Battling the Heat: Fiberglass vs. Spray Foam Attic Insulation

Alright, let's get one thing straight right out of the gate: picking attic insulation isn't about finding the 'best' material in a vacuum. It's about matching the material to your specific attic's constraints, your budget's reality, and your crew's skill level. I've burned through time and material on both sides of this fence, so I'm going to walk you through the comparison the way I wish someone had for me back in 2019 — by looking at the key dimensions that actually matter.

So, what are we actually comparing?

We're looking at two heavyweights in the Johns Manville lineup for residential attics: fiberglass blown-in insulation (like their ComfortTherm or Spider lines) versus closed-cell spray polyurethane foam (SPF) (like their ProGlas or Corbond lines). Not batts, not open-cell foam — specifically these two workhorses. (I should add that I'm focusing on attics here, not crawlspaces or walls. Different animals entirely.)

The popular oversimplification is, 'Spray foam is better because it has a higher R-value per inch.' It's tempting to think you can just compare R-values. But identical specs on paper can result in wildly different outcomes once you factor in the house's envelope and the installer's attention to detail.

Dimension 1: Air Sealing & Thermal Performance

Spray Foam: The Airtight Contender

Spray foam is the undisputed champion of air sealing and thermal performance in one shot. It expands to fill every crack, crevice, and weird corner of an attic. An R-value of about 6.5 per inch for closed-cell means you can get a high R-value in a thinner profile. This was true when it first became popular 15 years ago, and it's still true today. It creates a monolithic, airtight thermal envelope.

Fiberglass: The Misunderstood Workhorse

Here's where the legacy myth comes in. 'Fiberglass is drafty.' This was true 20 years ago when we were slapping un-faced batts between ceiling joists and hoping for the best. Today, a properly installed blown-in fiberglass attic insulation system — installed to the correct depth and density — is one of the most cost-effective solutions. But — and this is the critical point the 'R-value only' crowd misses — fiberglass does not air seal. It's a thermal material, not an air barrier. You need a separate air sealing step (caulking, foam on top plates, etc.) for it to perform as intended.

In our testing on a 2,400 sq ft ranch home in Denver: We retrofit the old, settled fiberglass with new blown-in (JM Spider). We did not air seal first — a mistake I made in my first year (2017). The homeowner complained of drafts. We went back, spent another $800 on air sealing materials and labor. The result? Their energy bill dropped another 18%. The fiberglass itself was never the problem; the missing air sealing was.

The verdict on this dimension: Spray foam wins on air sealing + thermal performance in a single step. Fiberglass wins only if you commit to a separate, thorough air sealing process. Skipping that step is a false economy.

Dimension 2: Installation Complexity & Risk

Fiberglass: Forgiving, but Demanding

Blowing fiberglass into an attic is physically demanding, but it's technologically forgiving. You're rocking a hose, making sure you hit the right depth marks on the trusses. The material itself is stable. The biggest risk? Uneven coverage, not catastrophic failure. I once ordered 30 bags of fiberglass for a small attic job — checked it myself, approved the delivery, processed the invoice. We caught the issue when the insulation inspector flagged low spots in the corners. That cost me a Saturday and a $450 lost profit from the redo. Learn the lesson: always use depth markers and verify after the blower leaves.

Spray Foam: Results-Dependent on the Pro

Spray foam is a chemical reaction happening in real-time inside your attic. The quality depends entirely on the operator. The temperature of the components, the mix pressure, the substrate temperature, the humidity — if any of these are off, you get a compromised result. In September 2022, I watched a crew install closed-cell foam on a 95°F day. The foam gelled too fast. It pulled away from the roof deck, leaving cryptic air channels — voids you couldn't see until you cut into it a week later. The homeowner's energy bill went up, not down. The fix required removing the foam and starting over — a $3,200 disaster for the installer.

The verdict on this dimension: Fiberglass is lower risk for the average crew. Spray foam's quality is binary — perfectly installed, it's magical. Poorly installed, it's a money pit. A specialist who knows their limits is worth more here than a generalist who overpromises.

