R-value and RSI: reading insulation values on a Canadian job without mixing them up
A supplier offers an R-20 batt against a wall note reading RSI 3.52. The numbers appear equivalent after conversion, but the assembly schedule calls for an effective value. The estimator needs to distinguish a units match from evidence that the proposed whole wall meets its requirement.
Read the units and the basis together. A correct conversion is useful, but it cannot turn a product’s nominal rating into the effective performance of a complete assembly. Record what the drawing requires and what the submitted value represents before comparing them. Leave the actual assembly calculation and approval with the responsible project process.
Check both the number and what it describes
- Copy the requirement with its basis. Record the current drawing or specification reference, assembly location, stated units and whether the value is nominal or effective. Ask for clarification if the note and schedule disagree. Do not silently choose the easier number. Keep separate walls or roof zones distinct when their assemblies and requirements differ.
- Convert the units consistently. Natural Resources Canada gives RSI = R ÷ 5.678. Reverse that relationship with R = RSI × 5.678. R-20 converts to approximately RSI 3.52; RSI 3.52 converts to approximately R-20. Keep sensible precision and the original number on the record. This is a unit conversion, not a calculation of the proposed wall’s effective resistance.
- Identify the submitted value’s scope. NRCan distinguishes the insulation’s nominal rating from effective resistance that accounts for other assembly components and their proportions. Obtain the actual product thickness and rated value, then identify the proposed assembly and its supporting calculation or reference. Matching a bag label to an effective requirement skips the question the submittal needs to answer.
- Record a comparison that can be reviewed. Put the requirement, converted units, proposed product and assembly basis beside one another. Refer any shortfall, changed component or unclear assumption to the responsible reviewer. Keep the answer traceable to the applicable location and revision. An accepted product for one assembly does not by itself establish acceptance for another configuration.
Common mistakes
- Comparing R and RSI as if they were the same units.
- Treating nominal product resistance as effective wall resistance.
- Adding a universal framing deduction without the actual assembly basis.
- Dropping the original units or drawing reference from the question.
Checklist
A useful insulation comparison records
- Current requirement and assembly location.
- Original value, units and nominal or effective basis.
- Conversion using R ÷ 5.678 or RSI × 5.678.
- Proposed product and actual assembly assumptions.
- Responsible clarification and received approval reference.
Check your understanding
An R-20 bag converts to RSI 3.52. Does that prove an effective RSI 3.52 wall requirement is met?
Show the answer
It proves the unit conversion only. The bag’s nominal product value and the wall’s effective assembly value have different scopes. Review the actual framing, other components and supporting assembly calculation through the project’s submittal process before treating the requirement as satisfied.
