Use effective RSI (accounting for thermal bridging) from the Thermal Resistance Converter. Multiply result by surface area (m²) to get total heat loss in watts.
Conversion Factors
1 BTU/hr·ft² = 3.15459 W/m² = 0.003155 kW/m²
Heat flux (W/m²) = ΔT (°C) / RSI (m²·K/W)
Total heat loss (W) = heat flux (W/m²) × surface area (m²)
Building Envelope Heat Flux Reference (Canadian Design Conditions)
| Assembly / Condition | ΔT (°C) | RSI (m²·K/W) | Heat Flux (W/m²) | BTU/hr·ft² |
|---|---|---|---|---|
| R-20 wall, Vancouver (-4°C outdoor, 21°C indoor) | 25 | 3.52 | 7.1 | 2.25 |
| R-20 wall, Toronto (-16°C outdoor, 21°C indoor) | 37 | 3.52 | 10.5 | 3.33 |
| R-20 wall, Calgary (-29°C outdoor, 21°C indoor) | 50 | 3.52 | 14.2 | 4.50 |
| R-40 attic, Toronto (-16°C outdoor, 21°C indoor) | 37 | 7.04 | 5.3 | 1.68 |
| R-40 attic, Calgary (-29°C outdoor, 21°C indoor) | 50 | 7.04 | 7.1 | 2.25 |
| Double-pane window, Calgary (U=2.0 W/m²·K) | 50 | 0.5 | 100 | 31.7 |
| Triple-pane window, Calgary (U=1.0 W/m²·K) | 50 | 1.0 | 50 | 15.9 |
| Slab-on-grade edge, Calgary (RSI 1.76 perimeter) | 50 | 1.76 | 28.4 | 9.0 |
Radiant Panel and Solar Irradiance Reference
| Source / Application | W/m² | BTU/hr·ft² | Notes |
|---|---|---|---|
| Radiant floor heating (typical) | 40 - 80 | 12.7 - 25.4 | Comfortable occupied areas |
| Radiant floor heating (max comfort) | 100 - 120 | 31.7 - 38.1 | Floor surface max ~29°C |
| Radiant ceiling panel | 150 - 300 | 47.6 - 95.1 | Higher flux possible overhead |
| Radiant wall panel | 100 - 200 | 31.7 - 63.4 | Vertical surface comfort range |
| Peak solar irradiance (Canada, summer) | 800 - 1000 | 253.6 - 317.0 | Clear sky on horizontal surface |
| Diffuse solar (overcast) | 50 - 200 | 15.9 - 63.4 | Depends on cloud cover and season |
| Solar gain through clear glass | 300 - 600 | 95.1 - 190.2 | South-facing window, summer midday |
| Human body heat flux (sedentary) | 40 - 60 | 12.7 - 19.0 | Contributes to internal gains |
How to Use the Heat Flux Converter
Type a heat flux value in W/m², BTU/hr·ft², kW/m², or kcal/hr·m². All other units update instantly. Use the quick presets for common HVAC reference points: a typical radiant floor output, a peak solar irradiance value, or the heat flux through a standard Canadian wall assembly.
Enter a temperature difference and an effective RSI value in the Envelope Heat Flux Calculator. The tool computes heat flux through the assembly in both W/m² and BTU/hr·ft², and populates the main converter. Use the Thermal Resistance Converter to get the effective RSI for your assembly.
The result panel identifies what type of heat transfer your converted value represents -- building envelope, radiant panel, or solar-scale flux. This helps quickly flag whether a value is in a realistic range for the application being designed.
Multiply heat flux (W/m²) by the surface area of the assembly (m²) to get total heat loss in watts for that surface. Sum across all envelope surfaces for total building heat loss, which drives equipment sizing. Use the Wall Heat Loss Calculator for a complete surface-by-surface analysis.
Heat Flux in Canadian HVAC Building Envelope Work
Heat flux -- the rate of heat transfer per unit area -- is one of the most fundamental quantities in building science. It connects thermal resistance (RSI or R-value) and temperature difference to actual heat flow rates, and it's used to compare building assembly performance, size radiant heating panels, and evaluate solar gains through glazing.
