The short answer
Blown-in fiberglass or cellulose is usually the cheaper way to insulate an accessible vented attic floor, after the ceiling is air-sealed; HomeGuide puts blown-in at about $1.00 to $2.80 per square foot installed. Spray foam costs more but can air-seal and insulate in one layer, and it is the usual choice for an unvented attic at the roof deck. The two often go in different places, so compare finished assemblies, not unit prices.
If your attic already has a sound roof, functional ventilation and an accessible ceiling plane, air sealing plus blown fiberglass or cellulose is usually the first option worth pricing. If your project calls for a correctly designed unvented attic at the roof deck, a complex air-sealing detail or insulation in limited depth, spray polyurethane foam may justify its higher cost.
The mistake: Comparing an attic-floor blown-in quote with a roof-deck spray-foam quote as if they were interchangeable. They insulate different surfaces, often cover different areas and create different attic conditions.
US pricing and technical guidance reviewed October 19, 2026. Actual insulation requirements depend on climate zone, the locally adopted code and the completed building assembly.
Spray Foam vs Blown-In Insulation at a Glance
| Comparison | Spray foam | Blown-in insulation |
|---|---|---|
| Main products | Open-cell and closed-cell polyurethane foam | Loose-fill fiberglass and cellulose |
| Installation | Reactive foam sprayed onto compatible surfaces | Loose fibres distributed using blowing equipment |
| Typical attic location | Attic floor or approved roofline assembly | Vented attic floor |
| Air sealing | Can create an air barrier at tested thickness and with continuous detailing | Not an air barrier by itself |
| Material R-value per inch | Open-cell ~R-3.6 to R-4.5[1]; closed-cell ~R-5.8 to R-6.9[1] | Depends on product, installed density and settled thickness |
| Initial cost | Usually higher | Usually lower for accessible attic floors |
| Moisture implications | Foam type and roof/wall assembly determine drying behavior | Requires separate air/moisture control and a dry installation |
| Retrofit flexibility | Adhered foam is difficult to remove | Loose-fill is often easier to supplement, move or remove |
| Main benefit | Insulation plus air sealing in suitable assemblies | Economical coverage and high attic R-values |
| Best choice | Job- and assembly-dependent | Job- and assembly-dependent |
Quick verdict: For a conventional vented attic, blown-in insulation installed after ceiling air sealing is a strong-value default. For a planned unvented roofline, evaluate spray foam as part of a properly specified assembly, not merely as a more expensive loose-fill alternative.
What Is the Difference Between Spray Foam and Blown-In Insulation?
Spray Foam Insulation
Spray polyurethane foam (SPF) is formed when reactive components are mixed and sprayed onto a prepared surface. The foam expands and cures in place. It comes in two major categories:
Open-cell spray foam is typically lower-density, offers around R-3.6 to R-4.5[1] per inch and is more water-vapor-permeable.
Closed-cell spray foam is denser, often provides approximately R-5.8 to R-6.9[1] per inch and generally has lower water-vapor permeance.
Either product can help restrict air leakage at the correct installed thickness when the air-control layer is properly connected. Neither is a substitute for repairing roof leaks or designing a moisture-safe assembly.
Blown-In Insulation
Blown-in insulation is usually loose-fill fiberglass or cellulose, installed using a blowing machine. The installer distributes the insulation across an attic floor to a specified depth and coverage density.
Blown fiberglass is made from fine glass fibres. Blown cellulose is commonly made from treated recycled paper. Both reduce heat transfer by holding air in small spaces within the insulation.
Loose-fill insulation is particularly useful on accessible attic floors and around framing irregularities, provided installers maintain consistent coverage, protect ventilation channels and achieve the correct installed and settled depths.
Neither blown fiberglass nor loose-fill cellulose replaces ceiling air sealing. The Department of Energy recommends sealing gaps and penetrations before installing loose-fill attic insulation. See DOE Building America: Blown Insulation in an Existing Vented Attic[2].
Spray Foam vs Blown-In Insulation Cost in 2026
HomeGuide’s US cost guide reports approximately $1.00–$2.80 per square foot installed for blown-in insulation. Its comparison table lists $1.00–$4.50 per square foot installed for spray foam, with pricing affected by the type and job scope[3].
These broad square-foot bands aren’t necessarily quotations for the same installed R-value or surface. That’s particularly important because spray foam is frequently priced by board foot, which includes thickness.
