Spray foam and blown-in insulation are not the same tool for the same job. Spray foam is an applied liquid that expands into a rigid or semi-rigid mass, air-seals as it cures, and delivers the highest R-value per inch of any common attic insulation, but it costs the most and must be installed by a professional. Blown-in (loose-fill) insulation is fiberglass or cellulose blown in place, is far cheaper, can be a DIY job on an open attic floor, but it does not air-seal on its own and can settle or lose R-value over time. If your priority is the strongest thermal-and-air barrier in the least thickness and budget is flexible, spray foam generally wins. If your priority is covering a large attic floor for the lowest cost, blown-in usually wins.
Most comparisons pit these two as if one is simply “better.” In practice they behave differently, cost very differently, and are often chosen for different parts of a home. Below is what each material actually is, a side-by-side comparison of cost, R-value, lifespan, and installation, and a decision framework that starts with the problem you are trying to solve.
What spray foam insulation is (and is not)
Spray polyurethane foam (SPF) is applied as a liquid that expands on contact and cures into a solid. It comes in two forms. Open-cell foam is lighter and softer, with an R-value commonly cited around R-3.5 to R-3.8 per inch, and it is vapor-permeable. Closed-cell foam is denser and rigid, with an R-value often cited near R-6 to R-7 per inch, and it also acts as an air barrier and, at sufficient thickness, a vapor retarder. Because the foam expands into gaps and cracks, it seals air leaks as part of the same step that adds R-value, which loose-fill materials do not do on their own.
What spray foam does not do is install itself cheaply or by hand. It requires professional equipment, protective gear, and a cure period during which the space should be unoccupied, and an improperly mixed or applied batch can cure poorly. It is also the most expensive option per square foot. Homeowners weighing it for the attic often start with our overview of spray foam attic insulation, which covers open-cell versus closed-cell and where each belongs.
What blown-in insulation is (and is not)
Blown-in, or loose-fill, insulation is a material blown into place with a machine so it fills the cavity as loose fibers or particles. The two common materials are fiberglass, with an R-value generally cited around R-2.2 to R-2.7 per inch, and cellulose, made largely from recycled paper, generally cited near R-3.2 to R-3.8 per inch. It is the standard choice for covering an open attic floor because a blower can spread a large area quickly and fill around joists and irregular framing. For material specifics, see our guide to blown-in insulation, and if you are choosing between the two loose-fill materials specifically, our cellulose vs fiberglass attic insulation comparison goes deeper on that decision.
What blown-in does not do is stop air movement by itself. Loose fibers slow conductive heat transfer but air can still pass through them, so a blown-in attic usually needs separate air-sealing at penetrations, top plates, and the attic hatch to perform well. Loose-fill can also settle over time, which reduces installed depth and R-value, and if it gets wet it can compress or, in the case of cellulose, take longer to dry. To size a job, R-value targets are worth checking against an insulation R-value chart.
Spray foam vs blown-in insulation: side-by-side comparison
The two systems differ on nearly every axis, from price to whether they seal air. The table below summarizes the trade-offs. Cost and R-value figures reflect widely published 2026 industry pricing and manufacturer ranges; actual numbers vary by material type, thickness, attic size, accessibility, and region, so treat them as planning estimates rather than quotes.
| Factor | Spray foam (SPF) | Blown-in (loose-fill) |
|---|---|---|
| Material | Open-cell or closed-cell polyurethane, applied as expanding liquid | Loose fiberglass or cellulose fibers, machine-blown |
| R-value per inch | About R-3.5 to R-3.8 (open-cell); about R-6 to R-7 (closed-cell) | About R-2.2 to R-2.7 (fiberglass); about R-3.2 to R-3.8 (cellulose) |
| Air sealing | Yes, seals air leaks as it expands; closed-cell also resists vapor | No, slows conductive heat only; needs separate air-sealing |
| Typical installed cost | Higher, commonly cited around $2 to $5+ per sq ft (closed-cell at the top of the range) | Lower, commonly cited around $1 to $2 per sq ft |
| Installation | Professional only; specialized equipment and cure time | Professional, or DIY on an open attic floor with a rented blower |
| Lifespan / durability | Long-lived; does not settle; often cited at the life of the structure | Can settle and lose depth over time; may need topping up; degrades if wet |
| Moisture behavior | Closed-cell resists water; can trap moisture if misapplied | Fiberglass does not absorb much water but loses R-value wet; cellulose holds moisture longer |
| Best for | Air-sealing, tight spaces, roof-deck / conditioned attics, high R in thin build-up | Covering large open attic floors at the lowest cost |
Which should you choose?
Start with the job, not the label. The right pick depends on where the insulation goes, how much air leakage you are fighting, and your budget. Work through these questions in order.
- Is air leakage a main problem? If you have drafts, uneven temperatures, or a leaky attic plane, spray foam addresses insulation and air-sealing in one step. Blown-in can match the thermal target but generally needs a separate air-sealing pass to perform as intended.
- Are you insulating a large open attic floor? For simply piling R-value onto an accessible attic floor, blown-in is usually the most cost-effective route and covers area fast. This is the most common case where loose-fill wins on value.
- Do you need high R-value in limited depth? Where thickness is constrained, such as a rafter bay or a roof deck you want to insulate directly, closed-cell spray foam delivers more R per inch than loose-fill can, so it can hit the target where blown-in would not fit.
- Are you creating a conditioned or unvented attic? Bringing the attic inside the thermal envelope by insulating at the roofline typically points to spray foam. Loose-fill on the attic floor keeps the attic vented and outside the envelope.
- What is the budget, now versus later? Blown-in has the lower upfront cost and can be DIY on an open floor. Spray foam costs more upfront but does not settle and bundles in air-sealing, which can matter over the life of the home.
- Is moisture or an irregular cavity a factor? In damp-prone or hard-to-reach assemblies, closed-cell foam’s moisture resistance can be an advantage; for open, dry attic floors with lots of framing to fill, blown-in flows around obstructions easily.
These are general guidelines, not a code determination. Local climate zone, ventilation strategy, and fire and vapor-retarder requirements can change the right answer, so confirm the assembly with a qualified installer or building official before committing. For the broader menu of options, our roundup of the best insulation for an attic puts these two in context with batts and rigid board.
Can you use spray foam and blown-in insulation together?
Yes, and a hybrid is common. One frequent approach uses a thin layer of closed-cell spray foam to air-seal and add a moisture-resistant base, then blown-in loose-fill on top to reach the target R-value at a lower cost than all-foam. This “flash-and-fill” or “flash-and-batt” style pairs foam’s air-sealing with loose-fill’s cheaper R-value. When combining materials, follow manufacturer and code guidance on vapor retarders and ignition barriers, and keep the two layers compatible for the assembly and climate zone. Neither material replaces proper attic ventilation where the attic is designed to be vented, or a deliberate air-sealing plan where it is not. For homeowners still mapping the whole attic decision, start with our general guide to attic insulation.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.