Attic insulation comparisons usually get argued as an R-value contest, and R-value per inch is only half the story. The other half is air movement. Insulation slows heat transfer through the ceiling; it does not stop warm air from leaking up through the ceiling in the first place. That's why the cheapest material, installed after proper air sealing, often outperforms the premium material sprayed or blown over a leaky attic floor. Here's how the three main types actually stack up — and if you're prepping for the cold season, this pairs with the rest of our winterize your home checklist.
| Fiberglass | Cellulose | Spray foam | |
|---|---|---|---|
| R-value per inch | ~R-3.1 to 3.8 (batts at the high end, blown at the low end) | ~R-3.2 to 3.8, blown | Open-cell ~R-3.5 to 3.7; closed-cell ~R-6 to 7 |
| Air sealing | None on its own | Slows air movement somewhat; not a seal | Yes — foam is its own air barrier |
| Moisture behavior | Doesn't absorb water, but loses R-value while wet and dries slowly | Absorbs and releases moisture; borate treatment resists mold and pests | Closed-cell resists water and acts as a vapor retarder; open-cell absorbs water |
| DIY-friendly | Batts yes; blown yes with a rental blower | Yes, with a rental blower (often free with a minimum purchase) | Closed-cell and large jobs: no. Small kits only for tiny areas |
| Relative cost | Low to mid | Usually the lowest per R-value installed | Highest by a wide margin |
Fiberglass comes as batts — the pink or yellow rolls and pre-cut blankets — and as loose fill blown through a hose. Batts are the most DIY-friendly insulation there is: unroll, cut to length, lay between or across the joists. Their weakness is fit. Every gap around a wire, pipe, or junction box is a leak in your thermal blanket, and batts stuffed or compressed into odd spaces lose the loft that gives them their rating. Blown fiberglass fills those odd spaces better but settles at a lower R-value per inch than batts, so you need more depth for the same result. Fiberglass also does nothing about air leaks, and it irritates skin and lungs during install — gloves, sleeves, a real mask.
Cellulose is recycled newsprint, shredded, treated with borates, and blown in as loose fill. Per inch it's comparable to fiberglass, and per dollar it's usually the best buy in an attic — it flows around obstructions, fills the corners batts leave open, and its density slows air movement through the insulation layer itself better than loose fiberglass does. The borate treatment makes it unappealing to insects and resistant to mold. The honest downsides: it settles over time (installers compensate by blowing it deeper than the target), it can hold moisture if a roof leak soaks it, and a sloppy install with no depth markers leaves thin spots you'll never see from the attic hatch. In the typical scenario — an existing attic that needs more insulation — cellulose blown over what's already there, after air sealing, is the job most pros would quote first.
Spray foam is the odd one out because it's usually applied to the underside of the roof deck rather than the attic floor, moving the insulation boundary to the roofline. Closed-cell foam is the high performer: R-6 to R-7 per inch, rigid, an air barrier and a vapor retarder in one, and it adds some structural stiffness. Open-cell foam is softer, cheaper, roughly fiberglass-level R per inch, and seals air without blocking moisture. Roofline foaming makes real sense for attics with ducts and air handlers in them, cathedral ceilings with no room for batts, and new construction. For a vented attic with nothing in it but insulation, foam at the roofline is usually the most expensive way to solve a problem blown insulation solves for a quarter of the price. It's also a pro job: the chemicals, the respirators, and the off-ratio risk of a bad mix are not homeowner territory.
Many northern attics target around R-38 to R-49 at the ceiling; milder climates often land near R-30. Translate that into inches and the types diverge fast. Cellulose at ~R-3.5 per inch needs about 11 inches for R-38. Blown fiberglass, at the low end of its range, may need 13 to 16 inches. Fiberglass batts get there with two crossed layers. Closed-cell foam hits R-38 in about 6 inches — which is exactly why foam wins in tight roof cavities and loses on price in open attics where depth is free. Whatever you choose, install depth markers and check the far corners; thin spots near the eaves are the most common failure in real attics. If you haven't settled on a target number for your climate yet, our guide to what R-value insulation you need breaks the recommendations down by climate zone.
Compressing batts to fit. A batt rated R-19 squeezed into a shallower space doesn't give you R-19 — you get the R-value of its compressed thickness. Blocking the soffit vents. Insulation pushed into the eaves chokes attic ventilation and invites moisture problems; baffles keep a channel open. This is also why exhaust fans must duct outside, never into the attic — the moisture math in our look at venting a bathroom fan into the attic shows what trapped humid air does up there. Covering recessed lights. Older non-IC-rated cans can't be buried; keep clearance or use rated covers. Ignoring a roof leak. New insulation over a slow leak hides the leak and feeds mold. Adding insulation over knob-and-tube wiring. Old wiring that dissipates heat into open air can't be buried safely — an electrician evaluates that first, no exceptions.
For the standard job — a vented attic, some insulation already there, a homeowner budget — blown cellulose is the value pick, with blown or batt fiberglass a close second, both after air sealing. Choose closed-cell foam when the space itself forces it: ducts in the attic, a roofline that must be insulated, a shallow cavity that can't reach target R any other way. Decide by the attic you have, not by the highest R-per-inch on the bag. And once the insulation is in, the rest of the cold-weather prep still matters — our step-by-step on how to winterize your house picks up where the attic leaves off.
It depends on climate. Cold-climate attics commonly target R-49 to R-60 at the ceiling, moderate climates around R-38, and warm climates near R-30. Your local code and energy program will have a specific number; when in doubt, more depth of blown insulation is cheap insurance since labor is the same trip.
Yes, and it's the normal approach — as long as the old layer is dry, not moldy, and not compressed flat. Blown cellulose or fiberglass goes right over existing batts or fill. If the old insulation is soaked, contaminated by pests, or hiding knob-and-tube wiring, deal with that first.
Sometimes. It's worth pricing when your ducts or air handler sit in the attic, when the ceiling is a cathedral with shallow rafters, or in new construction. For an empty vented attic, blown insulation plus air sealing usually reaches the same comfort for far less money.
Cellulose and open-cell foam dampen sound better than fiberglass batts, thanks to their density and fit. It's a side benefit rather than a reason to choose them — in an attic, thermal performance and air sealing matter far more.
Cellulose settles the most — roughly a fifth of its blown depth over the years is a common planning figure, and installers compensate by blowing extra. Blown fiberglass settles a little; batts and foam essentially don't. Settling is planned for, not a defect, when the install starts deeper than target.