Dimension 3: Long-Term Performance & Maintenance

Fiberglass: Will It Settle?

Yes. Blown-in fiberglass settles over time, typically losing 5-8% of its installed height in the first few years. This is a known, predictable behavior. When we sized our attic for a rental property, we built in an extra 2 inches of depth specifically to account for this. The product's performance is stable; it just requires understanding its physical properties. It doesn't degrade chemically, it doesn't off-gas (once installed), and it's not a food source for pests — though mice will nest in it if they have a point of entry.

Spray Foam: The Aging Question

The 'lifetime' promise of spray foam is being debated more intensely than ever. The problem is that high-performance closed-cell foam has only been widely used in attics for about 20 years. We don't have 50-year data. It can become brittle and shrink if the formula wasn't perfect. It can be a fire hazard if not covered with a thermal barrier. And — here's the big one — it makes future renovations or electrical work a nightmare. Want to add a light fixture? You're sawing through cured foam. Want to inspect a roof leak? The foam blocks the view. The vendor who told me 'spray foam is a permanent solution and never needs maintenance' lost my trust. There is no 'permanent' in building construction. Everything needs to be accessible.

The verdict on this dimension: Fiberglass is predictable and serviceable. It's boring, but it works. Spray foam offers incredible immediate performance, but it introduces significant long-term serviceability and chemical aging risks. You're betting on chemistry.

Dimension 4: Cost & Return on Investment

Fiberglass: The Upfront Winner

The total cost of ownership for a blown-in attic insulation project includes the material, the air sealing, and the labor. Real numbers from a 1,500 sq ft attic (R-49 target) based on late 2024 pricing:
- Material (JM blown-in fiberglass): ~$0.70 – $0.90 per sq ft
- Air sealing materials (caulk, foam, weatherstripping): ~$0.15 – $0.30 per sq ft
- Labor (professional crew, not DIY): ~$0.50 – $0.75 per sq ft
Total estimated cost: $1.35 – $1.95 per sq ft, or roughly $2,000 – $2,900 total.

Setup fees for the blower are negligible for pro crews. The payback period is typically 2-4 years through energy savings.

Spray Foam: The Premium Performance

Closed-cell spray foam is significantly more expensive due to material cost, specialized equipment, and certified labor. Using a similar project:
- Material (JM closed-cell SPF): ~$1.50 – $2.50 per sq ft (at 4-inch depth)
- Labor: ~$1.00 – $1.50 per sq ft
Total estimated cost: $2.50 – $4.00 per sq ft, or roughly $3,750 – $6,000 total.

Setup fees are often included. The payback period is longer — typically 5-8 years, depending on local energy prices. The value isn't in the speed of the payback; it's in the certainty of the performance and increased home value for a discerning buyer.

So, Which One Should You Choose?

This is where the 'professional boundary' comes in. I can't tell you one is universally better. I can tell you what works in what scenario.

• Choose blown-in fiberglass if:
  - You're on a strict budget and need a proven, predictable solution.
  - You or your crew are willing to spend the upfront time on proper air sealing.
  - You want a material that's easy to modify, service, and eventually remove.
  - The attic is unconditioned and ventilation is already good.
• Choose spray foam if:
  - You have an unconditioned attic and you want to bring it into the thermal envelope (creating a conditioned attic).
  - You have severe air leakage issues that are difficult to seal mechanically.
  - You have limited height to work with and need maximum R-value per inch.
  - You're working with a certified, experienced installer with a track record — and you have a budget that can handle the premium.

One last thing: I used to think 'spray foam is the best' was the end of the conversation. After the 2022 disaster and subsequent testing on 15+ projects, I've learned that a well-executed fiberglass job often outperforms a sloppy foam job — and is far easier to fix. The best material is the one that's installed correctly on a house that's properly designed for it. Feel free to check the Johns Manville technical bulletins on air sealing vs. foam performance for the latest data; I've linked to them on our team's checklist. Just don't learn this one the hard way like I did.