You’re sitting on the couch, wrapped in a blanket, and you can literally feel the draft crawling across the floor. Your thermostat says 72 degrees, but your toes are telling a different story. It’s frustrating. You know the culprit is likely those hollow, cold exterior walls, but the thought of tearing down drywall to fix it feels like an absolute nightmare. This is exactly why foam insulation for existing walls has become the "holy grail" for homeowners living in older builds.
It sounds perfect on paper. Someone comes in, drills a few small holes, pumps in some magic goo, and suddenly your house is a thermos. But honestly? It’s more complicated than the brochures make it out to be. There are real risks, specific chemistry involved, and a whole lot of nuance regarding whether your house can even handle it.
If you’ve got a home built before the 1980s, you’re likely dealing with empty cavities or, worse, settled fiberglass that’s doing basically nothing. Adding foam can fix that, but you have to know which type of foam you’re actually getting. It isn't all the same stuff.
The Massive Difference Between Spray Foam and Injection Foam
People use these terms interchangeably. They shouldn't. If a contractor tells you they’re going to "spray foam" your finished walls, they’re either lying or about to cause a disaster.
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Standard spray foam—the stuff you see in new construction or open attics—is usually polyurethane. It expands rapidly. If you pump high-expansion spray foam into a closed wall cavity, it will find the path of least resistance. Usually, that’s your drywall. I’ve seen photos of living rooms where the walls literally bowed out and cracked because the pressure was too high. It’s a mess.
For foam insulation for existing walls, you’re almost always looking for injection foam.
This is a totally different beast. It has the consistency of shaving cream when it’s first pumped in. Brands like RetroFoam or USA Insulation’s Premium Foam are common examples. This stuff is designed to flow around wires, pipes, and old "handfuls" of fiberglass without creating massive pressure. It fills the gaps and then slowly hardens. It doesn't expand with the violent force of polyurethane.
Why R-Value Isn't the Only Number That Matters
Most people obsess over R-value. They want the highest number possible. While injection foam usually lands somewhere between $R-3.5$ and $R-5.0$ per inch—which is great—the real magic is the air seal.
Traditional insulation like fiberglass or cellulose is like wearing a wool sweater on a windy day. It keeps you warm, but the wind blows right through it. Injection foam is more like a windbreaker. It stops the air movement. According to the Department of Energy, air leakage accounts for a massive chunk of energy wasted in typical U.S. homes. By filling that 3.5-inch cavity in a standard 2x4 wall, you aren't just adding "stuff"; you’re stopping the house from breathing in the wrong places.
The Installation Process: Holes, Plugs, and Dust
It’s not invisible. Let’s be real about that.
If you have siding—vinyl, aluminum, or wood—the installers usually pop a piece off, drill a hole through the sheathing, and inject the foam from the outside. Once they're done, they plug the hole and put the siding back. You’ll never know they were there.
But if you have brick? That’s where it gets interesting.
The installers have to drill through the mortar joints. Usually, it's a 5/8-inch or 1-inch hole. They’ll mix up some new mortar to fill it when they're finished, but matching 40-year-old weathered mortar is an art form, not a science. You might see those "dots" for a while until they weather in.
And if you’re doing it from the inside?
- They drill through the drywall.
- It gets dusty. Very dusty.
- You’re left with a bunch of circles that need to be mudded, sanded, and repainted.
Most people choose the exterior route for a reason.
Let’s Talk About the "Fishy" Smell and Off-Gassing
This is the elephant in the room. You’ve probably read a horror story on a forum somewhere about a house that smelled like chemicals for six months after a foam job.
Is it a real risk? Yes. Is it common? No.
Injection foam is typically a tripolymer or phenolic-based resin. It’s water-based. When mixed correctly at the truck, it’s safe. But if the technician messes up the mix—if the "part A" and "part B" are out of whack or the temperature is wrong—it won't cure properly. That’s when you get odors.
The most common complaint is a slight "shaving cream" or chemical smell that lasts for 24 to 48 hours. That’s normal. If it smells like a dead fish or sour socks after a week, the chemistry was wrong. This is why you don't hire the guy with the cheapest bid who just bought his machine off eBay. You’re paying for the technician’s ability to calibrate that machine on the fly.
