Insulation Calculator

It depends on your climate zone and which part of the house you are insulating. As a rough orientation, attics run from about R-30 in the hottest zones to R-49 to R-60 in the coldest, wall cavities from R-13 to R-21, and floors from R-13 to R-30. The table above breaks that out by zone.

Two things matter more than picking the exact number. First, your local code is the authority, and it varies by which edition of the energy code your jurisdiction adopted. Second, air sealing usually buys you more comfort per dollar than the last few points of R-value, because insulation slows heat conduction but does nothing about the air pouring through a gap around a chimney chase.

Divide the target by the R per inch of the material, and you get a range rather than a single figure. R-30 needs about 8.8 to 9.7 in of fiberglass batt at 3.1 to 3.4 per inch, about 7.9 to 9.4 in of blown cellulose at 3.2 to 3.8, about 11.1 to 13.6 in of blown fiberglass at 2.2 to 2.7, and only 4.3 to 5 in of closed-cell spray foam at 6 to 7.

That is why the material choice matters so much in a shallow cavity. In a 5.5 in 2x6 wall, fiberglass batt tops out around R-17 to R-19 while closed-cell foam gets you to R-33 to R-38 in the same space, at several times the price per square foot.

Yes. Batt insulation works by trapping still air in the loft of its fibers, so squeezing the loft out takes the performance with it. An R-19 batt is about 6.25 in thick, and a 2x6 wall is 5.5 in. Compress that batt into the cavity and it performs at roughly R-18, not R-19. Manufacturers publish compressed values for exactly this reason.

The right answer is not to squash a thicker batt in, it is to buy a batt made for the depth you have: an R-21 high density batt is designed for a 5.5 in 2x6 cavity and beats a compressed R-19. Note that compression is not the same as filling the cavity completely, which is what you want. What you are avoiding is forcing in more material than the space is designed for.

Read the bag. Every bag of loose fill prints a coverage chart giving the square feet it covers at each settled depth, along with the minimum weight per square foot, and that chart is the only authoritative number, because it is tied to the density that product is tested at.

What this calculator gives you is the physics underneath it: cubic feet of loose fill = area x depth in feet. A 1,000 sq ft attic at 12 in deep needs 1,000 cubic feet of settled material, whatever brand you buy. Enter the coverage figure from your bag and it turns that into a bag count. Always blow deeper than the target to allow for settling, and staple depth rulers to the joists before you start so you can check.

Batts are the cheapest per square foot and the easiest to DIY in open framing, but they punish sloppy fitting: gaps, compression around wires and unfilled corners lose real performance. Blown loose fill is the usual answer for an attic floor, since it flows around framing and over obstructions and can go as deep as you want, and it is much better than batts at filling awkward shapes.

Spray foam buys the most R-value per inch and air-seals as it goes, which is why it wins in shallow cavities and at rim joists, but it costs several times as much, is unforgiving of mistakes and mostly wants a professional. A common sensible mix is closed-cell foam at rim joists and problem areas, batts or blown everywhere the space is generous.

In an attic, usually yes, and the R-values add. Blowing 8 in of cellulose over an existing 6 in of settled fiberglass gives you roughly the sum of the two. The one firm rule: do not put a second vapor barrier over the first. If the existing batts are faced, either use unfaced batts on top or slash the facing on the new material, otherwise you trap moisture between two barriers.

Two other cautions. Keep insulation clear of soffit vents with baffles, or you block the ventilation path and trade one problem for another. And keep it off recessed lights that are not rated for insulation contact, off chimney and flue clearances, and out of the way of anything that gets hot.

Because they genuinely differ. R per inch varies with density, with the specific product, and with the test temperature, so an honest answer is a range. Blown fiberglass at 2.2 per inch and at 2.7 per inch are both correct for different products. Anyone quoting one decimal figure for a whole material category is rounding away real variation.

Two materials also change over time or temperature. XPS is sold at R-5.0 per inch but its blowing agent escapes slowly and the aged value drifts below the label. Polyiso is the highest performing common board at room temperature but loses R-value as it gets cold, which is precisely the condition a cold climate roof lives in. Neither fact is hidden; both are simply missing from most calculators.

No, and it says so on purpose. This tool reports cavity R-value, which is the insulation between the framing. Wood is roughly R-1.25 per inch, so every stud and joist is a thermal bridge running straight through the assembly, and the whole-wall R-value of a framed wall is meaningfully lower than the cavity figure once framing, plates, headers and corners are counted.

Cavity R-value is still the right number for shopping, because it is how the product is labeled and how code tables for cavity insulation are written. If you want to close the gap, that is what continuous exterior foam does: a layer of rigid board over the studs is not interrupted by framing, which is why code language increasingly appears in the form "R-20 or R-13+5".