Urethane vs. Silicone Sealants for Waterproof Construction

When sealing the deal on a construction project, the choice between silicone and urethane sealants is more than just a matter of price. 

The decision can impact a project’s durability, aesthetics, and functionality. In comparing urethane sealants vs. silicones, there are many criteria to consider – all will affect at least one of those three desirable traits. 

Whether you're sealing skyscraper windows or patching up a pavement, knowing these differences is crucial for any project manager's toolkit.

8 Criteria for Choosing Urethane Sealants vs. Silicones

When debating urethane vs. silicone as sealants for construction-grade work, consider these eight criteria:

1. Flexibility
2. Other mechanical properties
3. Durability in environment
4. Sustainability
5. Turnaround time
6. Adhesion
7. Ability to coat
8. Cost

In a hurry? We rated each trait from one to three “stars”:

1. Flexibility

Buildings aren’t always the immovable objects you think they are. Their structural components expand, contract, and face other outside forces that cause movement.

Silicone is highly elastic, so products made with it are great for structures that experience movement or settling. 

An example is an 8x8 panel for a building facade – those squares create joints, and you should seal them with silicone.

Urethane can be unusually rigid, which is in some ways a great thing for structural applications. However, certain urethane products can become so rigid under certain conditions (more on that below) that they’re ineffective for moving joints.

A more recent urethane offering comes with acrylic mixed in, with the goal of competing with silicone in movement capability. Unfortunately, it comes with a shorter warranty than competing silicone products.

Winner: Silicone

2. Other Mechanical Properties

If it’s mechanical sturdiness you seek, then first you should know that only silicone has structural options available. It’s also great for vibratory and deflective environments. Urethane products are merely OK at dealing with the dangers of torque.

Where urethane excels is at reducing tear and abrasion damage. Its durometer hardness rating shows it’s safe to use in applications with heavy foot, vehicle, or equipment traffic.

With the battle split down the middle, there are clearly specific use and don’t-use cases for silicone and urethane when it comes to mechanical properties. For example:

• Use silicone in cities with seismic activity.
• Use urethane in parking garages.
• Use neither in a nonmoving joint. (Use epoxy instead.)

Winner: Tie

3. Durability in Environment

Think of joints as the Achilles' heel of a building envelope – they're exposed and vulnerable, yet crucial to functionality and structural integrity.

Silicone sealants, when you use them correctly, will last about 3x longer than urethane equivalents. Because they’re inorganic, they don’t degrade during exposure to:

• Ultraviolet (UV) light
• Moisture
• Extreme temperatures – even during application
• Chemicals

Urethane sealants often fail in a variety of applications. There’s a tendency to blame poor workmanship, damp conditions, or adjacent materials. Those can be valid reasons, but studies by product makers show the #1 factor in urethane sealant failure is long-term exposure to UV. 

Extreme temperatures are also a factor. There are additives you can put into urethane for resisting cold air, but it still can’t go as low as silicone without degrading.

On the plus side, urethane is resistant to a range of chemicals, making it ideal for industrial settings. It is, however, prone to moisture during curing. 

Consider your environment’s potential impact on the product you choose. For example:

• Silicone can handle metal-on-metal applications in warm climates, as it’s able to accommodate the movement such weather causes.
• Never try to make urethane bond to asphalt; silicone is easier to “gun” out of the tube and performs better in cold weather.
• Dow offers a silicone joint sealant that bonds to concrete and asphalt, and is jet fuel-resistant
• With proper technique, you can apply high-performance, moisture-curing silicone and polyurethane sealants at temps down to -20 °F.

(Garvin Tip: Store water-based sealants at 40° or higher. Moisture-cured (one-part) sealants solidify at a slower pace while the temperature drops. Warm up primers and sealants for 24 hours before use so they’re easy to apply and perform as advertised.)

Winner: Silicone

4. Sustainability

Here’s some good news: Both silicone and urethane construction materials are nontoxic and recyclable. It’s more so the overall quality of each product line makes one better than the other for sustainability.

Silicone weather-sealing products make it easy for contractors, architects, and property owners to build or restore sustainably. Silicone inherently reduces waste because it’s more durable in the elements and you won’t have to replace it as often.

The general higher quality of silicone weatherproofing products also means they’re more energy-efficient. For example, when you use silicone for structural glazing and sealing windows, it's like giving your building a better-fitting thermal jacket. Multiple major material manufacturers say that their silicone products keep the heat in (or out) better than those with mechanically fixed, gasket-based glazing.

Winner: Silicone

5. Turnaround Time

If you’re the type who has to resist standing over your build with a stopwatch, you probably care deeply about sealant application speed.

