What Insulation Do I Need?

Best material:

Blown-in cellulose ($0.60–$2.30/sq ft) for open accessible attics. Achieves R-49 in ~14", R-60 in ~17" settled depth. Flows around wiring, plumbing, junction boxes better than batts. DIY-friendly with rented blower from Home Depot/Lowe's.

Targets:

• Zone 1: R-30 minimum
• Zones 2–3: R-49 minimum
• Zones 4–8: R-60 minimum

When to consider spray foam:

Conditioned attics (HVAC equipment in attic) where the roof deck is insulated instead of the floor. 2× the cost of blown-in but eliminates duct losses in unconditioned attic air.

Air seal first:

Air sealing the attic floor before insulating saves an additional 15–25% on energy bills per DOE field studies. Top plates, wire/pipe penetrations, recessed lights, attic hatch, duct boots.

New construction:

Fiberglass batts sized to cavity (R-13 in 2×4, R-19 or R-21 HD in 2×6). In zones 4–8, add continuous exterior rigid foam (R-5ci or R-10ci) to reduce thermal bridging.

Code targets:

• Zones 1–2: R-13 cavity
• Zones 3–4: R-20 or R-13+5ci
• Zones 5–8: R-20+5ci or R-13+10ci

Retrofit (no demo):

Dense-pack cellulose blown through small holes drilled in siding/drywall, packed to 3–3.5 lb/ft³. Adds R-12–R-13 to a 2×4 cavity plus significant air-sealing benefit. Pro-only job ($1.50–$3.00/sq ft).

Avoid:

DIY consumer blowers can't reach proper dense-pack density. Half-density installations settle within a year and leave the top 18" of every wall empty.

Best for moisture:

XPS rigid foam directly on the foundation wall (1.5–3" for R-7.5 to R-15) is moisture-tolerant, frame a 2×4 wall over it for finishing. Or 2–3" closed-cell spray foam for built-in vapor control.

Code targets:

• Zones 1–2: Not Required
• Zone 3: R-5ci or R-13
• Zone 4: R-10ci or R-13
• Zones 5–8: R-15ci or R-19

Highest-ROI add-on:

Rim joists. 2" closed-cell spray foam ($800–$2,000 for a typical home) seals one of the largest air leakage points in the building envelope. Blower-door measurements show 0.5–1.0 ACH50 reductions from rim joists alone.

Avoid:

Fiberglass batts directly against concrete walls. They wick moisture, lose R-value, and grow mold. If you must use batts, they go in a framed wall over rigid foam, not against concrete.

Modern best practice:

Encapsulate: seal vents, install heavy-duty vapor barrier on dirt floor, insulate walls with 2" closed-cell spray foam or rigid foam. Crawl space becomes part of conditioned envelope. $3,000–$15,000 depending on size and access.

Traditional approach:

Insulate floor above crawl space (R-19 to R-30 batts between joists, supported with insulation wires). Keep vents open. Less expensive but moisture issues are common.

Avoid at all costs:

Fiberglass batts in vented crawl space, unsupported. They sag within years, absorb moisture, grow mold. The most commonly documented insulation failure mode in residential energy audits.

Vapor barrier first:

A 6+ mil polyethylene barrier on the dirt floor is non-negotiable. Soil moisture creates continuous vapor drive upward through the crawl space without it.

Attached garage walls:

Walls between garage and conditioned space need fire-rated batts (mineral wool R-15 in 2×4, R-23 in 2×6) plus 5/8" Type X drywall on the garage side. Detached unheated garages generally don't need wall insulation.

Floor over garage:

Match the wall requirement for the climate (R-19 to R-30 batts between joists). Best practice: 2" closed-cell spray foam on the underside (R-12–R-14) plus batts above for total R-30. Air-seals and prevents CO migration into living space.

Garage doors:

Most heat loss in attached garages is through the door, not the walls. A new insulated steel garage door (R-12 to R-18) costs $800–$2,500 and pays back faster than wall insulation in most cases.

Heating decision:

If you're NOT going to heat the garage, just insulate the shared wall and the floor above. Insulating the exterior walls of an unheated garage is wasted money.