Field calculator
Insulation R-value thickness calculator
Hitting a required R-value comes down to how much R each inch of the chosen insulation delivers. The thickness needed is simply the target R-value divided by the material's R-value per inch. Enter the target R and the product R-per-inch to get the thickness. The R-per-inch varies widely by material: fiberglass and mineral wool batt sit lower, while polyiso, extruded and expanded polystyrene, and closed-cell spray foam sit higher, and polyiso de-rates in cold temperatures, so always use the specific product datasheet rather than a generic number. Remember the value this gives is for the insulation layer alone. The whole-assembly R is pulled down by framing and other thermal bridging and added to by the sheathing, air films, and other layers, so for a roof, wall, or ceiling, confirm the code-required assembly R for the climate zone against the energy code, not just the insulation R.
Result
Insulation thickness for a target R-value: thickness = target R divided by the material R-value per inch. Enter the target R and the product R-per-inch. R-per-inch varies a lot by material (fiberglass and mineral wool batt are lower; polyiso, XPS, EPS, and closed-cell spray foam are higher, and polyiso de-rates in the cold), so use the specific product datasheet. The whole-assembly R is also pulled down by framing and thermal bridging and added to by the other layers and air films, so confirm the required assembly R for the climate zone against the energy code.
Worked example
A roof assembly needs R-49, using closed-cell spray foam at R-6.5 per inch.
- Target R-valueR-49
- Material R per inch6.5
- Thickness = target R ÷ R per inch = 49 ÷ 6.5 = 7.5 in of foam.
About 7.5 in of closed-cell foam hits R-49. Batt at R-3.7/in would need ~13.2 in, which rarely fits the cavity.
anvilfield.com/calculators/insulation-r-value-thickness-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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R-value thickness FAQ
What are the types of roofing systems?
Roofing systems group by slope. Steep-slope shedding systems include asphalt shingle, metal, clay and concrete tile, slate, and wood shake. Low-slope membrane systems include single-ply (TPO, EPDM, PVC), built-up roofing, modified bitumen, and spray polyurethane foam. The slope decides the family, and the building's use, budget, and climate pick the covering.
What is the difference between steep-slope and low-slope roofing?
Steep-slope roofing sheds water with overlapping materials on a slope steep enough to keep water moving off the surface. Low-slope roofing holds water back with a continuous, sealed membrane on a roof too flat to shed. Steep roofs leak at laps, nails, and flashing; low-slope roofs leak at seams and penetrations. The slope is the line between them.
What roofing is best for a flat roof?
A flat or low-slope roof needs a continuous membrane, not a shedding material. Single-ply (TPO, EPDM, or PVC) is the common commercial choice, with PVC for grease or chemical exposure. Built-up and modified bitumen suit redundancy or heavy traffic, and spray foam suits insulation and recover work. The building's exposure and use pick among them.
How do you choose a roofing system?
Run the conditions in order. The slope comes first and sets steep-slope or low-slope. Then the building type, the structure, the budget, the wanted life, the climate, the foot traffic, the energy and code requirements, and the look narrow it to a covering. Match the system to the building's real conditions, and weigh installed cost against warranted life.
What slope is considered low-slope or flat?
A roof is generally considered low-slope below roughly 2:12, meaning 2 in of rise per 12 in of run, where a continuous membrane is needed instead of shedding materials. The 2:12 to 3:12 band is a transition where some shedding materials are allowed only with extra detailing. Confirm the limit against NRCA, the manufacturer, and the adopted code.