How much insulation do I need?
Start with the code minimum for your climate zone, not with what fits. Under the 2021 IRC, attics need R-30 in zone 1, R-49 in zones 2–3, and R-60 in zones 4 and above. Walls need R-13 in zones 1–2 and R-20 in zone 3 upward.
Then check what the cavity can actually hold. A 2×4 wall tops out around R-15 with high-density batts, and a 2×6 around R-21. In zone 4 or colder, where walls want R-20, a 2×4 wall physically cannot get there on cavity insulation alone — that is why continuous exterior foam exists.
Attics are the opposite problem: there is unlimited depth, so hitting R-60 is a matter of blowing 16 inches of cellulose rather than fighting the framing. Attic insulation is also the cheapest R you will ever buy, which is why it is always the first job.
How the calculation works
Blown insulation is sold by the bag with a coverage chart that varies by target depth: the deeper you blow, the fewer square feet a bag covers. Cellulose gives about R-3.7 per inch and fibreglass about R-2.5, so R-60 is roughly 16 inches of cellulose or 24 of blown fibreglass.
Batts are sold by bag area for a given cavity. Bag coverage drops as the batt gets thicker, which is why an R-21 bag covers less wall than an R-13 bag of the same physical size.
Batt width must match the framing. Fifteen-inch batts are for 16 in on centre; 23 in batts are for 24 in on centre. Using the wrong one leaves a gap in every single bay, and a 5% gap in coverage costs far more than 5% of the performance.
None of this gives you the assembly R-value. Wood framing is roughly R-1.25 per inch against R-3.5 for fibreglass, so studs are thermal bridges — a 2×4 wall at 16 in on centre is about 25% framing by area, and the whole-wall R lands well below the number on the bag. That gap is the argument for 24 in spacing and exterior foam.
Typical insulation cost, mid-2026
| Material | Unit | Typical range |
|---|---|---|
| Fibreglass batts | per sq ft | $0.65 – $1.50 |
| Mineral wool battsBetter fire and sound performance | per sq ft | $1.30 – $2.60 |
| Blown fibreglass | per sq ft | $0.50 – $1.10 |
| Blown celluloseHigher R per inch | per sq ft | $0.60 – $1.30 |
| Closed-cell spray foamR-6.5 per inch, air seals as it goes | per sq ft | $1.50 – $4.50 |
| Rigid foam board (R-5/in)Continuous exterior layer | per sq ft | $0.80 – $2.00 |
| Blower rentalOften free with enough bags | per day | $0.00 – $100 |
| Air sealing before insulating | per house | $300 – $1,200 |
Air sealing first is not optional. Insulation slows conduction; it does nothing about air moving through it. An unsealed attic loses more heat through gaps than through the insulation.
What goes wrong on this job
- Air seal before you insulate, always. Top plates, wire penetrations, recessed lights and the attic hatch leak more than the insulation ever conducts, and once the attic is full of loose fill you cannot reach any of it.
- Do not compress batts to fit. A compressed R-19 batt performs like an R-13, and the space you squashed it into does not get the R back. Cut the batt to the bay instead.
- Split batts around wiring rather than jamming them behind. Peel the batt into two layers, put half behind the cable and half in front — stuffing it creates a void the size of your fist.
- Mark the joist depth on a ruler stapled to the attic before blowing. Loose fill looks level and is not, and settling is real — blow to the settled-depth figure on the bag, not to the depth that looks right.
- Leave a gap at the eaves and use baffles. Blocking the soffit vents with insulation is the most common attic mistake and it turns a ventilation problem into a moisture and ice-dam problem.
Frequently asked questions
What R-value do I need in my attic?
Under the 2021 IRC: R-30 in climate zone 1, R-49 in zones 2–3, and R-60 in zones 4 through 8. Older codes said R-38, so many existing attics are well short of current minimums.
How many inches of insulation is R-60?
About 16 inches of blown cellulose at R-3.7 per inch, or roughly 24 inches of blown fibreglass at R-2.5.
Can a 2x4 wall meet R-20?
Not on cavity insulation alone — a 2×4 cavity tops out near R-15. Meeting R-20 needs continuous exterior foam, closed-cell spray foam, or 2×6 framing.
Is blown-in better than batts for an attic?
Usually yes. Blown insulation self-levels around joists, wiring and odd framing, whereas batts leave a gap at every obstruction — and gaps cost far more performance than the nominal R suggests.
What size batts do I need for 24 inch on centre framing?
23 in wide batts. The common 15 in batts are sized for 16 in on centre and will leave a gap in every bay.