Attic Insulation Fiberglass Cellulose or Spray Foa

I was in an attic last week that had fiberglass batts installed in 1985. They were dirty, compressed, and full of mouse droppings. The homeowner, a woman named Patty, asked me if she should replace them with cellulose or spray foam. She had read that spray foam was the best. But she also heard it was expensive. She wanted to know if the extra cost was worth it.

I told her the first rule of attic insulation: anything is better than nothing. But if you’re going to spend money, spend it wisely. Fiberglass, cellulose, and spray foam each have pros and cons. The best choice depends on your attic’s condition, your budget, and your climate.

Let me start with fiberglass. It’s the most common attic insulation in America. Comes in batts or rolls. Pink, yellow, or white. Easy to install. Inexpensive. About $0.50 to $1.00 per square foot for material. Installation adds another $0.50 to $1.00. So total $1.00 to $2.00 per square foot for R‑13 to R‑19. For a 1,500 square foot attic, that’s $1,500 to $3,000.

The problem with fiberglass is that it doesn’t stop air movement. Air can flow right through it. If your attic has leaks — and every attic has leaks — the fiberglass will not prevent warm air from rising into the attic in winter. That warm air carries moisture. In cold climates, that moisture can freeze on the roof sheathing, causing ice dams. Fiberglass also loses R‑value when it gets dirty or compressed. Over time, it settles. And mice love it.

Patty’s attic had compressed fiberglass from years of storage boxes. The R‑value had dropped from maybe R‑19 to R‑10. It was useless.

Cellulose is made from recycled newspaper. It’s treated with borates for fire resistance and pest control. It’s blown in as loose fill. It costs about $1.00 to $1.50 per square foot for R‑38, installed. For a 1,500 square foot attic, that’s $1,500 to $2,250. Cellulose is denser than fiberglass. It does a better job of blocking air movement. It also fills gaps and crevices better. It’s more environmentally friendly because it’s recycled. And it doesn’t settle as much as fiberglass.

The downside of cellulose is that it absorbs moisture. If you have a roof leak, the cellulose will get wet and clump. Wet cellulose loses R‑value and can grow mold. It’s also heavy. A fully insulated attic with cellulose can add several hundred pounds. That’s fine for most attics, but if your ceiling is old plaster, it could be a problem.

Patty’s house was built in 1950. The ceiling was plaster over lath. It could handle the weight. But she had a minor roof leak near the chimney. I told her to fix the leak before adding any insulation.

Spray foam is the premium option. It comes in two types: open cell and closed cell. Open cell is lighter and cheaper, about $1.50 to $2.50 per square foot for R‑21. Closed cell is denser, about $2.50 to $4.00 per square foot for R‑21. For R‑38, you need about 5 inches of closed cell foam, so cost is higher. For a 1,500 square foot attic, closed cell spray foam could cost $7,000 to $12,000.

The advantage of spray foam is that it air seals and insulates at the same time. It expands to fill every crack and gap. It creates a continuous thermal barrier. It doesn’t settle. It doesn’t absorb moisture. It resists pests. And it has a higher R‑value per inch than fiberglass or cellulose. Closed cell foam has about R‑7 per inch. Open cell has about R‑3.6 per inch. Fiberglass is about R‑3 per inch. Cellulose is about R‑3.5 per inch.

So if you have limited attic height, spray foam can get you to a higher R‑value with less thickness. That’s useful in knee walls or sloped ceilings.

But spray foam is expensive. And it requires professional installation. You can’t DIY spray foam safely. The chemicals are toxic during application. You need a certified installer.

Patty got three quotes for spray foam. The lowest was $8,500 for closed cell at R‑38. That’s a lot of money. Her annual heating bill was about $1,200. Even if the spray foam cut her bill in half — which is optimistic — the payback would be 14 years. That’s longer than the life of many roofs.

I suggested a different approach. Air seal first. That’s cheap. A few cans of spray foam and some caulk. Then blow in cellulose. That would give her air sealing plus insulation for about $2,000. She would save about $400 per year on heating. Payback 5 years. She liked that.

We air sealed the attic floor. We used spray foam in a can to seal around wire penetrations, plumbing stacks, and the attic hatch. We used caulk for smaller gaps. That took about four hours. Cost $50 for materials. Then we blew in cellulose to R‑49. Cost $1,800. Total $1,850. Her heating bill the next winter dropped from $1,200 to $800. That’s $400 saved per year. Payback 4.6 years.

She was thrilled. She said she didn’t miss the spray foam.

So when should you choose spray foam? When you have a difficult attic. Hard to access. Lots of obstructions. Limited headroom. When you have a roof leak that you’ve fixed. Closed cell foam won’t absorb moisture. Also when you have a conditioned attic. That means you want the attic itself to be part of the living space. In that case, spray foam on the roofline is the best solution. But that’s a different project.

For most attics, cellulose is the best value. It’s cheap. It’s effective. It’s environmentally friendly. It air seals better than fiberglass. And it’s easy to install.

Fiberglass batts are still useful for crawl spaces, between floor joists, and for DIY projects. But for attics, I rarely recommend them.

Now, there’s one more option: blown‑in fiberglass. It’s like cellulose but made of fiberglass. It’s lighter, doesn’t absorb moisture, and has a higher R‑value per inch. But it’s more expensive than cellulose and doesn’t air seal as well. It’s a middle ground.

Patty’s neighbor chose blown‑in fiberglass. He paid $2,200 for R‑49. His bill dropped from $1,300 to $900. Payback 5.5 years. Slightly worse than cellulose, but still good.

The Department of Energy recommends attic insulation of R‑38 to R‑60 in most of the U.S. Zone 1 (Florida, Texas) needs R‑30 to R‑49. Zone 5 (Colorado, Nebraska) needs R‑38 to R‑60. Zone 7 (Minnesota) needs R‑49 to R‑60. Check your zone.

If your attic has less than R‑20, you’re under‑insulated. Adding insulation is one of the best energy investments you can make. The payback is usually 3‑7 years.

I have a tool that helps you calculate the ROI.

The tool also includes air sealing benefits. For cellulose and spray foam, it assumes a 10‑20% air sealing bonus.

For people who want to measure their own attic, the Home Energy Bill Breakdown can show how much you spend on heating.