Can It Cause Rot? The Moisture Debate
There is a subset of building science experts who worry about "moisture sandwiches." The idea is that if you fill a wall with foam, you’re changing how the wall handles water.
In an old, drafty house, moisture gets in, but the wind blowing through the walls dries it out. When you add foam insulation for existing walls, you stop that airflow. If you have a major leak in your siding or around a window, that water might get trapped against the wood studs.
However, most modern injection foams are "breathable" in the sense that they allow vapor to pass through, even if they stop liquid water and bulk air. Joe Lstiburek of Building Science Corporation has written extensively on this. The consensus is generally that as long as your "bulk water" (rain) is kept out by good siding and flashing, the foam isn't going to rot your house down. But you absolutely must fix window leaks before you insulate.
The Cost: Is It Actually Worth It?
This isn't a $500 weekend DIY project. You’re likely looking at $2,500 to $6,000 for an average-sized home, depending on your geography and the complexity of the job.
You’ll see claims of "saving 50% on your energy bill." Honestly? Take that with a grain of salt. While some people see massive drops, a 15% to 25% reduction is a more realistic expectation for most.
The real value—the thing you can't always put a price on—is the comfort. It’s the ability to sit near a window in January without feeling like you’re in a walk-in freezer. It’s the fact that your AC doesn't have to run 22 hours a day in July just to keep the upstairs habitable.
Knocking Down the Myths
I hear this one a lot: "I already have fiberglass in my walls, so I can't do foam."
Actually, you can.
When they inject the foam, it doesn't just sit on top of the fiberglass. It’s under pressure, so it compresses the old batts against the back of the cavity and fills the voids around them. It's not as "clean" as an empty wall, but it’s still significantly better than leaving the sagging fiberglass to do its own thing.
Another one: "The foam will mess up my electrical wiring."
Unless your house has "knob and tube" wiring—those old ceramic posts and cloth-covered wires from the early 1900s—you’re fine. Modern Romex wiring is rated to be buried in insulation. But if you do have knob and tube, do not—I repeat, do not—inject foam. Those wires need air space to dissipate heat. Burying them in foam is a legitimate fire hazard. A reputable contractor will check your outlets with a circuit tester or look in your attic before they start.
What to Check Before You Sign a Contract
Don't just take the salesperson's word for it. They're there to sell a job. You need to be the expert for your own home.
First, ask for the Technical Data Sheet (TDS) of the specific foam they use. You want to see the "flame spread" and "smoke developed" ratings. Most injection foams are Class A fire-rated, which is what you want.
Second, check their "plug" policy. If they are drilling through your siding or mortar, exactly how are they filling those holes? Ask for photos of previous jobs. If the mortar matches look like a checkerboard, you might want to reconsider.
Third, ensure they use an infrared camera. A pro will walk around your house with a thermal imager after the job is done. This shows the "holdup" areas—spots where a stud or a fire block prevented the foam from filling a pocket. If they find a cold spot, they should drill a new hole and fill it right then and there. If they don't use thermal imaging, they're just guessing.
Final Steps for Homeowners
If you're tired of the drafts, start with these specific moves.
- Audit your walls: Use a simple incense stick or a damp hand on a windy day. Run it along your outlets and baseboards. If the smoke dances or your hand feels a chill, you’ve got air infiltration.
- Check for Knob and Tube: Pop off a few outlet covers (turn off the breaker first!). Look at the wires. If you see black cloth or wires disappearing into porcelain tubes, call an electrician before an insulator.
- Fix the leaks first: If you have peeling paint or soft wood around your windows, your "building envelope" is failing. Foam won't fix a rot problem; it might actually hide it until it's much worse.
- Get three quotes: This industry has huge price swings. Some companies are franchises with massive overhead; others are local outfits. Make sure you’re comparing the same type of foam (injection resin vs. pourable foam).
- Ask about rebates: Check the Database of State Incentives for Renewables & Efficiency (DSIRE). Many utility companies offer massive rebates for wall insulation because it lightens the load on the power grid. You might get $1,000 back just for making your house more efficient.
The goal isn't just to save a few bucks on the gas bill. It's to make your home feel like a sanctuary again, rather than a place where you're constantly fighting the elements. Foam insulation for existing walls is arguably the single most effective way to "retrofit" an old house into the modern age without a sledgehammer. Just make sure you're hiring someone who treats the chemistry with the respect it deserves.