Information · Condensation Resistance
Condensation Resistance
Condensation forms on any surface colder than the dew point of the air touching it. Indoors, that air is warm and carries moisture from people, cooking, showers and plants. When it meets a cold surface, it cools, and the water it can no longer hold appears as fog, droplets or, in deep cold, frost. The window is usually the coldest surface in the room, so it is where condensation shows first.
Condensation is not a defect in itself. It is the meeting of two things: how cold the window's inside surface gets, which the window controls, and how humid the room is, which the building and its occupants control. A good window keeps its coldest point warm enough that ordinary indoor humidity never reaches it.
Dew point: the number that matters
The dew point is the temperature at which the air becomes saturated. It depends on room temperature and relative humidity (RH). At a typical indoor temperature of 70 °F (21 °C):
| Indoor RH | Dew point | Typical situation |
|---|---|---|
| 25% | 33 °F (0 °C) | Heated home in deep cold, dry climate |
| 30% | 37 °F (3 °C) | Well-ventilated home in winter |
| 40% | 45 °F (7 °C) | Comfortable winter target in mild climates |
| 50% | 50 °F (10 °C) | Humidified home, offices, new construction drying out |
| 60% | 55 °F (13 °C) | Kitchens, bathrooms, occupied classrooms |
| 70% | 60 °F (15 °C) | Pools, spas, museums, some hospital spaces |
If any part of the window's inside face drops below the dew point, water will form there. The goal of window design is simple to state: keep the coldest point above the dew point for the climate and the room.
Where condensation forms on a window
Condensation almost never starts in the middle of the glass. It starts where heat escapes fastest:
- The bottom edge of the glass. Cool air sinks down the pane and pools at the sill, and the edge of the insulating glass is colder than its center. The bottom inch or two is where fog appears first.
- The spacer. The spacer separates the panes around the edge. A metal spacer is a direct thermal path from outside to inside and can make the glass edge several degrees colder, often 5 to 10 °F, than a warm-edge spacer.
- The frame and sash. Aluminum without a thermal break can run near outdoor temperature. Corners, hardware and joints are the usual cold points.
- Hidden surfaces. Warm, moist air leaking out through gaps around or within the window can condense inside the wall, the rough opening or the frame cavity, where it cannot be seen or dried. This is the most damaging kind.
How cold does the glass get?
The better the glass insulates, the closer its inside face stays to room temperature. Calculated center-of-glass temperatures at 0 °F (−18 °C) outside and 70 °F (21 °C) inside, with standard interior air films:
| Glazing | Center-of-glass U (W/m²·K) | Inside surface | Condensation starts above about |
|---|---|---|---|
| Single glass | 5.8 | 17 °F (−8 °C) | Below 10% RH (frost likely) |
| Double, clear, air-filled | 2.7 | 45 °F (7 °C) | 40% RH |
| Double, low-e, argon | 1.4 | 57 °F (14 °C) | 63% RH |
| Triple, two low-e coatings, argon | 0.7 | 64 °F (18 °C) | 80% RH |
| ZERO 04 hybrid vacuum glass | 0.25 | 68 °F (20 °C) | Over 90% RH |
These are center-of-glass figures. The edge of the glass and the frame always run colder, which is why spacer and frame design decide real-world condensation performance far more than the glass alone. Values are typical calculations, not product ratings.
How condensation resistance is rated in the United States
NFRC ratings. The National Fenestration Rating Council publishes the condensation rating found on many residential window labels. Under NFRC 500 the rating was the Condensation Resistance (CR), a number from 1 to 100 based on how much of the window's inside surface falls below the dew point at 30%, 50% and 70% indoor RH. ANSI/NFRC 500-2020 replaces it with the Condensation Index (CI), also from 1 to 100, based on the coldest temperatures of the frame, the edge of glass and the center of glass at 0 °F (−18 °C) outside and 70 °F (21 °C) inside. The lowest of the three sets the rating. CI is calculated with the LBNL THERM and WINDOW software, with a physical test only where a product cannot be simulated. CR and CI are both relative ratings and are not interchangeable. Higher is better in both.
AAMA CRF. Commercial specifications often call for the Condensation Resistance Factor (CRF) under AAMA 1503. It is measured in a hot box at 0 °F outside and 70 °F inside, with thermocouples on the frame and glass, and reported separately for frame and glass:
CRF = [ (coldest weighted surface temperature − outdoor temperature) ÷ (indoor temperature − outdoor temperature) ] × 100
A CRF of 50 means the weighted cold surfaces sit halfway between outdoor and indoor temperature. The lower of the frame and glass values is the product's CRF. AAMA publishes a selection chart that converts a winter design temperature and an indoor RH into the minimum CRF to specify.
Codes. U.S. energy codes regulate U-factor, SHGC and air leakage, but neither the IECC nor the International Residential Code sets a minimum condensation rating. Condensation resistance is a specification choice, which makes it easy to overlook. For buildings with high indoor humidity (pools, hospitals, labs, museums, food processing), it should be calculated, not assumed.
How condensation resistance is rated in Canada
CSA A440.2 Temperature Index (I-value). In Canada, window thermal performance is rated under CSA A440.2/A440.3, the companion to the NAFS performance standard. It includes an optional condensation rating called the Temperature Index, or I-value. Like the CRF, it expresses the coldest relevant inside surface temperature as a percentage of the distance between outdoor and indoor temperature, at −18 °C outside and 21 °C inside. It is based on either the coldest frame temperature or the average of glass sensors placed 50 mm in from the sightline. NFRC's newer Condensation Index was modeled on this Canadian approach, so the two systems are converging.
