Insulation R Value Calculator

Purpose and model boundary

This calculator estimates the insulation to buy for a floor plan you draw, and whether it reaches the R-values you are aiming for. It covers the exterior walls (batts in the wall cavities), the attic (blown loose-fill insulation) and, if you choose, the floor over an unheated space such as a crawlspace (batts between the joists). You draw the outline, the interior walls, the windows and the doors, set one wall height for the plan, check each opening's height, mark each door Exterior Yes or No, and the spreadsheet returns the areas, the packages and bags to buy, and the R-value each surface reaches against your target. It is not an energy model, a code-compliance certificate or a cost estimate, and it prices nothing.

Inputs and units

The drawing is the first input. The drawing tool finds the rooms and writes one row per room (its ID, floor area and enclosing-wall length), one row per wall side of every room, and one row per window and per door with its width and the room it belongs to. The outline is also sent as its corner points, and up to five islands or columns inside the floor as exclusions. Each exclusion has a height, set by double-clicking it on the drawing: blank is full height, as for a column or a closet, and a height at or above the wall height counts as full height. The attic covers the ceiling over an exclusion lower than the walls, such as a kitchen island, but not over a full-height one.

The heights and the Exterior answers are set on the drawing or in the tables under it: one wall height for the whole plan, floor to ceiling, set with the drawing's Wall height button; each window's height and sill height; each door's height; and, for each door, Exterior: Yes for a door in an outside wall, No for a door between rooms. The drawing cannot tell an entrance door from an interior one, so every door needs an answer, written exactly Yes or No. Every window is in an outside wall, because the drawing tool places windows on the outline only. Lengths and heights are in feet under US and metres under Metric. The spreadsheet takes a wall height from 6 to 20 ft (1.80 to 6.00 m); on the page the drawing accepts the heights valid in both unit systems, 6 to about 19.68 ft (1.83 to 6.00 m), so a unit switch never leaves the wall out of range; in the same way it asks for a window at least 1 ft (0.3048 m) and a door at least 5 ft (1.524 m) tall in both systems, so in Metric it refuses a 0.30 m window or a 1.50 m door that the spreadsheet alone would take. The wall height is one physical height and converts exactly on a switch (8 ft is 2.4384 m). Each unit system keeps its own opening heights; a new opening, and after a switch or when a worked example loads an opening with no height in the current system, gets the drawing's default heights: a door 6 ft 8 in or 2.03 m, no higher than the wall, and a window 4 ft or 1.20 m on a 3 ft or 0.90 m sill, the sill lowered as needed so the window fits below the wall. A door's Exterior answer is not a length, so it carries across a switch. The 3D view shows the same inputs and computes nothing.

The inputs beside the drawing are:

• Wall cavity depth: the depth the wall batt fills, 3.5 in under US and 9 cm under Metric by default.
• Batt R-value per inch (per centimetre under Metric): 3.7 per inch, or 0.257 m²·K/W per centimetre.
• Framing factor: 0.8 by default, a rule-of-thumb allowance for the studs and joists (see the R-value model below).
• Wall target R-value: R-13, or 2.29 m²·K/W.
• Wall batt coverage per package: 40 sq ft, or 3.7 m².
• Attic insulation depth: 10 in, or 25 cm of blown insulation.
• Blown R-value per inch (per centimetre): 3.7, or 0.257.
• Attic target R-value: R-38, or 6.69 m²·K/W.
• Bag coverage at that depth: 40 sq ft, or 3.7 m², from the bag's coverage chart.
• Insulate the floor: No by default. With Yes, the floor joist cavity depth (9.25 in, a 2×10 joist; 23.5 cm is 9.25 in rounded), the floor target R-value (R-19, or 3.35 m²·K/W) and the floor batt coverage per package (40 sq ft, or 3.7 m²) are used. The floor fields stay on the page while the answer is No, and the spreadsheet does not read them then.
• Insulation waste: 5%, added once to the batts and the bags.

