Insulation FAQ

Start with area and target R-value. Multiply your square footage by the R-value gap you need to close, then divide by the coverage rating on the bag. Our attic and blown-in calculators do exactly that arithmetic for you.

Attics: R-30 in the far south, R-49 through the middle of the country, R-60 from the Midwest north. Walls: R-13 to R-21 depending on framing depth. Floors over unconditioned space: R-19 to R-38.

Blown-in attic work runs about $1.70 to $1.85 per square foot installed at R-49 — cellulose at the lower end, fiberglass just above it. Batts in open walls are around $1.75 at R-15. Closed-cell spray foam is the expensive one, roughly $3.60 per square foot at the 2 inches it is usually sprayed at.

Those are 2026 national averages for a contractor-installed job. Labor is most of the number, so the same work moves about 30% either side of it between the cheapest and priciest markets. The cost-per-square-foot guide breaks it down by material, and the state pages adjust for where you are.

Blown-in attic insulation and unfaced batts in open framing are very DIY-friendly — the machine is free with a bag minimum at most stores. Dense-pack in closed walls and large spray foam jobs are not; leave those to a contractor with the right rig.

In cold climates, yes on the warm-in-winter side. In hot-humid climates a vapor barrier on the inside can trap moisture and cause more harm than good. Air sealing matters more than vapor barriers almost everywhere.

The mistake that actually damages houses is having two: a poly sheet on the inside and a foil facing or foam board on the outside traps any moisture that gets between them, with no direction to dry in. One retarder, on the correct side, or none at all.

In an attic, usually yes, and it is the cheapest R-value you will ever buy. Blow or lay unfaced material straight over what is there. The one rule is that the new layer must be unfaced — a second vapor retarder buried mid-stack is exactly the sandwich described above.

Replace rather than top up if the existing material is wet, compacted flat, smells of rodents, or is contaminated. Removal runs about $1.55 per square foot on top of the new work, which is why it so often doubles a quote.

Depth follows from the R-value target and the material, because each material delivers a different R-value per inch. Blown fiberglass is about R-2.5 per inch, blown cellulose about R-3.6, fiberglass batts about R-3.2, closed-cell spray foam about R-6.5.

So R-49 is roughly 20 inches of blown fiberglass, about 14 inches of cellulose, or under 8 inches of closed-cell foam. Measure to the rating, not to a number of inches someone quoted for a different material.

Loose-fill does, and manufacturers account for it. Bag coverage ratings are published at settled depth, so a correctly blown attic hits its rated R-value after settling rather than before. The failure mode is under-blowing to start with, not settling.

This is why the depth ruler stapled to a joist matters. Blow to the installed depth on the bag chart, not to the depth you want the finished layer to be.

Because they stop different things. Insulation slows conduction — heat moving through material. Air sealing stops convection — heated air physically leaving through gaps around recessed lights, the attic hatch, plumbing stacks and top plates.

Blowing insulation over unsealed penetrations does not close them; air moves through the fibres. An attic that is air sealed first and then insulated to R-38 usually outperforms one blown to R-60 over open holes, for less money.

Topping up an under-insulated attic usually does, and quickly. Heat loss through a surface is inversely proportional to its R-value, so the first inches do most of the work: going from R-11 to R-19 cuts loss through that surface by about 42%, while going from R-49 to R-60 cuts it by about 18%.

That is the honest shape of it. There is a lot of value in fixing a bad attic and very little in chasing the last few R-points on a good one. The energy savings calculator will run your own numbers, including a payback period.

Yes, wherever there are soffit vents. Insulation pushed into the eaves blocks the airflow the roof relies on, which in cold climates is how ice dams start. Baffles hold the channel open and cost very little.

Keep insulation off anything that gets hot too — recessed lights that are not IC-rated, and the flue. Both need a dam built around them rather than material packed against them.