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Can You Use Expanding Foam Under Door Threshold? Here’s How

Maintaining your home can be a daunting task.

One aspect of home maintenance is the upkeep of your door thresholds.

Over time, door thresholds deteriorate and their gaps increase.

Expanding foam is a quick and effective way to fill the gap, but does it work for filling door thresholds?

Premade expanding foam is designed for filling cracks and holes in walls, floors and ceilings.

The cans are filled with a thick liquid that expands to several times its original volume.

The thick liquid is then sprayed into the cracks or holes in large quantities, filling them entirely and making a sturdy patch.

When the expanding liquid dries, it leaves behind a hard, white foam that is about as strong as the material around it and can be painted or stained to match.

Why expanding foam is ideal for sealing gaps in a door threshold

Older door thresholds often develop gaps that allow water to get inside the house.

Expanding foam fills the gap in its entirety, which means that water will have no nook or cranny to get a foothold.

Any gaps should be filled with expanding foam before installing new door thresholds.

Once you’ve sealed up the gaps in the door threshold, it is important to make sure that moisture isn’t able to work its way into the gap and cause mold or mildew to grow.

The best way to keep moisture out is by keeping the door threshold in good shape.

Expanding foam is a great option for filling gaps, but as your door threshold ages it will likely need more than just expanding foam to keep water out of your house.

Uneven surfaces higher than ½ inch (1 centimeter) will need a sealant as well.

One thing to keep in mind is that not all expanding foam is the same: The thickness of the finished product varies from brand to brand and even between batches from the same company.

Not only does the thickness affect how big an area you can fill with one can, the thickness will also affect how long it takes for the expanding foam to dry.

Shopping for expanding foam on a per-can basis is not advised because of these discrepancies in quality and thickness.

Instead look for a kit that has all of the tools you need (hoses, nozzles, sealants).

How to use expanding foam to seal gaps in a door threshold

The first thing you will need to do is prep the area where you’re going to apply the expanding foam.

You should trim away any mold or mildew, then clean the surface with soap and water.

It’s important that this step is done thoroughly because no additional sealant will adhere to mold or mildew.

Next, you’ll need to make sure that the area is completely dry.

Finally, decide where you want to apply the expanding foam and prepare your equipment accordingly.

The can of expanding foam should be shaken for about 30 seconds before use so that the liquid inside is evenly mixed.

You can use either an upright or gravity gun setup for applying expanding foam.  If you are going to use a gravity gun, set up your equipment near the edge of where you plan on spraying so that you don’t waste any foam.

Before you begin spraying, make sure that the nozzle is deep into the can so that no air bubbles are introduced into your foam.

There should be a 3 second delay between shaking the can and applying the expanding foam. This allows for a small amount of time for all of the liquid to settle. It is important to not spray before this time delay has elapsed.

The first thing you should be aware of when applying expanding foam is the desired thickness of the finished product.  

For sealing door thresholds, 1/4 inch is ideal, but you will need to adjust your nozzle and gun height accordingly.

Do you have to prime the door threshold before applying expanding foam?

Kits for applying expanding foam include a specialized primer that will seal the door threshold, making it water resistant.

The primer works by penetrating the small cracks in the door frame and filling them up.  

This is important because if moisture can seep into those spaces it will have a place to hide from any foam you apply.

After spraying on a coat of primer, wait 15 minutes, then apply a second coat.  The goal here is to fill all of the nooks and crannies.

Once you have allowed for both coats of primer to dry, spray on one or two layers of expanding foam sealant.

It’s important that you don’t get any bubbles in your foam.  If you do, there are a number of things that you can do to get them out:

  • Try to vibrate the nozzle while applying the expanding foam. This will stir up any air bubbles and allow them to come to the surface so they can be popped
  • You can try to tap the nozzle on a solid object so that the force will pop any bubbles
  • If you are having trouble getting rid of a bubble, spray on another layer.  If you successfully get rid of it in this second coat, the problem is solved.  If not, repeat the process again with even more than two layers *Remember: Adding additional layers does not increase the strength of the initial sealant layer.

How long does it take to apply expanding foam?

Wet foam will take longer to dry than a foam that is allowed to dry naturally.  This is because wet foam can trap air in its matrix.

Drying times can vary greatly depending on the weather, humidity levels, shakiness of your hands, and other factors (e.g., how long you spray for).

A good general rule of thumb is to allow for at least 30 minutes of dry time before walking on the surface.

After that, you can go ahead and follow up with a layer or two of polyurethane or polyacrylate finish.

