RSI = thickness (m) / k (W/m·K) | R-value = thickness (in) / k (BTU·in/hr·ft²·°F). Enter k-value above first, then thickness.
Conversion Factors
1 W/m·K = 0.57779 BTU/hr·ft·°F
1 W/m·K = 0.86002 kcal/m·hr·°C
1 BTU·in/hr·ft²·°F = 0.14423 W/m·K
RSI (m²·K/W) = thickness (m) / k (W/m·K)
R-value = thickness (in) / k (BTU·in/hr·ft²·°F)
Insulation Material Thermal Conductivity Reference
| Material | k (W/m·K) | k (BTU·in/hr·ft²·°F) | R/inch (Imperial) | Notes |
|---|---|---|---|---|
| Fiberglass batt (standard) | 0.036 - 0.042 | 0.250 - 0.291 | 3.4 - 4.0 | 2x4 = R-12, 2x6 = R-19 |
| Mineral wool / Rockwool batt | 0.033 - 0.040 | 0.229 - 0.277 | 3.6 - 4.4 | Better fire resistance, higher R/inch |
| Blown fiberglass (loose fill) | 0.038 - 0.044 | 0.263 - 0.305 | 3.3 - 3.8 | Settles; common for attic top-up |
| Blown cellulose (loose fill) | 0.040 - 0.043 | 0.277 - 0.298 | 3.4 - 3.6 | Recycled content; good for attic |
| EPS rigid foam (Type II) | 0.036 - 0.040 | 0.250 - 0.277 | 3.6 - 4.0 | White expanded polystyrene board |
| XPS rigid foam (extruded) | 0.029 - 0.033 | 0.201 - 0.229 | 4.4 - 5.0 | Best for below-grade; blue/pink/grey |
| Polyiso rigid foam | 0.022 - 0.028 | 0.153 - 0.194 | 5.2 - 6.5 | Highest R/in; degrades slightly in cold |
| Open-cell spray foam | 0.036 - 0.040 | 0.250 - 0.277 | 3.6 - 4.0 | Air barrier; vapour-permeable |
| Closed-cell spray foam | 0.022 - 0.027 | 0.153 - 0.187 | 5.3 - 6.5 | Vapour barrier; structural strength |
| Rigid mineral wool board | 0.033 - 0.040 | 0.229 - 0.277 | 3.6 - 4.4 | Non-combustible; exterior continuous |
Building Material Thermal Conductivity Reference
| Material | k (W/m·K) | k (BTU·in/hr·ft²·°F) | Notes |
|---|---|---|---|
| Air (still) | 0.025 | 0.173 | Basis for insulation effectiveness |
| OSB / plywood sheathing | 0.13 | 0.901 | Structural sheathing; lower than solid wood |
| Wood framing (softwood) | 0.12 | 0.832 | 2x4/2x6 studs; creates thermal bridging |
| Gypsum board (drywall) | 0.17 | 1.179 | 5/8-in drywall has RSI ~0.06 (R-0.32) |
| Brick (common) | 0.72 | 4.994 | Facing brick; varies by density |
| Concrete (normal weight) | 1.0 - 1.5 | 6.93 - 10.40 | Higher density = higher conductivity |
| Concrete block (lightweight) | 0.19 | 1.317 | Hollow core; effective k lower |
| Glass (window) | 1.0 | 6.93 | Glass itself; frame and gas fill dominate window RSI |
| Steel / carbon steel | 50 | 346.7 | Major thermal bridge through steel studs |
| Aluminium | 205 | 1421 | Curtain wall framing thermal bridging |
| Copper | 385 | 2669 | ACR pipe; excellent heat transfer |
| Mineral wool pipe insulation | 0.036 | 0.250 | Pipe and duct insulation wrap |
k-values are at approximately 20-25°C. Thermal conductivity varies with temperature, moisture content, and density. Always use manufacturer test data for design calculations. Use the R-Value / RSI Converter for insulation compliance checks.
How to Use the Thermal Conductivity Converter
Type a thermal conductivity value in W/m·K, BTU·in/hr·ft²·°F, BTU/hr·ft·°F, or kcal/m·hr·°C. All other units update instantly. Use the quick presets to load common insulation and building material k-values for Canada.
With a k-value entered, type insulation thickness in mm or inches. The R-Value / RSI Calculator instantly shows RSI (metric R), Imperial R-value, R per inch, and the assembly U-value. Entering thickness in mm auto-fills the inch field and vice versa. This is the fastest way to verify that a proposed insulation layer meets NBC minimum RSI requirements.
The result panel identifies what category of material your k-value corresponds to: high-performance insulation, standard insulation, building materials, or metals. A k-value of 50 W/m·K flagging as carbon steel, for instance, immediately confirms you're working with structural members rather than insulation -- preventing unit or material mix-ups.
Two reference tables cover 10 common insulation products and 12 building materials with k-values in both unit systems and R/inch ratings for insulation. Use the insulation table alongside the R-Value / RSI Converter and Wall Heat Loss Calculator for complete envelope thermal analysis.
Thermal Conductivity in HVAC and Building Science
Thermal conductivity (k) is the rate at which a material conducts heat per unit thickness, per unit area, per unit temperature difference. Low k-value means a good insulator; high k-value means a good conductor. In Canadian building and HVAC work, W/m·K is the SI standard while BTU·in/hr·ft²·°F appears on North American insulation product datasheets, with laboratory-tested k-values published through research such as the National Research Council's building envelope testing. Converting between them is routine for envelope design and compliance work.
