Insulation R-value calculator

How the R-value, the loss and the table check are worked out

This page does two separate jobs. It adds the layers of a wall, ceiling or floor into a total R-value, with the wood framing as a second path in parallel, and turns that into a U-factor and a heat loss in BTU/h. The page also compares the insulation you typed, the cavity R and the continuous R, with the minimums of the 2021 IECC R-value table for the climate zone. That table counts installed insulation only, so the total R of the assembly never enters the comparison. The table cells come from a Department of Energy reprint, with the zone 3 wall cell as the ICC errata print it, and not from the ICC code text, and the page says so.

Layers in series

R of a path = inside film + interior finish + air space + cavity insulation or framing + continuous insulation + sheathing and other layers + outside filmResistances of layers in the same path add. The Building America worked wall adds an inside film of 0.68, half-inch drywall at 0.45, an R-19 batt, 0.63 of sheathing and a film of 0.68 to R-21.44 through the insulation. This page uses 0.17 for the outside film of a wall, from a state energy code table.

Framing path

framing R = 1.25 x framing depth (in.)The Building America example gives 5.5 inches of framing as R-6.88, which is R-1.25 per inch. This page opens on a 2x6 wall with 25.4 percent of the area in framing, the share that example cites for walls. No sourced share for joists is used here, so it is a typed number for ceilings and floors.

Parallel path

U = (1 - f) / R(path through insulation) + f / R(path through framing), R of the assembly = 1 / UHeat is taken to flow through the insulated area and the framed area independently. The worked wall with paths of R-21.44 and R-9.32 and 25.4 percent framing comes to R-16.1 as a whole on this arithmetic. That tool is for wood framing and says not to use it for metal framing.

Heat loss

heat loss (BTU/h) = U x area (sq ft) x temperature difference (F)UA, the U-factor times the area, is the heat lost per degree. The default wall of 1,000 sq ft at a 70 degree difference loses 4,514 BTU/h at a U-factor of 0.0645, which is a UA of 64.5 BTU/h per degree.

Table comparison

reached when the cavity R is at or above the first number and the continuous R is at or above the second number of any option in the cellIn the table 13+5ci is R-13 cavity plus R-5 continuous. A Zone 5 wood frame wall reads 30 or 20+5ci or 13+10ci or 0+20ci. R-19 in the cavity with no foam reaches none of those, and R-10 of foam added reaches 13+10ci. Ceilings and floors list one number that cavity R plus continuous R must reach.

Worked examples

The default case is a 2x6 wood frame wall with an R-19 batt, half-inch drywall, 0.63 of sheathing, the standard air films and 25.4 percent framing. The path through the insulation is R-20.9 and the path through the framing is R-8.8, so the whole wall is R-15.5, which is 25.9 percent below the insulated path. Over 1,000 sq ft at 70 degrees F the loss is 4,514 BTU/h. The table for Zone 5 reads 30 or 20+5ci or 13+10ci or 0+20ci, and R-19 in the cavity with no foam reaches none of those.

The table cells used by this page, by climate zone. Ceilings and floors list one number of installed insulation, and walls and basement walls list options of cavity and continuous (ci) insulation.

Eight cases below run through the same functions the form uses. Foam changes the table result and also the total R. R-20 in the cavity with R-5 of foam reaches 20+5ci in Zone 5 and comes to R-21.7, and in Zone 3 the R-19 wall needs R-5 of foam to reach 13+5ci. The attic at R-38 is R-22 short of the ceiling value.

Data sources and references

• Building America Solution Center (U.S. DOE, PNNL), 2009-2021 IECC and IRC Minimum Insulation Requirements for New Homes - The cells of Table R402.1.3 of the 2021 IECC by climate zone for ceilings, wood frame walls, floors and basement walls, last updated in September 2021. It also gives the notes that 13+5 is R-13 cavity plus R-5 continuous insulation and that the installed R-value must reach the table value where a cavity is thinner than the label thickness.
• International Code Council, 2021 International Energy Conservation Code errata, Table R402.1.3 - The zone 3 wood frame wall cell as 20 or 13&5ci or 0&15ci. Where the reprint above prints 0+15 without the ci, this page follows the errata.
• Building America Solution Center, Parallel Path R-Value - The parallel-path method for wood framing, UA as the U-factor times the area, and a worked wall with paths of R-21.44 and R-9.32, 25.4 percent framing, 0.68 and 0.45 for the inside film and drywall and 0.63 for sheathing.
• Washington State Energy Code, WAC 51-11C-61015, Table A101.5 Default R-values for Building Materials - Air films of 0.17 outside and 0.68, 0.61 and 0.92 inside for vertical surfaces and for heat flow up and down, gypsum board at 0.45 and 0.56, a wood subfloor at 0.94, and an unventilated air space at 0.91. It gives no credit to a cavity open to outside air.
• ENERGY STAR (EPA and DOE), Recommended Home Insulation R-Values - The definition of R-value as the ability of insulation to resist heat traveling through it, where a higher R-value means better thermal performance.

