Practical guide · stated assumptions and checkable examples
Insulation estimates: resistance, thickness and climate are different inputs
Published · Sources checked
Source and worked-example review; not professional advice or approval. Examples are synthetic unless explicitly identified otherwise. Follow the cited authority for current eligibility and legal requirements.
A useful insulation estimate answers a limited question: how much material and entered cost are associated with a stated area and thickness? It should not quietly invent the resistance required by a climate zone or the suitability of an assembly.
Separate thermal resistance from dimensions
The Department of Energy describes R-value as resistance to heat flow and notes that effectiveness depends on installation and the assembly. A thickness measurement alone does not establish the R-value of an unspecified product. Use the manufacturer’s declared performance, and distinguish a material value from the performance of a wall or roof containing framing and thermal bridges.
References: US Department of Energy — Insulation
Do not mix US R-value and SI resistance
Numerically identical resistance values in different unit systems are not equivalent. One US R unit is approximately 0.176110 m²·K/W. Thus an R-20 material corresponds to approximately RSI 3.5222, not RSI 20. Write the unit beside the value before comparing products, and avoid changing only the unit label while leaving the number untouched.
Calculation: RSI (m²·K/W) = US R-value × 0.176110
References: NIST — Guide for the Use of the International System of Units
A volume and cost example
For a synthetic 80 m² installation at 0.10 m thickness, net material volume is 8 m³. A chosen 10% allowance gives 8.8 m³. At an entered 120 per m³, material cost is 1,056 before labor, delivery and excluded works. A packaged product may instead be sold by area at a fixed thickness, so its order calculation should use pack coverage and whole-pack rounding. Do not multiply by thickness twice.
Climate does not remove the need to inspect the assembly
DOE’s existing-home guidance considers the current insulation and the areas being upgraded. A retrofit must also consider moisture, ventilation and installation constraints. This worksheet does not approve an assembly, fire performance or a local minimum. Obtain the applicable requirement and product detail, then enter those values rather than treating a national default as a building-code decision.
References: US Department of Energy — Adding insulation to an existing home
Keep the saving estimate conditional
If a separate heat-loss model predicts lower heating demand, check that its boundary includes the part of the building being insulated. The material-volume calculation itself does not calculate infiltration, occupants’ behavior or annual weather. List the assumptions behind any saving and test more than one energy-price scenario before using it in payback.
Questions about this guide
Does R-20 mean 20 centimetres of insulation?
No. R-value measures thermal resistance, not thickness. The relationship depends on the product and its installation.
Will twice the insulation always halve my bill?
No. That conclusion does not follow from a material-quantity calculation. Whole-building heat flows and non-heating charges remain outside that calculation.