Expanding foam is one of those materials that can seem almost magical. You spray it into a gap, and it grows, hardens, and seals up spaces like a pro. It’s used for insulation, filling cracks, and even construction projects. But a common question people ask is: is expanding foam waterproof?
What Is Expanding Foam?
Expanding foam is a polyurethane-based material that expands when exposed to air. It’s commonly used for insulation, sealing gaps, and preventing drafts. You’ll find it in DIY projects and large-scale construction work alike. Some people even use it for crafts or unusual fixes around the home. But before relying on it for waterproofing, it’s worth understanding how it reacts to water.
Does Expanding Foam Repel Water?
Expanding foam does have water-resistant properties, but that doesn’t mean it’s completely waterproof. Once it cures, the outer surface forms a hardened shell that can resist moisture to some degree. However, if constantly exposed to water, especially over a long period, it can start to break down.
Some types of expanding foam are labelled as “closed-cell,” meaning they have a more compact structure that resists water better. Others are “open-cell,” which are softer, less dense, and more likely to absorb moisture. If you need a waterproof solution, the type of foam you choose makes all the difference.
Open-Cell vs. Closed-Cell Foam: Which One Handles Water Better?
Open-Cell Foam
This type of foam is lightweight and flexible. It expands more than closed-cell foam, which makes it useful for filling irregular gaps. But it also has a porous structure, meaning it absorbs water like a sponge. If exposed to moisture, it can degrade over time, making it unsuitable for areas prone to heavy water exposure.
Closed-Cell Foam
Now, this is the option you want if water resistance is a priority. Closed-cell foam is much denser and doesn’t allow water to pass through easily. It’s commonly used in marine applications, roofing, and exterior insulation. While not entirely waterproof, it holds up much better against moisture compared to open-cell foam.
Can Expanding Foam Be Used Underwater?
If you’re thinking of using expanding foam for underwater applications, you’ll need to be careful. Standard expanding foams won’t hold up if they’re submerged for long periods. However, some specialised polyurethane foams are designed specifically for wet environments. These are often used in construction projects involving water tanks, ponds, or even boats.
For example, certain two-part polyurethane foams are created to withstand continuous water exposure. These are more expensive but offer a reliable solution when waterproofing is non-negotiable.
What Happens When Expanding Foam Gets Wet?
If you use open-cell foam and it comes into contact with water, it will likely absorb the moisture, weaken, and potentially become mouldy. On the other hand, closed-cell foam can handle short-term exposure without much trouble, but over time, if water manages to seep into any gaps or cracks, it can lead to deterioration.
If you’re sealing areas that are prone to water exposure, applying a waterproof coating over the foam is a smart move. Many professionals use waterproof sealants or paints to provide extra protection.
Where Should You Avoid Using Expanding Foam?
While expanding foam is incredibly useful, it’s not suitable for every situation. Here are some places where you might want to reconsider using it:
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Basements prone to flooding – Unless you’re using waterproof-grade foam, prolonged exposure to moisture could cause problems.
- Outdoor applications without UV protection – Sunlight can break down expanding foam over time, so it needs to be painted or sealed if used outside.
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Areas exposed to extreme heat – Some foams aren’t designed to handle high temperatures and could break down under intense heat.
How to Make Expanding Foam More Waterproof
If you’ve already used expanding foam but need it to be more resistant to water, there are a few ways to improve its durability:
- Apply a waterproof sealant – Using a silicone-based or polyurethane coating over the foam can add extra protection.
- Use a waterproof paint – Some paints are specifically designed to create a waterproof barrier, making them a good option for sealing foam.
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Choose the right foam from the start – If you’re working on a project where water exposure is a concern, go for closed-cell foam or a waterproof-specific product.
Alternative Waterproof Sealants
If expanding foam isn’t quite what you need, there are other materials that might be a better fit:
- Silicone sealants – Great for waterproofing joints and small gaps.
- Epoxy coatings – Often used for industrial waterproofing, offering a hard and durable finish.
- Rubberised sprays – These create a flexible, waterproof layer and are easy to apply.
The Short Answer
Expanding foam has plenty of benefits, but if you’re relying on it for waterproofing, you need to be careful. Standard foams are water-resistant to an extent, but they’re not fully waterproof unless specifically designed to be. Closed-cell foams offer better moisture resistance, while open-cell varieties absorb water and break down over time.
For the best results, consider using a waterproof sealant over the foam or opt for alternative materials if water exposure is a major concern. Expanding foam is fantastic for many jobs, but knowing its limits helps ensure your project stands the test of time.