Open Cell vs Closed Cell Spray Foam for a DFW Attic

The Answer: Open Cell for the Vented Floor, Closed Cell for the Roof Deck

Open cell for a vented attic floor. Closed cell for a roof deck or unvented space. That's the short version, and it holds for nearly every DFW home. The deciding factor isn't R-value or price — it's how the attic handles moisture. If the question "open cell vs closed cell spray foam for attic" is what brought you here, that one rule answers it before you read another paragraph.

Let me give you the raw numbers first. Open cell runs R-3.5 to R-4.0 per inch. Closed cell runs R-6.0 to R-6.5 per inch. So closed cell gets you the same R-value in roughly half the thickness. That's why it's the tool for thin, hard-to-reach spaces like rim joists and roof decks.

The Cost Difference, in Dollars

Open cell runs $1.50 to $2.20 per square foot installed. Closed cell runs $2.50 to $4.00. On a 1,500-square-foot attic, that's about $2,300 to $3,300 for open cell and $3,500 to $6,000 for closed cell, both to R-38. A five-thousand-dollar job against a three-thousand-dollar one is worth pausing over.

The Moisture Lesson That Changes Everything

Here's the part I want you to remember. Closed cell is a vapor barrier. It does not let moisture pass. In our humid DFW climate, that's a double-edged sword. On a roof deck, it protects the framing. But spray closed cell on a vented attic floor and you trap the moisture that rises from the living space. That's how mold blooms behind the foam.

Open cell breathes. It lets vapor move through, which is exactly what a vented attic needs. That's not a small detail in a climate where attics hit 140 degrees and the humidity is brutal.

When Open Cell Is the Right Call

Open cell wins for a standard vented attic floor. It air-seals, it insulates, and it lets the attic breathe, all for about half the price of closed cell. If you're just cutting cooling bills and the attic isn't living space, open cell to R-38 is the smart buy.

When Closed Cell Is the Right Call

Closed cell wins where you need maximum R-value in a thin space or you need a vapor and air barrier. That's a roof deck on an attic conversion, a cathedral ceiling, a rim joist, or a crawl space. Closed-cell spray foam is the workhorse for those. It also adds a little structural rigidity, which engineers like.

Why You Don't Always Need Spray Foam at All

This is where I'm honest even when it costs me a job. For many DFW attics, blown-in fiberglass to R-38 does the same job for about half the cost. Spray foam's real advantage is the built-in air seal. Air sealing plus blown-in often matches that performance for less. Spray foam isn't automatically better — it's different, and the difference only pays when you need it.

The Choice in One Paragraph

Ask yourself one question: is this attic becoming living space, or is it vented storage? Living space means closed cell on the roof deck. Vented storage means open cell on the floor — or honestly, blown-in fiberglass and a good air-sealing pass. That one question answers the whole thing without a spec sheet.

The Cost Reality You Won't Find on a Spec Sheet

Spec sheets give you R-value per inch. They don't give you the install reality. Closed cell at $2.50 to $4.00 per square foot sounds close to open cell's $1.50 to $2.20 until you're talking about a thousand square feet, where the gap becomes a real number. When I quote a job, I price both options from the same attic measurement so the homeowner sees the actual delta instead of a marketing table. More often than not, that honest number is what settles the argument.

What the DOE Says About R-Value Targets

The Department of Energy and ENERGY STAR both publish the attic insulation target for climate zone 3 — DFW's zone: R-38 minimum, R-49 recommended. Neither source cares whether you meet that with open cell, closed cell, or fiberglass. They set the destination; the material is the debate. Keeping that in mind stops the foam-type argument from hijacking the real question, which is whether you've hit the target for the conditioned space you're actually building.

Frequently Asked Questions

Call me, Neal, at (469) 895-2695 and I'll tell you which foam — or whether you need foam at all — for your DFW attic. The straight answer isn't always the one that sells the most foam, but it's the one that gets you the right result.