Insulation Calculator

How to use the insulation calculator

Start with the area. For an attic that is the length and width of the floor you are covering, not the roof surface, since insulation on an attic floor lies flat. For a wall run, take the length times the height of the section you are filling and ignore the framing for now.

Then set the target R-value. The reference table on this page carries the ceiling and wall minimums the US Department of Energy publishes for the 2021 IECC, by climate zone, so find your zone and read the ceiling column. Those are minimums to meet code, not a ceiling on what is worth doing.

The input that changes the answer most is the one people leave at zero: the R-value already up there. Insulation layers in series add, so an attic already at R-19 that you want at R-49 needs R-30 of new material, not R-49. The calculator does that subtraction and the difference is large.

Blown-in bags print coverage as square feet at a stated R-value, because a bag holds a fixed amount of material: blow it deeper and it covers less floor. That is why the calculator asks for the R-value your coverage figure is quoted at, and why the bag count falls when you enter existing insulation.

Batts work differently. A package covers its stated area once, at the batt nominal R, so reaching a higher target means laying more than one layer. An R-49 target from R-30 batts is 2 layers, which is 50 packages on the default attic, and the second layer should run across the joists rather than along them.

R-values add, so a top-up only closes the gap

Thermal resistances in series add, which is the single most useful fact in this whole subject. Six inches of existing loose fill at R-19 plus new material at R-30 gives an R-49 assembly. You do not have to strip anything out, and you do not have to buy R-49 of new insulation to reach R-49.

Measuring what is already there is a ruler job, not a guess. Push a rule down to the ceiling drywall in several places, average the readings, and multiply by the R per inch of whatever the material is. Old insulation settles and thins, so the depth you find is usually less than the depth someone installed.

Why each added R buys less than the one before it

Conductive heat flow through an assembly is proportional to 1 divided by its R-value. That is the whole story. Because the relationship is a reciprocal and not a straight line, equal additions of R remove steadily smaller amounts of heat flow, and you can watch it happen with three equal steps.

Read it in absolute terms and the fall is steeper still. Flow at R-11 is 0.0909 in these units, at R-30 it is 0.0333, at R-49 it is 0.0204 and at R-68 it is 0.0147. The first R-19 removes 0.0576 of flow, the second removes 0.0129, and the third removes 0.0057, roughly a tenth of the first.

None of which is an argument against insulating. It is an argument for spending the first dollars on the assembly that is worst insulated rather than piling more onto the one that is already good. An R-11 attic and an R-49 attic are the same product and the same labour, and one of them is worth about five times as much.

The formula, worked line by line

Two ideas carry this calculator, and both of them are arithmetic rather than opinion. The first is that thermal resistances in series add, so the R-value of a finished assembly is the sum of its layers and a top-up only has to make up the difference between the target and what is already installed.

The second is that heat flow is proportional to 1 divided by R. That reciprocal is why insulation has diminishing returns: the same added R-19 removes a big share of the flow through a poorly insulated assembly and a small share of the flow through a good one, with no judgement call involved.

The product arithmetic on top of that is deliberately thin, because it depends on figures that are printed on the bag and vary between products. Depth is the R you are adding divided by the R per inch you enter. Bag counts come from the coverage you enter, quoted at the R-value you enter it against.

R to add = target R - existing R, never below 0
finished R = existing R + R added
blown depth in = R to add ÷ R per inch from the bag
blown bags = area × R to add ÷ (bag coverage × the R it is quoted at)
batt layers = R to add ÷ batt R, rounded UP
batt packages = area × layers ÷ package coverage, rounded UP
relative heat flow = 1 ÷ R
flow removed = 1 - existing R ÷ finished R

The depth figure is settled depth, which is what matters once the material has stopped moving. Loose fill goes in fluffed and drops, so blow to the installed depth the bag calls for rather than to the number here, and mark the target on rulers stapled to the joists so you can see the level as you go.

The flow figures are conductive only. A real ceiling also loses heat through air leakage and through framing that bypasses the insulation, and those two are why air sealing before insulating is worth more than the last R-10 of depth. No calculator on this page models either of them.

The R-values in the table are code minimums for new work in that climate zone, from the Department of Energy. They are not a recommendation for your particular house, which depends on the existing assembly, the fuel you heat with and what the rest of the envelope is doing. A local energy assessment settles that.

Questions people ask