Spray foam insulation is a great way to fill small holes, gaps, and cracks that rob your home of conditioned air. Approximately 30% of the heat lost from a typical home is caused by drafts that literally suck the warm air from your home in winter.

Energy prices are still rising, so many homeowners look for ways to seal these air leaks without hiring a professional. If you don’t mind getting a little dirty, applying spray foam insulation to your home can save you money now and in the future.

Here we will discuss why spray foam is used to fill holes, gaps, and cracking around the framing, masonry, and crawlspace of your home. We’ll offer a few tips to ensure a successful project and keep yourself safe using spray foam .

What Types Of Holes Can You Fill With Expanding Spray Foam?

Spray foam insulation was designed primarily to seal small penetrations in the framing and foundation of a residential home. No other insulation material seals and protects these areas better than spray foam, as it requires no fasteners, adhesive, or vapor barrier.

Generally, spray foam insulation is available in two distinct forms, which are open cell foam and closed cell foam. Open cell spray foam expands about three times its original size, while closed cell foam only expands about twice its original size.

Open cell foam is often best for small holes no more than 1” in diameter or smaller. Closed cell spray foam (also known as low-rise spray foam) is typically best for remodeling or new construction projects where the home will be insulated entirely with spray foam.

Where Can I Use Spray Foam to Fill Holes?

Spray foam is ideal for sealing holes in wood and masonry, like those found in attics and crawl spaces. Spray foam seals holes around pipes, conduit, wires, and other common penetrations around a foundation without requiring fasteners.

Spray foam will seal holes around the roof as well. Eaves, gables, and cornices tend to create small holes around the roof that allow animals and insects into the attic. Spray foam is perfect for sealing these holes, especially when wire mesh is installed over the hole as well.

Here are a few common scenarios where spray foam insulation is usually the best solution.

Scenario 1. Holes Causing Window Drafts

Drafts are noticeable when window dressings move when the window is closed. Often these drafts are caused by a gap between the window rough opening behind the window casing.

In these instances, often the easiest solution is to gently remove the casing and apply open cell spray foam between the window frame and the rough opening. Be very careful to apply as little spray foam as possible as overfilling the space could break the glass.

Scenario 2. Sealing Holes Created By a Remodeling Project

Remodeling projects are good candidates for closed cell spray foam insulation because they tend to create new holes between the old construction and the new. Pipes, wires, gas lines, and communication wiring is often shared with the original structure, creating new openings in attics and crawlspaces.

In most instances, the original home was insulated with loose fill or batts, which are not as effective as spray foam at stopping drafts caused by the new holes. Both open cell and closed cell foam are ideal for closing these new holes without risking damage to the pipe or wire.

Scenario 3. Holes in Attics and Crawlspaces

Attics and crawlspaces are perfect for insulating with either open cell or closed cell spray foam, as most drafts are caused by small holes and gaps. However, when these drafts accumulate, they cause a significant loss in energy efficiency. 

Most draft-causing holes in an attic are caused by pipe, wire, or gas line penetrations. As the wood in these areas ages, twisting and curling of the boards is common. Professionals will look for holes in these areas first and fill them as needed with spray foam to stop any drafts.

The same applies to crawlspaces, except that closed cell spray foam can be applied directly onto the exterior masonry walls. Spray foam can also be installed between the floor joists or trusses to maintain a constant floor temperature above.

Scenario 4. Unsealed Holes In New Construction

Even in new homes, small gaps and holes can appear as the home settles into its foundation. Normally, these holes change in size due to the relative humidity and temperature of the ambient air, so caulk tends to detach quickly. Spray foam offers more flexibility as the wood expands and contracts, preventing new holes from forming.

The lumber used to build new homes may warp and twist during construction, causing small holes to develop. Because spray foam retains a small amount of flexibility, builders often use it instead of caulk to seal small holes in the framing. Spray foam usually lasts at least twice as long as caulk, so the holes remain sealed for much longer without additional maintenance.

What Is the Difference In R Value Between Closed Cell and Open Cell Spray Foam Insulation?

Open cell spray foam provides about R-3 when installed to a thickness of one inch. Closed cell spray foam insulation provides about R-7 per inch, making closed cell foam ideal for 2” x 4” walls, between floor trusses, and on the surface of masonry walls. 

When Should I NOT Use Spray Foam to Fill Holes?

Professionals will use spray foam to fill holes in attics, crawlspaces, and even outdoors. However, spray foam is not appropriate around any source of open flame or high heat. Spray foam also requires some form of protection for durability, so it should be covered.

Here are a few scenarios where other types of insulation like fiberglass batts and mineral wool are a better solution than spray foam.

Scenario 1. Furnaces and Ductwork

You can use spray foam insulation to fill holes around furnaces, air handlers, and compressor units, but stay at least 24” away from pilot lights and furnace doors. Although spray foam has a high ignition point, it will burn.

Use mineral wool around these areas instead, as mineral wool is essentially fireproof. Mineral wool insulation is made from the slag produced when stone is melted, so mineral wool is perfect for insulating around gas furnaces, gas water heaters, and other sources of open flame.

Scenario 2. Grease and Water

Neither open cell nor closed cell spray foam will adhere to moisture or oil. Spray foam requires dry, solid contact with the surface to achieve an effective bond. Garages, factories, and restaurants should not be insulated with spray foam anywhere grease or water will collect on the surface to be insulated.

Scenario 3. Over Moisture or Rot

When open and closed cell spray foam adheres to a surface it will prevent any surface damage from receiving ventilation. Don’t use spray foam to cover up damage or wood rot because it traps moisture, leading to even faster rot.

Always make the appropriate repairs before applying any insulation including spray foam. Skipping this step usually leads to repeating the work because the original problem was never resolved. Spray foam will stick to wood framing, but if the framing rots away the spray foam will go with it.

Scenario 4. Attic Vents and Baffles

Use caution when using spray foam insulation around attic vents and baffles, if the HVAC system uses them. These devices physically move to perform their function and spray foam in the wrong place can either block the passage of air, prevent the movement, or both.

Professionals often use mineral wool or fiberglass batts around attic vents because they retain their shape and stay in place. Spray foam applied too close to a baffle door can over-expand and prevent the baffle from moving. Spray foam applied near a soffit vent might over-expand as it cures and prevent the vent from operating.

How Big Of A Hole Can You Fill With Spray Foam?

¼” to 1” Holes

Generally, if the hole is less than ¼” closed cell spray foam will work best because it will not significantly overfill the cavity. If the hole is larger than ¼” and smaller than 1”, open cell spray foam is often the best choice when price, ease of use, and effectiveness are considered.

1” to 2” Holes

You can fill spaces up to 2” wide with some spray foams, but be sure and check the label. Some open cell spray foams boast of 3x expansion, which helps fill larger gaps and holes. However, when holes are larger than 2” wide, a more permanent solution is usually required.

Example Scenario: 

Let’s say a hole was cut in a firewall to accommodate an 8” duct, but a 6” duct was actually installed. You can use open cell foam as a temporary repair to block the hole, but eventually you will need a stronger repair.

Ductwork moves as the air enters and leaves the duct due to the varying air pressure. Over time, spray foam will fatigue from the constant movement and likely detach. In these instances, sheet metal (often aluminum) is installed to close the space and spray foam is applied to the joint for sealing.


Justin's been in construction for over 20 years in both new build and renovation. With experience in both commercial and residential construction, he specializes in healthier and more energy-efficient homes.

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