Termite foam is an insecticide mixed with a foaming agent to create a shaving-cream-like consistency that can be injected into wall voids, foundation gaps, and wood galleries where liquid treatments can’t easily reach. It’s designed for targeting active infestations in concealed spaces, expanding to coat surfaces that would otherwise go untreated. Most products use one of two active ingredients: fipronil or imidacloprid, both of which kill termites through nervous system disruption.
How Termite Foam Works
The foam starts as a liquid termiticide diluted in water with a foaming agent. When injected through a small hole into a wall void or termite gallery, it expands to fill the space, coating interior surfaces with a thin layer of insecticide. The foam itself breaks down quickly, but it leaves behind an active chemical residue on every surface it touched. That residue is what actually kills the termites.
The two most common active ingredients work differently. Fipronil (found in Termidor Foam) blocks chloride channels in the termite’s central nervous system, causing overstimulation and death. Imidacloprid (found in Premise Foam) targets receptors in the nervous system that handle chemical signaling between nerve cells. Both are effective, but fipronil has a notable advantage: termites that contact it can transfer the chemical to nestmates through normal grooming and social contact, spreading the poison deeper into the colony than the foam itself ever reaches.
Expansion Ratios and Why They Matter
Not all termite foam expands the same way. The expansion ratio, which measures how much foam you get from a given amount of liquid, determines where and how the product spreads. There are three categories:
- Wet foams expand at ratios below 3:1 (three gallons of foam from one gallon of liquid). These penetrate downward well and spread somewhat to the sides, making them useful for treating soil or filling cavities below the injection point.
- Medium foams expand at 6:1 to 10:1 ratios, offering a balance of penetration and spread.
- Dry foams expand at 25:1 to 50:1 ratios. These are “stackable,” meaning they can push horizontally and even upward through voids. They’re ideal for treating wall cavities and overhead spaces where a liquid would simply run to the floor.
Choosing the right expansion ratio depends on the geometry of the space you’re treating. A wall void that extends vertically calls for a dry foam that can rise and spread. A gap along a foundation where you need the chemical to soak into soil benefits from a wet foam with more liquid content.
When Foam Is the Right Choice
Termite foam fills a specific niche: treating active infestations in spaces you can’t easily access with liquid. Wall voids, hollow block foundations, behind baseboards, inside door frames, and ceiling cavities are all scenarios where foam excels. Liquid termiticide poured into these spaces would pool at the bottom and miss most surfaces. Foam expands to coat the entire interior.
For subterranean termites, foam is typically used as a supplement to a full perimeter liquid barrier or bait system, not a replacement. It handles the spots where termites have already entered the structure while the barrier or bait system addresses the colony’s access routes through the soil. For drywood termites, which live entirely inside wood rather than traveling from the ground, foam can work as a standalone spot treatment. The challenge with drywood species is finding the galleries. You need to locate the small “kickout holes” where termites push out their pellet-shaped droppings, then inject foam directly into those openings. If the foam doesn’t reach the galleries where termites are actively living and feeding, it won’t work.
Foam vs. Liquid Barriers vs. Bait Stations
Liquid soil treatments create a chemical barrier in the ground around your home’s foundation. Termites that cross through treated soil pick up the insecticide and die, often spreading it to nestmates first. This is the standard approach for subterranean termite prevention and works well for broad perimeter protection. The limitation is that liquid can’t effectively reach inside walls or above-ground voids where termites may already be active.
Bait stations take a different approach entirely. Installed in the ground around the structure, they contain cellulose laced with a slow-acting toxin. Foraging termites find the bait, carry it back to the colony, and share it. Colony elimination can take weeks to months, but when it works, it eliminates the source rather than just defending the perimeter.
Foam fits between these two strategies. It’s a targeted, localized treatment for active infestations inside the structure. Many of the same insecticides used in liquid barriers can also be formulated as foam, so the chemistry is similar. The difference is delivery: foam goes where liquid physically cannot. A comprehensive treatment plan for a serious infestation often combines all three, using a liquid barrier in the soil, bait stations to attack the colony, and foam to knock out termites already inside walls.
How Long Foam Lasts
Termite foam residue remains active for up to six weeks after application. That sounds short compared to liquid soil treatments, which can last years, but the purpose is different. Foam is meant to eliminate an active infestation, not prevent future ones. Six weeks is enough time for termites in the treated area to contact the residue and, in the case of transferable chemicals like fipronil, spread it through the colony. Once the infestation is cleared, long-term prevention falls to perimeter barriers or bait systems.
DIY Products and Cost
Ready-to-use termite foam cans are available to homeowners and generally run between $20 and $60. FUSE Foam, one of the most affordable options, costs around $22. Premise Foam (imidacloprid) runs about $30. Termidor Foam (fipronil), widely considered the professional standard because of its transfer effect, costs roughly $40 per can. These aerosol cans come with injection tips designed to fit into small drill holes or existing gaps.
Professional foam treatments cost significantly more because they include inspection, identification of termite species and entry points, strategic placement, and often combination with other treatment methods. For DIY users, the critical factor is accurate placement. Injecting foam into a random wall void accomplishes nothing. You need to confirm active termite presence, ideally with visible mud tubes, frass pellets, or damaged wood, and inject directly into the galleries or voids where termites are traveling.
Application Tips
Drilling small holes (typically 1/4 to 1/2 inch) into the wall or wood surface at the site of activity gives you access to inject the foam. Space holes every 6 to 12 inches along the suspected gallery path. Insert the applicator tip and fill the void until foam begins to seep back out, which tells you the space is fully coated. Seal the holes afterward with wood putty or caulk.
For drywood termites in exposed wood, like attic framing, you can inject directly into kickout holes without drilling. The goal is always the same: get the foam into the galleries where termites are present. Surface application on the outside of wood does very little because the termites are inside, and the foam won’t penetrate solid wood on its own. Permeating the internal galleries is the only way to make contact with the colony members doing the damage.