it dry completely
- Apply the correct product for that substrate from scratch
There is no shortcut here. "Caulk over old caulk" is the sentence that precedes a failed joint every time.
Warning: Do not caulk weep holes. The small slots or plugged holes along the bottom exterior edge of window frames are intentional drainage channels. Sealing them traps water inside the frame, which freezes, expands, and fractures vinyl welds or rots wood framing. Caulk the top and two sides of the frame. Leave the bottom weep holes clear. (For a full explanation of weep hole function and what happens when they fail, see drafty windows and caulking failures.)
A Comparison Table Worth Keeping
| Property | 100% Silicone | Polyurethane | Acrylic / "Siliconized Acrylic" |
|---|---|---|---|
| Best substrate | Glass, vinyl, aluminum (non-porous) | Concrete, brick, masonry, wood (porous) | Interior trim only |
| UV resistance | Excellent — does not degrade | Poor — must be painted or shaded | Poor |
| Flexibility range | -40°C to +200°C | -20°C to +80°C | Brittle below -10°C |
| Paintable? | No | Yes (after full cure) | Yes |
| Shrinkage on cure | None | Minimal | Up to 30% (causes cracking) |
| Adhesion to porous surfaces | Weak | Strong | Moderate |
| Typical exterior lifespan | 20+ years | 5–15 years (with UV protection) | 3–5 years |
| Canadian brand examples | Mulco Supra Expert, GE SilPruf | Tremco Vulkem 116, Sikaflex-1a | Various — avoid for exterior use |
[Image Idea: Side-by-side close-up of a properly tooled silicone bead on vinyl vs. a painted polyurethane bead against brick, with labels]
Does Weather Affect Which Caulk You Can Use?
Yes — and in Toronto's climate, timing matters more than most homeowners realize.
Both silicone and polyurethane cure by reacting with atmospheric moisture. The reaction rate slows dramatically in cold and dry conditions. Below +5°C surface temperature, adhesion is compromised regardless of which product you use.
The freeze-thaw problem specific to masonry: Even when the air temperature is above freezing, brick and concrete can be colder than the air. If there is any absorbed moisture in the masonry — which is nearly always the case after a Toronto winter — that surface moisture can be at or near 0°C when the air is at +8°C. Applying polyurethane to a near-frozen brick surface gives you temporary adhesion. The spring thaw pulls the sealant off the face of the brick.
The reliable application window in Toronto is late April through early October — when surface temperatures stay above +10°C and overnight frost is not a factor. In 2026, with the late freeze-thaw swings the GTA has been seeing, erring toward May as your start date is sensible.
One weather-related note specific to silicone: high-humidity conditions (like a Toronto July) actually accelerate silicone cure time. A bead applied on a humid summer day may skin over in 30 minutes and fully cure in 12 to 24 hours. The same bead in dry spring conditions might take 48 to 72 hours. Do not disturb or rain-expose a fresh silicone joint until it has had time to fully cross-link.
Application: The Steps That Actually Matter
These are the steps that separate a joint that lasts 20 years from one that fails in 18 months.
1. Remove Everything First
New sealant over old sealant is not a repair. It is a delay. Use a utility knife to score the edges of the old bead, then a 5-in-1 tool or caulk removal tool to pull it out in strips. For silicone on glass, a plastic scraper prevents scratching.
2. Clean the Substrate
Wipe both surfaces with isopropyl alcohol (70% or higher). Silicone residue, dust, and moisture contamination are the top three causes of adhesion failure. The substrate must be visually dry before you open a tube.
3. Use Backer Rod on Wide Gaps
Any gap wider than 6 mm (1/4 inch) needs a backer rod — a closed-cell foam cylinder stuffed into the gap before caulking. Backer rod serves two functions: it controls the depth of the sealant bead (you want a depth roughly half the width), and it prevents three-sided adhesion. Three-sided adhesion — when the sealant bonds to both sides of the joint and the back of the gap — prevents the joint from stretching properly and causes it to tear at the bond line. Backer rod breaks that third bond point.
