R-value chart

The chart is the reference. The notes underneath are the part that saves money.

• Fibreglass batt — R-3.1/in (R-3.7/in for high-density). Cheapest per R, unforgiving of sloppy installation.
• Blown fibreglass — R-2.6/in loose in an attic. Settles about 2 to 3 percent.
• Blown cellulose — R-3.6/in. Denser, fills around obstructions better, settles 10 to 20 percent if it is dry-blown flat, which is why installers over-blow to a marked depth.
• Mineral wool batt — R-4.2/in. Holds its shape, does not slump, non-combustible, and it is what I would use in a cathedral ceiling or any rim joist I could not foam.
• Open-cell spray foam — R-3.7/in. Air-seals, vapour-open, cheaper than closed-cell.
• Closed-cell spray foam — R-6.3/in. Air barrier, vapour retarder at 1.5 in and up, and it adds racking strength. Three to four times the price of batt per R.
• EPS board — R-4.0/in. Stable, drainable, cheapest board.
• XPS board — R-5.0/in labelled, but it drifts down toward R-4.5 over decades as the blowing agent leaves.
• Polyiso board — R-6.5/in at 75°F, and it drops as it gets colder. At 25°F mean temperature the effective figure is nearer R-5.0. That is fine over a Georgia roof deck and a real problem on the outside of a Minnesota wall.

IECC zones, uninsulated existing house, DOE recommendations:

Zone 1 (south Florida, Hawaii) — attic R-30 to R-49, wood-frame wall R-13 to R-15, floor R-13. Zone 2 (Gulf coast, central Florida, south Texas) — attic R-49 to R-60, wall R-13 to R-15 plus R-4 to R-6 continuous, floor R-13. Zone 3 (Atlanta, Dallas, Los Angeles) — attic R-49 to R-60, wall R-20 or R-13 plus R-5 continuous, floor R-19. Zone 4 (Nashville, Baltimore, Seattle) — attic R-60, wall R-20 plus R-5 continuous, floor R-19 to R-25. Zone 5 (Chicago, Denver, Boston) — attic R-60, wall R-20 plus R-5 to R-10 continuous, floor R-25 to R-30. Zone 6 (Minneapolis, Vermont) — attic R-60, wall R-20 plus R-10 continuous, floor R-30. Zones 7 to 8 (northern Minnesota, Alaska) — attic R-60 and up, wall R-20 plus R-15 continuous or a double-stud assembly, floor R-38.

Basement and crawlspace walls run R-10 to R-19 continuous from zone 3 north, and in a conditioned crawlspace the insulation belongs on the perimeter wall, not stapled between the floor joists where it falls down in five years.

The recommended R is meaningless if the depth is not there. Actual framing depths:

• 2x4 wall, 3.5 in — R-13 fibreglass, R-15 high-density, R-15 mineral wool, R-21 closed-cell.
• 2x6 wall, 5.5 in — R-19 or R-21 fibreglass, R-23 mineral wool, R-34 closed-cell.
• 2x8 rafter, 7.25 in — R-25 batt, and you still owe 1 in of ventilation gap if the roof is vented.
• 2x10 joist, 9.25 in — R-30 batt.
• 2x12, 11.25 in — R-38 batt.

Attic floors are the easy case because depth is free. R-60 in blown cellulose is about 17 in; in blown fibreglass about 22 in. Both are far above the top plate, so mark the depth on the trusses before the crew starts and check it after they leave.

Getting to R-20-plus in a 2x4 wall means going outside the framing with continuous board. Which is the right answer anyway, because the studs themselves are only R-4.4 and they are 23 percent of the wall.

Compression destroys the rating and the compression is usually yours. An R-19 batt is 6.25 in of material designed for a 6 in cavity. Squeeze it into a 5.5 in 2x6 bay and it performs at about R-18. Squeeze R-19 into a 2x4 bay, which people do because the store had it, and you get roughly R-13 for double the price of an R-13. Same the other way: a batt stuffed behind a wire or bunched around a box leaves a void, and a 5 percent void in the insulated area costs you far more than 5 percent of the R.

R-value is measured at 75°F mean temperature. ASTM C518, steady state, no air movement, no sun. A Florida attic in August runs 130°F, and the standard cannot describe what happens up there — where radiant gain and duct leakage dominate and a radiant barrier or a sealed, insulated duct run buys more comfort than another R-11 of blown fibreglass. Polyiso is the specific case where the number moves the wrong way in the cold, as above.

Past code minimum, air sealing beats added R. A house at R-38 in the attic with an unsealed top plate, open chases and a leaking attic hatch loses more heat through air movement than through the insulation. Foam the top plates, gasket the hatch, seal the can lights and the plumbing penetrations, then blow. Going R-38 to R-60 on a leaky attic is buying the last 15 percent while ignoring the first 40. A blower door test at $300 to $600 tells you which house you have.

A wall labelled R-13 is not an R-13 wall. Whole-assembly R accounts for the studs, plates, headers and rim, which is why energy codes now specify things like "R-20 or R-13 plus R-5ci".

Rough numbers for a 2x4 wall at 16 in on centre, 23 percent framing factor: - R-13 cavity only, no exterior board: assembly around R-11. - R-15 cavity plus R-5 continuous: assembly around R-17. - R-15 cavity plus R-10 continuous: assembly around R-22.

The continuous layer does disproportionate work because it is the only part of the wall the studs cannot short-circuit, and it moves the sheathing's dew point outward, which is a moisture argument as much as an energy one. Get the ratio wrong in a cold zone — thin foam over a full cavity — and you condense inside the wall. Building America publishes the minimum exterior-R ratios by zone and they are worth checking before ordering board.