Insulation R-Value per Inch: Use the Product and Application

US R uses ft²·°F·h/Btu. SI RSI uses m²·K/W. RSI = US R × 0.1761101838; a numerical R-20 corresponds to approximately RSI 3.522. U is reciprocal resistance only in matching units, so 1 / 20 is US U-factor rather than an SI U-value.

For a homogeneous SI layer, RSI = thickness in metres / conductivity in W/(m·K). Conductivity must describe the actual product and relevant conditions. A layered or framed assembly needs more than one material’s per-inch figure.

If a product is entered at US R-3.5 per inch, 6 in gives an estimated R-21 by a linear thickness model. If a custom target is R-30, the corresponding estimated thickness is 30 / 3.5 = 8.571 in. These are mathematical examples, not generic properties assigned to a category.

Loose-fill charts can specify installed density, installed depth and settled depth. Use the chart rather than manufacturing a bag curve from a broad R-per-inch estimate. Compression, voids and application conditions can also make nominal linear assumptions unsuitable.

Framing provides a parallel heat path, while common continuous layers are in series with each path. Do not label cavity insulation resistance as whole-wall resistance. Record the source, product thickness and application beside every thermal input. NRCan’s material guidance gives context; the actual product documentation determines the properties to enter.