Window Ratings Decoded: U-Value, R-Value, SHGC & ER
Published on: March 7, 2026
4 min read
Why Should You Care About Window Ratings?
When you shop for windows, every manufacturer throws around numbers — U-value, R-value, SHGC, ER. They sound technical, but they answer a simple question: will these windows keep your home comfortable and your energy bills low?
In Quebec, where temperatures swing from +35 °C in summer to -30 °C in winter, choosing the wrong window costs you thousands over its lifetime. This guide breaks down each rating so you can compare products on a level playing field.
U-Value — How Much Heat Escapes
The U-value (also called U-factor) measures how easily heat passes through the entire window assembly — glass, frame, spacer, everything. It is measured in W/m²·K (watts per square metre per degree Kelvin).
Lower is better. Less heat escapes in winter; less heat enters in summer.
| Window Type | Typical U-Value | Verdict |
|---|---|---|
| Single-pane (old) | 5.0 – 6.0 | Very poor |
| Standard double-pane (North America) | 1.6 – 2.0 | Code minimum |
| Quality double-pane, Low-E, argon | 1.2 – 1.4 | Good |
| European triple-pane (Maxima) | 0.97 | Exceptional |
Maxima windows achieve a U-value as low as 0.97 W/m²·K — nearly half the heat loss of a standard North American window. See our windows →
R-Value — The Insulation Score
The R-value is simply the inverse of the U-value (R = 1 / U). While U-value dominates window specifications, R-value is what most Canadians know from wall insulation.
- A standard 2×6 wall with fibreglass: R-20
- A typical North American window: R-2 to R-3
- A Maxima triple-pane window: R-5 to R-6
The gap matters: a window with R-2 is a thermal hole in an R-20 wall. Upgrading to R-5+ means your windows finally keep pace with your insulation.
SHGC — How Much Sun Gets In
Solar Heat Gain Coefficient measures the fraction of solar energy that passes through the glass. It ranges from 0 to 1.
- High SHGC (0.50+): More free solar heat in winter — great for south-facing windows in cold climates.
- Low SHGC (below 0.30): Blocks summer heat — better for west-facing windows or warm climates.
In Quebec, the ideal strategy is:
- South-facing windows: Higher SHGC to capture winter sun.
- North, east, west-facing: Lower SHGC to limit overheating.
Maxima windows offer selectable Low-E coatings so you can optimise SHGC by orientation — not one-size-fits-all.
ER — Energy Rating (Canada-Specific)
Energy Rating (ER) is a Canadian metric that combines U-value, SHGC, and air leakage into a single number. The higher the ER, the better the window performs in a Canadian heating climate.
- ER below 25: Minimum code (not great)
- ER 25–34: Good
- ER 34+: High-performance (ENERGY STAR Most Efficient)
Maxima windows routinely exceed ER 40, qualifying them among the most efficient windows available in Canada.
Air Leakage — The Silent Thief
Air leakage is measured in L/s/m² (litres per second per square metre of window). This tells you how much outside air sneaks through the sealed window.
- Standard windows: 0.50 – 1.50 L/s/m²
- High-performance European windows: below 0.10 L/s/m²
Even a window with a great U-value will underperform if air leaks through the seals. European tilt-and-turn hardware creates a compression seal on all four sides, which is why air leakage is dramatically lower than slider or single/double-hung windows.
Learn about our tilt-and-turn technology →
How to Read an NFRC Sticker
Every certified window carries an NFRC label with these values. Here is what to look for:
- U-Factor — Lower is better (look for < 1.2, ideally < 1.0)
- SHGC — Context-dependent (match to window orientation)
- VT (Visible Transmittance) — How much light gets through (higher = brighter)
- Air Leakage — Lower is better (< 0.30 is excellent)
Download our NFRC simulation results →
What This Means for Your Home
If you are replacing windows in Quebec, do not shop by price alone. Compare:
- U-value (target < 1.0 for real comfort)
- Air leakage (target < 0.20)
- ER (target 34+)
Windows with these numbers will noticeably reduce drafts, eliminate condensation, lower heating bills, and increase street noise reduction.
Ready to Compare?
Book a free consultation and we will walk through the numbers for your specific project — orientation, climate zone, and budget.
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