Can You Fill A Hollow Core Door With Foam?

If you want to turn a hollow door into a solid one, improve its insulation, or soundproof it, you might be wondering if you could fill it with foam.

It is possible, but carrying it out successfully isn’t a simple job and will require you to be get it right the first time. Plus, it does have some risks that could damage your door and need you to purchase a new one anyway. 

Before attempting to fill your door with foam, please make sure you are fully aware of what is required. Keep reading to find out all you need to know.

Spray Foam

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How can you fill a hollow door with foam?

You can start by making small holes on the door’s edges. The foam should be applied starting from the lowest point on the door while allowing it to expand and solidify.

To minimize any problems, you should ideally take the door off of its hinges by removing the hinge pins and laying it on a flat surface.

Then, you can place a sheet of plywood on top of the door along with around 100 pounds of weight. Leave it for at least two days to allow the foam to set. This sandwiching process ensures that the foam does not bulge the doors.

What to check before filling the door with foam

It is important to keep in mind that hollow doors are mostly filled with cardboard in a honeycomb-like arrangement. So, it is likely that the foam won’t go all the way into the door. It would be a good idea to drill a hole into the top edge of the door as a test. You can look into the hole with a flashlight to check how hollow it is. 

If the door really is hollow, you can go ahead. You might want to use a foam that expands less aggressively for this purpose. Also, make sure to keep some newspaper or cardboard around your area of work as foam can leak and become very messy and difficult to remove. 

Read more to find out what is a hollow core door.

Are there any problems with filling a hollow door with foam?

Filling a hollow door with foam is not always considered a good idea for several reasons. Many people would suggest replacing a door altogether rather than filling it with foam.

Firstly, a hollow door is not always actually hollow. As mentioned earlier, it does have an internal structure, usually a cardboard support between the panels. This structure looks like a large honeycomb and it can prevent the foam from spreading evenly throughout. There are also chances that the foam simply won’t be able to fill the core. 

If you have a luan plywood door, it will be impossible to fill it with spray foam. As the veneer of the material is not very thick, the door won’t be able to take the force of the expanding spray foam.

You need to understand the project properly and have a proper understanding of how it works before undertaking a project like this.

Remember, the spray foam will need a lot of atmospheric air to properly react and expand in the door, which is tricky when you are trying to fill something like a door.

In most cases, it’s most definitely easier, to order a new door or just get insulated doors.

What is spray foam?

You should also understand how spray polyurethane foam works. This is actually a heat-activated polymer that can work as a foam insulation. Essentially, it is a spray-applied plastic made by mixing two ingredients; isocyanate and polyol resin. 

The foam is sprayed with the help of tube to insulate a surface or seal any gaps or cracks. As the two ingredients react, the liquid turns foamy and expands, and then hardens up.

Why is spray foam so popular?

Spray foam is widely used as an insulation method as it can take care of air leakages very well. Once you spray it into place, it will fill up cavities or small holes effectively. It can also be applied to vertical and horizontal surfaces as an air barrier. 

Plus, it is a synthetic material and does not attract insects or rodents. With proper application, spray foam can be a good way of improving energy efficiency too.

Should you use high expanding foam for doors?

Expanding foam is commonly used for sealing cracks or gaps around doors and windows. However, highly expansive foam can push the door jamb inward. This can make it impossible to open the door.

For this reason, it is better to use a foam that expands minimally. Make sure to only use a foam that is specifically labeled for using on doors and windows.

Wipe away the foam mess immediately

One downside of using foam is that it can create a horrible mess. In case the foam goes somewhere you didn’t want, you should know what to do.

Wiping the foam immediately can spread the goo and make it harder to clean up. While it’s still wet, you can clean it using a solvent. Alternatively, you can let the foam harden and then scrape it off. You can use a utility knife to cut off excess foam.

Do you have a door that’s not closing properly? Find out if you can trim a hollow core door.