What R-Value Do You Need?
Typical targets by climate
| Climate | Attic | 2x4 walls | 2x6 walls | Floors over unheated space |
|---|---|---|---|---|
| Hot — Gulf South, desert Southwest | R-30–49 | R-13 | R-19 | R-13 |
| Mixed — Mid-Atlantic, lower Midwest, inland West | R-38–49 | R-13–15 | R-19–21 | R-19–25 |
| Cold — Upper Midwest, Northeast, mountains | R-49–60 | R-13–15 plus exterior sheathing | R-19–21 | R-25–30 |
Ranges in the spirit of the DOE recommendations. The enforceable number is your local energy code, which works from IECC climate zones — check it for new work and additions.
What R-value actually is
R measures resistance to heat flow: double the R, halve the heat loss through that assembly. Two consequences follow directly:
- R adds in layers. R-19 over R-19 in an attic is R-38. That is why topping up an attic is such an easy job — new unfaced batts laid across the old.
- Returns diminish. Going from R-0 to R-19 cuts loss enormously; from R-38 to R-57 saves a third of what is left of an already small number. This is why nobody builds R-100 anything, and why the right question is "where am I thinnest", not "how high can I go".
Why the attic is first, every time
- It is usually the biggest loss surface facing the biggest temperature difference — heat rises to the ceiling and leaves through it.
- Depth is free up there. A wall caps out at its framing depth; an attic takes as much material as you can pile in, no demolition.
- The work is cheap and reversible — lay batts or blow loose fill over what exists.
Walls, by contrast, are capped by cavity depth (R-13/15 in a 2x4, R-19/21 in a 2x6 — a fatter batt compressed in loses R rather than gaining it), and getting past that cap means exterior foam or deeper framing, i.e. a renovation. Retrofit priority is almost always: attic → air sealing → rim joist and floors → walls last.
The asterisk that matters: air sealing
R-value assumes still air. Gaps around top plates, ceiling penetrations, recessed lights and the attic hatch move heat by bulk airflow, which insulation does not stop — R-60 with a leaky attic hatch underperforms R-38 with a sealed one. Caulk and foam the penetrations before burying them under new insulation, because afterwards nobody ever digs them out.
Reading the numbers on the product
| Product | R per inch | Note |
|---|---|---|
| Fibreglass batt | ~3.1–3.7 | The default; must fill the cavity |
| Mineral wool batt | ~4.0–4.3 | Denser, fire-resistant, cuts cleanly |
| Blown fibreglass / cellulose | ~2.2–3.8 | Attics; bag chart gives depth per R |
| EPS / XPS / polyiso foam board | ~4–6.5 | Exterior sheathing, rim joists, basements |
| Closed-cell spray foam | ~6–7 | Highest R per inch, highest cost |
For batts the arithmetic is done for you — buy the R printed for your framing depth. For blown attic insulation the bag chart converts target R to inches of depth and bags per square foot, and the insulation calculator's bag-coverage field takes that figure directly.
Work out how many bags of batt insulation a wall or attic needs. The tool deducts the framing — batts go between the studs, not over them — and divides by the coverage printed on your bag.
Frequently asked questions
What R-value should an attic be?
R-30 to R-49 in warm climates and R-49 to R-60 in cold ones, per the usual DOE-style guidance — your local energy code gives the enforceable figure. If the existing insulation is below joist height, topping up is almost always worth it.
What R-value fits in a 2x4 wall?
R-13 or R-15 — that is what 3.5 inches of cavity holds. A 2x6 wall takes R-19 or R-21. Compressing a thicker batt into a thinner wall reduces its performance instead of improving it; going beyond the cavity means exterior foam board.
Is a higher R-value always better?
Returns diminish: each doubling of R halves what is left of an already-reduced loss. The efficient strategy is raising the thinnest surfaces first — usually the attic — and sealing air leaks, rather than pushing one assembly to extremes.
Does insulation work if air leaks through?
Poorly. R-value measures resistance to conduction through still material; bulk airflow around and through it carries heat regardless. Seal penetrations, top plates and the attic hatch before adding insulation — afterwards they are buried.