Limitations of this calculator

• The table cells are read from the Building America Solution Center reprint of the 2021 IECC table and not from the ICC code text. A jurisdiction adopts a particular edition with its own amendments, and many are on other editions, so read the table the permit office enforces. In the reprint the zone 3 wall cell reads 20 or 13+5ci or 0+15 with no ci after the 15. The ICC errata to the 2021 IECC print that cell as 0&15ci, so this page prints 0+15ci and reads that number as continuous insulation.
• Only wood frame walls, ceilings, floors and basement walls are compared. Mass walls, slabs, crawl space walls, windows, doors and skylights are not on this page, and neither are the U-factor and total UA compliance paths of the code.
• A lone number in a wall or basement cell, such as 13 or 30, is read as cavity insulation of that R, from the note that the first value of a pair is cavity insulation. Ceiling and floor cells are one number compared with the cavity R plus the continuous R.
• The table compares installed insulation. Framing, sheathing, drywall and air films do not count toward it, and the page does not use the total R of the assembly for the comparison.
• The parallel-path method is for wood framing. Metal framing, headers, corners, plates and other thermal bridges are not modeled beyond the framing share you type, and the 25.4 percent default is the wall figure of one source, not a figure for joists.
• Film, board and air space values are defaults from one state energy code table. The air space is credited at 0.91 only when it is unventilated, a basement wall takes no outside film, and soil is not credited. Sheathing, siding and subfloor R is typed.
• The cavity R is the installed value you type. Compression, gaps, voids, wind washing and air movement inside the assembly are not modeled, and the page does not check the R against the depth of the cavity.
• Footnotes and special provisions of the code text, such as those for particular attic or framing conditions, are not applied. The comparison covers the main cell of each table row.
• Heat loss is conduction through one assembly at one temperature difference. Air leakage, ground contact, solar gain and the other assemblies of the house are left out, so the figure is not a Manual J load.
• This is a preliminary check and not a permit submittal. A compliance tool such as REScheck, the adopted code and a licensed designer decide the assembly.

Common R-value mistakes

1. Putting the total R in the table comparison. The table lists installed insulation, so the total R of the assembly does not belong in the comparison. A default wall comes to R-15.5 in total, and the table is read with the R-19 in the cavity and the R-0 of continuous insulation. Zone 5 asks for 30 or 20+5ci or 13+10ci or 0+20ci.
2. Reading the batt label as the wall. An R-19 batt in a 2x6 wall is sold as R-19. With 25.4 percent framing, the films, the sheathing and the drywall, the assembly comes to R-15.5, and the path through the insulation alone is R-20.9. The framing path is only R-8.8, so the whole is 25.9 percent below the insulated path.
3. Leaving the framing out of the heat loss. Setting the framing share to 0 gives R-20.9 and 3,344 BTU/h for the default wall, against R-15.5 and 4,514 BTU/h with 25.4 percent framing. The studs carry heat the batt does not, and the difference is 25.9 percent of the insulated path.
4. Treating R-19 as R-20 in Zone 3. The Zone 3 wall cell reads 20 or 13+5ci or 0+15ci. R-19 in the cavity falls short of the R-20, and it reaches no other option on its own. Adding R-5 of continuous foam reaches 13+5ci, and the wall then comes to R-21.2 in total.
5. Stopping an attic at R-38 in Zone 5. A Zone 5 ceiling is listed at R-60. R-38 of blown insulation gives R-39.2 for the whole assembly, but the table is read on the R-38 alone, which is R-22 short. The films and drywall do not make up the gap.
6. Mixing up the basement wall pairs. The Zone 5 basement wall cell reads 15ci or 19 or 13+5ci. R-13 batts alone reach none of the three, and R-5 more continuous insulation reaches 13+5ci. In Zone 3 the cell reads 5ci or 13, and R-5 of foam alone reaches it.

Common questions about R-value and the IECC table

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