The Short Answer for Tennessee Homeowners
For most Franklin, Brentwood, Nolensville, and Thompson's Station homes, we recommend closed-cell spray foam for crawl spaces, basement rim joists, and exterior walls in humid areas, and open-cell spray foam for interior walls, attic ceilings, and soundproofing where budget matters and moisture isn't a concern. Tennessee's humid subtropical climate — hot, sticky summers and freeze-thaw winters — makes moisture control just as important as thermal resistance, and the two foams behave very differently on that front.
Both products are sprayed as a liquid that expands into a rigid or semi-rigid foam, sealing gaps that traditional fiberglass and cellulose can never reach. The right choice depends on where it's being installed, your R-value target, and how much you want to invest per square foot.
Open-Cell vs Closed-Cell: Side-by-Side Comparison
Open-cell spray foam has a soft, spongy texture with tiny air-filled cells that make it lighter and less dense (about 0.5 lb/ft³). Closed-cell foam is dense and rigid (about 2 lb/ft³) with cells that are completely sealed off from one another, giving it structural strength and a built-in moisture barrier.
Here is how the two compare on the metrics that matter most in Tennessee:
• R-value per inch — Open-cell: about R-3.5 per inch. Closed-cell: about R-6.5 to R-7 per inch. Closed-cell delivers nearly double the thermal resistance in the same cavity depth.
• Moisture barrier — Open-cell is vapor-permeable and will absorb water. Closed-cell is a Class II vapor retarder at 2 inches, ideal for crawl spaces, basements, and any assembly at risk of condensation.
• Air seal — Both create an excellent air seal, dramatically reducing infiltration compared to batt insulation.
• Sound dampening — Open-cell is superior for reducing airborne noise between rooms, floors, and from HVAC equipment.
• Structural strength — Closed-cell adds meaningful racking strength to walls and roof decks; open-cell does not.
• Cost — Open-cell typically runs $0.44–$0.65 per board foot. Closed-cell runs $1.00–$1.50 per board foot. Closed-cell costs roughly 2x more but delivers 2x the R-value, so cost per R-value is comparable.
• Expansion — Open-cell expands 100x its liquid volume and fills irregular cavities beautifully. Closed-cell expands 30–40x and holds its shape.
Humidity and Freeze-Thaw Are the Deciding Factors
Middle Tennessee averages 70%+ relative humidity for much of the year, and Williamson County soil holds moisture long after storms. That environment is punishing for insulation that absorbs water. In crawl spaces, on the underside of subfloors, along basement rim joists, and on the interior of exterior walls in older Franklin homes, closed-cell spray foam is the safer, longer-lasting choice because it doubles as a vapor barrier.
For attic ceilings (the underside of the roof deck in a conditioned attic assembly), open-cell is often the better fit — it's less expensive, expands to fill every truss cavity, and its permeability actually helps because it lets a small amount of drying occur in either direction, which reduces the risk of trapping moisture against sheathing. This is a common recommendation from spray foam manufacturers for Climate Zone 4A, which covers most of Middle Tennessee.
For interior walls where the goal is sound control between a bedroom and a home theater, or between a nursery and a laundry room, open-cell wins on price and acoustic performance.
Cost Breakdown for a Typical Franklin, TN Home
A 2,000 sq ft Franklin home with a crawl space, exterior walls, and an unfinished attic typically breaks down like this when we specify each foam where it performs best:
• Crawl space walls and rim joists (closed-cell, ~2 inches): $3,500–$5,500
• Attic roof deck (open-cell, ~5.5 inches for R-19+): $4,500–$7,000
• Exterior wall retrofit (closed-cell, ~2 inches in 2x4 cavities): $4,000–$6,500
• Interior soundproofing walls (open-cell, full cavity): $1.20–$2.00 per sq ft of wall
Mixing the two foams strategically is almost always more cost-effective than defaulting to one for the whole house. The upfront investment pays back through lower utility bills — many of our Franklin and Brentwood customers see 30–50% reductions in heating and cooling costs after a whole-home spray foam project.
Get the Right Recommendation for Your Home
Every home is different. A 1920s bungalow near downtown Franklin has different needs than a new build in Nolensville or a hillside home in Brentwood. BST Spray Foam of Franklin will inspect your attic, walls, and crawl space, take moisture and thermal readings, and recommend the exact combination of open-cell and closed-cell foam that fits your home and budget.
Call 629-245-2588 or request a free estimate online. We serve Franklin, Brentwood, Nolensville, Thompson's Station, and the surrounding Middle Tennessee communities with licensed, insured spray foam installation.