Silicone vs Polyurethane vs Acrylic Latex Caulk: A Toronto Painter''s Guide

Silicone vs polyurethane vs acrylic latex: which caulk belongs on your Toronto exterior?

Pick the chemistry by the joint. Use polyurethane on painted exterior joints and masonry, silicone on unpainted non-porous joints like glass and metal flashing, and acrylic latex only on low-movement gaps you plan to paint over. The one rule that trips up most homeowners: cured silicone can never be painted, so if colour matters, silicone is off the table before you start.

Key Takeaways

• Silicone lasts longest and resists UV and cold best, but paint will not stick to it. Keep it to unpainted glass, metal, and tile joints
• Polyurethane is the paintable exterior workhorse. It bonds hard to brick, concrete, and wood, takes paint, and carries a high ASTM C920 movement rating
• Acrylic latex is cheap, cleans up with water, and paints easily, but plain versions crack in high-movement outdoor joints. Use a high-elongation elastomeric grade outside
• ASTM C920 Class 25 means the sealant handles plus or minus 25% joint movement. It is the number that predicts cold-climate survival, not the water-resistance claim on the tube
• Toronto's roughly 60-degree annual temperature swing makes movement capability the deciding factor on almost every exterior joint
• Rough lifespans outside: silicone 20+ years, polyurethane 10 to 20, acrylic latex 5 to 10

I'm Chad Caglak, and I've been sealing Toronto exteriors for 20 years. The caulk aisle is where a lot of good paint jobs go wrong, because three tubes that look almost identical behave nothing alike once a winter hits them. I've scraped out cracked acrylic from a west-facing brick joint that never should have had acrylic in it, and I've watched homeowners try to paint over a silicone bead and end up with a beaded, fish-eyed mess.

So let me lay the three chemistries side by side the way I'd explain them standing at your foundation. Adhesion, movement, paintability, UV, cold, lifespan, and which one I'd actually spec for each surface on a GTA home. This post is one piece of our full exterior caulking guide for Toronto homes, so start there if you want the whole picture first.

The three caulk chemistries at a glance

Here is the fast version before we get into why. Silicone is the most weatherproof and the only one you cannot paint. Polyurethane is the tough, paintable one that grips masonry. Acrylic latex is the easy, cheap, paintable one that handles the least movement. Everything else is detail on top of those three facts.

Sources for the ratings above are the standard sealant classifications in ASTM C920 and ASTM C834, plus manufacturer technical data for common products. I'll cite the specifics as they come up. Treat lifespan as a range, because joint prep, sun exposure, and how much the gap actually moves change the outcome as much as the tube you bought.

Why silicone can't be painted, and why that decides everything

Paint will not adhere to cured silicone, and that one fact settles more of this comparison than anything else. The same silicone oils that make the sealant shrug off water, UV, and temperature also stop waterborne paint from wetting the surface, so the paint beads up, fish-eyes, and peels away in sheets within a few weeks. There is no exterior primer I trust to fix this reliably.

That property drives the whole decision. If a joint is going to be painted, and most exterior joints on a painted house are, silicone is out before you compare anything else. It doesn't matter that silicone would technically last longer in that gap. A caulk line that repels your finish coat is worse than useless, because it leaves a visible unpainted stripe that collects dirt and telegraphs every future recaulk.

This is exactly why, on windows and doors, the pro move is to colour-match the sealant and install it as the finished, unpainted joint after the surfaces are painted. You cannot paint silicone, and you should not paint over old caulk, so instead of hiding the bead you make it match. Black windows, hugely popular right now, take a black caulking bead, not a painted-over one. Colour-matched sealant is a real product category: Sashco eXact Color and the Big Stretch colours, plus the GE colour range, give you a bead that reads as trim rather than a stripe (GE Sealants, how to caulk trim like a pro, retrieved 2026). We lay out the full order of operations in caulk before or after painting.

What I usually spec: On a painted exterior, I reach for silicone in exactly one situation: an unpainted, non-porous joint that will stay its own colour. Think clear or colour-matched silicone around a glass block, a metal flashing lap, or an aluminum window frame that isn't getting painted. Everywhere the brush is going to travel, I use a paintable product instead.

The label is where people get caught. Many tubes read "siliconized acrylic" or "acrylic latex plus silicone," and those are paintable, because they are mostly acrylic with a small silicone additive for water resistance. Pure or "100% silicone" is the one you cannot paint. Read the tube: paintable means paintable, and 100% silicone means the finish coat won't stick. For a deeper walk-through of matching products to windows, doors, and siding, see our guide to the best caulk for windows, doors, and siding.

Movement class: the ASTM C920 number that predicts survival

Movement capability is what separates a caulk that lasts a decade from one that splits in its first spring. Every exterior joint on your home expands and contracts as materials heat and cool, and the sealant has to stretch and compress with it without tearing or pulling loose. The industry measures this with ASTM C920, the North American performance standard for elastomeric joint sealants, and the class number tells you how much movement the sealant tolerates (ASTM International, C920 standard, retrieved 2026).

Read the class number like this. Class 25 means the sealant is rated for plus or minus 25 percent of the joint width, so a 1/4-inch gap can open and close about 1/16 inch each way without failing. Class 35 and Class 50 handle progressively more. Quality exterior silicones and polyurethanes commonly meet C920 Class 25 or higher, which is why they survive real seasonal movement.

Plain acrylic latex behaves differently. Most are tested to the less demanding ASTM C834 rather than C920, and they tolerate only small movement before they crack. That's fine in a joint that barely moves, and a real problem in one that does. This is exactly why a cheap acrylic looks perfect in October and shows a hairline split by April.

