If you're trying to figure out the difference between closed and open cell spray foam for your next project, you probably just want to know which one stops drafts better without blowing your entire renovation budget. It can feel like a bit of a chemistry lesson once you start digging into technical specs, but the choice usually comes down to where you live, what part of the house you're insulating, and how much you're willing to spend.
At its simplest, both types are made of polyurethane. They both start as a liquid and expand into a foam that seals up those annoying gaps where air leaks out. But that's pretty much where the similarities end. Let's break down what actually sets them apart so you don't end up with the wrong stuff in your walls.
What Are We Actually Looking At?
Think of open cell foam like a very dense sponge. When it's sprayed, the tiny cells within the foam don't completely close. They stay "open," which allows the foam to be incredibly flexible and soft. If you poke it with your finger, it has some give to it. Because those cells aren't sealed shut, the foam expands a massive amount—sometimes up to 100 times its liquid size. This makes it a beast at filling every little nook and cranny in a standard wall cavity.
Closed cell foam is a completely different animal. The cells are packed tight and filled with a gas that helps the foam expand, but once it sets, those cells are totally sealed. This makes the foam much denser and much harder. If you try to poke closed cell foam, it'll feel like a piece of plastic or a hard surfboard. It doesn't expand nearly as much as open cell, but it's far more "solid" once it cures.
The R-Value: Which One Actually Insulates Better?
In the world of insulation, "R-value" is just a fancy way of saying how well a material resists heat flow. The higher the number, the better it is at keeping your heat inside during the winter and the AC inside during the summer.
This is a major difference between closed and open cell spray foam. Closed cell foam is the heavyweight champion here. It typically has an R-value of around 6.5 to 7 per inch. Open cell foam usually sits somewhere between 3.5 and 3.8 per inch.
What does that mean in the real world? It means if you have a narrow wall cavity—like a 2x4 stud wall—you can get a much higher total R-value using closed cell because you're packing more "insulating power" into a thinner space. If you used open cell in that same 2x4 wall, you'd have a lower total R-value, though it would still likely outperform traditional fiberglass batts because it creates an airtight seal.
Moisture and the Dreaded Mold Issue
This is probably the most important part of the conversation. If you're insulating a basement, a crawlspace, or anything below grade, you have to care about moisture.
Closed cell foam is basically waterproof. Because the cells are sealed tight, water can't get through it. In fact, it's often used as a vapor barrier. If you live in an area prone to flooding or high humidity, closed cell is usually the way to go because it won't soak up water like a sponge.
Open cell foam, however, can hold water. Since the cells are open, moisture can move through it. This isn't always a bad thing—for instance, if you have a roof leak, open cell foam will let the water through so you can actually see the leak and fix it. If you had closed cell on the underside of your roof, the water might stay trapped against the wood for a long time before you ever notice a problem, which could lead to rot. That said, you generally don't want open cell in a damp basement where it can absorb ground moisture and get funky.
Soundproofing and Quiet Homes
If your main goal is to stop hearing your teenager's drum set from the next room, the difference between closed and open cell spray foam swings in favor of the open cell.
Because open cell is softer and less dense, it's actually much better at absorbing sound waves. Closed cell is so rigid that sound can sometimes travel right through it or even bounce off it. If you're insulating interior walls—like between a bathroom and a living room—open cell is a fantastic choice for keeping things quiet.
Structural Strength: A Hidden Bonus
Here's something people often overlook: closed cell spray foam is incredibly strong. Because it turns into a hard, plastic-like substance, it actually adds structural integrity to your walls.
In some cases, it can increase the "rack strength" of a wall by up to 300%. If you're building a pole barn, a metal building, or a house in a high-wind zone (like a hurricane-prone coastal area), that extra glue-like strength can be a huge benefit. Open cell, being soft and squishy, doesn't add any structural strength to the building at all.
Let's Talk About the Cost
We can't ignore the price tag. Generally speaking, closed cell foam is more expensive. It requires more raw material to cover the same amount of space because it's so much denser.
Open cell is the more budget-friendly option. Since it expands so much, you use less liquid product to fill the same wall cavity. If you have a massive attic to do and you're on a tight budget, open cell might be the only way to make the numbers work. However, you have to weigh that lower upfront cost against the fact that you're getting a lower R-value per inch.
Where Should You Use Which?
To make it easier, here is a quick "cheat sheet" on where these materials usually end up:
Use Open Cell if: * You're insulating interior walls for sound dampening. * You have deep wall cavities (like 2x6 or 2x8) where you can fit enough foam to get a good R-value. * You're on a tighter budget. * You're doing an attic and want to be able to spot roof leaks easily.
Use Closed Cell if: * You're insulating a basement or crawlspace where moisture is a concern. * You have limited space (shallow walls) and need the highest R-value possible. * You're insulating a metal building or a van/RV. * You want to add structural strength to the building. * You live in a flood-prone area.
The Installation Experience
Regardless of which one you choose, the installation process is pretty similar, but the "mess factor" varies. Both involve a professional (usually) wearing a hazmat-looking suit and using a spray gun.
Open cell expands so fast that it often grows way past the wall studs. The installers then have to go through with a big saw or a "scrubber" to shave the excess foam off so the drywall can sit flat. This creates a lot of scrap material. Closed cell is a bit more controlled; because it doesn't expand as much, installers can usually spray it more precisely, often leaving a bit of a gap between the foam and the edge of the stud.
Making the Final Call
At the end of the day, the difference between closed and open cell spray foam really comes down to your specific environment. If you're in a freezing cold climate with 2x4 walls, you'll probably want the punch of closed cell. If you're in a temperate climate and just want to stop the drafts in a standard attic, open cell will do the trick and save you some cash.
Don't feel like you have to pick just one for the whole house, either. Plenty of people use closed cell in the basement and rim joists where moisture is an issue, and then switch to open cell for the upstairs walls and attic to keep costs down. It's all about using the right tool for the job.
Whatever you decide, moving away from old-school fiberglass to any type of spray foam is usually a massive upgrade for your home's comfort. You'll notice the lack of drafts almost immediately, and your HVAC system will certainly thank you.