Separately, Angi’s spray-foam guide reports broad installed rates of $0.60–$1.60[4][5] per board foot for open-cell and $1.30–$3.10[4][5] per board foot for closed-cell. One board foot means one square foot at one inch thick.
| Published price reference | Unit | Reported installed range |
|---|---|---|
| Blown-in insulation, HomeGuide | Per treated square foot | $1.00–$2.80 |
| Spray foam, HomeGuide | Per treated square foot | $1.00–$4.50 |
| Open-cell foam, Angi | Per board foot | $0.60–$1.60 |
| Closed-cell foam, Angi | Per board foot | $1.30–$3.10 |
Sources: HomeGuide blown-in insulation cost[3]; Angi spray foam insulation cost[4]. Different providers use different thickness, project and inclusion assumptions. Do not add or average these ranges as if they were identical surveys.
Example: A 1,000-Square-Foot Vented Attic Floor
If the attic-floor area is 1,000 square feet, HomeGuide’s published blown-in installed price band produces an illustrative $1,000–$2,800 insulation estimate.
But the completed job may also require ceiling air sealing, insulation removal, ventilation baffles, attic-hatch treatment, electrical corrections or other work. Those items may be outside the quoted insulation price.
Example: 1,000 Square Feet of Spray Foam at Different Depths
For spray foam:
Board feet = treated square feet × applied thickness in inches.
Suppose the same 1,000 square feet of surface are treated:
| Example | Board feet | Calculated foam-volume estimate |
|---|---|---|
| Open-cell at 3.5 inches | 3,500 | $2,100–$5,600 |
| Open-cell at 5.5 inches | 5,500 | $3,300–$8,800 |
| Closed-cell at 2 inches | 2,000 | $2,600–$6,200 |
| Closed-cell at 3 inches | 3,000 | $3,900–$9,300 |
Each figure is a mathematical example using Angi’s installed board-foot rates. The depths are illustrative, not building-code recommendations. The rows are not equivalent-R-value quotations.
Most importantly, if the spray foam is going on the roof deck rather than the attic floor, 1,000 square feet may no longer be the correct area. Sloping roof planes and gables can make the roofline surface larger.
Use the spray foam insulation cost calculator for your measured surface and proposed depth. For wider budgeting, see our main spray foam cost guide.
R-Value: Which Insulation Performs Better?
Closed-cell spray foam generally provides more nominal R-value per inch than loose-fill insulation. But in an open attic floor with plenty of depth available, blown-in insulation can achieve a high total R-value simply by using a thicker layer.
R-value is resistance to heat flow. A higher number is better insulation performance under the stated test conditions, but the entire attic’s results also depend on air leakage, framing, thermal bridging and installation quality.
| Material | General R-value characteristic | What to check |
|---|---|---|
| Open-cell spray foam | Roughly R-3.6 to R-4.5[1] per inch | Declared aged/product R-value and installed depth |
| Closed-cell spray foam | Roughly R-5.8 to R-6.9[1] per inch | Product data, required depth and vapor behavior |
| Blown cellulose | Commonly around R-3.2–R-3.8[6] per inch | Coverage chart, installed density and settled depth |
| Blown fiberglass | About R-2.2–R-2.7 per inch for loose-fill (This Old House)[6]; product-dependent | Manufacturer’s coverage chart and final depth |
These are illustrative material ranges, not whole-assembly ratings. Manufacturer declarations and your local energy-code requirements take precedence.
Example: Reaching Nominal R-38
At assumed R-3.7 per inch, an open-cell foam layer would require roughly 10.3 inches for nominal R-38.
At assumed R-6.2 per inch, closed-cell would require roughly 6.1 inches for nominal R-38.
For blown-in products, calculate the required depth from the actual manufacturer’s coverage chart rather than forcing every fiberglass or cellulose product into a universal number.
The takeaway isn’t that spray foam is always more effective. It’s that per-inch performance matters most where space is limited; on a tall attic floor, inexpensive additional loose fill may be a better financial route.
Air Sealing: The Biggest Difference
Loose-fill fiberglass and cellulose slow heat transfer but do not create a reliable continuous air barrier on their own.
The DOE’s Air Sealing and Insulating Ceilings in Vented Attics[7] guide explains that porous fibrous insulation does little to stop conditioned air escaping through gaps around fittings, flues, wiring and top-plate transitions.
The correct approach is to establish the air barrier at the ceiling plane first, then install blown insulation above it.
What Good Attic Air Sealing Includes
A qualified installer should inspect and address attic hatches, plumbing penetrations, electrical chases, ceiling-to-wall top plates and larger open soffits or cavities.
Air sealing around chimneys, flues and recessed lights requires approved materials and appropriate clearances. Do not pack insulation or combustible sealants against heat-producing components.
Does Spray Foam Seal the Air Automatically?
