The short answer
Fiberglass costs less, about $0.30 to $1.50 per square foot against $1 to $2 for spray foam in Angi's comparison, and performs well when paired with a separate air barrier. Spray foam insulates and air-seals in one layer, and closed-cell gives more R-value per inch. For an accessible vented attic floor, air sealing plus fiberglass is usually the better value; spray foam earns its premium at rim joists, shallow cavities and designed roof-deck assemblies.
For most accessible, conventional vented attic floors, price professional air sealing plus blown fiberglass or well-installed fiberglass insulation before paying for a full spray-foam system. For awkward framing, rim joists and selected roof-deck or shallow-cavity assemblies, spray polyurethane foam may justify its higher cost. For exterior walls, compare the whole wall, not a material’s R-value per inch.
Bottom line: Spray foam isn’t automatically better, and fiberglass isn’t automatically a budget compromise. The winning option is the lowest-cost suitable finished insulation and air-sealing system for the building assembly.
US guide reviewed October 19, 2026. Pricing is based on published national guides, not an on-site contractor bid. A qualified professional should verify moisture, fire-protection and code requirements for the actual project.
Spray Foam vs Fiberglass: Side-by-Side Comparison
| Feature | Spray polyurethane foam | Fiberglass insulation |
|---|---|---|
| Common formats | Open-cell or closed-cell, spray applied | Batts, rolls or blown loose-fill |
| Nominal R-value per inch | Open-cell ~R-3.6 to R-4.5[1]; closed-cell ~R-5.8 to R-6.9[1] | About R-2.2–R-2.9/in (HomeGuide) to R-3.1–R-4.3/in (This Old House) for batts[2][3] |
| Air sealing | Can serve as an air barrier at tested depth and continuous installation | Fiberglass itself is not an air barrier |
| Upfront cost | Usually higher | Usually lower |
| DIY suitability | Two-component systems carry significant application and exposure risks | Batts are more accessible to capable DIY installers with appropriate precautions |
| Moisture | Depends strongly on foam type and assembly | Needs appropriate air/vapor control; wet material must be evaluated |
| Installation quality risks | Off-ratio or poorly adhered foam, voids, insufficient depth | Gaps, compression, missing support and wind washing |
| Future changes | Adhered foam can be difficult to remove | Batts can generally be removed more easily |
| Fire considerations | Foam plastic normally requires an approved protective assembly | Product facings and full assembly still must satisfy code |
| Best-value scenarios | Irregular air-leakage details, limited depth, selected roofline/perimeter systems | Accessible attic floors and well-detailed framed cavities |
Figures describe broad product categories rather than individual brand specifications. Read the manufacturer data and compare the completed assemblies.
Which Is Cheaper: Spray Foam or Fiberglass?
Fiberglass usually wins on initial installed cost. Angi’s July 2026 direct comparison cites approximately $0.30–$1.50 per square foot for fiberglass and $1–$2 per square foot for spray foam under that article’s assumptions[4]. Angi’s and HomeAdvisor’s spray-foam cost guides report a different set of installed unit rates measured per board foot: $0.60–$1.60[5][6] for open-cell and $1.30–$3.10[5][6] for closed-cell. These price estimates use different scopes and units and must not be mixed without specifying insulation thickness.
That distinction matters because one board foot = one square foot at one inch thick. A 1,000-square-foot wall at three inches uses 3,000 theoretical board feet of spray foam, not 1,000.
Example: Spray Foam for 1,000 Square Feet
For 1,000 square feet of actual spray surface:
| Illustrative foam system | Installed thickness | Board feet | Cost using Angi’s board-foot bands |
|---|---|---|---|
| Open-cell | 3.5 in | 3,500 | $2,100–$5,600 |
| Closed-cell | 2 in | 2,000 | $2,600–$6,200 |
These figures are mathematical examples, not site-specific bids, equal-performance alternatives or prescribed insulation depths. They exclude separately charged work and minimum crew charges.
By contrast, applying the published fiberglass $0.30–$1.50/sq-ft rate to 1,000 square feet produces a very broad $300–$1,500 illustrative insulation figure under that source’s assumptions. It does not establish equal R-value or include every necessary air-sealing, removal, access or finishing cost.
The valid buying comparison is between proposals for the same area, code-appropriate thermal target and full scope, including separate air sealing for fiberglass.
For detailed foam calculations, use the spray foam insulation cost calculator and our spray foam cost guide.
Why Fiberglass’s Headline Price Can Mislead
A cheap batt installation that leaves open bypasses, unsealed ceiling penetrations or poorly fitted corners may underperform. Pay separately for an effective air barrier, correct depth and proper installation.