Reading the I-value. The I-value lets you match a window to a climate and a room. As outdoor design temperature falls, or indoor humidity rises, a higher I-value is needed. A window that stays dry all winter in Vancouver may fog on the coldest nights in Winnipeg or Edmonton at the same indoor humidity, so the I-value should be chosen for the location, not just the product.
Design temperatures. The National Building Code lists a January 2.5% design temperature for every Canadian locality. Typical values:
| City | January design temperature |
|---|---|
| Vancouver | about −7 °C |
| Halifax | about −16 °C |
| Toronto | about −20 °C |
| Montréal | about −23 °C |
| Ottawa | about −25 °C |
| Calgary and Edmonton | about −30 °C |
| Winnipeg | about −33 °C |
Check the NBC or your provincial code for your exact location. These colder design points are why triple glazing, warm-edge spacers and insulating frames are standard practice across most of Canada.
Indoor humidity in cold weather. Canadian codes require mechanical ventilation in new homes, which controls indoor humidity. A widely used rule of thumb lowers the indoor RH target as it gets colder outside:
| Outdoor temperature | Maximum indoor RH (typical guidance) |
|---|---|
| 0 °C (32 °F) | about 40% |
| −10 °C (14 °F) | about 35% |
| −20 °C (−4 °F) | about 25% |
| −30 °C (−22 °F) | about 15 to 20% |
A better window raises these limits. With a high I-value window, a home can stay more comfortable and less dry through a cold snap without fogging glass.
Passive House: the temperature factor
The Passive House Institute uses a temperature factor, fRsi, which works the same way as the I-value and CRF but as a decimal. For certified components, the coldest inside surface must reach at least fRsi 0.70 in cool-temperate climates and 0.75 in cold climates. That keeps the surface above the threshold for mold growth, not just above visible condensation. Our GR85 system is certified at fRsi 0.77, with its coldest inside surface at 13.2 °C when it is −10 °C outside.
How the rating is calculated today
Hot-box testing under AAMA 1503 and ASTM C1199 is still used, but most condensation ratings are now calculated with two-dimensional heat-flow software, chiefly LBNL THERM and WINDOW, validated against physical tests. Simulation shows the temperature at every point on the section, so designers can see exactly where the cold spots are and fix them before the window is built. This is how NFRC's Condensation Index, the CSA I-value and Passive House fRsi values are produced.
What makes a window resist condensation
- An insulating frame. Our fiberglass conducts heat more than a thousand times less than aluminum. A polyurethane fiberglass frame keeps its inside surface close to room temperature with no thermal break needed.
- A warm-edge spacer. We use Quanex Super Spacer® Premium, a silicone-foam spacer with no metal, which raises the glass-edge temperature where condensation starts.
- Better glass. Low-e coatings, argon, a third pane or vacuum glass each warm the inside surface. Vacuum glass keeps the inside face within a few degrees of room temperature.
- Deep glass bite. Setting the glass edge deeper into an insulating frame shelters the coldest part of the glazing.
- Airtight, continuous seals. Multiple gaskets keep moist room air out of the frame and the rough opening.
The room matters as much as the window
- Humidity sources. Showers, cooking, drying laundry, plants, aquariums and unvented gas appliances all add moisture. A family of four can add 10 or more liters of water to indoor air a day.
- Ventilation. Exhaust fans, an HRV or ERV, and a properly set humidistat are the main controls. Turn humidifiers down in very cold weather.
- Air movement at the glass. Heavy drapes, tight blinds, cellular shades and deep sills trap still air against the glass and make it colder. Leave a gap, and let heating registers wash the window with warm air.
- New construction. Concrete, drywall, plaster and lumber release large amounts of water during the first heating season. Expect more condensation in the first winter and ventilate generously.
Installation and hidden condensation
A window can carry an excellent rating and still suffer condensation if it is installed poorly. Gaps between the frame and the rough opening let warm, moist indoor air leak outward, where it condenses inside the wall. The result is mold, rot and stained finishes, often found only years later. A continuous interior air seal around the frame, insulation that fills the shim space, and a window set in line with the wall's insulation layer are as important as the window itself. See Air Barrier Interface and Installation.
Outside condensation is a good sign
On cool, clear mornings, very well insulated windows sometimes fog on the outside. Because so little heat escapes, the outer pane cools to the night sky and drops below the outdoor dew point, just like a car windshield or a lawn. It clears as the sun comes up and means the glass is keeping heat inside. In hot, humid climates such as Florida, the same can happen in summer when the glass is chilled by air conditioning, which is another reason to choose low-conductance glass and frames there.
Specifying for condensation resistance
- Decide the indoor RH the building must hold in winter, and find the local winter design temperature.
- Calculate the dew point, and the minimum CRF, CI, I-value or fRsi that keeps the coldest surface above it, with a margin.
- Ask for rated or simulated values for the whole window, frame and edge of glass included, not center-of-glass alone.
- For pools, hospitals, museums, labs and other high-humidity rooms, request a project-specific THERM analysis.
Our technical team can run a condensation analysis for your project and conditions. Contact us with your location, indoor design conditions and window schedule.