Where spray foam pays its way in a Lowcountry house and where blown insulation does the job with less fuss: open-cell or closed-cell, foam under the roof deck, rim joists, the state's termite rule and the safety guidance.
Spray foam does two jobs in one pass. It insulates, and because it bonds to the framing and expands into cracks, it also blocks air. In a climate where August humidity averages 79% and the air conditioner runs from spring well into fall, that second job is a large part of what you pay for.
Foam is also hard to undo, and it can cover up a roof leak or a termite trail. So the first question is not open-cell or closed-cell but whether foam belongs in your house at all. On an open attic floor, blown insulation over a sealed ceiling usually reaches the target R-value (see attic insulation). Foam earns its place where there is no room for depth, no attic floor to work on, or ductwork that should be brought inside. The free assessment sorts out which, and the photographs stay with you either way.
Professional spray foam arrives as two liquids mixed at the gun, and the Consumer Product Safety Commission notes that one side contains isocyanates. Once it cures, the two common kinds behave very differently. EPA's Safer Choice program summarized the high-pressure foams this way:
| Open-cell | Closed-cell | |
|---|---|---|
| Density | About half a pound | About 2 pounds (3 for roofing grades) |
| Rough R per inch | About 3.5 | About 6.5 |
| As it sprays | Expands aggressively | Expands less |
| Once cured | Soft | Rigid |
Run those per-inch figures against the state's R-30 ceiling and open-cell needs about 8.5 inches while closed-cell needs about 4.6. That gap decides a lot of jobs: in a shallow rafter bay or a 2x4 wall, closed-cell may be the only foam that reaches the number. Treat EPA's figures as rough; each product's data sheet gives its own.
Moisture is the other difference. PNNL's Building America guide calls open-cell foam "vapor open": water vapor passes through it, so it cannot act as the vapor control layer by itself, and where a code calls for one, a coating or retarder has to be added.
The strongest case for spray foam in Charleston is an attic that holds the air handler and the ductwork. PNNL's Building America guidance does not recommend ducts in a vented attic, and its guide to unvented attics lays out the alternative: foam sprayed on the underside of the roof deck, so the attic joins the conditioned house and the equipment sits inside the thermal envelope, where its heat losses shrink.
The residential code's unvented-attic section, as PNNL's guide sums it up, sets the basic conditions:
The foam now does the work the attic floor insulation used to do, so its depth is planned against the same R-value target. Any gas appliance left in the attic should be a sealed-combustion, direct-vent model, as PNNL's guide for existing roofs advises. PNNL's model house for climate zone 3A with an unvented attic also adds dehumidification to the attic space, so ask how yours will be kept dry.
Once foam is bonded to the underside of the deck, a slow roof leak is much harder to spot from inside the attic. PNNL's guide for foaming existing roofs starts with the roof itself: look for damage, water stains and active leaks, fix them and repair the damage before spraying, and think about replacement if the roof is near the end of its service life. The decking should be dry and free of dust and debris when the foam goes on.
The same guide gives a number worth knowing: roof sheathing can be damaged when its wood moisture content stays above 18%. It also explains why the code keeps plastic sheeting off the ceiling side: a Class I vapor retarder there would stop the deck from drying inward after a leak.
A few more things get checked before any foam goes up. Wiring that will be buried matters, because the electrical code does not permit concealed knob-and-tube where foamed-in-place insulation encloses the conductors. Bath fans have to reach the outdoors. Anything on the attic floor that looks like vermiculite stays undisturbed; EPA says to hire an asbestos professional before any job that could stir it up. And if a new roof is planned in the next few years, it usually makes sense to do the roof first.
Where foam adds little: an open attic floor with room for depth, or a wall cavity that is already full.
The City of Charleston and Folly Beach both rate termite damage as "very heavy" in their code design tables. For areas with that rating, South Carolina's Building Codes Council wrote its own version of the residential code's foam plastic protection section, S.C. Code of Regulations 8-1216. In plain terms:
The two exceptions are buildings whose walls, floors, ceilings and roofs are framed entirely in noncombustible materials or pressure-treated wood, and the inside face of basement walls. For a spray-foam job this mostly touches rim joists and crawl space walls: the band at the sill is left bare on purpose. A termite bond is worth a call to the pest control company before any foam goes near the foundation. The crawl space page covers the rest of the work under the floor.
Foam is a plastic, and the building code treats it as one. PNNL's guide puts the requirement in two parts: give the foam the fire resistance the code requires, and check the current state and local codes for when a thermal barrier or an ignition barrier has to cover it. It also notes that not every spray foam needs a barrier everywhere; that depends on the product's tested properties, which is why its documentation belongs in your job file.
