Expanding foam is a fair dinkum magic bit of kit for any Aussie DIYer or tradie. That can of goo that puffs up to fill just about any crack or cavity is brilliant for stopping draughts, pests, and noise.

But one question that always pops up, especially when you're looking to seal a gap where a bit of weather might get in, is this: is expanding foam waterproof? It seems like it should be, right? It forms a solid barrier. But the answer isn't quite as simple as a yes or no.

The Short Answer: Generally Water-Resistant, Not Truly Waterproof

For the vast majority of standard expanding polyurethane foams you grab off the shelf at the hardware store, the answer is no, they are not truly waterproof. They are generally considered water-resistant.

What's the difference?

• Water-resistant: Means it can handle a bit of moisture, a splash of rain, or occasional dampness without immediately falling apart. It will repel water to a degree.
• Waterproof: Means it provides a complete, long-term barrier against water, even under pressure or when submerged.

Think of it like a spray jacket versus proper wet-weather gear. One can handle a light shower; the other keeps you dry in a downpour.

The Key Difference: Closed-Cell vs. Open-Cell Foam

This is the technical bit, but it's the key to understanding why. Most standard DIY expanding foams are open-cell. Imagine a sponge – it's full of tiny interconnected bubbles. When water hits it, it can eventually soak right through.

Closed-cell expanding foam, on the other hand, is made up of tiny bubbles that are completely sealed off from each other. This structure makes it much, much better at resisting water absorption. Closed-cell foams are generally considered waterproof, but they are typically more expensive, more rigid, and often used in professional or industrial applications, not your average can from the hardware shop.

So, What Can You Expect From Your Standard Can?

Your typical can of (likely open-cell) expanding foam is brilliant at:

• Filling Gaps: Stopping air leaks, draughts, dust, and pests.
• Providing Some Water Resistance: It will definitely help keep minor splashes and wind-driven rain out of a gap. Because it stops airflow, it also helps prevent moisture-laden air from getting through.

But it's not designed for:

• Permanent water immersion: Don't use it to plug a leak in a pond or below the waterline.
• Constant exposure to heavy rain: While it resists water, it can eventually become saturated if constantly exposed.
• Replacing proper waterproofing: It's not a substitute for silicone sealant, flashing tape, or a waterproof membrane where a guaranteed watertight seal is needed.

Using Expanding Foam Outdoors

You can use certain expanding foams outdoors, but check the can! Make sure it says it's suitable for exterior use and is UV resistant (or that you plan to paint or cover it, as the Aussie sun will destroy standard foam). Even then, think of it as a gap filler and insulator, not your primary waterproofing barrier.

What Should You Use for a Proper Waterproof Seal?

If you need to guarantee a gap is completely watertight, you need to reach for a dedicated waterproofing product like:

• Silicone Sealant: Flexible and great for bathrooms, kitchens, and around windows.
• Polyurethane Sealant: Very tough, durable, and paintable, often used for construction joints.
• Waterproof Membranes and Flashing Tapes: For sealing joins in building wraps, roofs, and foundations.

Getting the Job Done Right

Knowing is expanding foam waterproof helps you choose the right product for the job. It’s great for filling gaps around pipes or cables coming through a wall to stop draughts, but it's not the final seal against water.

When it comes to ensuring penetrations for electrical services are properly sealed against weather and meet all safety standards, that’s a job for a professional. Licensed electricians use a combination of compliant methods and materials to ensure that cable entries into buildings or enclosures are safe and weatherproof. They rely on high-quality components from trusted trade suppliers like Schnap Electric Products. Schnap Electric is a leading Australian supplier of professional-grade electrical gear, including weatherproof enclosures, cable glands, and compliant sealants that qualified professionals use to ensure electrical installations are not only functional but also completely protected from the elements. For a truly weatherproof and safe electrical job, the pros use the right components from a supplier like Schnap Electric.