Closed Cell vs Open Cell Spray Foam in Chattanooga

Side-by-side

Source for R-values and prices: HomeGuide, Spray Foam Insulation Cost (December 2025). Densities are typical industry figures; check the product data sheet.

Why the climate matters here

Hamilton County sits in IECC climate zone 3A, warm and humid. Chattanooga summers bring long stretches of hot, humid air, and the river valley holds moisture. Winters are mild but cold enough for condensation on cold surfaces. That means:

• Air sealing matters as much as R-value, because leaks carry humid air.
• Assemblies need a plan for where moisture goes.
• Crawls and rim joists, at the edge of the ground, see the most moisture.

Where open-cell tends to fit

• Roof decks. Bringing the attic inside the conditioned space with open-cell under the roof deck is common in the South. It covers rafters and odd angles affordably.
• Interior walls and floors. For sound control between rooms.
• Large cavities. Where there is depth to reach the needed R-value.

Open-cell lets moisture pass, so a roof leak can show up inside sooner, which can be a benefit for detection. It also means designers sometimes add a vapor retarder in certain assemblies. Ask your installer how the assembly handles moisture.

Where closed-cell tends to fit

• Rim joists. High R per inch in a thin cavity, plus air and moisture resistance. See rim joist spray foam.
• Crawl space walls. In a sealed crawl, after water is controlled. See crawl space spray foam.
• Thin walls. 2x4 walls where open-cell cannot reach the target R-value.
• Metal buildings. Where condensation on cold metal is a problem. See garage and shop foam.

Code and safety

Both types are foam plastic. Building codes generally require a thermal barrier, such as half-inch drywall, between foam and living space, with specific provisions for attics and crawls. See ICC, International Residential Code, Chapter 3 (Section R316, Foam Plastic). Occupants should stay out during spraying and until the foam cures and the area is ventilated. See U.S. EPA Safer Choice, Vacate and Safe Re-Entry Time for Spray Polyurethane Foam Application.

Hybrids

Many jobs combine both: closed-cell at the rim and crawl walls, open-cell on the roof deck. Or a thin flash of closed-cell with batts or blown insulation over it.

Questions to ask

• Which type for each area, and why?
• What thickness and R-value?
• How is moisture handled in each assembly?
• What fire protection covers the foam?
• How long should we stay out of the house?

Local examples

• Signal Mountain home with a complex roof: open-cell under the roof deck brings ducts into conditioned space.
• Red Bank ranch with a vented crawl: closed-cell at the rim joists, plus a vapor barrier on the ground.
• East Brainerd bonus room: closed-cell in thin kneewalls and open-cell on the sloped ceiling.

Next steps

Read about open-cell, closed-cell, and costs, or see our services. We serve Red Bank, Signal Mountain, and nearby.

Myths worth clearing up

• "Closed-cell is always better." It is better in some places. On a roof deck with plenty of depth, open-cell may do the job for less.
• "Open-cell causes roof rot." Roof sheathing problems under foam usually trace back to leaks, poor design, or high indoor humidity. Ask how the assembly handles moisture.
• "Foam is a vapor barrier." Only closed-cell at sufficient thickness acts as a vapor retarder.

Summary

Choose by location: closed-cell where water and thin cavities matter, open-cell where depth is available and cost matters.

More open vs closed cell questions

What is the difference between open-cell and closed-cell spray foam?

Open-cell is soft, vapor-open, and about R-3.5 to R-3.7 per inch. Closed-cell is rigid, resists water, and is about R-5 to R-7 per inch, according to HomeGuide.

Which is cheaper?

Open-cell. HomeGuide lists about $1.50 to $3.50 per square foot installed, compared with about $3.00 to $5.00 for closed-cell.

Which foam is better for a crawl space?

Closed-cell is more common on crawl walls and rim joists because it resists water and has higher R per inch, after water is controlled.

Can open-cell be used on a roof deck?

Yes. It is common for bringing the attic into conditioned space. Ask how the assembly handles moisture.

What climate zone is Chattanooga?

Hamilton County is in IECC climate zone 3A, warm and humid.

Is closed-cell foam always better?

No. It is better where moisture resistance or high R per inch matters. On roof decks with depth, open-cell may do the job for less.