Why Spray Foam’s Headline Price Can Mislead
A quote may exclude demolition, old-insulation removal, substrate repairs, attic ventilation changes, thermal/ignition protection, mechanical ventilation or project minimums. In existing finished walls, opening and repairing drywall may cost a substantial amount on top of the foam.
Ask both contractors for the price of the completed assembly, not a bag of fiberglass versus a gallon of foam.
R-Value: Is Spray Foam Better Than Fiberglass?
Closed-cell spray foam generally has higher R-value per inch than conventional fiberglass. Open-cell foam and some fiberglass products can be closer in nominal thermal resistance. The correct figure depends on the exact product.
| Material | Illustrative nominal R-value per inch | Important limitation |
|---|---|---|
| Open-cell spray foam | R-3.6 to R-4.5[1] | Product and installed depth affect performance |
| Closed-cell spray foam | R-5.8 to R-6.9[1] | Use declared aged R-value and appropriate assembly |
| Fiberglass batts | About R-2.2–R-2.9 (HomeGuide) to R-3.1–R-4.3 (This Old House)[2][3] | Actual performance affected by fit, compression and gaps |
| Loose-fill fiberglass | Varies by product and installed density | Follow coverage/depth specifications |
R-value measures resistance to heat flow, not airtightness. Higher per-inch R-value can matter in a thin cavity, but a whole wall or attic’s effective performance depends on framing, continuous insulation, air leakage and installation quality.
Example: Nominal R-20 From the Insulation Layer
If an open-cell product provides R-3.7 per inch, reaching nominal material R-20 takes approximately 5.41 inches.
If a closed-cell product provides R-6.2 per inch, it takes approximately 3.23 inches.
A fiberglass product would need the installed thickness specified on its label to reach the same nominal R-20. It is not valid to treat every fiberglass product as having one universal R-value per inch.
These are material-only calculations. A wall with R-20 between wood studs is not automatically an R-20 whole-wall assembly because studs conduct heat.
Does Higher R-Value Always Mean Lower Bills?
No. Reducing unwanted heat flow is useful, but the extra energy savings from upgrading an already well-insulated area can be smaller than those from sealing major leaks. Equipment efficiency, climate, energy prices and the quality of the full enclosure determine real savings.
Air Sealing: Spray Foam’s Biggest Practical Advantage
Fiberglass insulation alone does not stop air leakage. The US Department of Energy’s Building America guidance explicitly warns that fibrous insulation is not an air barrier and recommends sealing penetrations before insulating.
Spray polyurethane foam can create a continuous air-control layer when the selected product is applied correctly at an approved thickness and connected to adjacent air-barrier components. This is particularly valuable around irregular framing, roof transitions and rim joists.
But spray foam does not automatically make an entire home airtight. Missing areas, poorly detailed junctions, unsealed windows and other pathways remain possible.
Can Fiberglass Perform Just as Well?
Yes, as part of a correctly designed and installed system. A ceiling sealed with durable materials at penetrations and top plates, then covered with sufficient fiberglass insulation, can perform very well. Fiberglass’s job is thermal resistance; the separate air barrier manages air movement.
DOE’s Air Sealing Existing Attics[7] guide specifically recommends sealing attic-floor penetrations before insulation is installed.
If someone claims fiberglass “can’t seal air, therefore it is bad insulation,” they’re comparing a material with an entire assembly.
Spray Foam vs Fiberglass for Attics
The attic’s thermal boundary decides the better solution.
Vented Attic: Fiberglass Often Has the Cost Advantage
In a typical vented attic, insulation goes at the ceiling plane and outside airflow is intentionally maintained above it. Properly air-seal ceiling penetrations, install enough fiberglass insulation to meet the requirement, keep soffit ventilation pathways open and protect heat-producing fixtures appropriately.
In this scenario, fiberglass is often the economically stronger choice because it can achieve the target thermal resistance without converting the attic into a more complicated roofline assembly.
Unvented Attic: Spray Foam May Be Worth Pricing
An unvented roof-deck approach places the air and thermal boundary at the roofline. It can be useful where ducts or HVAC equipment sit in a hot attic or where roof geometry complicates ceiling-level insulation.
Spray foam is one possible system, but conversion involves moisture control, roof sheathing drying, vent modifications and attic humidity management. A specialist should compare an approved foam roofline design against retaining a vented attic.
Read attic spray foam insulation and vented vs unvented attic before deciding.
Can You Combine Fiberglass and Spray Foam in an Attic?
Yes. In a vented attic, spray foam or sealants can address selected leakage areas before fiberglass is placed on the attic floor. In certain designed roof assemblies, spray foam and fiberglass can form an approved hybrid system.
However, combining materials at a roof deck is a moisture- and code-design question. Don’t assume that spraying a thin layer beneath sheathing and packing the remainder with fiberglass is acceptable in every climate.