Not across the entire house. Spray foam can become part of a continuous air-control layer, but it still needs adequate installed depth, full coverage and sound connections at adjoining construction details.
Missed framing joints, poorly prepared substrates and unsealed transitions can leave air leaks despite a large foam bill.
A good air-sealed blown-in attic can outperform a poorly detailed spray-foam job.
Attic Floor vs Roof Deck: The Decision That Changes Everything
The best insulation choice is usually clearer once you decide where the attic’s thermal boundary belongs.
Vented Attic: Ceiling Plane Insulation
In a conventional vented attic, the conditioned rooms are below the ceiling. The attic above remains ventilated to outdoors, and the insulation typically covers the attic floor.
For this setup, air sealing followed by blown-in fiberglass or cellulose is often the simpler and cheaper solution.
DOE guidance calls for roof-leak repairs, proper vent baffles, exterior termination of bathroom exhaust ducts and consistent loose-fill coverage before work is considered complete.
Unvented Attic: Roofline Insulation
A correctly designed unvented attic places the thermal and air-control boundary at the roofline. Spray foam applied against the underside of roof sheathing is one way to achieve this, although approved rigid/hybrid roof assemblies may also be used.
This changes roof drying, ventilation and humidity requirements. It is not simply a matter of spraying insulation onto plywood and closing soffit vents.
A roofline assembly can be worth investigating when HVAC equipment and ductwork are located in the attic or the roof geometry makes ceiling-level treatment difficult.
Read attic spray foam insulation, spray foam roof-deck insulation and vented vs unvented attic before selecting a system.
Blown-In Fiberglass vs Blown-In Cellulose
Not all blown-in insulation is the same material.
Blown Fiberglass
Loose-fill fiberglass is a common attic product and is installed according to a manufacturer’s coverage chart. Proper depth, uniform distribution and baffles to preserve soffit ventilation are crucial.
Its performance can be compromised by voids, shifting, wind effects at the eaves, or incorrect application density. It still relies on a separate ceiling air barrier.
Blown Cellulose
Cellulose insulation is made primarily from treated paper fibre. It can conform to irregular attic-floor features when correctly installed and can offer a higher R-value per inch than some loose-fill fiberglass products.
Cellulose must be installed to the coverage and settled-depth specifications of the actual product. Like fiberglass, it is not a substitute for sealing major air leaks or fixing roof moisture problems.
Which Blown-In Material Is Better?
Compare two quotes for the same installed/settled R-value, verifying total coverage bags, depth markers, ventilation baffles, air sealing and any removal work.
Fiberglass-versus-cellulose is a separate product decision within the broader attic-floor strategy. Neither needs a blanket “always better” label.
Can You Combine Spray Foam and Blown-In Insulation?
Yes. A hybrid attic-floor system can use a controlled spray-foam air-sealing layer with blown insulation above it.
DOE Building America specifically describes an approach for existing vented attics that uses a minimum one-inch layer of closed-cell spray foam on a properly prepared ceiling plane, followed by additional loose-fill or batt insulation where needed to meet the required thermal performance. This is a documented example assembly, not permission to use one inch of any foam anywhere.
Source: DOE Spray Foam Applied to Existing Attic Floor[8].
The benefit is combining a continuous air barrier with the lower incremental cost of blown fiberglass or cellulose.
When Hybrid Insulation Makes Sense
It may suit an attic where many small cracks and penetrations are difficult to seal one by one, provided the floor can be exposed and cleaned adequately.
The installer must maintain required vent pathways and ensure the combined layers meet code. Old contaminated insulation or debris should not simply be buried under fresh foam.
Be Careful With Hybrid Roof Decks
A roof-deck assembly with spray foam plus fibrous insulation has different requirements from a vented attic-floor hybrid. Climate-specific condensation control, insulation ratios and vapor behavior may govern which combinations are allowed.
Don’t translate an attic-floor hybrid into a roofline solution without a proper roof-assembly design.
Moisture, Mold and Condensation
Neither blown-in insulation nor spray foam fixes a leaking roof. The primary defence against bulk water is sound roofing, flashing, drainage and proper construction details.
Blown-In Attic Moisture Risks
Loose-fill insulation can become damp from roof leaks or condensation. Wet areas may need investigation, drying and replacement. Ceiling air leaks can transport humid indoor air upward, increasing the risk of condensation on cold roof surfaces.
Vented attic insulation should preserve functioning ventilation pathways, but ventilation is not a substitute for air sealing.
Spray Foam Roofline Moisture Risks
Open-cell foam is more vapor-permeable; closed-cell foam generally restricts vapor movement and inward drying more strongly. Either can work in suitable roof assemblies, but applying foam against roof sheathing changes the roof’s moisture behavior.