Silicone products are easy to apply, often requiring just a single coat. These sealants come in both one-part and two-part varieties.

Urethane sealants usually require multiple coats, increasing labor time. They also are available in one- and two-part variants.

As for curing speed, material type doesn’t matter. (For example, weighing urethane vs. silicone caulk for windows is pointless.) it’s all about the distinction between one- and two-part sealants. 

One-part sealants are ready to use straight from the tube or cartridge, and they cure upon exposure to moisture in the air. They're user-friendly and great for quick projects, but not necessarily ideal for major construction projects. Two-part sealants require you to mix a base and an activator before you apply them. This type affords stronger bonds and more control over curing time, making them suitable for demanding applications.

Note that cure rate will always be at least somewhat dependent on temperature, as well as dew point, wind chill, and substrate.

Winner: Silicone (two-part)

6. Adhesion

Adhesion ensures a tight seal, which is crucial for maintaining the structural integrity and energy efficiency of a building.

Silicone, a synthetic material based on silica stone, is ideal for several surface types:

• Glass-to-glass
• Glass-to-metal
• Porous surfaces like concrete & masonry

Silicone sealants retain their adhesion during exposure to UV radiation at the bond line and in applications with scorching heat. Still, these products don’t adhere well to every surface without a treatment, so make sure to check product compatibility.

Note that, in structural applications, four-sided adhesives are an option with silicone sealants.

Urethane sticks well to most surfaces, making it a slightly more versatile option. You can use it successfully on:

• Concrete
• Metal
• Wood
• Plastic

In any application, consider an adhesion test to make sure you’ve got the right product. 

Where polyurethane won’t stick is to silicone. In the case of replacement glazing adhesive, you can perform a quick self-check to see whether the existing material is polyurethane or silicone adhesive bead. Give the existing material a good, hard press with your fingernail. If it's polyurethane or a direct glazing MS (basically, modified silicone or hybrid) polymer, it’ll be tough to leave even a slight dent. If it’s silicone (or butyl), it should be possible to press at least halfway through.

Garvin Tip: Adhesives better maintain tack if you keep them warm prior to application. 

Winner: Urethane

7. Ability to Coat

This trait somewhat depends on how well you want the weatherproofing product to perform its other duties.

Painting silicone sealants is possible in some cases, but it's often the lower-quality products that offer this feature. Higher-end silicones typically resist paint; this performance level is a drawback when a project calls for a coated finish.

However, it's not all black and white. There are custom color options for silicone sealants, although you'll need to plan for extra lead times. 

Garvin Tip: For those in a pinch, Pecora Corp. in Pennsylvania provides a unique solution – a field-tintable silicone. This product allows contractors to mix in color on-site, but it’s literally the sole exception to the rule (for now).

One significant advantage of silicone is its ability to retain color in intense sun exposure or harsh weather conditions.

Urethanes are always ready to take on a coat of paint, making them a versatile choice for exterior building work. 

Imagine you're out there, recaulking a crack or an expansion joint, and you need to cover your tracks with an architectural wall coating. Urethane won't let you down – it's ready to blend seamlessly into your design with a broad color palette. You'll usually find the perfect urethane aesthetic in a shorter timeframe than a silicone one.

Winner: Urethane

8. Cost

A sealant failure in a structure’s exterior can lead to expensive and time-sucking repairs. That’s why cost is last on this list – sticker price shouldn’t always be a high priority, but lifetime value should be.

Silicone construction products cost more money, especially high-quality versions. That’s because the process to convert and refine silica takes a ton of energy. 

However, that’s a short-sighted answer. Silicone doesn't require reapplications, providing ~2x the life cycle value.

Urethane products come with about 50% of the up-front price. However, they also come with about half the lifetime value because of the high maintenance costs associated with their use. Recaulking an eight-story building because you used urethane the first time costs tens of thousands of dollars you could’ve saved with silicone.

Winner: Silicone (in total value)

Urethane Sealant vs. Silicone: The Winner

Your decision to reach for that waterproof silicone sealant vs. a urethane counterpart will hinge on which properties matter most to your project.

Are you looking for a construction product that’s durable and flexible, and OK with a slightly higher price tag up-front? A commercial silicone sealant is your best bet. 

If you need a material that sticks well to various surfaces and has an easy-to-customize look, urethane could be the way to go. There are professional-grade polyurethane sealants that stand out from their lower-performing friends.

Ultimately, you usually can’t skimp on price when you need construction-grade sealants and waterproofing products. Remember: If you do the project well, your building will stay snug, sealed, and a signal of quality for years to come.

Click below for a list of manufacturers of building products. Always consult with your supplier before choosing a new urethane or silicone construction sealant.