Quick answer: spray foam seals air as well as insulating, delivers roughly R-5.66 per inch against fiberglass batt’s R-2.9–3.8, and resists moisture — which makes it the better long-term performer. Fiberglass costs a third to half as much and is genuinely the right choice in some situations. The decision comes down to whether air leakage and moisture are problems in your building, and what your budget is.
Choosing insulation affects your energy bills, your comfort and your maintenance costs for decades. Spray foam and fiberglass are the two most common options, and they fail in different ways — so the comparison is worth doing properly rather than assuming the more expensive one wins.
Spray Foam vs Fiberglass: The Numbers
| Closed-cell spray foam | Open-cell spray foam | Fiberglass batt | Blown-in fiberglass | |
|---|---|---|---|---|
| R-value per inch | ~R-5.66 | ~R-3.5 | R-2.9–3.8 | R-2.2–2.7 |
| Air sealing | Yes — seals as it expands | Yes | No | No |
| Moisture | Vapour barrier at sufficient thickness | Absorbs water | Absorbs and holds water | Absorbs and holds water |
| Settling | None | None | Sags if poorly fitted | Settles over time, losing R-value |
| Installed cost | $1.50–$3.50/sq ft | $0.50–$1.50/sq ft | $0.40–$1.00/sq ft | $0.80–$1.50/sq ft |
| Service life | Decades, no degradation | Decades | 15–25 years typical | Shorter — settling reduces performance |
| DIY-friendly | Yes, with a kit | Yes | Yes — easiest of all | Needs a blower |
Not sure which spray foam type applies to you? See open-cell vs closed-cell spray foam.
The Real Difference Is Air, Not R-Value
R-value measures resistance to heat conduction. It says nothing about air moving through an assembly — and that is where most energy is actually lost.
The U.S. Department of Energy attributes up to 30% of a typical home’s heating and cooling costs to air leaks. Fiberglass does not stop air; it is a fibrous mat that air passes through, which is why a fiberglass-insulated wall can still feel draughty and why the material needs separate air sealing and vapour barriers to reach its rated performance.
Spray foam expands into the cavity and bonds to the surfaces around it, sealing gaps around wiring, plumbing penetrations and framing joints in the same pass. That is the mechanism behind most of the energy difference — not the R-value gap.
What About Blown-In Fiberglass?
Blown-in (loose-fill) fiberglass is often proposed as the middle ground: cheaper than foam, better coverage than batts. It does fill irregular cavities more completely than batts, and it is a reasonable choice for open attic floors.
But it shares the core weakness — it does not seal air — and adds one of its own: settling. Loose-fill compresses under its own weight over time, and depth loss means R-value loss. An attic blown to R-38 does not stay at R-38 indefinitely.
If you are choosing between blown-in fiberglass and spray foam, the question is the same as with batts: does this building have an air leakage problem? If yes, blown-in will not fix it.
Moisture: Different Failure Modes
Fiberglass absorbs and holds water. Wet fiberglass loses most of its insulating value and stays wet, which is how insulation becomes a mould problem rather than just an efficiency one. It does not cause the leak, but it makes the consequences worse.
Closed-cell spray foam resists water and acts as its own vapour barrier at sufficient thickness, which is why it suits basements, crawl spaces and roof decks. Open-cell foam does not — it absorbs water like fiberglass and belongs in dry interior assemblies only. For the full distinction, see is spray foam insulation waterproof.
Cost: Where the Money Actually Goes
Fiberglass is the cheaper material and the cheaper installation. Spray foam typically runs two to four times the installed cost per square foot.
But most of a professional foam quote is labour and equipment, not material. A DIY kit removes that portion, which narrows the gap considerably for homeowners doing their own attic, garage or rim-joist work. For a full breakdown including how to calculate how many cans you need, see the cost of spray foam insulation.
When Fiberglass Is the Right Choice
Foam is not automatically the answer. Fiberglass makes more sense when:
- The budget is the binding constraint. Insulating a whole house in fiberglass beats insulating a third of it in foam. Coverage matters more than material.
- The cavity is open, regular and dry. Standard stud bays in a new interior wall are exactly what batts are designed for.
- You may need access later. Batts can be pulled out and put back. Cured foam cannot — removing it is a job in itself.
- The assembly is already well sealed. If air sealing has been handled separately, fiberglass’s main weakness is already covered.
- You are insulating an attic floor with easy access. Blown-in fiberglass over a sealed ceiling plane is a cost-effective, well-proven approach.
The Hybrid Approach: Flash and Batt
You do not have to choose one for the whole building. A common professional method is flash and batt: a thin layer of closed-cell spray foam applied directly to the sheathing to seal the assembly and control vapour, then fiberglass batts over it to build up R-value more cheaply.
You get the air seal where it matters and the bulk insulation at fiberglass prices. It is often the best value option in a wall cavity where full-depth foam would be overkill.
Which Should You Choose?
Choose spray foam if the building has draughts, the space is damp or below grade, the cavity is shallow and you need maximum R-value per inch, or you want the air sealing and insulation handled in one step.
Choose fiberglass if budget is the limiting factor, the cavity is dry and regular, or you need to keep the assembly accessible.
Consider both if you want the seal without the full foam cost — flash and batt, or foam in the problem areas and fiberglass elsewhere.
Frequently Asked Questions
Is spray foam really worth the extra cost over fiberglass?
In a leaky or damp building, usually yes — the air sealing addresses a loss fiberglass cannot touch. In a tight, dry, already-sealed assembly the advantage narrows considerably, and fiberglass may be the better value.
Can I put spray foam over existing fiberglass?
It is not recommended. Foam needs to bond to the substrate to seal properly, and spraying over fiberglass traps the old material and any moisture in it. Remove the fiberglass first.
What is the R-value of fiberglass compared to spray foam?
Fiberglass batts run about R-2.9–3.8 per inch and blown-in fiberglass about R-2.2–2.7. Closed-cell spray foam is roughly R-5.66 per inch, so it reaches a target R-value in roughly half the depth.
Does fiberglass insulation go bad?
It does not decompose, but it degrades in practice: batts sag, loose-fill settles, and any water exposure sharply reduces performance. Expect 15–25 years of useful life in normal conditions, less if it gets wet.
Is fiberglass safer than spray foam?
Both are safe once installed. Fiberglass irritates skin and lungs during handling; spray foam requires ventilation and protective gear during application and cure. Neither poses a meaningful risk once the job is finished and cured.
Which is better for soundproofing?
Open-cell spray foam absorbs sound best; fiberglass is decent and closed-cell foam is the weakest of the three at absorption. See spray foam for soundproofing.
The Bottom Line
Spray foam wins on performance: better R-value per inch, air sealing built in, and moisture resistance in the closed-cell form. Fiberglass wins on price and accessibility. If your building leaks air or sits in a damp space, foam solves a problem fiberglass cannot. If it does not, fiberglass may be the smarter spend.
If you are ready to start, see the ixirpro Spray Foam Insulation Kit.