The Conversion: W/m·K to BTU·in/hr·ft²·°F
Multiply W/m·K by 6.9335 to get BTU·in/hr·ft²·°F. Fiberglass batt at k = 0.040 W/m·K is 0.040 x 6.9335 = 0.277 BTU·in/hr·ft²·°F. Closed-cell spray foam at k = 0.024 W/m·K is 0.024 x 6.9335 = 0.166 BTU·in/hr·ft²·°F. Note that BTU/hr·ft·°F (without the "in") is a different unit -- used for dense materials and metals -- where the factor is 0.57779: 1 W/m·K = 0.57779 BTU/hr·ft·°F. Always check which unit a datasheet uses before converting, using the exact factor set out in the NIST Guide to the SI, Appendix B.
From k to R-Value: Connecting Conductivity and Resistance
Thermal resistance (R-value / RSI) equals thickness divided by conductivity: RSI = t(m) / k(W/m·K) and R = t(in) / k(BTU·in/hr·ft²·°F). This means R-value per inch = 1/k in Imperial units. XPS foam with k = 0.029 W/m·K gives k(Imperial) = 0.201 BTU·in/hr·ft²·°F, so R/in = 1/0.201 = 4.97 -- the well-known "R-5 per inch" for XPS. Polyiso at k = 0.022 gives k(Imperial) = 0.153, R/in = 6.5. Use the R-Value calculator on this page to compute R-value for any k and thickness combination, then verify against NBC minimums with the R-Value / RSI Converter.
Thermal Bridging and Steel
Steel's thermal conductivity of 50 W/m·K is over 1000 times higher than fiberglass insulation at 0.040 W/m·K. This means a steel stud in a wall assembly creates a thermal bridge that bypasses the insulation almost entirely. A 2x6 wood stud wall with R-19 cavity insulation has an effective clear-field assembly RSI of about 3.0-3.3 due to wood thermal bridging. The same wall with steel studs has an effective RSI of only about 1.0-1.5 -- the steel bridges 15% of the wall area and devastates the overall thermal performance. This is why exterior continuous insulation is essential in steel-framed buildings to maintain NBC compliance. Use the Wall Heat Loss Calculator for parallel-path assembly calculations including bridging effects.
Thermal Conductivity in Pipe and Duct Insulation
HVAC piping insulation is selected based on the pipe surface temperature and the required thermal resistance to prevent condensation or energy loss. The minimum insulation thickness depends on the pipe diameter, the fluid temperature, and the insulation's k-value. Fiberglass pipe insulation and mineral wool pipe insulation both have k values around 0.033-0.040 W/m·K. Closed-cell elastomeric foam (like Armaflex) has k of about 0.036-0.040 W/m·K. The Pipe Insulation Heat Loss Calculator works from these k-values to size the correct wall thickness. For refrigerant suction lines, vapour barrier integrity is as important as thermal resistance -- condensation on cold suction lines indicates inadequate insulation or vapour barrier failure.
Frequently Asked Questions
Multiply W/m·K by 6.9335 to get BTU·in/hr·ft²·°F. So fiberglass batt at k = 0.040 W/m·K equals 0.277 BTU·in/hr·ft²·°F. XPS foam at 0.029 W/m·K equals 0.201 BTU·in/hr·ft²·°F. To go the other way, divide by 6.9335 or multiply by 0.14423. Watch out for the similar-looking unit BTU/hr·ft·°F (without "in") -- that uses factor 0.57779 and is used for denser materials and metals, not insulation.
R-value equals thickness divided by thermal conductivity: R = thickness (inches) / k (BTU·in/hr·ft²·°F). So R/inch = 1/k. XPS foam at k = 0.201 BTU·in/hr·ft²·°F gives R/inch = 1/0.201 = 4.97 (the well-known R-5/inch). In metric: RSI = thickness (m) / k (W/m·K). Use the R-Value calculator on this page to compute R-value for any k and thickness, then verify against NBC minimums with the R-Value / RSI Converter.
Common Canadian insulation k-values: fiberglass batt 0.036-0.042 W/m·K, mineral wool batt 0.033-0.040 W/m·K, EPS rigid foam 0.036-0.040 W/m·K, XPS rigid foam 0.029-0.033 W/m·K, polyiso 0.022-0.028 W/m·K, open-cell spray foam 0.036-0.040 W/m·K, closed-cell spray foam 0.022-0.027 W/m·K. Lower k-value means better insulation per unit thickness. Polyiso and closed-cell spray foam provide the highest R-value per inch of any common Canadian insulation product.
R-value per inch equals 1 divided by k (in BTU·in/hr·ft²·°F). XPS with k = 0.029 W/m·K converts to 0.201 BTU·in/hr·ft²·°F, giving R/inch = 1/0.201 = 4.97. Polyiso with k = 0.022 W/m·K gives 0.153 BTU·in/hr·ft²·°F and R/inch = 6.5. Fiberglass batt at k = 0.040 W/m·K gives 0.277 BTU·in/hr·ft²·°F and R/inch = 3.6. Use the R-Value calculator on this page -- enter k from the converter and a thickness to get R-value and RSI immediately.