Spray Foam vs Fiberglass for Exterior Walls
Both can work in above-grade framed exterior walls. Spray foam can conform to awkward cavity shapes and reduce air leaks. Fiberglass batts can be economical when correctly cut to fit and paired with a continuous air barrier.
For a new wall with exposed framing, compare bids using the same full-wall performance requirement. Include any continuous exterior insulation needed to manage thermal bridging and meet code.
For existing closed walls, conventional two-component spray foam usually requires exposure of cavities. Some products marketed as injection foam are different systems; don’t assume drilling and filling a closed wall is equivalent to spraying conventional polyurethane SPF into an open stud bay.
Our exterior-wall spray foam guide explains the retrofit distinction.
Which Is Better for 2×4 Walls?
Closed-cell’s high R-value per inch may help when cavity depth is limited. But that does not mean it is always the most economical code-compliant wall design. Continuous exterior insulation, air sealing and vapor requirements may dictate a different combination.
Which Is Better for 2×6 Walls?
Deeper cavities can make fiberglass or open-cell foam cost-effective. Compare the exact products’ thermal values, correct cavity fit, air barrier and local code requirements before selecting.
Crawl Spaces, Basements and Rim Joists
These locations make simplistic comparisons especially dangerous because moisture behavior matters as much as price.
Rim joists: Spray foam can be attractive for irregular air-sealing transitions. Properly cut rigid foam with sealed edges is another strong option.
Vented crawl-space floors: Correctly supported fiberglass can work when it remains dry and is installed tightly against the subfloor air barrier, but conditions are often challenging. Spray foam may better conform to complicated framing.
Sealed crawl spaces: Perimeter insulation, ground vapor control and humidity management are central. Neither fiberglass nor spray foam solves standing water or foundation drainage issues.
Basement foundation walls: Closed-cell spray foam or suitable rigid foam may be preferable to exposed fibrous insulation directly against damp masonry. The specific foundation and drainage conditions govern.
See crawl space spray foam, rim joist insulation and spray foam basement walls for assembly-specific guidance.
Moisture, Mold and Vapor Barriers
Neither spray foam nor fiberglass is a waterproofing system. Bulk water entry through roofs, wall flashing, foundations or plumbing must be repaired separately.
Fiberglass can be damaged or lose performance when wet or displaced; a correctly designed fibrous insulation system also relies on adequate air and vapor control for the climate.
Spray foam behaves differently by type. Open-cell is generally more vapor-permeable; closed-cell usually has lower vapor permeability and restricts drying more. This makes each suitable for some assemblies and problematic in others.
A low-permeance foam layer can be valuable for condensation control, but can also limit the drying of sheathing or masonry if water enters. The correct design considers the entire wall, roof or foundation.
Never cover visibly wet or damaged materials merely to make a room feel warmer.
Soundproofing: Which Insulation Wins?
Neither material guarantees a soundproof room. Insulation can absorb or reduce sound energy within a wall cavity, but real acoustic performance depends on the whole assembly: drywall mass, framing, resilient isolation, sealed gaps and flanking paths.
Fiberglass batts are frequently used in acoustic assemblies. Open-cell spray foam can also absorb sound. Closed-cell’s density does not automatically make it the best acoustic treatment.
If noise reduction is the goal, compare tested wall or ceiling assembly ratings, not unsupported claims that one material reduces noise by a fixed percentage.
Safety, Fire Protection and Health
Fiberglass and spray polyurethane foam have different installation hazards.
Fiberglass: Handling or disturbing loose fibres can irritate skin, eyes and the respiratory tract. Installers should use appropriate protective equipment and follow manufacturer guidance. Existing insulation of unknown composition should be identified before disturbance.
Spray foam: Two-component SPF involves reactive chemicals during application, including isocyanates. The US EPA[8] recommends trained applicators, work-area isolation, ventilation, personal protective equipment and product-specific re-entry precautions. There is no single safe re-occupancy period for every foam product or job.
Fire protection: Foam plastics commonly require approved thermal or ignition protection depending on application and code. Fiberglass products, especially faced batts, also require correct installation and assembly fire details.
A low price is not a reason to accept an installer who cannot provide the product documentation and occupant protection plan.
Lifespan and Repairability
Both can remain effective for decades when installed correctly and kept within suitable environmental conditions. There is no credible universal rule that every foam installation lasts 100 years or that fiberglass routinely fails after a short period.
Spray foam is adhered to surfaces, so later removal can be difficult when wiring, roof decking or framing needs work.
Fiberglass batts are usually easier to move or replace. However, batts that are compressed, sagging or contaminated may need correction.
Plan for inspection and future changes, especially on roofs and foundations where concealing a leak can be expensive.
