Best Insulation for New Construction: What to Spec and Why

Why new construction is the easiest time to get this right

In a retrofit, you're often working around existing framing, vapor barriers, and constraints that limit your options. In new construction, the cavity is open and you can install exactly what the job requires. The cost difference between meeting code minimums and exceeding them by one product tier is usually small — but the performance difference lasts the life of the building.

This guide covers the standard spec for each assembly in residential and light commercial new construction, with notes on where upgrading makes sense.

Above-grade exterior walls

2×4 framing: R-13 is code minimum in most zones; R-15 (high-density) is the preferred upgrade. Both at 3.5" thick. In climate zones 4+, consider adding continuous insulation (CI) on the exterior to meet the full requirement — cavity-only R-13 may not satisfy code in colder climates.

2×6 framing: R-21 is the standard spec for zones 4–8. It's high-density, thinner than R-19 (5.5" vs 6.25"), and satisfies R-20+ code requirements. R-19 is also used but R-21 is increasingly the default given the same installation effort.

For fire-rated walls (commercial, multi-family): Mineral wool R-15 (2×4) or R-21 (2×6) is specified when non-combustibility is required by code.

Attics

Attics are the highest-ROI assembly in any building. Codes have steadily increased requirements here:

• Zones 1–2: R-30 minimum; R-38 recommended
• Zone 3: R-38 minimum; R-49 recommended
• Zones 4–8: R-49 minimum; R-60 is increasingly specified in colder climates

Blown-in fiberglass is the most common spec for new construction attics. It fills irregularities around can lights (using approved covers), HVAC ducts, and framing intersections in a way that batts can't. It also achieves higher R-values per inch than standard batts.

Floor cavities

Over unconditioned crawl spaces and basements, R-19 is the code standard in most jurisdictions for 2×8 and 2×10 floor framing. Some colder-climate codes require R-25 or R-30 — verify locally.

For floor assemblies, the facing faces up toward the living space. Use unfaced batts in conditioned crawl spaces or when the crawl space itself is being conditioned with spray foam perimeter insulation.

Basement walls

Below-grade basement walls are often overlooked in new construction. In zones 4+, codes typically require R-10 to R-15 for below-grade walls. Options include:

• Continuous rigid insulation on the exterior of the foundation (most common in commercial)
• Mineral wool batts on the interior (R-15 in 2×4 framing) — preferred over fiberglass because mineral wool's hydrophobic properties handle moisture better in below-grade environments

Interior partition walls — don't forget sound

Building codes don't require insulation in interior partitions, but they're still worth insulating for sound. In bedrooms, bathrooms, and between units in multi-family construction, R-11 or R-13 batts (or mineral wool) in interior partition walls make a measurable difference in occupant comfort.

For multi-family party walls where STC ratings are required, mineral wool is the standard specification. Its density and rigidity give better mid-frequency sound absorption than fiberglass.

Spec sheet summary by assembly

The most common new construction mistake

Specifying R-values that barely meet code minimums, then discovering mid-project that the local AHJ requires something higher or that the energy modeler needs more to achieve the HERS target. The cost to upgrade from R-19 to R-21 during framing is minimal. The cost to remediate after drywall is significant.

When in doubt, go one tier above code minimum on walls and two tiers on attics. The material cost difference is small at the time of construction. The long-term performance difference is real and your clients will notice it on their utility bills.