Under Metric the R-values are in m²·K/W, often written RSI. The Metric R-value defaults are the US figures converted at NIST's factor (1 h·ft²·°F/Btu is 0.1761102 m²·K/W), to three significant figures. The Metric depths and coverages are separate figures from the worked examples, not conversions. Every default is a worked figure or an assumption to review; replace them with your own before you rely on the result.

Where the drawn values come from

Each room's floor area comes from the drawing tool's room detection, and each window's and door's width and room come from the drawing. The exterior wall length is computed by the spreadsheet from your outline points, so interior walls and partitions never count. The spreadsheet checks that the room areas add up to the floor area it computes from the outline, and that the walls listed go all the way round every room and the outline.

Governing relationships

The exterior wall length is the outline's perimeter, measured from its points, closing side included. Every side of the outline bounds exactly one room, so this is the total of every room's outside walls.

Gross exterior wall = exterior wall length × wall height

Windows and exterior doors = each window's width × height + each door marked Exterior Yes: width × height

Net exterior wall = gross exterior wall − windows and exterior doors

Attic area = the rooms' floor area plus every exclusion lower than the walls; a full-height exclusion (blank, or at or above the wall height) has no ceiling over it and is left out

Floor area = the rooms' floor area, when the floor is insulated

Wall R-value = wall cavity depth × batt R-value per inch × framing factor

Attic R-value = attic depth × blown R-value per inch

Floor R-value = floor cavity depth × batt R-value per inch × framing factor

Each surface meets its target when its R-value is at least the target. Its shortfall is the target less the R-value reached, or 0 when it meets it.

Extra attic depth = attic shortfall ÷ blown R-value per inch

Wall batt packages = net exterior wall × (1 + waste) ÷ wall batt coverage, rounded up

Attic bags = attic area × (1 + waste) ÷ bag coverage, rounded up

Floor batt packages = floor area × (1 + waste) ÷ floor batt coverage, rounded up

Waste is added once, at the count. Nothing else is rounded: the shown figures are the display of the full values. Every count and every comparison with a target is taken at six decimals first, so a figure that is whole on paper stays whole and a surface exactly at its target reads Yes with no shortfall.

The R-value model

The depth times the R-value per inch is the material's rated resistance at that thickness. The framing factor is a rule-of-thumb allowance for the studs and joists, which insulate less than the cavity: the cavity's R-value is multiplied by it. It is 0.8 by default, an assumption to review, and it is not the parallel-path whole-wall method. The attic is not derated, because blown insulation covers the ceiling joists. A batt compressed into a shallower cavity does not reach its labelled R-value, so enter the cavity depth the batt fills. Walls and floors have no extra-depth figure: their cavities are full, so a shortfall there needs deeper framing or continuous insulation.

Buying the material

Batt packages state the area they cover in square feet, and loose-fill bags carry a coverage chart (16 CFR 460.12): use the coverage at your depth. Recommended levels vary by climate zone (ENERGY STAR); the targets are yours to set.

Calculation sequence

1. The spreadsheet measures the outline's perimeter from the points and takes the gross exterior wall at the wall height.
2. It takes off every window and every door marked Exterior Yes, and finds the attic area and, when the floor is insulated, the floor area.
3. It works out each surface's R-value, compares it with the target and finds the shortfall, and the extra attic depth.
4. It counts the wall batt packages, the attic bags and the floor batt packages, with the waste added once.

The default plan works through as follows: one 20 by 15 ft room with 8 ft walls and no openings has a 70 ft exterior wall, 70 × 8 = 560 sq ft gross and net. The walls reach 3.5 × 3.7 × 0.8 = 10.36 against R-13, so No, 2.64 short, and take 560 × 1.05 ÷ 40 = 14.7 → 15 packages. The attic is 300 sq ft and reaches 10 × 3.7 = 37 against R-38, so No, 1 short and 1 ÷ 3.7 = 0.27 in deeper to reach it, and takes 300 × 1.05 ÷ 40 = 7.875 → 8 bags.

