Silicone vs Urethane vs Polyurethane Sealants: When to Use Each on a Commercial Building
Pick the wrong sealant for a joint and it can fail years early, even if the install was perfect. The good news: the choice is simpler than the product aisle makes it look.
Short answer: Urethane and polyurethane sealants are the same chemistry, so the real decision is between two families. Use silicone on glass, metal, curtain walls, and sun-exposed joints that will never be painted. Use polyurethane (urethane) on concrete, masonry, traffic-bearing joints, and anywhere the sealant must be painted to match the wall. When a joint needs paintability and better UV resistance than a standard urethane, a hybrid (silyl-terminated polyether) sealant is often the middle ground.
Key Takeaways
- “Urethane” and “polyurethane” are interchangeable names for the same sealant family. Don’t pay for a three-way comparison that is really two-way.
- Silicone wins on UV, heat, and movement. Some building-grade silicones are rated for +100/-50% joint movement.
- Polyurethane wins on toughness, paintability, and concrete. It handles foot and vehicle traffic far better than most silicones.
- Nothing sticks well to cured silicone except fresh silicone. That one fact drives a lot of repair decisions.
- Read the ASTM C920 label, not the marketing. Class, grade, and “Use” codes tell you where a product actually belongs.
Are Urethane and Polyurethane Sealants Different?
No. “Polyurethane” just means a polymer made of many urethane links, and in construction and industrial use the two names describe the same material. Manufacturers and specs use them interchangeably. Sikaflex-1A, one of the most common joint sealants on commercial jobs, is sold as a one-part polyurethane, and nobody on a jobsite would call it anything but “urethane.”
So where does the third option come in? The distinction that actually matters on a commercial building is:
- One-part vs. two-part polyurethane. Two-part (multicomponent) urethanes cure faster and are common on plaza decks and parking structures.
- Hybrid sealants, sold as MS polymer, STPE, or STPU. These use a polyether or polyurethane backbone capped with silane groups, which is why they are often described as combining the strength of polyurethane with the weathering of silicone.
We will cover hybrids below, because they now show up on a lot of façade specs.
Silicone vs Polyurethane Sealant at a Glance
The core difference is chemistry. Silicone has a silicon-oxygen backbone, while polyurethane is carbon-based. According to sealant maker Pecora, the Si-O bond is far more UV stable than the carbon-oxygen bond in urethanes. Almost every practical trade-off below traces back to that.
| Property | Silicone | Polyurethane (Urethane) | Hybrid (STPE / MS Polymer) |
| UV and heat resistance | Excellent | Fair; breaks down with long sun exposure | Good to very good |
| Movement capability | Up to +100/-50% (ultra-low-modulus grades) | Typically ±25% to ±50% | Typically ±25% to ±50% |
| Paintable | No | Yes | Yes |
| Adhesion to concrete and masonry | Good with the right grade, sometimes primed | Excellent, often unprimed | Very good, often unprimed |
| Adhesion to glass and metal | Excellent | Good; not for structural glazing | Good |
| Abrasion and traffic resistance | Low to moderate | High | Moderate |
| Typical exterior service life | 15 to 20+ years | 5 to 10 years | Varies by product |
| Best known for | Curtain walls, glazing, sunny façades | Concrete joints, decks, painted walls | Mixed substrates that need paint |
Service life depends heavily on exposure. Our guide on how often commercial joint sealants need replacing in Florida breaks that down by façade and climate.
How to Read the ASTM C920 Label
Every serious building sealant is tested to ASTM C920, the standard for elastomeric joint sealants. The rating string on the data sheet tells you more than the product name.
| Code | What it means |
| Type S / Type M | Single-component (ready to gun) or multicomponent (mixed on site) |
| Grade NS / Grade P | Non-sag for vertical joints, or pourable (self-leveling) for horizontal joints |
| Class 12.5, 25, 35, 50, 100/50 | Movement capability as a percent of joint width |
| Use T / NT | Traffic (decks, walkways) or non-traffic (walls, windows) |
| Use I | Rated for joints continuously immersed in water |
| Use M, G, A, O | Tested adhesion to mortar, glass, aluminum, and other substrates |
Two real examples show how this plays out. DOWSIL 790, a silicone building sealant, is rated Type S, Grade NS, Class 100/50, meaning it can stretch to double its joint width and compress by half. Sikaflex-1A, a polyurethane, is rated Class 35 with Use T and NT, and it is paintable with water-, oil-, and rubber-based paints. Different tools for different jobs.
When to Use Silicone Sealant
Silicone is the default wherever the joint sees steady sun, wide thermal movement, or glass. Use it on:
- Curtain wall and storefront glazing joints, including structural glazing, which requires silicone
- Window and door perimeters on metal or glass that will not be painted
- Metal panel joints and sun-baked south and west elevations
- Precast and tilt-up panel joints where a high-movement, low-modulus grade is specified
- Roof penetrations and details on a silicone-coated roof, since fresh silicone is about the only thing that bonds to cured silicone
The trade-offs are real. Silicone cannot be painted, it tends to collect dirt on smooth beads, and it is softer under foot or wheel traffic. On porous stone, pick a grade that has passed ASTM C1248 non-staining testing, because some silicones can leave a halo on marble or limestone. If your roof is coated, our comparison of silicone, acrylic, and urethane roof coatings explains why the sealant should match the coating chemistry.
