Blown-in insulation and spray foam solve the same problem from two different places in the attic. Blown-in insulation is loose fiberglass or cellulose spread across the attic floor, right on top of your ceiling. The attic above it stays vented and hot, and the insulation keeps that heat out of the rooms below. Spray foam is a liquid that expands into a solid foam. In most attic jobs it's sprayed onto the underside of the roof deck, which seals the attic and pulls it inside the home's insulated shell.
So the real spray foam vs. blown-in question isn't which material is stronger. It's where you want the insulation boundary to be. Blown-in on the floor is the more common and lower-cost approach for a typical attic. Spray foam at the roofline is a bigger change that makes the most sense in specific situations, especially when your air conditioning equipment and ductwork sit in the attic.
Neither option is the right answer for every home. The rest of this article walks through how each one works, what it costs to plan for, and the questions that usually decide it.
Quick Comparison
| Blown-in (fiberglass or cellulose) | Spray foam | |
|---|---|---|
| Where it usually goes in an attic | Attic floor, over the ceiling | Underside of the roof deck |
| What happens to the attic | Stays vented and unconditioned | Vents are sealed; attic becomes part of the conditioned space |
| Air sealing | Does not seal air leaks by itself | Acts as an air barrier at the right thickness |
| R-value per inch | Roughly R-2.2 to R-3.8 depending on material and product | About R-3.8 (open-cell) to R-7.0 (closed-cell) for the products cited below |
| Installation | Blown with a machine; no chemical curing | Two chemicals mixed on site; occupants leave during installation |
| Typical planning cost | Lower | Higher |
How Blown-In Insulation Works
Blown-in insulation is a loose material blown through a hose until it covers the attic floor to a set depth. The two common materials are fiberglass and cellulose. ENERGY STAR notes that most contractors use blown-in loose-fill on attic floors because it installs quickly with the right equipment. It also fills around wiring, framing, and other obstructions better than precut pieces do.
Because it sits on the attic floor, blown-in insulation works with a vented attic. Outside air still moves through the soffit and roof vents, and the insulation's job is to slow the heat trying to come down through the ceiling. That's why keeping the soffit vents clear matters so much with this approach. ENERGY STAR calls blocking the eaves the most common homeowner mistake and recommends rafter vents (baffles) to keep air flowing past the insulation.
One important limit: fibrous blown-in insulation slows heat, but it doesn't stop air leaking through gaps around light fixtures, wiring, plumbing, and the tops of walls. Those leaks should be sealed before new insulation goes on top of them.
If you're weighing the two blown-in materials, our pages on blown-in fiberglass insulation and blown-in cellulose insulation cover how each one is installed.
How Spray Foam Works
Spray foam is made by mixing two liquid components at the spray gun. The mixture expands, fills gaps, and cures into foam that sticks to the surface. There are two main types:
- Open-cell spray foam is lighter and softer. Johns Manville rates its open-cell product at about R-3.8 per inch and air-impermeable at 3.75 inches thick.
- Closed-cell spray foam is denser and rigid. Johns Manville rates its closed-cell products at about R-7.0 per inch and says one inch works as an air barrier and 1.5 inches as a vapor barrier. The same manufacturer also notes that closed-cell may not be appropriate in warmer climate zones, because it can deliver more R-value than code requires there.
Spray foam attic insulation at the roofline
Most spray foam attic insulation is installed along the roof, not the floor. The Building America Solution Center describes this as an unvented attic. The vents are closed, and the insulation, air barrier, and moisture control all move up to the roof deck. The attic then becomes a tempered space inside the home's insulated envelope instead of an outdoor-temperature space above it.
Building codes set a minimum R-value for the air-impermeable insulation used in these roof assemblies, and the requirement depends on your climate zone and local code. Your installer should confirm this for your home before the job is priced.
For more on how we approach foam, see our spray foam insulation service page.
Why Ductwork in the Attic Changes the Answer
In many homes the air conditioner's air handler and ducts run through the attic. With blown-in insulation on the floor, that equipment sits in the hottest part of the house.
Research from the U.S. Department of Energy's Building America program found that putting ducts in unconditioned, vented attics increases heating and cooling energy use by about 10%. The same research found that duct air leakage commonly exceeds 20% of conditioned airflow. Moving the insulation to the roofline puts the ducts inside the insulated space, which is the main reason spray foam at the roof is considered.
That doesn't mean every home with attic ducts needs spray foam. Sealing and insulating the ducts themselves is a separate option, and the right choice depends on the duct layout, their condition, and the budget. But if your ducts live in the attic, it's a question worth raising during an inspection.
Spray Foam Attic Insulation Cost vs. Blown-In
Spray foam almost always costs more than blown-in insulation for the same attic. Our insulation cost calculator uses these planning ranges, based on published installed-price data for the San Antonio market (checked September 2026). They're for planning only and aren't a quote:
| Attic option | Planning range per sq ft of attic floor |
|---|---|
| Blown-in fiberglass | $1.00 to $2.50 |
| Blown-in cellulose | $1.00 to $2.50 |
| Spray foam at the roofline | $2.00 to $4.50 |
A few things push the cost of spray foam for an attic up or down:
- Roof area, not floor area. Foam goes on the underside of the roof, which is larger than the attic floor on most roofs. A steep or complicated roof has more surface to cover.