Outputs and interpretation

• Insulation to buy: the wall batt packages and the attic bags.
• R-values against your targets: the wall and attic R-values reached, whether each meets its target, the shortfall, and the extra attic depth.
• Exterior walls and attic: the exterior wall length, the gross area, the windows and exterior doors, the net wall area, and the attic area.
• Floor: the floor batt packages, the R-value reached, whether it meets its target and the shortfall; 0 and Not included while the floor is not insulated.
• The floor's net area, gross area and the area of each island or column, from the drawing.
• Each surface: one row per surface with its area, the R-value reached and its target, the shortfall, and how much to buy. The floor row is blank when the floor is not included.
• Each room: each room's floor area and the window and exterior-door area in its walls. The wall length and wall area are given for the whole plan only: the drawing does not say which of a room's walls are outside walls.

Validation and status logic

The status reads OK, CHECK with what to look at, or NOT VALID with what to fix. While it is NOT VALID every quantity reads zero, the verdicts read NOT VALID and the Each surface and Each room rows are blank; the drawn areas still show. CHECK shows every figure. The first problem found is reported, in this order:

1. NOT VALID: choose US or Metric
2. NOT VALID: draw at least three outline points
3. NOT VALID: every outline point needs both coordinates
4. NOT VALID: every point of exclusion 1 needs both coordinates
5. NOT VALID: every point of exclusion 2 needs both coordinates
6. NOT VALID: every point of exclusion 3 needs both coordinates
7. NOT VALID: every point of exclusion 4 needs both coordinates
8. NOT VALID: every point of exclusion 5 needs both coordinates
9. NOT VALID: each exclusion needs zero or at least three complete points
10. NOT VALID: outline and exclusion points must fill consecutive rows, with no blank row between them
11. NOT VALID: the floor outline encloses no area
12. NOT VALID: the exclusions leave no floor area
13. NOT VALID: room, wall, window and door rows must be complete, with lengths above zero and no negative area, and fill consecutive rows with no blank row between them
14. NOT VALID: the room areas do not add up to the floor area drawn
15. NOT VALID: every room must be listed once, and every wall, window and door must belong to a listed room
16. NOT VALID: the walls listed do not go all the way round every room and the outline drawn
17. NOT VALID: wall, window and door IDs must each be listed once, and no ID or room ID may contain *, ? or ~ or begin with <, > or =
18. NOT VALID: the wall height must be from 6 to 20 ft (US) or from 1.80 to 6.00 m (Metric)
19. NOT VALID: every window needs a height of at least 1 ft (US) or 0.30 m (Metric)
20. NOT VALID: every window needs a sill height of zero or more
21. NOT VALID: every door needs a height of at least 5 ft (US) or 1.50 m (Metric)
22. NOT VALID: a window or door reaches above the wall height
23. NOT VALID: an exclusion height must be greater than zero; leave it blank for full height
24. NOT VALID: an exclusion height is given for an exclusion that is not drawn
25. NOT VALID: choose Yes or No under Exterior in the doors table for every door
26. NOT VALID: the windows and exterior doors cover more than the exterior walls
27. NOT VALID: wall cavity depth must be greater than zero
28. NOT VALID: the batt R-value per unit of thickness must be greater than zero
29. NOT VALID: enter a framing factor greater than 0 and no more than 1
30. NOT VALID: the wall target R-value must be greater than zero
31. NOT VALID: wall batt coverage must be greater than zero
32. NOT VALID: attic insulation depth must be greater than zero
33. NOT VALID: the blown R-value per unit of thickness must be greater than zero
34. NOT VALID: the attic target R-value must be greater than zero
35. NOT VALID: bag coverage must be greater than zero
36. NOT VALID: choose Yes or No for including the floor
37. NOT VALID: floor cavity depth must be greater than zero
38. NOT VALID: the floor target R-value must be greater than zero
39. NOT VALID: floor batt coverage must be greater than zero
40. NOT VALID: enter a waste between 0% and 100%
41. CHECK: an island or column is drawn: the floor area leaves out the area under it, the attic leaves out a full-height one, and the walls around it are not counted as exterior wall; add them if they are insulated