These sprays will improve the durability and strength of your sealant substantially.

You should consider following up with a layer of finish after about 24 hours.  

This will help to add extra water resistance to your door threshold, as well as reduce the chance that it could scratch off in the future.

What color should I apply?

The number 1 question that people have when applying expanding foam is usually what color they should use.  

The answer to this is rather simple: You should match the color of your foam to whatever surface you are applying it to.

For example, if you are applying the foam to a brick wall, it is best to use a brick colored foam.

The same goes for wood or concrete surfaces like floors and walls.  White or beige foams should only be used indoors in an area that doesn’t get much sunlight (and even then I’d avoid using them).

Darker colors tend to have better thermal qualities, and that (along with other factors) is why they are used so often for outdoor projects.

How long will it last?

As long as you use a high quality spray foam sealant, your door threshold should be good for at least decades before having any problems.

The only thing to worry about is if the foam has been applied poorly and is starting to deteriorate.  

The best way to avoid this is by following a kit’s instructions (which can be found on the box or online).

Here are what causes expanding foam to fail, so that you can try to avoid them:

  1. Foam that has been applied to areas where it is exposed to UV radiation can break down faster. This includes locations where it’s used in the shade, but also surfaces directly exposed to sunlight (e.g., on your roof).  
  1. Areas where water hits the door threshold often.  This can cause premature breakdown of your foam’s adhesive properties.
  1. Things with a high PH level (i.e., acids and other chemicals).  Acids will destroy foam immediately, while milder chemicals can cause it to start deteriorating over time.
  1. Temperature extremes.  These can deform the foam and cause it to fail catastrophically.

How do I make my sealant stronger?

Spray on another layer of foam.  

This will increase the durability of your sealant, and help you achieve a greater degree of water resistance (which is especially important when working with natural stone materials like brick).

If you are working with a spray foam that is meant to be used for wood, I’d recommend wet sanding the surface before applying another layer of sealant to smooth out any rough patches from the first coat.

What if I accidentally get it on something?

If you have gotten expanding foam on an object that you do not want it to be on, you will need to remove it.

This can be done by using a paper towel and some mineral spirits.  

Clean the area thoroughly, and make sure that there is no residue left behind.

You will have to use mineral spirits every time you clean off an expanding foam mistake in order for it to completely disappear.  

Other cleaning solutions (like soaps and detergents) tend to leave behind a residue that will prevent the foam from bonding properly.  

If you have any doubt about whether or not your cleaning solution is okay for using with expanding foam, just stick with pure mineral spirits.

Other alternative products as good as expanding foam for filling door thresholds

Older products like spackling paste and putty are not recommended for filling door thresholds, because they do not have good UV resistance or durability.

For filling door thresholds, you generally want to go with a spray foam sealant that will last and not turn white or crumble in high heat.

This is especially true for any exterior surfaces that are directly exposed to the sun’s radiation (which happens a lot more often than you may think).

There are lots of other ways to fill door thresholds with other products, such as:

  1. Grout – this is a good method, but I would recommend making sure that the surface of your door threshold has been cleaned beforehand to avoid any messes or accidents.
  1. Cork – this is a popular alternative to foam sealants, although it does not provide much of a thermal buffer from the elements.
  1. Wood – some people choose to use wood as an alternative, but it is not absorbed into the surface of your door threshold like foam sealants are.
  1. Rubber – you may want to consider rubber if you need a sealant with high UV resistance.

Final thoughts

Walls and door thresholds are important parts of a building’s structure. When renovating or remodeling, it is best to check with your contractor first to see if there are supporting beams that the expanding foam could interfere with.

Not only will it keep your house standing strong, but it will also give you peace of mind for years to come.

Foam sealant can help keep moisture out, so if you’re in need of a solution to a drafty doorway, you may want to consider it.

If you’ve noticed that your doorway is letting in cool air during the winter or hot air during the summer months, foam sealant might just be what you need to keep your home warm all year long.

Sealant is available at most hardware stores, and it’s a relatively quick and cost-effective solution to your drafty doorway.

Laying the foam in place can be both messy and time-consuming, but once it dries, you’ll have a nice, moisture-tight seal.

Hiring a contractor to install foam sealant at the bottom of your door threshold is another option.

Maintaining your home may not be a top priority when you’re juggling family, work and other responsibilities, but it’s an investment in your future that you’ll want to be sure you don’t neglect.

So if you’ve been wondering if foam sealant is something worth considering, we’d say that it’s definitely worth a try.