How to Insulate an Attic

Measure the attic floor footprint (length × width of each rectangle) and note the depth of anything already up there. Old fiberglass that has settled to 5 inches is roughly R-15 — nowhere near current targets.

The Department of Energy recommends R-49 to R-60 in attics for most of the country, and R-30 to R-49 in the warmest zones. You are topping up to that number, not starting from zero, so subtract whatever the existing layer is worth.

Insulation slows heat conduction; it does not stop air movement. If you bury leaks under 16 inches of cellulose you have insulated a chimney. Every top plate, plumbing stack, wiring hole, bath fan housing and dropped soffit is a hole into the attic.

Use canned foam for gaps under an inch, fire-rated caulk or sheet metal plus high-temp sealant around flues and chimneys, and rigid foam capped with foam for large chases. This is the step that turns a mediocre insulation job into a noticeable one.

Staple baffles into every rafter bay above the soffit vents so incoming air has a clear channel to the ridge. Blocked soffits are the number one cause of ice dams and damp sheathing after an insulation job.

Build a dam of rigid foam or metal flashing around non-IC-rated recessed lights, chimneys and flues, keeping insulation 3 inches away. Fit an insulated cover over the attic hatch or pull-down ladder — an uncovered hatch can leak as much as a small open window.

For batts: lay them tight against each other with no gaps at the ends, friction-fit between joists, and run a second layer perpendicular across the joists to cover the thermal bridge the framing creates. Never compress a batt to make it fit — compressed fiberglass loses R-value fast.

For blown-in: rent a blower (usually free with a bag minimum), have a helper feed the hopper, and work from the far corner back toward the hatch. Set depth markers — cheap rulers stapled to joists — so you can see when you have hit the target inches without guessing.

Walk the perimeter and check that the insulation reaches the top plate over the exterior walls without stuffing the soffit vents. Eaves are almost always the thin spot.

Note the final depth and R-value on the framing near the hatch. Future you — and any home inspector — will thank you.

Skipping air sealing, blocking soffit vents, burying knob-and-tube wiring, compressing insulation with stored boxes, and installing a vapor barrier on top of the insulation in a cold climate. Any one of these can cancel most of the benefit.