The Core Relationship: Flux, RSI, and Temperature
Heat flux through any building assembly is: q (W/m²) = DeltaT (°C) / RSI (m²·K/W). This is the same relationship as Fourier's law of heat conduction, expressed in convenient HVAC units. A wall with RSI 3.52 (R-20) separating 21°C indoors from -29°C outdoors in Calgary (DeltaT = 50°C) has a heat flux of 50/3.52 = 14.2 W/m². Multiply by the wall area -- say 20 m² -- to get 284 W of heat loss through that wall section alone. The National Research Council's building envelope research is where much of the underlying thermal performance testing behind these assembly values comes from. The Thermal Resistance Converter handles RSI and R-value conversions for assembly analysis.
Converting W/m² to BTU/hr·ft²
The conversion factor is 1 W/m² = 0.31699 BTU/hr·ft² (or equivalently, 1 BTU/hr·ft² = 3.15459 W/m²). For quick estimates, rounding to 0.317 and 3.15 is accurate enough for most field work, and the exact figures trace back to the BTU-to-joule and foot-to-metre relationships in the NIST Guide to the SI, Appendix B. Canadian engineering uses W/m² throughout; US envelope performance datasheets and some Canadian product literature use BTU/hr·ft², making this conversion a routine part of comparing window and wall assembly specifications.
Radiant Heating Panel Sizing
Radiant floor, ceiling, and wall panels are sized based on output heat flux -- the heat they must deliver per unit of panel surface area to meet the room heating load. A common approach: calculate the room heat loss in watts, divide by the available panel area in m² to get the required flux in W/m², then verify this falls within the comfortable design range. For radiant floors in occupied areas, a practical upper limit of roughly 100 to 120 W/m² applies to keep surface temperatures comfortable underfoot. The Radiant Floor Heat Calculator works through this full sizing process from room load to panel area. Ceiling panels can operate at 150 to 300 W/m² or more since the surface isn't in contact with occupants.
Solar Irradiance and Cooling Loads
Solar irradiance drives cooling loads through windows and roofs. Peak solar irradiance at the Earth's surface under clear-sky summer conditions in Canada reaches 800 to 1000 W/m² on a horizontal surface. South-facing glazing at midday may receive 600 to 800 W/m² of direct solar radiation, much of which enters the building as heat gain. Shading coefficients, solar heat gain coefficients (SHGC), and overhangs reduce this to lower effective gains, which the Solar Heat Gain Calculator works through for a specific window and orientation. The heat flux into a surface equals the irradiance multiplied by the relevant solar gain factor for the glazing or surface in question.
Frequently Asked Questions
Multiply W/m² by 0.31699 to get BTU/hr·ft². So 100 W/m² = 31.7 BTU/hr·ft² and 300 W/m² = 95.1 BTU/hr·ft². To go the other way, multiply BTU/hr·ft² by 3.15459. This conversion is routine in Canadian building envelope work since assembly analysis uses W/m² while some window and product datasheets use BTU/hr·ft².
Heat flux is the rate of heat transfer per unit area (W/m² or BTU/hr·ft²). In HVAC it's used to analyse building envelope heat loss (q = DeltaT/RSI), size radiant heating panels (watts of output per m² of panel surface), evaluate solar gain through glazing, and check heat exchanger surface loading. Multiply heat flux by surface area to get total heat transfer in watts or BTU/hr.
Heat flux depends on temperature difference and thermal resistance: q = DeltaT/RSI. An R-20 wall (RSI 3.52) in Calgary (-29°C outdoor, 21°C indoor, DeltaT = 50°C) has q = 50/3.52 = 14.2 W/m² (4.5 BTU/hr·ft²). The same wall in Vancouver (-4°C outdoor, DeltaT = 25°C) has only 7.1 W/m². A poorly insulated double-pane window in Calgary can reach 100 W/m² or more -- roughly 7x the R-20 wall -- showing why windows dominate envelope heat loss in cold climates.
Radiant floor heating systems in Canada typically output 40 to 120 W/m² (12.7 to 38.1 BTU/hr·ft²) depending on floor construction, tube spacing, and water temperature. Keeping floor surface temperature below roughly 29°C (84°F) in occupied areas limits practical output flux to about 100 to 120 W/m². The required flux equals the room heat loss (W) divided by the available floor panel area (m²) -- if this exceeds 120 W/m², supplemental heating or additional panel area is needed.