Insulation Calculator

The short answer

Batt insulation is specified by R-value and thickness together: R-13 is about 3.5 inches, R-19 about 6.25 inches and R-30 about 9.5 inches. Bags are sold by the area they cover at a given R-value, and an R-13 bag typically covers about 88 square feet. The walls of a 12 by 14 foot room are about 416 square feet gross, so roughly five bags before deducting openings.

What it depends on

The target R-value depends on climate zone, and the figures that suit a cold northern climate are different from a warm southern one — local energy code sets the minimum. Wall cavities with existing insulation, wiring and plumbing reduce the coverage. Attic targets are usually higher than wall targets, and air sealing before insulating changes the result more than adding material does.

Where this stops being useful

This is a quantity estimate. Vapour control, air sealing, ventilation and fire blocking are all part of a compliant assembly, and getting the vapour retarder on the wrong side of a wall in a cold climate causes moisture problems. Local energy code governs the required values. General information and planning only, not engineering, structural or code advice.

Insulation is rated by R-value, which is resistance to heat flow, and the number you need depends on your climate and the part of the building you are insulating. A wall, a ceiling and a floor all have different requirements, and the same bag of material will not be right for all three. Measure by area rather than by volume — insulation batts are sold by square foot of coverage per bag — and remember that compression reduces the R-value, so do not cram a high-number batt into a cavity built for a lower one.

Insulation is cheap relative to what it saves, and the difference between a good installation and a poor one is far larger than the difference between insulation products. The calculation tells you how much you need; the installation determines whether it works.

Insulation performance depends on installation as much as on the material. Gaps at the edges, around pipes and above the top plate can lose more heat than the entire wall assembly, so sealing before insulating matters more than the R-value you choose. Vapor barriers and air barriers are different things and are often confused; in most climates you need the air barrier continuous, and the vapor barrier position depends on climate. Check whether the walls are already sheathed or whether you are creating a cavity, because the required assembly is different.

Insulation is measured by its resistance to heat flow, the R-value, and by its depth, and the two are linked by the material: each inch of a given insulation adds a fixed amount of R. Fibreglass batts add around three and a half per inch, blown cellulose a little less, and rigid foam boards considerably more. That is why reaching a target R-value can take very different thicknesses depending on what you install. Depth matters because the R-value is only realised when insulation is thick enough. Settled or compressed insulation loses performance — squashing a batt into a thinner cavity lowers its R-value rather than improving it — and blown insulation settles over the years, so a slightly generous initial depth is normal. In an attic, insulation must also stay clear of vents and not block airflow from the eaves, which would create moisture problems. Choosing a target depends on climate, the part of the building, and what is already there. Attics in cold climates commonly target R-38 to R-60, walls less, because of the space available. Adding insulation to an under-insulated attic is usually the single most cost-effective improvement a house can make, since heat rises and the area is usually easy to access compared with walls or floors. This calculator turns a target R-value into a depth using the material's R per inch, and a quantity of bags from their stated coverage at that depth. It ignores existing insulation, so subtract what is already present before ordering, and it does not account for compressed or settled material. Check the coverage printed on the product, since it varies with the depth you are installing and the R-value you are targeting.

This calculator returns estimates for planning and general information only. It is not engineering, architectural, structural, electrical or code advice, and it cannot replace a qualified professional who has seen the site. Material quantities depend on the actual dimensions, the product you buy and local building codes, so confirm the figures before ordering materials or starting work, and always follow local regulations.