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Insulation

What it's for

Buying insulation for an attic, a wall or a floor, and checking the R-value you picked against the energy code minimum for your climate zone. You walk away with bags to buy (or board feet of spray foam), the depth to install, a within-or-below result against the IRC table, and a section drawing. The check is against the 2021 IRC table the screen names. Your town may have amended it.

Step by step

The example is the screen as it opens: a 12' by 10' attic in climate zone 5, blown cellulose to R-60.

Insulation inputs at their defaults: AREA TYPE Attic (Wall and Floor beside it), INSULATION TYPE with Blown Cellulose on its own line under the label, CLIMATE ZONE Zone 5 with an info button, the Find my zone on the map button, TARGET R-VALUE R-60, then AREAS with AREA 1 and its trash button, LENGTH 12' 0", WIDTH 10' 0", ADD AREA, and WASTE FACTOR 10%. The answer bar reads 12 bags, a green At or above the R-60 IRC 2021 N1102.1.3 min, zone 5 attic — check local code, and R-60 target · R-60 code min (zone 5 attic). 1 2 3 4 5
Where it goes, what it is, your zone, and the R-value you are aiming for.
  1. AREA TYPE — Wall, Attic or Floor. The code minimum is different for each. Leave it at Attic.
  2. INSULATION TYPE — Batt/Roll, Blown Cellulose, Blown Fiberglass, Spray Foam (Open-Cell) or Spray Foam (Closed-Cell). Leave it at Blown Cellulose.
  3. CLIMATE ZONE — your energy-code zone, 1 for the hottest through 8 for the coldest, with Marine 4 between 4 and 5 for the coastal Northwest. It sets the code minimum. Zone 5 here.
  4. If you do not know your zone, tap Find my zone on the map. See below.
  5. TARGET R-VALUE — what you intend to install. Pick it from the menu. R-60 here.
  6. Under AREAS, measure each rectangle you are insulating: LENGTH (12' 0") and WIDTH (10' 0"). For an attic or a floor that is the plan size. For a wall, use the wall's length and its height.
  7. Tap Add Area for each further rectangle. The trash can removes one.
  8. WASTE FACTOR — the extra you buy, in steps of 5%. It opens at 10%.
  9. Read RESULTS: 12 bags, R-60 target · R-60 code min (zone 5 attic), Depth ≈ 1' 5-1/8", 132 sq ft incl. waste. For a wall the code min lists every option the table gives, such as R-30 or R-20+5ci or R-13+10ci or R-0+20ci in zones 4–8: the cavity alone, or less cavity plus continuous insulation.
  10. Read the check under the facts. Green here: At or above the R-60 IRC 2021 N1102.1.3 min, zone 5 attic — check local code.
  11. Tap BASIS & WHAT WAS ASSUMED for where the minimum comes from.
  12. Check the section drawing.
  13. Read MATERIALS, then save or share the job.
Insulation scrolled to the attic drawing. The answer bar keeps 12 bags and the green check, cut off at min…. The ATTIC SECTION drawing shows the ceiling and joists under a blown layer marked ≈ 17-1/8" REQ, joists at 16" and 16", under the notes RULER 2" MARKS · 1" MIN EAVE AIR PER IRC R806.3; CEILING + JOIST DEPTH SCHEMATIC — NO R CREDIT; R-60 TARGET · R-60 IRC MIN ZONE 5 — AT OR ABOVE MIN; R-3.5/IN EFF · 14-1/2" BAY @ 16" OC ASSUMED; 12' 0" × 10' 0" · 120 SF +10% = 132 SF · 12 BAGS; and CAVITY R ONLY — THERMAL BRIDGING NOT MODELED, over the title strip ATTIC SECTION, BLOWN CELLULOSE · R-60, NOT TO SCALE. MATERIALS lists Blown Cellulose — Greenfiber 25-lb bag: 11.6 sq ft @ R-60 — check the bag chart, 12 bags; Eave baffles — one per bay at each vented eave, both long sides @ 16" o.c. (N1102.2.3) — verify, 18 pcs; and Attic depth markers — one per 300 sq ft, facing the attic access (N1101.10.1.1), 1 pcs. Under them a dimmed SHARE PDF with a list icon at its right end sits beside a lit SAVE. 9 10 12 13
The section drawing and the order. The answer bar under the title repeats the headline and the check.

Finding your zone on the map

The Insulation screen dimmed behind the CLIMATE ZONES sheet, with CANCEL at its top right: Tap your state. A state in one zone sets it; a state marked * spans several, so pick your county (2021 IECC Table R301.1). A grid of state tiles, each with its code and zone number and coloured from hot to cold, such as HI 1, AZ 2*, TX 2*, OK 3*, KY 4, PA 4*, NE 5, IA 5*, MN 6*, MT 6, VT 6 and AK 7*. Under the grid a legend starts: Zone 1 — ceiling R-30, walls R-13 or R-0+10ci; Zone 2 — ceiling R-49 …; Zone 3 — ceiling R-49, walls R-20 or R-13+5ci or R-0+15ci; Zone 4 — ceiling R-60 ….
The CLIMATE ZONES sheet from Find my zone on the map. A * on a tile means the state spans several zones.

