Types of Attic Insulation: Batts vs. Blown-In vs. Spray Foam
Updated: 4 hours ago
I’ve been crawling through attics as a HERS rater for a long time. The question I get most isn’t “what’s the best insulation?” It’s “why is my upstairs still miserable when I’ve got insulation up there?” Usually the answer has less to do with the type of insulation and more to do with how it went in.
So here’s the straight version: what the main types are, what each one is actually good at, and how much of it code expects you to have.
The Short Answer
For most existing homes with a regular open attic, blown-in cellulose or fiberglass is the best bang for the buck. It fills around wires, pipes, and odd framing in a way batts never will. Batts work fine when the attic is wide open, and someone takes the time to install them right. Spray foam solves specific problems, like cramped attics, HVAC equipment in the attic, and complicated rooflines. It’s not an automatic upgrade.
Fiberglass and Mineral Wool Batts
Batts are the pre-cut blankets most people picture. Fiberglass is the cheapest and most common. Mineral wool costs a bit more, but it’s denser, handles fire better, and holds its shape.
Where batts work: open attics with standard joist spacing and not a lot of obstructions. They’re also the easiest DIY option.
Where they fall short: batts only perform when they fill the cavity completely, with no gaps, no compression, and no voids around wiring. Every gap is a hole in your thermal blanket, and squishing a batt to make it fit lowers its R-value. When I see batts underperforming, it’s almost always installation, not the product.
If you’re adding a second layer over existing insulation, run the new batts perpendicular to the joists. That covers the tops of the joists and the seams in the first layer. Use unfaced batts for that top layer. A second facing creates a vapor trap you don’t want.
Blown-In (Loose-Fill) Insulation
Loose-fill is blown into the attic with a machine and comes in two main flavors.
Cellulose is mostly recycled paper treated with fire retardant. It’s a little more R-value per inch than blown fiberglass, and it’s dense enough to slow air movement somewhat. It does settle over time, so installers blow in extra depth to account for it.
Blown fiberglass is lighter, doesn’t absorb moisture as readily, and settles less. Because it’s fluffier, you need more depth to hit the same R-value.
Why I like blown-in for most retrofits: it flows around everything, fills the corners, and covers the joist tops in one continuous layer. It’s also easy to top off an existing attic without tearing anything out.
Watch for: blocked soffit vents. Baffles (rafter vents) need to go in at the eaves before anything gets blown in, or you’ll choke off your attic ventilation. Also check for depth markers so you can confirm you got what you paid for.
Spray Foam
Spray foam comes in two types:
Open-cell is lighter, cheaper, and good at air sealing, but it has a lower R-value per inch, and it lets vapor pass through.
Closed-cell is dense and rigid, with the highest R-value per inch of anything on this list. It also acts as a vapor retarder.
Spray foam is usually applied to the underside of the roof deck. That turns the attic into a conditioned (unvented) space instead of insulating the attic floor. That’s a big design change, not a simple insulation swap.
When it makes sense: ductwork or HVAC equipment in the attic (bringing it inside the thermal envelope is a real efficiency win), complicated roof framing, cathedral ceilings, or shallow spaces where you can’t fit enough depth of anything else.
The trade-offs: it’s the most expensive option, it needs a qualified installer, and it’s a pain to remove if you ever need to get at the roof deck or wiring. It’s also not magic. A poorly installed spray foam job with thin spots will underperform a good blown-in job.
Rigid Foam Board
Rigid boards (polyiso, XPS, EPS) show up in attics mostly on hatch covers, knee walls, and as continuous insulation on roof assemblies. They’re great for specific spots, but they’re not how you insulate a whole attic floor.
Radiant Barriers
A radiant barrier is a reflective foil that reduces heat radiating down from a hot roof. It doesn’t have an R-value like insulation does. It can help in hot, sunny climates, especially with ducts in the attic. In colder climates, it does very little. Think of it as a supplement, never a substitute.
How Much Attic Insulation Does Code Require?
This is the part most insulation articles skip. For new construction and additions, your ceiling insulation has to meet the energy code your state or jurisdiction has adopted, usually some version of the IECC.
A couple of things worth knowing:
There’s an eave allowance. Under the 2021 IECC, where R-60 is required, R-49 is allowed if it runs full height, uncompressed, over the wall top plates at the eaves. R-38 gets the same treatment where R-49 is required.
Your state may amend these numbers. Plenty of states adopt the IECC with changes, or are still on an older edition. Check your state’s requirements before you buy materials.
You don’t always have to hit the table exactly. A REScheck lets you trade off. Extra attic insulation can offset a window or wall that’s a bit below prescriptive values, and vice versa. That flexibility is exactly why builders use REScheck instead of the prescriptive table.
Don’t Skip Air Sealing
If you take one thing from this page: seal the attic floor before you add insulation. Fiberglass and cellulose don’t stop air. If warm air is pouring up through gaps around light fixtures, plumbing stacks, top plates, chases, and the attic hatch, it carries heat and moisture right through your insulation.
Seal those penetrations first. Weatherstrip and insulate the hatch. Make sure recessed lights are IC-rated and airtight, or covered properly. Then insulate. That order matters more than which type you pick.
Which Type Should You Choose?
Existing home, open attic, tight budget: blown-in cellulose or fiberglass, after air sealing.
Wide-open attic, DIY project: batts, installed carefully, with no gaps or compression.
Ducts or equipment in the attic, or complicated roof framing: consider spray foam at the roof deck.
Hot climate with ducts in the attic: add a radiant barrier as a supplement.
New construction: pick what meets your code R-value at the best installed cost, then prove it on your REScheck.
Frequently Asked Questions
Is blown-in or batt insulation better for an attic? For most existing attics, blown-in. It fills around obstructions and covers joist tops without gaps. Batts can perform just as well on paper, but only with a near-perfect install.
How many inches of attic insulation do I need? It depends on the R-value you’re targeting and the material. As a rough guide, R-49 is about 14 inches of cellulose or closer to 18–20 inches of blown fiberglass. Check the product’s coverage chart for exact depths.
Can I put new insulation over old insulation? Usually, yes, as long as the old insulation is dry, not moldy, and not contaminated by pests. Use unfaced batts or loose-fill on top. Don’t add another vapor retarder.
Does spray foam pay for itself?Sometimes. It makes the most sense when it brings ductwork into conditioned space or solves an air-leakage problem nothing else can. For a simple open attic, blown-in usually gets you a faster payback.
Do I need a REScheck for attic insulation? For new homes and additions in most jurisdictions, yes. Your building department will want to see that the envelope, attic included, meets the energy code. If you’re building and need one, we can turn it around quickly.
Building new or adding on? Your attic R-value is one line on your REScheck. Get the whole envelope checked before inspection. Order your REScheck here.
