Is Expanding Foam Waterproof & Using it for Water Leaks!

Wondering if Expanding foam can be used to fix water Leaks? Is it even Waterproof?

In this guide you will learn:

  • Just How Waterproof is Expanding Foam & What Happens when it gets wet,
  • How to Use Expanding Spray Foam for Water leaks
  • All about the Best Waterproof Expanding Foams, what to look for and their effectiveness.

If you find yourself in the unfortunate situation of having to deal with water leaks of any sort, many DIYers have turned to spray foam as of late as a quick, relatively inexpensive way to fix the problem.

While some spray foams can be helpful, others can create a false sense of security which can lead to disaster and even worse water damage.

Is Expanding Foam Waterproof

When it comes to expanding foams, there are two main types: open cell foam, and closed cell foam. 

Open cell foam can hold a great deal of water, but as a result is neither waterproof nor water resistant.

Most open cell spray foam will still dry after it gets wet, though, and due to the fact that it is inedible to mold you’ll never have to worry about it growing moldy.  So even if it does get wet, it shouldn’t sustain any real damage as long as it is allowed to dry properly.

On the other hand, high-density closed cell foams are highly water resistant, and in some cases can be waterproof. 

Be mindful of low-density closed cell foams, as they typically aren’t waterproof, and check the packaging of any spray foam before applying in areas that may get wet.

Other types, such as Polyurethane expanding foam, are specifically used for waterproofing in construction due to its ability to fill gaps and repel water.  These do not come in a can, though, as they require a mixing of chemicals as they are applied.

Water-Resistance Of Expanding Spray Foam

While being water resistant or straight up waterproof sounds like the way to go, there may be instances where open cell may be the way to go.

Typically used in interior settings, open cell spray foam will allow moisture to escape through it, which could be advantageous in certain situations but detrimental in others.

High density closed cell foam, while usually more expensive, also doesn’t absorb water like open cell foam does. 

As a result, high density closed foam is better suited for outdoor projects, and some brands are even specifically marketed for use on outdoor water features such as ponds and fountains.

Here’s What Happens When Expanding Foam Gets Wet…

Different spray foams will react differently to getting wet.  Open cell foams, for example, will absorb a certain amount of water, and can be dried out afterwards.

If open cell foams are continuously exposed to water or moisture, they will degrade over time and likely be ruined within a year.  Even a few days of exposure to direct sunlight will cause them to begin to yellow.

Medium to high density closed cell foams, however, will either mostly or entirely repel water due to the nature of their chemical composition. 

As a result, water will pool around them if it has nowhere to go, which can be problematic on roofs and other areas susceptible to water damage.

Water Damage after a Leak?

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How to Use Expanding Foam For Water Leaks

While spray foam can be incredibly useful for DIY water leak repairs, it’s important to ensure you’re not only using the correct spray foam for the job, but that you’re doing it properly.

The first thing you’ll want to do is select the right spray foam for the job, based on the severity of the leak.  Open cell spray foams should never be used to repair water leaks. 

You’ll want to find a good high density closed cell spray foam that guarantees itself as being waterproof.  From there, follow these steps.

1.Determine the source of the leak

First and foremost is a relatively simple one; you need to determine where the leak is coming from.  This can become difficult in some instances, for example if the leak is coming through an unfinished wall which would require locating the point of entry on the wall’s exterior.

In the instance of cracked pipes or damaged plumbing, you’ll need access to the area. Wear protective gear and use caution where necessary while doing so.

If the source of the leak is determined to be severe enough, spray foam may not be the best option. Significant damage to a foundation wall, for example, should be given immediate professional attention as opposed to the thoughtful application of canned spray foam.

2.Shut off the water if possible/necessary

While shutting off the water isn’t viable in every scenario, it may be necessary in others.  If water is still continuing to trickle or even pour through, spray foam isn’t going to be able to properly adhere.

Find the shut off valve for the water in your home, which is usually located either along the basement wall where the water meter is, or near the hot water tank.

If the water is coming in from the outside, it would be ideal to find a way to divert the water away from the area, at least for the time being.

3.Prepare the area

Make sure the area where the spray foam will be applied is clean and free of any debris.  While a damp surface will help the spray foam adhere to it, a wet one will make it difficult to ensure a good seal.

If there is dirt or any kind of residue, make sure it’s wiped clean. 

If you wipe water away and more appears, chances are the source of the leak hasn’t been properly dealt with, and you’ll want to go back and check on it.

