Insulation Calculator

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.