- Foam type and thickness. Closed-cell foam costs more per inch than open-cell. Thicker layers cost more.
- Prep work. Sealing the vents, dealing with existing floor insulation, and fixing any roof leaks all add to the job.
Blown-in costs mostly depend on attic size, target depth, access, and whether air sealing or removal is needed first.
Installation and Safety
Blown-in insulation is a physical material, not a chemical reaction. The main issues during installation are dust and heat in the attic. Installers wear protective gear, and the living space is generally unaffected once the attic hatch is closed.
Spray foam needs more planning. EPA guidance says building occupants should leave during installation, and re-entry should wait until the foam has cured and the space has been ventilated and cleaned. EPA notes that some manufacturers recommend 24 hours for their professional two-component systems, but the right time depends on the product. The Consumer Product Safety Commission's consumer guidance also recommends residents stay out for at least 24 hours. Ask your contractor for the specific re-occupancy time before the job starts.
Spray foam also needs a clean, dry, sound surface. Any roof leak should be fixed before foam is applied to the deck.
Which Should You Choose?
- Standard vented attic, ducts not in the attic, budget matters. Blown-in fiberglass or cellulose on the floor is usually the practical choice. Our attic insulation service covers this approach, including the air sealing that should come first.
- Air handler and ducts in the attic. Spray foam at the roofline is worth pricing alongside a blown-in option with duct sealing.
- Very little room at the eaves or a low-slope roof. The higher R-value per inch of spray foam can help, especially closed-cell.
- A roof with active leaks or near the end of its life. Fix the roof first. Spray foam on a deck that's about to be replaced or is still leaking creates problems.
- Topping off an attic that already has some insulation. Blown-in is designed for this. Spray foam at the roof is a different system, not a top-off.
Common mistakes
- Treating spray foam as "better insulation" instead of a different design. It changes how the attic works, and that has to be planned.
- Adding floor insulation and roofline foam without a plan. The attic should be designed as either vented or unvented.
- Skipping air sealing with blown-in. Leaks under loose insulation keep leaking.
- Not asking about re-occupancy time for spray foam. Plan where the family and pets will be during installation.
If you're not sure which approach fits your attic, an inspection answers the questions this article can't: where your ducts are, what's already up there, and what condition the roof deck is in. Get an Estimate and we'll price the options that actually apply to your home.
Frequently Asked Questions
Is spray foam or blown-in insulation better for an attic?
Neither is better in every case. Blown-in on the attic floor is simpler and costs less for a typical vented attic. Spray foam at the roofline is a bigger project that makes the most sense when ducts or HVAC equipment are in the attic, or when there isn't enough room for deep blown-in insulation.
Can you put spray foam over existing attic floor insulation?
Spray foam in an attic usually goes on the roof deck, not on top of the floor insulation. When an attic is converted to an unvented design, what happens to the existing floor insulation depends on its condition and the plan for the space. Some gets left in place and some gets removed. Your installer should explain which applies and why.
Is open-cell spray foam a good fit in San Antonio?
Open-cell foam is commonly used on the underside of attic roof decks. The foam type, thickness, and any code requirements for your roof assembly should be confirmed for your specific home. Closed-cell foam's higher R-value per inch isn't always needed in a warm climate, as one manufacturer notes.
Does blown-in insulation work in walls?
Yes. Blown-in fiberglass and cellulose can be dense-packed into closed wall cavities through small holes, which is a different technique from blowing loose insulation over an attic floor.
The Takeaway
The difference between blown-in insulation and spray foam is mostly about where the insulation goes:
- Blown-in insulates the attic floor, works with a vented attic, and is usually the lower-cost way to add insulation.
- Spray foam usually goes on the roof deck and turns the attic into a sealed, conditioned space. That can make sense when the air conditioning system lives up there, but it's a bigger decision with more planning, safety steps, and cost.
Start with how your attic is built and where your ducts are, and the right choice usually becomes clear.
References
- Johns Manville: JM Corbond Open-cell Spray Foam and JM Corbond Closed-cell and Open-cell SPF. jm.com
- U.S. Department of Energy, Building America Solution Center: Unvented Attic Insulation and Unvented Conditioned Attic with Spray Foam Insulation Below Roof Deck. basc.pnnl.gov
- U.S. Department of Energy: Unvented, Conditioned Attics, Building America Top Innovation. energy.gov
- U.S. Environmental Protection Agency: Vacate and Safe Re-entry Time for Spray Polyurethane Foam Application and Quick Safety Tips for Spray Polyurethane Foam Users. epa.gov
- U.S. Consumer Product Safety Commission: Spray Polyurethane Foam Insulation: Health and Safety Recommendations for Consumers. cpsc.gov
- ENERGY STAR: Well-Insulated and Sealed Attic and A Do-It-Yourself Guide to Sealing and Insulating with ENERGY STAR. energystar.gov