Insulation Calculator

How Insulation Quantity Is Calculated

Batt and roll insulation is sold by the bag, and every bag states the square feet it covers at a given R value and width. The math is simple once the area is known.

Area for an attic is the floor area of the space below it. Area for walls is the gross wall area minus windows and doors, though most estimators skip the deduction on small openings so there is material left for odd cavities and patching. Blown in insulation is different: it is sold by the bag with a coverage chart on the side that lists square feet per bag at each target R value, and the number of bags is what the machine actually consumes, so bag count is the number that matters, not depth alone.

Framing takes up space

A wall or joist bay is not the whole area. Studs at 16 in on center take roughly 15 percent of a wall's area at 24 in on center it is closer to 10 percent. Wood is a poor insulator compared with the cavity fill, and that thermal bridging pulls the whole assembly R value below the number on the batt. It does not change how many batts to buy, since the batts only fill the cavities, but it explains why an R21 wall performs like an R16 or R17 wall in practice. Continuous exterior foam sheathing is the usual fix.

R Values by Climate Zone

R value is resistance to heat flow, and more is better up to the point where the added cost stops paying back. The right target depends on the climate zone, which the US Department of Energy divides into eight bands running from the Gulf Coast to interior Alaska. Local energy code sets the legal minimum and it may be higher than a general recommendation, so confirm with the building department before ordering.

Attic insulation is where the money goes furthest in nearly every US climate, because heat rises in winter and an unshaded roof is the hottest surface on the house in summer. Most of the country lands somewhere between R38 and R60 in the attic and between R13 and R21 in the walls.

Batts, Blown In and Spray Foam

Batts and rolls

Fiberglass or mineral wool batts come precut to fit 16 in or 24 in on center framing. They are the cheapest option, they are easy to handle, and they are the only practical choice in an open wall or a floor joist bay that has to stay accessible. Their weakness is that they only perform when they fill the cavity completely with no gaps, no compression and no voids around wiring and boxes.

Blown in loose fill

Loose fill fiberglass or cellulose is blown from a hopper through a hose. It is the best option for an existing attic floor because it flows around joists, wires, ducts and irregular framing and self levels into corners a batt cannot reach. Dense packed cellulose can also be blown into closed wall cavities through small holes, which is how older houses get insulated without opening the walls.

Spray foam

Spray polyurethane foam expands in place and air seals at the same time it insulates. Open cell foam runs about R3.7 per inch and is vapor permeable. Closed cell foam runs about R6.5 per inch, adds structural rigidity and acts as a vapor retarder at around 2 inches thickness. Foam costs several times more per square foot than batts, needs trained applicators and correct mixing ratios, and can off gas badly if installed wrong, so it is worth using where its air sealing and high R per inch actually earn the price, such as rim joists, cathedral ceilings and tight retrofit cavities.

Depth Needed to Reach an R Value

Blown in insulation is specified by depth as well as by bag count, and depth is what an inspector measures. Fiberglass loose fill delivers about R2.6 per inch and cellulose about R3.7 per inch. Fiberglass batt is about R3.2 per inch and closed cell spray foam about R6.5 per inch.

Those depths assume the stated R per inch, and products vary. Check the coverage chart printed on the bag, which lists the minimum depth, the maximum square feet per bag and the minimum weight per square foot for each R value. All three have to be met for the installation to hit its rating.

Compression Destroys R Value

An R19 batt is R19 only at its full 6.25 in loft. Squash that same batt into a 5.5 in deep 2x6 bay and it delivers about R18. Force an R19 batt into a 3.5 in 2x4 wall and it drops to roughly R13, no better than the R13 batt that was designed for the space and costing more. The insulating value comes from still air trapped in the fiber, and compressing the fiber removes the air.

The same rule applies to the small stuff. A batt tucked behind a wire instead of split around it leaves a compressed lump and a void. A batt jammed into a bay that is slightly narrow bulges and leaves gaps at the edges. Cut batts to fit, split them around wiring and pipes, notch them around outlet boxes, and never staple the facing so tight that it dents the batt.

Air Seal Before Insulating

Insulation slows conductive heat flow. It does not stop air moving. An attic with R49 of loose fill over an unsealed top plate, open wire penetrations, an uncapped chimney chase and a leaky attic hatch loses far more energy than the R value implies, because air carries heat straight past the insulation.

Seal first, then insulate. The list that matters most: top plates where interior walls meet the attic, plumbing and wiring penetrations, recessed light housings, bath fan housings, the chimney chase with sheet metal and high temperature sealant, dropped soffits over cabinets, and the attic hatch itself with weatherstripping and a rigid foam cap. Use canned foam for gaps up to about 3 inches and caulk for anything smaller.

Two things must stay clear of the insulation: soffit vents, which need baffles to keep loose fill from blocking the airflow path along the roof deck, and any heat producing fixture that is not rated IC, meaning insulation contact.