Find my zone on the map opens a sheet titled CLIMATE ZONES: a grid of state tiles, each with the state's code and its zone number, coloured from hot to cold. Tap your state. CANCEL closes it without changing anything. Under the grid is a legend with the ceiling and wall requirement for each zone, and a line for Marine 4.

Zones are assigned by county, not by state. A state that sits in one zone sets that zone and the sheet closes. A tile marked with * spans several zones, so tapping it opens that state's counties, as the 2021 code's county table (IECC Table R301.1, IRC Table N1101.7) lists them, each with its zone. Type in Filter counties to find yours, and tap it. Its zone becomes the calculator's zone, and a note under CLIMATE ZONE reads, for example, From Allegheny, PA — 2021 IECC Table R301.1. All states takes you back to the map. Picking a zone from the menu afterwards drops the county.

Marine 4 is the code's zone 4C: the coast of Oregon and Washington (and two California counties). The code gives it the zone 5 minimums, so a floor there wants R-30, not zone 4's R-19. A 4C county sets Marine 4 for you.

Walls

With AREA TYPE on Wall, one more input appears: WALL FRAMING, 2x4 (3-1/2") or 2x6 (5-1/2"). Set it to the studs you really have. It is the depth of the bay the insulation has to fit in. In a wall, blown insulation is figured as dense-pack, and the material line says so.

Reading the results

The headline is what you buy: bags for batt and blown, board feet for spray foam. It also shows in a strip under the title, with the check beside it, whenever the results card is scrolled off screen.

The facts. The first line sets your target beside the code minimum for your zone and area type. Depth is how deep that insulation type goes in — a ≈ in front means the figure has been rounded for display. It is the depth that reaches your target, except where a wall bay caps spray foam: foam is trimmed flush, so there the depth is the bay, and the board feet on the MATERIALS row are figured on that same depth, not on the deeper one the target would have wanted. The section drawing still shows the deeper figure against the bay, which is what the overrun looks like. The area line includes your waste factor.

The checks. Within the table minimum is green. The check grades the R the bay can hold, not only the R you typed: where a wall bay caps spray foam, or a dense-pack bay is full before it reaches your target, the code line reads that lower R, because insulation in a cavity thinner than it needs must still meet the table. Below the minimum is red capitals:

BASIS & WHAT WAS ASSUMED opens the source: the minimums are 2021 IRC Table N1102.1.3 by climate zone, zones are assigned by county, and the map picks a state's predominant zone, so verify locally. When you picked a county it names it, and for Marine 4 it says the zone 5 row applies. For a batt wall it names where the batt thickness comes from (the maker's fact sheet), and for blown cellulose or fiberglass the dense-pack chart, with its density. For walls it adds that the wall figure is the table's cavity-only option, and that the continuous-insulation alternatives are not modelled. It ends with IRC 2021 — verify local code and a reminder that states and towns amend the code.

The drawing is a section through the work — for the example, an attic with the fill drawn to the required depth against a depth ruler. Its notes carry what was assumed: the effective R per inch, the bay and spacing, CEILING + JOIST DEPTH SCHEMATIC — NO R CREDIT, and CAVITY R ONLY — THERMAL BRIDGING NOT MODELED. The attic section also reminds you to keep air space at the eave, citing IRC R806.3. Pinch to zoom, or tap the expand button at the top right to open it full screen. See Reading the drawing.

Materials. The insulation line names the maker's bag its count comes from — here Greenfiber 25-lb bag: 11.6 sq ft @ R-60 — check the bag chart. Blown cellulose follows Greenfiber's SANCTUARY chart, blown fiberglass Owens Corning's AttiCat chart, batts the Johns Manville unfaced bag for the bay width, and dense-pack the maker's dense-pack chart. An R between two chart rows takes the higher row, so the count never comes up short. Bag yields differ a lot from product to product: compare the figure with the chart on the bag you are buying before you order. Spray foam prints its board feet with ≈, the same figure as the headline.

An attic adds Eave baffles, one per bay at each vented eave along both long sides at 16" on center (IRC N1102.2.3 wants a baffle wherever air-permeable insulation meets a soffit vent; 18 for the example), and blown fill adds Attic depth markers, one per 300 sq ft, facing the attic access (N1101.10.1.1). Spray foam gets neither.

For a wall in zones 5–8 or Marine 4, BASIS & WHAT WAS ASSUMED adds the vapor retarder the wall needs: Class I or II on the interior side (IRC R702.7); a kraft-faced batt is Class II. For an attic it adds that the 2018 and 2024 editions put attics one step lower than 2021.

SHARE PDF sends a sheet with the drawing, your inputs and the material list. The list icon at the right end of the same button sends the material list alone. SAVE puts the job in History. SHARE PDF and the list icon in it stay dim until you change at least one input. SAVE stays lit, but while every input is still a default a tap only answers Enter your job's numbers first and saves nothing.

Insulation is a Pro calculator, and sharing a PDF and saving to History are part of Pro too. This help is free. See Free and Pro.

Common mistakes