Permits follow the same desk-by-desk pattern as other insulation work. Charleston County's sub-trade permit form names spray foam outright, next to fiberglass. In the City of Charleston, a house in flood zone AE or VE needs a permit for every job, interior work included. Folly Beach requires permits for interior as well as exterior work on existing homes, and its remodel energy form asks for a REScheck report. Permitting is on us, and the written scope lists the foam type, the thickness and the R-value it reaches, line by line.
Spray foam reacts and cures in place, and the chemistry does not stop the moment the spraying does. The Consumer Product Safety Commission's fact sheet for homeowners gathers the federal and industry guidance into a short list:
If odors or breathing problems linger after the job, CPSC's advice is to get medical attention first, then work with the contractor and the manufacturer, and file a report on SaferProducts.gov.
Foam is priced by volume, so the main drivers are the area, the thickness and which foam: closed-cell reaches a given R-value in roughly half the depth of open-cell. Preparation adds to it: covering floors and finishes, closing attic vents, dealing with old insulation that has to come out, and any coating or barrier the code calls for. The written scope breaks those out so you can compare quotes line for line.
On the program side, Dominion's limited-time Check-up Plus offer pays toward attic insulation, air sealing and duct sealing for qualifying electric-heat homes, but only through Dominion's own approved contractor, so it does not run through us and the program chooses the method. Berkeley Electric's HomeAdvantage on-bill loan finances insulation and air sealing once a co-op energy audit recommends the work, and the co-op says improvements generally need to pay back in under about six years. The federal 25C credit does not apply to work placed in service in 2026.
Both are used at roof decks. Open-cell needs about twice the thickness for the same R-value by EPA's rough figures, and it lets water vapor through, so it cannot be the vapor control layer on its own. Closed-cell reaches the target in less depth. Rafter depth, the roof's condition and the budget usually decide.
No. Foam is not a termite treatment, and it can hide the wood an inspector needs to see. That is why South Carolina keeps foam off below-grade foundation faces in very heavy termite areas and requires an open inspection band of at least 6 inches at the sill in crawl spaces.
CPSC's guidance for homeowners is to leave during the installation and for at least 24 hours afterward, pets included, and to settle the exact re-entry time with the contractor beforehand, because it varies with the product, the house and who lives there.
It can make one harder to find. PNNL's guide for foaming existing roofs says to inspect the roof and fix leaks and damage before spraying, and to consider replacing a roof that is near the end of its life first.
Not always, but humidity needs a plan. PNNL's model house for climate zone 3A with an unvented attic includes dehumidification in the attic space. Whether yours needs its own unit depends on the air conditioning, the ductwork and how tight the rest of the house is.
Often, yes. Charleston County lists spray foam on its sub-trade permit form, and in the City of Charleston every job needs a permit. Some towns publish nothing either way, so the desk gets asked before the work starts. Permitting is on us.
Spray foam insulation across greater Charleston, from the peninsula and the sea islands out to Summerville, Goose Creek and Moncks Corner. Each city has its own page with the permit rule and code edition that apply there:
Sources: EPA Safer Choice (archived): spray polyurethane foam product types, density and approximate R per inch · CPSC: Spray Polyurethane Foam Insulation, health and safety recommendations for consumers (24 hours out) · PNNL Building America Solution Center: unvented conditioned attic, spray foam below the roof deck (R806.5 summary, open-cell vapor open, barriers) · PNNL Building America Solution Center: below-deck spray foam on existing roofs (inspect roof, 18% moisture, direct-vent appliances) · PNNL Building America Solution Center: ducts and air handlers in vented attics not recommended · PNNL climate zone 3A unvented attic assembly (attic dehumidification) · PNNL climate zone 3A sprayed-foam roof and floor over piers (protection board) · S.C. Code of Regulations 8-1216, IRC R318.4 foam plastic protection (termite rule) · City of Charleston code design criteria (termite damage: very heavy) · SC State Climatology Office: August relative humidity, Charleston · Charleston County sub-trade permit form (insulation: fiberglass, spray foam) · City of Charleston substantial improvement rules: permit for all work in flood zones AE and VE · City of Folly Beach building and planning (permits for interior and exterior work) · Dominion Energy SC Home Energy Check-up Plus program overview · Berkeley Electric Cooperative HomeAdvantage program summary · IRS: Energy Efficient Home Improvement Credit (25C) · NEC 394.12(5) knob-and-tube text, as reproduced by the City of Berkeley. Codes, programs and prices change; confirm the current rule with your city before you rely on it.
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