Spray Foam vs. Blown Fiberglass in North Carolina Attics and Crawlspaces
Insulation choice determines how well your home resists nine months of Carolina humidity and summer heat. The two most common options in custom home construction are blown fiberglass (loose fill) and spray polyurethane foam.
They perform fundamentally different jobs: blown insulation resists heat flow; spray foam both resists heat flow and blocks air movement. Understanding that distinction drives the right choice for each part of the house.
Blown Fiberglass in Vented Attics: The Proven Default
For a traditionally vented attic with insulation flat on the ceiling deck, blown fiberglass (R-49 to R-60 under the NC Energy Conservation Code) is cost-effective, does not trap moisture, and allows roof sheathing to dry toward the attic side.
Its weakness is air leakage: recessed lights, wiring chases, and attic hatches leak conditioned air into the attic unless each penetration is individually air-sealed with foam or caulk before blowing.
Closed-Cell Spray Foam: Sealing the Roofline
Spraying closed-cell foam (R-6.5 to R-7 per inch) directly against roof sheathing converts the attic into conditioned semi-conditioned space, keeping ductwork inside the thermal envelope—a major efficiency win in Carolina summers.
Foam's air-sealing performance is unmatched. But it changes the building's moisture physics: the roof deck can no longer dry to the interior, so the attic must be managed as a sealed assembly, and the NC Residential Code requires an ignition or thermal barrier over foam in accessible attics.
Quality depends entirely on installer workmanship; off-ratio or thin foam passes a casual glance and fails a blower-door test.
Crawlspace Walls: Where Foam Earns Its Cost
In sealed conditioned crawlspaces, rigid foam board or closed-cell spray foam on foundation walls outperforms fiberglass batts, which sag, absorb moisture, and host mold when they touch humid block walls.
Foam on crawlspace walls also permits the code-required 3-inch termite inspection strip at the sill in ways loose fill cannot.
The Hybrid Strategy AGCS Recommends
For most Piedmont custom homes: blown fiberglass at the flat attic ceiling with meticulous air-sealing, plus closed-cell foam or rigid board on crawlspace walls and rim joists. You get proven drying behavior at the roof, airtight foundations, and the best value per R-value dollar.
Related service: Build on Your Lot · More in Build on Your Lot Learning Center
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