Expanding Foam Tips: Dos and Don’ts for Filling Gaps Like a Pro

By Sherry Holmes

Mike’s Advice / DIY Projects

Monday, May 26th, 2025 @ 11:24am

How to Use Expanding Foam Correctly: Expert Dos and Don’ts

When installing new doors and windows, low-expansion foam is the way to go. The average house can lose up to 30 % of its heat because of drafts. So by weather sealing and insulating around your windows, you can help control indoor temperature and make your home more energy efficient. This means lower energy bills. Expanding foam can also be used to fill gaps, cracks, spaces around pipe penetrations.

What is Expanding Foam Used For? Common Uses

Whether you’re a seasoned DIYer or just tackling a weekend project, taking the time to apply foam the right way will help protect your home from drafts, moisture, and unwanted intruders. Expanding foam has a few functions:

Sealing Around Windows and Doors

Prevents drafts and water leaks without warping or bowing the frames.
Low expansion foam expands gently, avoiding pressure on delicate window/door jambs.

Filling Small Gaps and Cracks

Ideal for sealing small holes or gaps in walls and around plumbing.

Improving Energy Efficiency

Low-expansion foam helps insulate and block air leakage around doors and windows, reducing heating and cooling costs.

Soundproofing

Acts as a sound barrier when used inside wall cavities or around fixtures.

Pest Prevention

Blocks small openings that insects and rodents could use to enter a home. The Sika Pest Block contains EPA-registered pesticide that protects foam from attack by mice and other pests.

Around Pipe Penetrations

Seals gaps around ducts, pipes.

Here are some do’s and don’ts when using low-expansion foam.

How to Use Expanding Foam Correctly | The Dos

DO wear PPE (personal protection equipment). Safety glasses and gloves are a must!
DO check the expiration date of the foam before you start spraying it. The foam performs best when it’s fresh, so if you have a can lying around, make sure you check the expiration date before using it.
DO follow the instructions on the packaging exactly!

DO gather the required tools ahead of time. The product we use (Sika Boom) comes with a convenient dispensing straw, or you can use your own dispenser gun.
DO clean the substrate before installing the foam. It should be free from oils, grease, dust and loose or friable particles. Paint, cement oils and other poorly adhering contaminants must be removed.
DO pre-dampen the substrate with clean water. This ensures that the foam cures properly.
DO check the right temperature to use your expanding foam. Sika Boom is an all-season product.
DO pick a product that when cured, the foam may be painted or coated for outdoor applications.

Best Foam for Windows and Doors: Sika Boom has been our trusted low-expansion foam for installing new doors and windows. Dom Trimbo, our window and door installer, prefers Sika Boom as well! Easily available at your local hardware stores. It’s also Environmentally Friendly: CFC and HCFC-free

Avoid These Expanding Foam Mistakes | The Don’ts

DON’T keep your packaging near heat, hot surfaces, sparks, open flames and other ignition sources
DON’T use expanding foam around outlets and recessed can lights.
DON’T overapply! Read the instructions properly. When using Sika Boom, we fill about 30% of the cavity depending on the application, and the expansion takes care of the rest. Sometimes two layers are required, but you must let the first layer cure.
DON’T apply the foam in extreme temperatures (too hot or too cold). Sika Boom is an all-season expanding foam.
DON’T apply expanding foam around electrical boxes, or in areas where it could interfere with ventilation

Tips for Using Expanding Foam Correctly

Instructions for Using Sika Boom low-expansion foam:

Shake Sika Boom® AS can well for about 60 seconds before use. Repeat shaking after long interruptions of use.
Turn the Sika Boom® AS can upside down and screw the straw adapter firmly into place without pressing the valve.
Regulate the foam flow by applying more or less pressure on the valve/adapter.
Sika Boom® AS can only be used in the upside position, failure to invert the can will prevent the foam from dispensing properly.
Fill deep cavities in several layers. Take care to allow each layer to cure and expand sufficiently by spraying water between each layer or allowing sufficient waiting time between the layers.
Fill the cavity approximately 30% to allow for expansion. All building elements must be temporarily fixed until the foam has fully cured.

Can I Fill A Hole With Expanding Foam?

Sika Boom Big Gap Filler can be used for interior and exterior applications. The low expansion foam puffs up to seal cracks, stop drafts, and deter bugs and rodents. It can be a good product for homeowners and DIYers as well.

*Disclaimer: I only recommend products I would use myself and all opinions expressed here are our own. This post may contain affiliate links.