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Hemp insulation R-value per inch: what Canadian installers measure
Hemp insulation R-value per inch in Canada: what Nature Fibres sheets convert to, how hemp batt compares with mineral wool, and why the wall decides.
What to take away
- Canadian hemp batt data sheets convert to roughly R-3.4 to R-3.7 per inch, against R-3.8 to R-4.2 for mineral wool.
- Most disagreements between hemp quotes come from test thickness, density or moisture, not from a different fibre.
- Per-inch R describes a dry lab sample. Air leakage and framing decide how a wall performs in a Canadian climate zone.
- Choose hemp for vapour openness and regional supply, hemp-lime for new walls with depth to spare, and mineral wool for shallow cavities.
Where the Canadian per-inch numbers come from
Hemp batt sold in Canada carries a declared thermal conductivity, not a per-inch R-value, and one converts to the other by division.
Nature Fibres, the Quebec hemp processor, publishes conductivity in a band near 0.039 to 0.042 W/mK on its batt sheets. Converted at 25 mm, that lands near R-3.4 to R-3.7 per inch. Density, moisture content and test temperature move the figure by a few points either way.
US sheets, Hempitecture's included, often report R-3.7 per inch or slightly above, because they are tested at their own density and thickness. Setting a US figure beside a Canadian one is not a like-for-like comparison; material advice about interiors rarely travels cleanly across regions.
Mineral wool from Canadian plants sits near R-4.0 per inch across common cavity sizes, with fibreglass lower, around R-3.2 to R-3.8. Treat those bands as illustrative, because product lines differ.
Vapour control matters more than the third decimal place. The National Research Council's guidance on vapour barriers and insulation sets out how a natural fibre assembly should handle that layer.
The criteria that matter
| Criterion | Hemp batt | Hemp-lime, cast | Mineral wool |
|---|---|---|---|
| Declared per-inch R, illustrative | R-3.4 to R-3.7 | R-2.2 to R-3.0 | R-3.8 to R-4.2 |
| Depth for a nominal R-24 | 6.5 to 7 in | 8 to 11 in | 6 in |
| Vapour behaviour | open | open and hygric | open |
| Settling in a vertical cavity | low when friction fitted | none, it cures solid | low |
| Canadian supply | narrow, Quebec-led | project by project | broad |
Compare values you can fill from a dated data sheet, at the thickness you will actually buy. A per-inch figure measured on a 100 mm sample says little about a 140 mm cavity.
Vapour-open assemblies still need the right membrane on the warm side. The vapour barrier entry covers the Canadian position on where that line sits.
Per-inch numbers also say nothing about how a house feels. The comfort case sits in airtightness, which is why natural insulation upgrades get judged on draughts and bills rather than on conductivity.
Where each option wins
Hemp batt is the right answer for a retrofit of an existing 2x6 wall where you want a plant fibre, a vapour-open build-up and a supplier within a day's drive.
Hemp-lime is the right answer for new walls with depth to spare, where airtightness and thermal mass matter and the schedule can absorb a cure.
Mineral wool is the right answer for shallow cavities, fire and sound ratings, and jobs where price and stock decide the order of work.
Example: two quotes for one 2x6 wall
- Ask each supplier for declared conductivity in W/mK on a dated sheet.
- Convert that figure to per-inch R at the thickness you will install.
- Multiply by the nominal cavity depth, here 5.5 inches.
- Subtract an illustrative 15 to 25 percent for framing and installation losses.
Hemp at R-3.5 gives about R-19 nominal in that cavity. Mineral wool at R-4.0 gives R-22. Both effective values land lower, and the gap between them narrows once framing is counted. Stage-by-stage behaviour, from sealing to settling, is set out in inside natural insulation.
Per-inch R describes a fibre. Wall performance describes a system.
What none of them solve
None of these products fix thermal bridging through studs, air leakage at plates and penetrations, or the gap between nominal and effective R. All three are tested dry, at a fixed temperature, so a wet winter sits outside the measurement.
The National Building Code sets minimum effective R by climate zone rather than naming products. The NRC codes centre publishes those tables, and an assembly has to meet them however good the batt looks per inch.
Reading the quote, not the data sheet
A declared conductivity is easy to print. Vet a natural insulation contractor on whether they measure cavity depth, allow for settling, and seal the envelope before any batt goes in.
Common questions
Is hemp insulation R-value per inch lower than mineral wool? Yes, by a small margin. Canadian hemp batt converts to roughly R-3.4 to R-3.7 per inch, while mineral wool from Canadian plants sits near R-4.0.
Why do two hemp suppliers quote different per-inch numbers? Usually density, thickness or moisture at the time of testing. Ask for conductivity in W/mK and compare at one thickness.
Does the National Building Code of Canada allow hemp batt? The code sets performance targets rather than approved products. A hemp assembly qualifies when it meets the effective R required for its climate zone.
Does a higher per-inch R mean a warmer house? Not on its own. Air leakage, settling and thermal bridging can cost more heat than the difference between R-3.5 and R-4.0 per inch.