Fix known roof leaks before insulating. Verify the climate-specific vapor strategy and how the attic will be conditioned, ventilated or dehumidified.
Does Either Material Prevent Mold?
No. Proper water management, indoor humidity control, air sealing and sound construction are what prevent persistent dampness. Insulation alone cannot guarantee a mold-free attic.
Lifespan, Maintenance and Future Roof Repairs
Both systems can last for many years when correctly installed, dry and undisturbed. Avoid marketing claims that one product is guaranteed to last a specific number of decades regardless of conditions.
Blown-in insulation can often be topped up, redistributed or removed during ceiling and wiring work. However, it can be disturbed by trades, contaminated by pests or displaced by air movement.
Adhered spray foam is generally more difficult to remove and may complicate access to the underside of roof sheathing. That matters when future roof leaks or repairs need investigation.
If the roof covering is near replacement, consider completing roof repairs before committing to a roof-deck foam installation.
Noise Control and Comfort
Both blown cellulose/fiberglass and suitable spray-foam installations can reduce sound transmission as part of a complete ceiling or wall assembly. But acoustic performance depends on drywall, framing, sealing, isolation and flanking routes, not insulation material alone.
Blown-in insulation over an attic floor can improve comfort by reducing heat flow through the ceiling. Spray foam at the roofline can reduce extreme attic temperatures under suitable conditions, potentially helping ductwork located there.
Neither choice guarantees a particular temperature reduction, noise-rating improvement or energy-bill saving without measuring the existing house and intended design.
Environmental and Installation Considerations
Fiberglass commonly contains recycled glass content, while cellulose uses treated recycled paper. Their manufacturing and transport footprints vary by product and supplier.
Spray polyurethane foam’s environmental profile depends on chemicals, blowing agents, product quantities and the life-cycle impact of the entire assembly. Ask for product environmental declarations where available rather than assuming every foam formulation has the same climate impact.
There are also distinct installation considerations. Blown loose-fill projects require protective equipment and dust control. Two-component spray polyurethane foam introduces additional chemical exposure hazards during application and curing.
The US EPA’s ventilation guidance[9] recommends proper work-area isolation, appropriate respiratory and skin protection, ventilation and manufacturer-specific occupant re-entry procedures. There is no universal safe re-entry time for every foam product and worksite.
Which Insulation Is Better for Different Projects?
| Project | Usually priced first | Why |
|---|---|---|
| Existing vented attic, accessible floor | Air sealing + blown-in fiberglass/cellulose | Usually strong thermal value at lower initial cost |
| Vented attic with many air leaks | Detailed air sealing + blown-in; consider floor-level foam hybrid | Fix leakage before burying it under loose fill |
| Unvented attic with HVAC ducts inside | A properly engineered roofline insulation system | Brings ducts into a less extreme enclosure when designed correctly |
| Exterior wall new build | Complete wall alternatives, including cavity foam or conventional insulation | Whole-wall performance and moisture design determine value |
| Small rim-joist area | Closed-cell foam or cut-and-seal rigid board | Awkward framing can reward dedicated air sealing |
| Existing closed wall | Specialist cavity assessment | Loose-fill cavity injection and conventional sprayed SPF are different processes |
| Wet or leaking attic | Roof/moisture repair first | Neither insulation material fixes water intrusion |
For a head-to-head material comparison against fiberglass specifically, see spray foam vs fiberglass insulation.
Is Spray Foam Worth the Extra Cost Over Blown-In Insulation?
Only when the premium pays for something the less expensive system doesn’t deliver as effectively or conveniently.
If the goal is to increase R-value on an accessible vented attic floor, properly installed blown insulation with a reliable separate air barrier often provides the better return.
If the goal is an approved unvented roofline, reducing heat gain around attic ductwork or sealing difficult architectural details, spray foam may have additional value. But evaluate it against alternatives such as relocating or sealing ducts, improving ceiling air sealing, or using hybrid insulation.
An Illustrative Payback Calculation
Suppose complete quotes for one suitable project are:
- Air sealing + blown-in insulation: $2,800.
- More expensive foam-based alternative: $6,800.
- Upfront premium: $4,000.
If independently modelled additional savings from the foam option were $150 per year, simple payback on that premium would be $4,000 ÷ $150 = about 27 years.
These are hypothetical prices and savings used to illustrate the decision, not market averages or performance promises. If the systems deliver similar comfort and air tightness, the premium may never produce a compelling energy-only payback. If the foam solves a different problem, a basic bill-payback calculation may omit important benefits.
