Open Cell vs Closed Cell Spray Foam

If you’ve started looking into upgrading your home's insulation, you’ve probably realized pretty quickly that "spray foam" isn't just one product. Contractors are throwing around terms like "open-cell" and "closed-cell," and if you don't know the difference, it’s easy to get confused—or worse, get sold the wrong product for your home.

Here at Schertz Spray Foam Insulation, we believe in keeping it simple. Both types of foam are incredible upgrades over cheap fiberglass, but they do completely different jobs. If you use the wrong foam in the wrong place in South Texas, you are wasting your money.

Here is the blunt, honest breakdown of the difference between open-cell and closed-cell spray foam, and exactly where you should use them in the San Antonio and Schertz climate.

The Short Version

Don't have time to read the science? Here is the cheat sheet:

• Open-Cell Foam: It expands massively, it's spongy, it’s cheaper, and it stops air. Use it in the attic.
• Closed-Cell Foam: It’s dense, hard as a rock, entirely waterproof, and more expensive. Use it under the house (crawl spaces) or on exterior walls.

Now, let's look at why.

Open-Cell Spray Foam: The Attic King

Open-cell foam gets its name because the tiny cells (bubbles) inside the foam are intentionally left open. This makes the foam softer and more flexible—it actually feels a bit like a dense sponge once it cures.

Key Characteristics:

• Massive Expansion: When sprayed, it expands up to 100 times its original liquid volume. This makes it incredible at filling every tiny crack, gap, and crevice in a roof deck.
• Air Sealing: It creates an absolute, perfect air barrier. Drafts cannot get through it.
• Sound Dampening: Because it is thick and spongy, it absorbs sound waves brilliantly.
• Cost-Effective: Because it yields so much volume per drum of liquid, it is cheaper per square foot than closed-cell foam.

Why It Belongs in Your Texas Attic:

In South Texas, the biggest enemy in your attic is the 140-degree air baking your AC ductwork. You need to seal that air out and bring the attic temperature down. Open-cell foam is the undisputed champion for this job.

We spray open-cell directly to the underside of your roof deck. Because it expands so aggressively, it flawlessly seals around the complicated framing of your roof. It blocks the brutal radiant heat from the sun and stops the hot air from pushing down into your living space.

Wait, what about water? Open-cell foam is an air barrier, but it is not a moisture barrier. If your roof develops a leak in a rainstorm, water will eventually seep through open-cell foam. In an attic, this is actually exactly what you want! If you used a waterproof foam on your roof and a shingle failed, the water would pool up on the wood and rot your roof deck before you ever knew you had a leak. Open-cell foam lets the water pass through, so you can see the leak and fix your roof before structural damage occurs.

Closed-Cell Spray Foam: The Crawl Space Armor

Closed-cell foam is the heavyweight fighter of the insulation world. The cells inside the foam are completely sealed shut and packed tightly together. When it cures, it is incredibly dense and hard—if you punch it, you might break your hand.

Key Characteristics:

• Waterproof: It acts as a 100% perfect vapor and moisture barrier. Water cannot penetrate it.
• High R-Value: It provides roughly double the thermal resistance (R-value) per inch compared to open-cell foam.
• Structural Strength: Because it dries rigid, it literally glues building materials together, adding significant structural rigidity to walls and floors.
• Low Expansion: It only expands about 30 times its original volume, so you need more liquid material to fill a space, making it more expensive.

Why It Belongs Under Your House (Crawl Spaces):

If you have a pier-and-beam house in Schertz or San Antonio, your crawl space is a damp, humid nightmare. Ground moisture is constantly evaporating and pushing up into your wood floor joists, causing rot, mold, and cupped hardwood floors.

You absolutely cannot put open-cell foam in a crawl space. Because open-cell absorbs water, it would soak up the ground humidity like a sponge and rot your floors faster than having no insulation at all.

Closed-cell foam is the only answer. When we spray closed-cell foam under your floorboards, it completely encapsulates the wood. It blocks the freezing winter drafts, prevents the summer heat from rising, and most importantly, it acts as a waterproof shield against the humid Texas soil. Moisture simply bounces off of it.

The Bottom Line for Texas Homeowners

There is no "better" foam—there is only the right foam for the specific job.

• Doing the Attic? You want Open-Cell Foam. It gives you the perfect air seal to stop the Texas heat, costs less, and allows a roof leak to remain visible so you can fix it.
• Doing a Pier-and-Beam Crawl Space or Metal Building? You want Closed-Cell Foam. You need that dense, rigid moisture barrier to stop humidity and rot from destroying the bottom of your house.

Don't let a contractor talk you into the wrong product just because it's what they have in their rig. If you want a straight answer and a job done right the first time, give Schertz Spray Foam Insulation a call today. We’ll show you exactly what your home needs to beat the Texas heat and humidity once and for all.