4. Cut the Nozzle Correctly
A 45-degree cut. The opening should match the desired bead width — start small, you can cut more. Pull the gun toward you rather than pushing it; pulling gives a cleaner bead that seats against both surfaces simultaneously.
5. Tool Within 5 Minutes
Silicone and polyurethane both tool best while still wet. Dip a finger in soapy water and draw a smooth concave fillet along the bead. The soapy water prevents the sealant from sticking to your skin and allows a clean pass. One pass only — going back over a tooled bead tears it.
What About Hybrid Sealants?
Hybrid sealant — sometimes marketed as MS polymer, silicone-modified polyurethane, or SMP — is a third category worth knowing about. These products combine elements of both chemistries: they bond to porous and non-porous surfaces, can be painted, and offer UV resistance closer to silicone than standard polyurethane.
Tremco Dymonic 100 is the most widely specified hybrid in commercial window work across the GTA. For residential jobs that span multiple substrate types — say, a window frame that touches both vinyl casing and a concrete lintel — a hybrid is a legitimate single-product solution.
The tradeoff: cost. Hybrid sealants typically run $25 to $40 per 300mL tube versus $10 to $18 for single-chemistry products. For a full-house re-caulk where you are buying 20+ tubes, that gap adds up. For a single complicated joint, the premium is worth it.
If you are dealing with a window leak that involves multiple substrates and you are not sure which product to reach for, our window leak diagnosis guide walks through which failure modes each sealant type is prone to.
The Numbers: How Much Caulk to Buy
A standard 300mL tube yields approximately 25 linear feet (about 7.5 metres) of a 6mm (1/4 inch) bead. A standard casement window has a perimeter of about 4 linear metres — so one tube covers roughly two windows at the exterior joint.
For a full exterior re-caulk on a typical two-storey Toronto semi (10 to 14 windows), you are looking at 8 to 12 tubes, depending on gap width. Buy one or two extra. Returned unopened tubes are not a problem; running short mid-job and creating a dry-lap joint is.
Frequently Asked Questions
Can I use silicone caulk on brick or concrete around my window?
You can, but it's not ideal. Silicone does not penetrate porous surfaces well and can pop off masonry under freeze-thaw stress. Polyurethane bonds chemically into the pores of concrete and brick, making it the better choice for those substrate transitions.
How long does polyurethane caulk last on exterior windows in Canada?
Exterior polyurethane sealants typically last 5 to 15 years depending on UV exposure and joint movement. Products like Tremco Vulkem 116 or Sikaflex-1a can reach the upper end of that range when kept painted or shielded from direct sun.
What is the best caulk for vinyl window frames?
100% silicone is the standard for vinyl-to-vinyl and glass-to-vinyl joints. It matches the expansion rate of vinyl, stays flexible from -40°C to +200°C, and does not pull away at the edges the way acrylic does after a few winters.
Can silicone and polyurethane caulk be painted?
Polyurethane caulk is paintable once cured — required if it will be exposed to sunlight. 100% silicone cannot be painted. Paint sits on top and peels off within months. If the joint needs to be painted, polyurethane or a paintable hybrid like OSI Quad Max is the right call.
Is there a caulk that works on both glass and concrete?
Hybrid sealants (sometimes called MS polymer or silicone-modified polyurethane) are designed to bond to both porous and non-porous surfaces and can be painted. Tremco Dymonic 100 is a widely used example in commercial window installation in the GTA. For most residential jobs, though, matching the caulk to the specific substrate gives better long-term results than using one product everywhere.
Not sure which joint needs which product?
We do assessments as part of any window service call — and if it turns out re-caulking is all you need, we'll tell you that. No upsell, no pressure. If you want a set of eyes on your exterior seals before the spring rain season hits, get in touch with the Installix team for a no-obligation look.