4.Use the expanding spray foam in accordance with manufacturer instructions

This cannot be stressed enough.  Read the manufacturer’s instructions, and follow them closely.  If it says not to use in a poorly ventilated area, don’t use it in a poorly ventilated area.

Wear protective gear, especially gloves. A quick look through the review sections of numerous brands of spray foam yields the lament of dozens who wish they had worn gloves. 

Spray foam will bond to skin and nails to such an extent that soap and water will not remove them.  Don’t wear a nice pair, but definitely put some gloves on.

While filling gaps, leave room for the foam to expand.  Canned foam won’t expand as much as the kind that mixes as its sprayed, but it might still be a good idea to test a little on a safe surface to see how much room you’ll need.

Once sprayed, give the product ample time to dry and set properly.  Again, follow the instructions in this regard.

5.Clean up the area

Once the foam has had a chance to set and adhere properly, take a moment to trim away any excess and clean up any messes. 

Be careful when trimming foam away, so as not to damage the repair you just made.

6. Ensure source of leak has been addressed

Now that the foam has been applied and been given ample time to adhere and set, turn your water back on if it was turned off.  Let it run for a few moments to ensure that the foam has sealed the area properly, and do a quality check of your work.

If the area springs a leak again suddenly, turn the water back off and begin the process again, this time ensuring a fuller, more even coating.

Best Waterproof Expanding Spray Foams

When shopping for spray foam, there are a few brands that stand out among their peers.

Great Stuff Pond & Stone

Probably the most well-known and best reviewed of the truly waterproof spray foams, Great Stuff Pond & Stone is a polyurethane-based insulating foam sealant specifically designed for water features. 

Being polyurethane, it’s a closed cell spray that boasts being able to direct the flow of water in waterfall, pond, and streambed construction.

Great Stuff Gaps & Cracks

While not as waterproof as their Pond & Stone variant, Gaps & Cracks is great for filling in things like crevices in your foundation from the inside or out. 

It can also be used to seal doors, windows, or any area where water might penetrate otherwise.

Red Devil Foam & Fill

One of the best contenders outside the Great Stuff brand, Red Devil’s Foam & Fill boasts similar advantages to Great Stuff, and works great for indoor and outdoor applications. 

Flex Seal Spray Rubber Sealant

While not technically a spray foam, Flex Seal’s Spray Rubber Sealant has built a reputation for being one of the most waterproof and durable sealant sprays on the market. 

Flex Seal won’t expand like spray foams, but it will adhere to most surfaces, and when given time to properly dry and cure can last up to 30 years or more.

Using Expanding Foam For Roof Leak Repair

If you find yourself in the unfortunate situation of a leaking roof, expanding foam can provide a quick, effective repair that will last as long as your roof does.

While not just any spray foam will necessarily work, spray polyurethane foam can be used to create a permanent seal and repair to your roof.  Typically applied 1 to 2 inches thick, a thin coating of silicone coating over the top of the foam will protect it from UV rays.

Once properly applied and allowed to set and cure, expanding foam will effectively seal roof leaks quickly and easily, and is great in emergency situations where a long-term solution is needed.

If you are wondering if Expanding Foam Stops Damp…

In some cases, yes.  Given the nature of open cell spray foams, they can be useful in insulating an area while remaining breathable, allowing moisture to escape and preventing damp from occurring.

When looking to treat areas for mold or damp, spray foam insulation offers a long-term solution while simultaneously helping to lower energy costs, as well as maintaining the structural integrity of your home.

Where Not To Use Expanding Foam

By now you may be enamored with this miraculous substance known as expanding spray foam, but there are some instances where spray foam should NOT be used. 

Such situations include:

  • Areas close to electrical boxes

If foam gets inside the box, it can jam up the parts, and the potentially flammable chemicals could be a fire hazard as well.

  • Areas close to ceiling light boxes

Spraying too close to the light boxes could insulate and trap heat, increasing the risk of a fire.

  • Closed-cavity spaces

In areas such as between studs or enclosed cavities of walls or brick exteriors, injection foam is recommended due to its slower rate of expansion.

  • If you have skin conditions or respiratory illnesses such as asthma

Many expanding foams can contain harsh chemicals, which could exacerbate preexisting conditions such as asthma, COPD, or any skin irritations.

Other instances to keep in mind include using open cell spray foam on your roof, which will deteriorate quickly the first time it rains.  Once that happens, you’ll not only be back where you started in terms of the leaking roof, but you’ll also be out the cost of a can of spray foam.


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