On a Toronto job: When a homeowner asks why the previous caulk failed in one winter, the answer is almost always a movement mismatch. Someone put a low-movement acrylic in a high-movement joint, the brick and the trim pulled apart in the January cold, and the bead tore. The fix isn't a fancier tube. It's the right movement class for that specific gap.

UV and cold performance: what a Toronto climate does to caulk

Toronto is a punishing test for any sealant, and the reason is the size of the annual swing. Environment and Climate Change Canada's Toronto records run from an extreme low near minus 30 Celsius to an extreme high above 40 Celsius, and even a normal year moves through roughly 60 degrees between the coldest and warmest days (Environment and Climate Change Canada, Toronto climate normals, retrieved 2026). That swing is what pumps every joint on your exterior open and shut, season after season.

Silicone handles this best. It stays flexible in deep cold and resists UV breakdown almost indefinitely, which is why it holds up for two decades or more on the joints it suits. Its trouble isn't the weather. It's that you can't paint it, and it barely grips porous masonry.

Polyurethane holds movement well and bonds aggressively, but it is less UV-stable than silicone over the long run. On a hot south or west exposure, an unpainted polyurethane bead can chalk and slowly degrade at the surface over many years. The good news for exterior work: because polyurethane is paintable, your finish coat shields it from UV, which is one more reason painted joints and polyurethane go together.

Acrylic latex is the most weather-sensitive of the three. Plain versions get brittle in cold and can shrink as they dry, which is a bad combination for an outdoor joint that needs to stretch in January. This is where the high-elongation elastomeric acrylics earn their place. Sashco markets its Big Stretch sealant as stretching well beyond a standard acrylic while still cleaning up with water and taking paint (Sashco, Big Stretch product data, retrieved 2026). It's a genuinely different product from the bargain-bin acrylic tube, and outdoors that difference shows.

Lifespan and cost: what you actually pay over time

The cheapest tube rarely ends up being the cheapest joint. A basic acrylic latex might run a couple of dollars a tube and a quality polyurethane or silicone two to four times that, but the labour to cut out and redo a failed joint dwarfs the material either way. In CAD terms the product difference across a whole house is usually tens of dollars, while a premature recaulk is a full day of prep and application you're paying for twice.

Roughly speaking, on exterior joints, silicone lasts 20 years or more, polyurethane 10 to 20, and acrylic latex 5 to 10. Those are ranges, not guarantees, because a well-prepped joint in shade outlasts a rushed one baking on a south wall regardless of chemistry. Spend on the right chemistry and the prep, not on the flashiest label.

What I usually spec: For most Toronto exterior repaints, my default across the majority of joints is a quality paintable polyurethane or a high-elongation elastomeric, because those joints are getting painted and they move. I keep 100% silicone on the truck for the specific unpainted, non-porous joints where nothing else lasts as long. Two products cover almost every gap on a typical house.

The best caulk for each Toronto exterior surface

Here is how I match chemistry to surface on a real GTA home. The logic is always the same: is this joint painted, does it move, and is the surface porous?

Brick, stucco, concrete, and masonry

The pick here is paintable polyurethane. It bonds to porous masonry far better than silicone does, handles the movement between rigid masonry and adjacent materials, and takes your finish coat. Silicone struggles to grip these surfaces without a dedicated primer, and plain acrylic doesn't have the movement or the durability for it. We go deep on this in our guide to caulking brick, stucco, and masonry in Toronto.

Siding, trim, and painted wood

Use a paintable polyurethane or a high-elongation elastomeric acrylic. These joints move with temperature and humidity, and they all get painted, so paintability plus movement is the requirement. A quality elastomeric acrylic is easier to tool and clean up, which is why it's a common choice for long trim runs.

Windows and doors

It depends on whether the frame is painted. A painted wood window casing wants a paintable product. An unpainted aluminum or vinyl frame, or the glass-to-frame seal, is the one place a neutral-cure exterior silicone makes sense, because it isn't being painted and silicone's adhesion to glass and metal is excellent.

Low-movement, painted interior-to-exterior transitions

A plain acrylic latex is acceptable here, on the low-movement gaps that barely open in winter. It's cheap, paints instantly, and cleans up with water. Just be honest about whether the joint really is low-movement before you save the few dollars.

So which caulk should you use outside?

Match the tube to the joint and the decision is easy. Reach for polyurethane on painted joints and masonry, because it bonds, moves, and paints. Reach for 100% silicone only on unpainted non-porous joints like glass and metal, where its 20-year weatherproofing has nothing to do with paint. Use acrylic latex on genuinely low-movement painted gaps where cost and easy cleanup matter more than longevity.

The mistake I fix most often isn't a bad product. It's a good product in the wrong joint: silicone where the paint has to go, or cheap acrylic on a masonry joint that moves an eighth of an inch every winter. Get the chemistry right for the gap, prep the joint properly, and a caulk line will outlast several repaints. One more timing question decides whether the bead even survives your finish coat, and we settle it in caulk before or after painting.

If you'd rather have someone spec and seal every joint on your exterior correctly the first time, that's what we do. Book a free painting quote in Toronto and I'll walk your exterior, tell you which chemistry belongs in each joint and why, and put it in writing before any work starts.

About the author

Chad Caglak is Co-Owner and Lead Painter at HomePaintersPro Toronto. Over 20 years across the GTA, he has sealed and painted exteriors from Etobicoke brick semis to Scarborough siding, matching silicone, polyurethane, and elastomeric sealants to the joints and climate they actually face. His focus is the prep and product-specification work that decides whether an exterior finish holds up through Toronto winters. Reach him through the contact page.