What R-value do I need in an Iowa attic?

Iowa is in IECC Climate Zone 5. Current code targets R-49 in attics. With open-cell at R-3.7/inch that's about 13″ of foam; with closed-cell at R-7/inch, about 7″.

Does spray foam lose R-value over time?

Closed-cell sees a small first-year drop (~5–10%) as the blowing agent stabilizes, then stays put for decades. Open-cell uses water as the blowing agent and doesn't change. Both vastly outperform fiberglass, which loses R-value to settling and air movement starting day one.

Is more R-value always better?

Past a point, no. Diminishing returns kick in around R-49 in our climate — doubling that to R-100 doesn't double your energy savings. Air-sealing matters more than chasing extra R-value past code.

Spray Foam R-Value Explained: What the Numbers Actually Mean

R-value is the most-cited and least-understood number in insulation. Here's what it actually measures, why a higher number isn't always better, and what you really need in an Iowa home.

What R-value actually measures

R-value is a material's resistance to conductive heat flow per inch of thickness. Higher number = slower heat transfer. That's it. It does not measure air leakage, moisture resistance, or anything else. A material can have a great R-value and still let your house leak heat like a sieve — which is exactly the problem with fiberglass batts.

R-value by material

Closed-cell spray foam: R-7.0 per inch
Open-cell spray foam: R-3.7 per inch
Blown-in cellulose: R-3.5 per inch
Fiberglass batts: R-3.1–3.5 per inch (when perfectly installed; less in practice)
Rigid foam board (XPS): R-5 per inch

Iowa code: what you actually need

Iowa sits in IECC Climate Zone 5. Current residential code targets:

Attic: R-49
Wood-framed walls: R-20 cavity, or R-13 + R-5 continuous
Floors: R-30
Basement walls: R-15 continuous or R-19 cavity
Crawl space walls: R-15 continuous or R-19 cavity

New construction has to hit these. Retrofits don't have to, but if you're paying for the upgrade you might as well meet code. Most attic insulation jobs in Bettendorf and Davenport land at R-49 or just above.

Why "effective R-value" beats nominal R-value

A tag on a fiberglass batt says R-19. The actual installed performance is closer to R-11 once you account for compression, gaps, and air movement through and around the batt. Spray foam, because it air-seals while it insulates, delivers very close to its nominal R-value in real-world conditions.

Put differently: R-13 of spray foam often outperforms R-19 of fiberglass in actual energy bills, because the spray foam stops the air leakage that's robbing the fiberglass.

Thickness math for typical QC jobs

To hit R-49 (Iowa attic code):

Open-cell: ~13″ of foam
Closed-cell: ~7″ of foam
Blown-in cellulose: ~14″
Fiberglass batts: typically R-38 batt + R-11 = stacked

For most attic jobs in Moline, Rock Island, and the rest of the QC, contractors apply open-cell on the underside of the roof deck to about 10–12″, which lands you at R-37 to R-44. Combined with the air-seal, the effective performance beats an R-49 fiberglass attic every time.

The take-away

R-value matters, but it's only half the story. Air leakage is the other half, and it's the half where spray foam wins decisively. Don't get talked into more inches than you need — focus on hitting code, sealing the air, and you're done. Want a real plan for your house? Get a free estimate.