How to Compare Contractor Quotes Fairly
Ask for two finished-system quotes based on the same performance objective rather than comparing a quoted bag price against a foam-unit price.
| Specification | Ask each contractor to document |
|---|---|
| Insulation location | Attic floor/ceiling or roof deck |
| Actual treated area | Measured square feet |
| Product | Manufacturer, specific foam or loose-fill product |
| Insulation performance | Required and installed/settled R-value |
| Air sealing | Scope, penetrations, transitions and verification |
| Moisture control | Roof leaks, attic ventilation and humidity strategy |
| Existing insulation | Whether removed, retained or added to |
| Safety | Dust/chemical exposure and occupant protection |
| Finish/protection | Baffles, barriers, hatches, protection and cleanup |
| Final cost | Total installed price and explicit exclusions |
With blown-in insulation, ask for coverage charts, installed depth markers and final settled R-value. With spray foam, ask for product data, minimum installed thickness, air-barrier continuity and required fire protection.
Use our spray foam contractor selection guide to understand what a qualified bid should include.
Questions owners ask
Is spray foam better than blown-in insulation?
Not universally. Spray foam can insulate and air-seal together, but blown-in insulation is often cheaper and effective when used over a properly air-sealed ceiling in a vented attic.
Is blown-in insulation cheaper than spray foam?
Usually, particularly for accessible attic floors. HomeGuide reports broad installed pricing around $1–$2.80 per square foot for blown-in and $1–$4.50 for spray foam under different project assumptions. Local completed-scope bids are more meaningful.
Which has a higher R-value: spray foam or blown-in?
Closed-cell foam typically provides higher R-value per inch. Blown fiberglass and cellulose can still achieve high total attic R-values by using sufficient installed depth.
Does blown-in insulation seal air leaks?
No. Loose-fill insulation is not a substitute for a continuous ceiling air barrier. Seal penetrations before installing the insulation.
Can spray foam and blown-in insulation be used together?
Yes. DOE describes a documented vented attic-floor assembly using a continuous closed-cell foam air-control layer with fibrous insulation above. Follow the exact assembly, code and manufacturer requirements.
Is cellulose better than blown fiberglass?
Both can work well. Compare the same installed and settled R-value, coverage, product specifications, moisture suitability and total installation cost.
Can blown-in insulation be installed on the roof deck?
Loose-fill products generally require a suitable contained assembly rather than being blown freely onto the exposed underside of roof sheathing. A designed enclosed roof assembly is a different application from an open vented attic floor.
Sources
- Frequently Asked Questions, Spray Polyurethane Foam Alliance (SPFA). Accessed October 19, 2026. Industry trade association. Page metadata shows modified April 27, 2026. Typical density, R-value per inch, air-barrier thickness and vapor permeance ranges.
- Blown Insulation for Existing Vented Attic, U.S. Department of Energy, Building America Solution Center. Accessed October 19, 2026. DOE guide: air sealing, baffles, roof-leak repair and coverage depth. No date shown.
- Blown-In Insulation Cost, HomeGuide. Accessed October 19, 2026. Dated November 5, 2025. Blown-in $1.00–$2.80 per sq ft installed; comparison table lists spray foam at $1.00–$4.50; cellulose R-3.1–3.8 and fiberglass R-2.2–2.9 per inch.
- How Much Does Spray Foam Insulation Cost?, Angi. Accessed October 19, 2026. National consumer cost guide from a survey of Angi customers. No update date shown; title says "2025 Data". Its installed per-board-foot ranges match HomeAdvisor's page updated June 17, 2026.
- How Much Does Spray Foam Insulation Cost?, HomeAdvisor. Accessed October 19, 2026. National consumer cost guide, updated June 17, 2026. Installed cost per board foot; national ranges, not quotes.
- Understanding Insulation and R-Value, This Old House. Accessed October 19, 2026. Updated June 6, 2026. R-value per inch by material (cellulose about 3.2–3.8; fiberglass batts 3.1–4.3; loose-fill fiberglass 2.2–2.7; mineral wool 3.0–3.3).
- Air Sealing and Insulating Ceilings in Vented Attics, U.S. Department of Energy, Building America Solution Center. Accessed October 19, 2026. DOE guide. No date shown.
- Spray Foam Insulation Applied to Existing Attic Floor, U.S. Department of Energy, Building America Solution Center. Accessed October 19, 2026. DOE guide: minimum 1 inch of spray foam (closed-cell preferred) on the attic floor as the air control layer, optionally topped with fibrous insulation. No date shown.
- Ventilation Guidance for Spray Polyurethane Foam Application, U.S. Environmental Protection Agency (archived). Accessed October 19, 2026. Last updated November 5, 2020; archived EPA page.