Open-Cell Spray Foam in Chattanooga

What open-cell foam is

Open-cell spray foam is a light, soft foam in which the cells are not fully closed. It expands a lot when sprayed, filling irregular spaces. HomeGuide lists open-cell at about R-3.5 to R-3.7 per inch and about $1.50 to $3.50 per square foot installed.

Source: HomeGuide, Spray Foam Insulation Cost (December 2025).

Strengths

• Lower cost per square foot.
• Air barrier at sufficient thickness.
• Fills irregular framing like rafters, trusses, and odd angles.
• Sound dampening for interior walls and floors.
• Vapor-open, which can let assemblies dry.

Limits

• Lower R per inch, so it needs more depth.
• Absorbs water if soaked, so it is not used below grade or where it will get wet.
• Vapor-open, which in some assemblies may need a vapor retarder.

Where open-cell is common around Chattanooga

Roof decks

Open-cell under the roof deck brings the attic into conditioned space. In Chattanooga, where many homes have ducts and air handlers in the attic, this is a common choice. See attic spray foam.

Interior walls and floors

For sound control between bedrooms, home offices, and media rooms.

Deep cavities

2x6 walls and deep rafters where there is room to reach the needed R-value.

Moisture behavior

Because open-cell lets vapor pass, a roof leak may show up as a wet spot on the foam, which helps detection. In humid climates, designers sometimes specify a vapor retarder coating on open-cell under roof decks to limit moisture reaching the roof sheathing in winter. Ask your installer how they handle it.

Fire protection

Open-cell is foam plastic and generally needs a thermal barrier between it and living space, with specific rules for attics. See ICC, International Residential Code, Chapter 3 (Section R316, Foam Plastic).

Safety

Stay out during spraying and until curing and ventilation are complete. See U.S. EPA Safer Choice, Vacate and Safe Re-Entry Time for Spray Polyurethane Foam Application.

Questions to ask

• What thickness and R-value?
• Will there be a vapor retarder coating?
• How is the foam protected for fire safety?
• How long should we stay out?

Next steps

Compare with closed-cell foam, read open vs closed, and see costs. We serve Ooltewah, Collegedale, Apison, and nearby. See our services.

A worked example

A two-story home in Ooltewah has ducts and an air handler in the attic, and the upstairs bedrooms are hot in July. Open-cell foam goes under the roof deck and on the gable walls, soffit vents are sealed, and the attic becomes part of the conditioned space. The upstairs stays closer to the thermostat setting, and the HVAC system runs less on hot afternoons.

Thickness on a roof deck

To reach a target R-value with open-cell at about R-3.5 to R-3.7 per inch, a roof deck may need several inches of foam. Rafters and trusses must be deep enough, or the foam will extend past them. Ask your installer what thickness they plan and why.

Open-cell and roof leaks

Because open-cell is vapor-open and absorbs water, a roof leak may show up as a wet or discolored patch on the foam. Check your attic after heavy storms. Repair leaks and dry or replace wet foam.

Open-cell and sound

Open-cell's soft structure helps absorb airborne sound in interior walls between bedrooms, offices, and media rooms. It is not a full acoustic solution, but it helps.

Open-cell and pests

Foam does not provide food for pests, but rodents can tunnel through soft foam. Seal entry points before foam goes in.

When open-cell is not the right choice

• Below grade or in wet areas.
• In thin cavities where higher R per inch is needed.
• Where a vapor retarder is required and none is planned.

Comparing quotes

Ask each installer for the planned thickness, R-value, product, whether a vapor retarder coating is included, how the attic will be protected for fire safety, and how long you need to stay out of the house.

Summary

Open-cell foam is often the most cost-effective way to air seal and insulate a roof deck or deep cavities. It should stay away from wet areas and below-grade walls, and the assembly should have a plan for moisture.

After the job

Check the attic after the first heavy storm for wet patches, and note how upstairs rooms feel on the next hot afternoon.

More open-cell questions

What R-value is open-cell spray foam?

HomeGuide lists about R-3.5 to R-3.7 per inch.

How much does open-cell spray foam cost?

HomeGuide lists about $1.50 to $3.50 per square foot installed.

Is open-cell foam good for attics?

It is common on roof decks to bring the attic into conditioned space, especially where ducts are in the attic.

Does open-cell foam absorb water?

It can if soaked, so it is not used below grade or where it will get wet.

Does open-cell foam reduce noise?

Yes. It dampens airborne sound in interior walls and floors.

How thick should open-cell be on a roof deck?

It depends on the target R-value. At about R-3.5 to R-3.7 per inch, several inches may be needed. Ask your installer what they plan and why.

Can rodents get through open-cell foam?

Rodents can tunnel through soft foam. Seal entry points before foam goes in.

When is open-cell not the right choice?

Below grade or in wet areas, in thin cavities needing high R per inch, or where a vapor retarder is required and none is planned.

What should an open-cell quote include?

Planned thickness, R-value, product, any vapor retarder coating, fire protection, and re-occupancy time.

What should I check after an open-cell job?

Look in the attic after the first heavy storm for wet patches, and notice upstairs comfort on hot afternoons.