Utah has adopted the 2021 IECC. For climate zone 5, which covers all of Utah County, the prescriptive minimums are R-49 in the ceiling, R-30 in floors, R-21 in wood frame wall cavities, and R-15 for basement walls. Utah Valley Foam is an independent insulation referral service covering Utah County, and this page lays out what the code actually requires, assembly by assembly, in the depths those numbers translate to.
What R-Value Actually Means
R-value measures how well a material slows heat down. Bigger number, better insulation. That is the whole idea. Everything else is detail.
Two different R numbers get used, and mixing them up is where the confusion starts:
- R-value per inch describes the material itself. Closed cell foam is about R-6.5 per inch. Fiberglass is about R-3.2 per inch, so it takes roughly twice as much fiberglass to do the same job.
- Total R-value describes the finished job. R-49 is what Utah asks for in an attic ceiling.
You get from one to the other with simple multiplication:
Inches x R-value per inch = total R-value. So 8 inches of something rated R-6 per inch gives you R-48.
The practical takeaway is that R-49 is a destination, not a product. You can get there with about 8 inches of closed cell foam or about 16 inches of blown fiberglass. Same finish line, very different depth. This is why asking a contractor “how many inches” tells you nothing useful until you also know what material.
Utah R-Value Requirements by Assembly
These are the prescriptive values from Table R402.1.3 for climate zone 5. The depth columns show what it takes to hit each number in each material:
| Assembly | Required R-value | Closed cell depth | Open cell depth | Fiberglass depth |
|---|---|---|---|---|
| Ceiling / attic | R-49 | 7 to 8 inches | 13 to 14 inches | 15 to 16 inches |
| Wood frame wall | R-21 cavity | 3 to 3.5 inches | 5.5 to 6 inches | about 6.5 inches |
| Floor | R-30 | 4 to 5 inches | 8 to 9 inches | 9 to 10 inches |
| Basement wall | R-15 | 2 to 2.5 inches | Not used below grade | Not recommended below grade |
| Unvented attic (5B / 6B) | R-26 | about 4 inches | about 7 inches | Not permitted alone |
What Does R-21+0ci or 15+5ci Actually Mean?
The code writes wall and basement requirements as combinations, and the notation trips up almost every homeowner who reads it. The “ci” stands for continuous insulation, meaning a layer that runs unbroken across the framing rather than sitting between studs.
For a wood frame wall in zone 5, the code accepts any of these three:
- R-21+0ci. R-21 in the stud cavity, nothing continuous over the outside.
- R-15+5ci. R-15 in the cavity plus R-5 of continuous insulation outside the framing.
- R-0+15ci. Nothing in the cavity, R-15 continuous over the whole wall.
Basement walls follow the same pattern, accepting R-15+0ci, R-0+11ci, or R-11+5ci. The reason continuous insulation counts for more per R is thermal bridging: a stud is a hole in your insulation, and a 2×6 wall is roughly a quarter framing by area at corners and openings. Continuous layers cover what the cavity cannot.
Can You Insulate to Less Than R-49 Legally?
Yes. Section R402.2.1.1 permits an unvented attic assembly at R-26, with a maximum U-factor of 0.038, in climate zones 5B and 6B. U-factor is R-value measured backwards: it describes how much heat gets through rather than how much is stopped, so with U-factor a lower number is better. Utah County is 5B, with 6B on the higher benches, so both apply here. That is roughly 4 inches of closed cell or 7 inches of open cell against the roof deck instead of 13 to 14 inches across the attic floor.
The reduced R-value is only allowed when all four of these conditions are met:
- A blower door test result of 2.5 ACH50 or tighter. A blower door is a fan fitted to your front door that measures how leaky the house is. “2.5 ACH50” means roughly a third of the air in the house leaking out and back per hour under test pressure. It is a tight house, and most older homes fail it without work.
- A whole-house mechanical ventilation system. Seal a house up that tightly and it needs deliberate fresh air, rather than relying on draughts.
- All indoor equipment inside the thermal envelope. The thermal envelope is the insulated shell of your house. Your furnace and ducts have to sit inside it, not out in a cold attic.
- Roof rafters wrapped to at least R-3, unless they are covered with drywall. Bare timber conducts heat, so the rafters get covered too.
This pathway suits a home where the ductwork already runs through the attic, because pulling that equipment inside the envelope is the entire point. It is a poor fit for a leaky older house that will not pass a blower door test.
Prescriptive Path or Performance Path?
Everything above is the prescriptive path: hit each number, assembly by assembly, and you comply. The code also allows a performance path, where the whole building is modelled and a shortfall in one assembly can be offset by doing better somewhere else.
This matters on retrofits. If your 1970s Orem walls cannot physically hold R-21 without being opened up, that does not automatically make the project non-compliant. It usually means the design moves to the performance path, and the attic and air sealing carry more of the load. Which is convenient, because the attic is the cheapest R-value in the house anyway.
Does Code Apply to an Existing Home?
Generally no. Energy code binds new construction, additions, and permitted alterations. It does not force you to upgrade insulation in a house you already own and are not renovating.
So the R-49 figure is not a legal obligation hanging over your attic. It is the number the code considers correct for this climate, which makes it a sensible target rather than a requirement. Most Utah County homes built before 2010 sit well under it. If you are pulling a permit for an addition or a significant alteration, the new work does have to comply.
Code adoption and local amendments vary between jurisdictions. Confirm the requirements for your project with your city or county building department before you commit to a design. Separately, what you can claim back on the work has changed: see Utah insulation rebates and tax credits.
Utah Insulation Code FAQ
What does R-value mean?
R-value measures how well a material slows heat down, and a bigger number is better. R-value per inch describes the material, total R-value describes the finished job, and inches multiplied by R-value per inch gives you the total. R-49 is a target you can hit with 8 inches of foam or 16 inches of fiberglass.
What climate zone is Utah County in?
Utah County is climate zone 5B, with 6B on the higher benches and mountain areas. Zone 5B is the figure that applies to nearly all of the valley floor, including Provo, Orem, Lehi, Springville, Spanish Fork, and Payson.
How many inches of spray foam equal R-49?
Roughly 7 to 8 inches of closed cell foam, or 13 to 14 inches of open cell. Closed cell delivers R-6 to R-7 per inch and open cell R-3.6 to R-3.9, so closed cell reaches the target in about half the depth at a higher cost per board foot. A board foot is one square foot of foam at one inch deep, and it is the unit spray foam is quoted in.
Which code edition has Utah adopted?
Utah has adopted the 2021 International Energy Conservation Code. Individual jurisdictions can amend it, so confirm the specifics with your local building department before finalising a design.
Do I legally have to bring my attic up to R-49?
Not for an existing home you are not renovating. Energy code applies to new construction, additions, and permitted alterations. R-49 is the level the code considers correct for this climate, so it is the right target, but nobody is going to require it of a house you already live in.
Does closed cell foam count as a vapor barrier under code?
At about 2 inches, closed cell foam qualifies as a class II vapor retarder, meaning a layer that lets only a small amount of moisture pass through it. That is why it handles insulation, air sealing, and vapor control in one application. Open cell is vapor permeable and does not, which is why it is not used below grade or against concrete.
Working out which pathway fits your home is what the free assessment is for. Call (801) 691-7136 and describe the project, or read how this plays out in an attic, a crawl space, or with closed cell versus open cell foam.