Risk by Surface Type (Same Indoor Conditions)
Full Results Table
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How to Use This Calculator
Choose from common surface presets, from single-pane windows (lowest RSI, highest risk) to well-insulated walls. The presets are in RSI (m²K/W), with the imperial R-value in brackets, and the R value converter handles other units. Thermal bridges like uninsulated wall corners or rim joists often have much lower effective RSI than the surrounding wall field.
Enter your target or measured indoor conditions. These represent the room air that will be in contact with the cold surface.
Use your local winter design temperature from NBC Appendix C, not the coldest recorded temperature. This is a reasonable worst-case condition the surface will regularly experience each winter.
The calculator computes surface temperature from the temperature gradient across the assembly, then calculates surface RH at that temperature. Results flag risk above the 80% surface RH mold growth threshold and 100% (condensation).
Surface Condensation and Mold Risk in Canadian Buildings
Mold problems in Canadian homes almost always trace back to one physical reality: a surface got colder than the dew point of the air touching it. Understanding where and why this happens, and being able to calculate it rather than guess, is one of the most practically useful skills for diagnosing building envelope and ventilation problems together.
Why Surface Temperature Matters More Than Room RH
Room air relative humidity is an average condition. It says nothing about what's happening at any specific surface. When indoor air contacts a cold surface, a thin boundary layer of air cools to match the surface temperature. Since relative humidity rises as temperature drops (at constant moisture content), that boundary layer air can have a much higher RH than the room average. At 21°C and 45% RH, a surface at 16°C is at about 62% RH, and a surface at 10°C is at about 91%, above the mold threshold even though a hygrometer in the middle of the room reads a comfortable 45%. Health Canada's residential indoor air quality guideline for moulds says mould growth in homes may pose a health hazard and that the response is to control humidity, repair water damage promptly, and clean mould thoroughly. This is why two identical rooms with the same air RH can have very different mold outcomes depending on their wall construction and window quality.
The Temperature Gradient Calculation
The interior surface temperature equals the indoor temperature minus the total indoor-outdoor temperature difference times the interior film resistance (0.12 m²K/W) divided by the total resistance, which is the assembly RSI plus the air films (0.16 m²K/W in this calculator). For a well-insulated assembly nearly all of the temperature drop happens in the insulation, so the interior surface sits close to room temperature. As RSI drops, through thinner insulation, thermal bridges, or single-pane glass, a larger share of the drop happens at the surface. At 21°C indoors and -20°C outdoors, single-pane glass at RSI 0.16 (R-0.9) has a surface temperature of about 5.6°C, while a triple-pane unit at RSI 0.97 (R-5.5) sits near 16.6°C. That is why old windows frost over on cold mornings while the wall around them stays dry. Product literature quotes both imperial R and metric RSI (multiply RSI by 5.678 to get R), and the R value converter handles the conversion so the right number goes into the calculator.
Thermal Bridges: The Hidden Mold Risk
A thermal bridge is any point in the building envelope where the insulation is interrupted or bypassed by a more conductive material, such as steel studs, concrete lintels, uninsulated headers, or the geometric effect at corners where the interior surface area is smaller than the exterior surface area. Wall corners can have noticeably lower effective RSI than the wall field even with identical insulation, purely from this effect. Rim joists at the floor-to-foundation transition are notoriously under-insulated in older Canadian homes. These are the locations where mold problems concentrate even in otherwise well-built and well-humidified homes. See the humidity comfort calculator for the room-level humidity context.
Choosing a Design Temperature
Winter design temperatures vary significantly across Canada and should be used rather than average or record-low temperatures for practical risk assessment. Design values are tabulated for Canadian locations in Appendix C of the National Building Code as January 1% and 2.5% temperatures, and the design temperature lookup finds them for your city. The calculator's default of -20°C is a screening value. The design temperature matters because it sets the temperature difference: for the default wall corner at RSI 0.35 with 21°C and 40% RH indoors, the surface temperature is about 11.4°C at -20°C outside and about 9.0°C at -30°C, which takes the surface from roughly 74% RH to roughly 87%. Using standard design values rather than absolute record lows gives a realistic picture of the conditions surfaces will regularly face each winter.
Practical Interventions
When surface RH exceeds 80% at design conditions, there are three intervention categories: reduce indoor RH (dehumidification, increased ventilation, source control of moisture), improve surface RSI (window upgrades, addressing thermal bridges, added insulation), or accept a lower winter indoor RH target than would otherwise be comfortable. Health Canada's relative humidity factsheet recommends 30 to 35% in winter and advises reducing humidification if condensation appears on windows. For most Canadian retrofits, some combination of all three is used. Triple-pane windows dramatically reduce the surface RH risk compared to older glass: at 21°C, 40% RH, and -20°C outdoors, the calculator puts a single-pane window at about 5.6°C, below the 6.9°C room dew point, so it condenses, while a triple-pane window at about 16.6°C sits near 53% RH.
Frequently Asked Questions
Most common indoor mold species, including Cladosporium, Penicillium, and Aspergillus, need sustained surface relative humidity above roughly 80% to germinate and grow. Below 70% surface RH, growth is much less likely and slower. Between 70% and 80%, some mold species can grow slowly under favourable temperature conditions. The 80% threshold is why building science professionals focus on surface RH rather than room air RH: a room at a comfortable 45% RH can still have mold-supporting conditions on a cold surface where the local surface RH is much higher than the room average.
Relative humidity is temperature-dependent: colder air holds the same absolute moisture at a higher relative humidity than warmer air. When room air contacts a cold surface, like a window or a poorly insulated wall corner, the air immediately at that surface cools down, and its relative humidity rises even though the absolute moisture content hasn't changed. If a wall corner is 5°C colder than the room air due to poor insulation or a thermal bridge, the surface RH at that spot is roughly 15 to 18 percentage points higher than the room average at 40 to 50% room RH, and the gap widens as the surface gets colder. This is why insulation and thermal bridging matter as much as humidity control for mold prevention.
The RSI needed depends on the indoor-outdoor temperature difference and the target indoor RH. In this calculator, with 21°C indoors and -20°C outdoors, a surface needs an effective RSI of about 0.3 (R-1.7) to stay below 80% surface RH at 40% room RH, and about 0.5 (R-2.9) at 50% room RH. Field areas of insulated walls clear that easily. The problem areas are windows and thermal bridges such as wall corners, window headers, and structural connections, which often have effective RSI far below the wall field. Prescriptive insulation levels for walls are set by climate zone in the National Building Code, so check the current code for your location.
Window glass and frames typically have much lower R-values than insulated walls, making them the coldest surfaces in most rooms during Canadian winters. Condensation on glass indicates the surface temperature dropped below the room air dew point. While glass itself doesn't support mold, condensation running onto sills, frames, and surrounding drywall creates mold risk in those materials. Persistent window condensation is often the first visible warning that indoor humidity is too high for current outdoor temperature and window performance.
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