Environmental Considerations
Spray foam’s environmental profile depends on chemical formulation, blowing agent and manufacturing impacts. Some closed-cell products historically used high-global-warming-potential blowing agents; modern formulations vary, so ask for current product disclosures.
Fiberglass is made from glass materials and often contains recycled content. Its environmental profile still depends on manufacturing energy, binder chemistry, packaging and installation conditions.
A fair comparison also considers how much energy the finished building system saves, not simply the insulation material in isolation. For a product-specific claim, request an environmental product declaration where available.
Is Spray Foam Worth the Extra Money?
The premium is only worth paying where the proposed foam system does something a cheaper, properly air-sealed fiberglass system cannot accomplish as effectively or economically.
Good reasons might include: a complicated air-leakage junction, limited cavity depth, an engineered roofline conversion, or a foundation detail that needs particular air/vapor characteristics.
Weak reasons include: “spray foam always saves 50%”, “fiberglass is useless”, or “closed-cell makes every wall waterproof”. None of those is a sound project specification.
Simple Break-Even Calculation
Assume, purely for illustration:
- Properly installed fiberglass plus air sealing: $2,500.
- Comparable suitable spray-foam system: $5,500.
- Difference in initial cost: $3,000.
- Independently modelled additional annual savings from the foam option: $150.
Simple premium payback: $3,000 ÷ $150 = 20 years.
That is an example, not a measured savings forecast. Actual energy modelling may show a very different result, and the savings difference could be small when both assemblies are well designed.
For many homeowners, proper air sealing and an adequate insulation level matter more than upgrading from a well-installed inexpensive material to a premium one.
How to Compare Fiberglass and Spray Foam Quotes
Ask contractors for two complete, technically appropriate bids with the same agreed performance requirement.
| Required item | Why it matters |
|---|---|
| Actual surface area | House floor area is not the treated insulation area |
| Insulation product | Material category alone is not a specification |
| Installed thickness and R-value | Prevents comparisons of unequal performance |
| Air-sealing scope | Fiberglass needs a separate air-control solution |
| Moisture and vapor plan | Avoids inappropriate roof, wall or basement assemblies |
| Preparation and removal | Can materially change retrofit pricing |
| Fire protection and finishing | Must be included where required |
| Testing and inspection | Blower-door or installation checks can verify work |
| Safety and re-entry | Particularly important for spray polyurethane foam |
| Total price and exclusions | A headline square-foot rate isn’t the final bill |
Where both alternatives are suitable, prioritize total installed cost, expected whole-building performance, inspection quality and future maintainability.
To compare installer qualifications, use our spray foam contractor guide.
Questions owners ask
Is spray foam better than fiberglass insulation?
Not universally. Spray foam can combine insulation and air sealing, while fiberglass is often less expensive and performs well when installed with a proper air barrier. The best option depends on the specific building assembly.
Is spray foam more expensive than fiberglass?
Usually, yes. Angi's July 2026 comparison cites $0.30–$1.50 per square foot for fiberglass and $1–$2 for spray foam under its assumptions. Foam quotes also vary by thickness and board-foot pricing.
Does spray foam have a higher R-value than fiberglass?
Closed-cell spray foam commonly has higher R-value per inch. Open-cell foam and fiberglass products may be closer. Always compare specified products and full assemblies.
Can fiberglass insulation stop drafts?
Fiberglass alone is not an air barrier. Pair it with continuous air sealing using suitable materials at cracks, penetrations and transitions.
Which is better for an attic, spray foam or fiberglass?
For many conventional vented attic floors, air sealing plus fiberglass is a strong-value option. Spray foam may be worthwhile for properly designed roofline/unvented attic systems or difficult leakage details.
Can you install fiberglass over spray foam?
Sometimes, in properly designed hybrid assemblies. Roof-deck combinations may have climate-specific condensation-control requirements, so don't assume every foam-plus-fiberglass stack is acceptable.
Is fiberglass safer than spray foam?
They have different hazards. Fiberglass may irritate skin and respiratory tissue during handling, while reactive spray-foam installation requires chemical-exposure controls and product-specific occupant re-entry procedures.
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-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.
- 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).
- Spray Foam Insulation vs. Fiberglass, Angi. Accessed October 19, 2026. Updated July 8, 2026. Fiberglass $0.30–$1.50 and spray foam $1–$2 per square foot under the article's assumptions.
- 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.
- Air Sealing Existing Attics, U.S. Department of Energy, Building Science Education. Accessed October 19, 2026. DOE educational page. No date shown.
- Safer Workplace Practices for Spray Polyurethane Foam Installation, U.S. Environmental Protection Agency (archived). Accessed October 19, 2026. Archived EPA page.