Items 37 to 39 apply only when the floor is insulated. The height limits are the drawing tool's own: the spreadsheet holds a direct caller to the drawing tool's range for each unit system (items 18 to 22), and the page narrows the wall and the openings to the ranges valid in both systems. A surface below its target is not a CHECK: it is the answer, given by the Yes or No beside its R-value. The Exterior answer must be exactly Yes or No, with that case and no spaces. The spreadsheet checks the rows' completeness, IDs, room areas, wall coverage, heights and Exterior answers; it does not check the wall and opening widths, an opening's room or which doors are exterior against any real plan, which the drawing tool writes on the page.

Assumptions and limitations

• The exterior wall is worked out for the whole plan only; each room shows its areas and openings.
• One wall height applies to the whole plan; per-room heights are not used.
• Interior doors are not taken off the wall, because they are not in an outside wall; the Exterior answers are taken as typed.
• The floor area leaves out the area under an island or column, and the walls around an island or column are not counted as exterior wall: the status reads CHECK while one is drawn. The attic covers the ceiling over an exclusion lower than the walls and leaves out a full-height one: an outline enclosing 272 sq ft with an 18 sq ft island 3 ft high has a 272 sq ft attic and a 254 sq ft floor; with the island full height (its height blank) the attic is 254 sq ft.
• Not included: gable-end walls above the top plate, rim and band joists, knee walls, skylights, basement and crawlspace walls, continuous insulation, air sealing and vapour control.
• One batt material serves the walls and the floor, and one blown material the attic; a batt thicker than its cavity is not modelled.
• Nothing is saved between visits. Export keeps the drawing but not the heights or the Exterior answers, and the room areas and enclosing lengths come from the drawing tool's room detection.

Restrictions and non-computing states

The spreadsheet computes nothing while the status is NOT VALID: every quantity reads zero, the verdicts read NOT VALID, and the Each surface and Each room rows are blank. Edits on the page stay local until Calculate is pressed.

Errors and warnings

Each NOT VALID line above names the input to fix, and the CHECK line names what to look at. A height outside the drawing's limits, a blank opening height or a blank sill on a window that has a height is refused, because the opening must still fit the wall. A blank Exterior answer is not refused on the page: it is sent, and the spreadsheet refuses it with item 25 until the door is marked Yes or No.

Sources and further reading

• National Institute of Standards and Technology, NIST Guide to the SI (Special Publication 811), Appendix B.9, conversion factors. nist.gov. 1 °F·h·ft²/Btu (International Table) is 0.1761102 m²·K/W: the factor behind the Metric R-value defaults (R-13 is 2.29, R-38 is 6.69, R-19 is 3.35, and 3.7 per inch is 0.257 per centimetre).
• U.S. Federal Trade Commission, Labeling and Advertising of Home Insulation, 16 CFR § 460.12, Labels, on the Legal Information Institute. law.cornell.edu. Batt and blanket labels show the R-value, the thickness and the square feet in the package; loose-fill bags carry a coverage chart at stated R-values. That is where the batt coverage per package and the bag coverage at your depth come from.
• ENERGY STAR (U.S. Environmental Protection Agency), "Recommended Home Insulation R-Values". energystar.gov. Recommended attic and floor levels by climate zone; the floor target default, R-19, is one of them.

The cavity depths, the R-values per inch, the targets, the coverages and the 5% waste are the worked examples' figures or assumptions to review, and the framing factor is a rule-of-thumb allowance, not a figure from these sources. The height defaults and limits are the drawing tool's. These sources describe a unit conversion, product labels and recommended levels; they certify no particular result.