When to Use Polyurethane (Urethane) Sealant
Polyurethane earns its place where toughness and paint matter more than UV life. Use it on:
- Concrete expansion and control joints, including sidewalks and warehouse slabs
- Plaza decks, balconies, and parking structures, where a traffic-rated (Use T) grade is required
- Painted stucco, block, and tilt-wall façades where the joint has to disappear under the finish coat
- Masonry and EIFS joints with moderate movement
- Damp or recently poured concrete, since some urethanes are approved for green concrete
The weak spot is sunlight. Left bare in full Florida sun, urethane chalks, stiffens, and cracks sooner than silicone. Painting over it helps shield it. Deck and balcony joints also need to work with the waterproofing layer around them, which we cover in our post on plaza deck and balcony leaks.
Where Hybrid (STPE / MS Polymer) Sealants Fit
Hybrids solve a specific problem: a joint that needs paint but also sits in harsh sun. They cure like silicone but accept paint like urethane, contain no isocyanates, and usually bond to mixed substrates without primer.
They are not a universal upgrade, though. Many hybrids are rated lower for continuous water immersion, and movement ratings vary widely between products. Treat “hybrid” as a category, then check the ASTM C920 class and Use codes like you would for any other sealant. Honestly, this is where a lot of spec mistakes happen: someone reads “combines the best of silicone and polyurethane” and stops reading.
Quick Picker: Which Sealant Goes Where?
| Joint on a commercial building | Usual first choice | Why |
| Curtain wall, storefront glass | Silicone | UV stability, glass adhesion, structural options |
| Window perimeter, metal frame, unpainted | Silicone | Sun exposure and movement |
| Window perimeter in painted stucco | Polyurethane or hybrid | Must accept paint |
| Precast or tilt-up panel joints | High-movement silicone or polyurethane | Depends on movement and whether walls get painted |
| EIFS joints | Low-modulus silicone or approved polyurethane | EIFS face is soft; low stress on the bond line matters |
| Parking deck and plaza deck joints | Polyurethane (often two-part) | Traffic rating and abrasion resistance |
| Sidewalk and slab control joints | Polyurethane (self-leveling) | Pourable grade, foot and cart traffic |
| Roof curbs and penetrations on coated roofs | Match the coating chemistry | Adhesion between materials |
Always confirm against the manufacturer’s data sheet and the wall or roof system’s requirements. A good spec beats a rule of thumb.
Compatibility Mistakes That Cause Early Failure
Most sealant failures we see on older buildings are not product failures. They are compatibility or prep failures.
- Putting urethane over old silicone. Silicone leaves a residue that new urethane and most paints won’t grip. Joints need to be cut out and cleaned to bare substrate.
- Switching chemistries without a test. If a joint has always been urethane and you want silicone, run a field adhesion (pull) test first.
- Using silicone where paint is scheduled. The paint will bead, crawl, or peel off the joint. This is also one reason painting over existing coatings often fails.
- Skipping primer on porous substrates when the data sheet calls for it.
- Ignoring joint design. A proper backer rod and bond breaker let the sealant stretch in an hourglass shape. Three-sided adhesion tears the bead apart.
Why Florida Changes the Math
Central Florida puts sealants through intense UV, afternoon heat, and daily thermal cycling, then adds summer storms and wind-driven rain. That combination is exactly what pushes exposed urethane toward the short end of its service life. It is also why silicone ends up on so many sun-facing joints here, and why painted urethane joints need the paint maintained to keep protecting them.
When a joint fails, water usually finds the wall cavity before anyone notices. If you already see staining or bubbling paint near a joint, a moisture diagnostics inspection can show how far the water has traveled before you reseal. Sealant work is often one part of a larger commercial waterproofing plan for the building envelope.
FAQ
Can you put polyurethane sealant over silicone?
Not reliably. Polyurethane does not bond well to cured silicone or its residue. Remove the old silicone completely, clean the joint, and test adhesion before switching materials.
Which lasts longer, silicone or polyurethane sealant?
On exterior, sun-exposed joints, silicone typically lasts longer, often 15 to 20 years or more compared with 5 to 10 years for polyurethane. On traffic-bearing horizontal joints, polyurethane usually holds up better because it resists abrasion.
Is silicone sealant paintable?
Standard silicone is not. Paint won’t adhere to cured silicone. If the joint must match a painted wall, use a polyurethane or hybrid sealant instead.
What sealant is best for concrete expansion joints?
Polyurethane is the common choice for concrete expansion and control joints, especially horizontal ones with traffic. For vertical precast panel joints with large movement, an ultra-low-modulus silicone is also widely specified.
Choosing the right chemistry is half the job. The other half is joint prep, primer, backer rod, and tooling done right the first time. For a closer look at how we handle this on commercial and institutional buildings, see our commercial sealants and caulking services, carried out by Lamphier & Company’s in-house crews across Central Florida.
