Visible transmittance (VT) measures the fraction of visible light that passes through a window’s glazing, expressed as a number from 0 to 1. A VT of 0.40 means 40% of the visible light striking the glass reaches the interior. Higher VT gives you a brighter room; lower VT blocks more light, which can reduce glare but also darkens the space.
- Visible transmittance (VT) values range from 0 (no light) to 1 (all light passes through); most residential windows fall between 0.40 and 0.70.
- Low-E coatings reduce VT slightly compared to uncoated glass: a clear double-pane with no coating typically has a VT of 0.79 to 0.82, while a Low-E unit runs 0.48 to 0.70.
- ENERGY STAR™ certified windows don’t have a minimum VT requirement, so a window can be certified while still appearing noticeably tinted.
- VT and solar heat gain coefficient (SHGC) are separate metrics: a window can block solar heat (low SHGC) while still allowing plenty of visible light (high VT).
- The light-to-solar gain (LSG) ratio (VT divided by SHGC) measures how efficiently a window transmits daylight relative to solar heat: a higher LSG is better for rooms where you want light without overheating.
What Is Visible Transmittance?
Visible transmittance is a measure of how much of the visible portion of the solar spectrum (wavelengths of roughly 380 to 780 nanometres) passes through the glazing system and into the room. It doesn’t include the infrared (heat) or ultraviolet portions of sunlight, just the light your eyes can see.
A clear uncoated single pane of glass has a VT of about 0.90: it lets through almost all visible light. Add a Low-E coating, a second pane, and an argon fill, and VT drops to somewhere between 0.48 and 0.70 depending on the coating specification. Tinted or reflective glass for commercial applications can drop VT below 0.30, though this is rare in residential windows.
Why Does Visible Transmittance Matter in a Home?
Natural daylight affects mood, productivity, and comfort. Research cited by Natural Resources Canada consistently links access to natural light to occupant wellbeing. From a practical standpoint, a room with high-VT windows needs less electric lighting during the day, which reduces energy use. In a GTA winter where daylight hours are short, maximising VT on north-facing and east-facing windows helps keep living spaces feeling bright even on overcast days.
The trade-off is glare: very high VT on south and west-facing windows can create uncomfortable glare on screens and work surfaces in summer afternoons. Choosing a slightly lower VT or using window coverings on those exposures is the usual solution.
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How VT Compares Across Window Types
| Glass specification | Typical VT | Appearance from inside |
| Single-pane clear | 0.88 to 0.92 | Crystal clear |
| Double-pane clear (no coating) | 0.79 to 0.82 | Very clear |
| Double-pane, standard Low-E | 0.60 to 0.70 | Slight green-blue tint |
| Double-pane, Low-E 366 | 0.48 to 0.55 | Noticeable neutral tint |
| Triple-pane, Low-E coatings | 0.40 to 0.55 | Somewhat darker than double-pane |
| Tinted glass (bronze/grey) | 0.20 to 0.50 | Visibly tinted |
| Electrochromic (switchable) | 0.03 to 0.60 | Varies by electrical setting |
Triple-pane windows tend to have lower VT than comparable double-pane units because there are three glass surfaces each absorbing and reflecting a small fraction of incoming light, plus the coatings needed to achieve the lower U-factor.
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VT vs Solar Heat Gain Coefficient: What’s the Difference?
VT and solar heat gain coefficient (SHGC) are often confused but measure different things.
SHGC measures all solar energy that passes through the window: visible light, near-infrared, and the radiant heat produced by the glass warming up. A low SHGC blocks solar heat (useful in summer on west-facing glass), but it also blocks warmth from winter sun on south-facing glass.
VT measures only visible light. A window can have a low SHGC (blocking heat) but a relatively high VT (letting in plenty of light). This is the goal of “spectrally selective” Low-E coatings like Low-E 366: they’re engineered to block solar heat more aggressively than visible light.
| Metric | What it measures | Better value for |
| VT | Visible light transmission | Higher for brighter rooms |
| SHGC | Total solar energy entering the room | Lower to block summer heat; varies for winter solar gain |
| LSG ratio (VT / SHGC) | Daylight per unit of solar heat | Higher means better daylight-to-heat efficiency |
What Is the Light-to-Solar Gain Ratio?
The light-to-solar gain (LSG) ratio is calculated by dividing VT by SHGC. It tells you how efficiently the window delivers daylight without solar heat. An LSG above 1.0 is generally considered good for residential windows: it means you’re getting more visible light than solar heat gain per unit of glazing.
For example, a window with VT of 0.55 and SHGC of 0.35 has an LSG of 1.57, which is excellent. A clear uncoated pane with VT of 0.90 and SHGC of 0.87 has an LSG of only 1.03: you’re getting plenty of light but almost the same amount of solar heat too, which is a problem on west-facing glass in summer.
“For rooms where clients want brightness without glare and without overheating, we specify a spectrally selective Low-E coating,” says a senior product engineer at TotalSeal. “The LSG ratio is the metric that actually captures what the client wants: lots of light, controlled heat.”
How Low-E Coatings Affect VT
Low-E coatings are the main factor that reduces VT below the uncoated glass baseline. The trade-off is intentional: by adding the coating, you dramatically improve the window’s U-factor (less heat loss in winter) and reduce SHGC (less solar heat gain in summer), at the cost of a modest reduction in VT. Different coating types make this trade-off in different proportions.
Soft-coat Low-E (also called sputter-coat or pyrolytic): applied in a vacuum chamber after the glass is made. Offers a better VT-to-performance trade-off than hard coats.
Hard-coat Low-E: baked onto the glass during manufacture. More durable but transmits a higher percentage of the solar spectrum, including heat.
Spectrally selective Low-E (such as Low-E 366): engineered to block a large proportion of solar infrared and UV while transmitting a high proportion of visible light. Achieves high LSG ratios.
Choosing VT for Different Rooms
There’s no single “right” VT for all windows. The right number depends on room use, orientation, and the balance between daylight and solar heat you want.
| Room / orientation | Recommended VT | Rationale |
| North-facing rooms (all types) | 0.55 to 0.70 | Maximise light from a low-heat-gain exposure |
| South-facing living areas (winter solar gain wanted) | 0.50 to 0.65 | Balance daylight with useful winter heat gain |
| West-facing rooms (afternoon glare concern) | 0.40 to 0.55 | Reduce glare and summer overheating |
| Home office (screen glare concern) | 0.40 to 0.55 | Comfortable for screen work |
| Bedroom (privacy and glare less critical) | 0.50 to 0.70 | Personal preference; higher VT for a brighter room |
“North-facing rooms are where high VT really earns its keep,” says a lead installation manager at TotalSeal. “You’re not getting much solar heat from that direction anyway, so the priority is to get as much natural light in as possible. We consistently specify the highest VT glazing available for north exposures.”
Visible Transmittance and UV Protection
VT measures visible light only. UV light (which causes fading of fabrics, flooring, and artwork) is a different part of the spectrum. Most Low-E coatings block 85 to 95% of UV even on high-VT specifications, so choosing a high VT window doesn’t mean accepting high UV exposure. If UV protection is a specific concern (for hardwood floors, artwork, or textiles near the window), ask your window supplier for the UV transmittance rating separately from the VT.
What VT Should You Look For?
For most GTA homes, a VT of 0.50 to 0.65 strikes the right balance: it’s bright enough to make rooms feel open and airy while still offering the Low-E coating performance needed to meet ENERGY STAR™ North zone energy criteria. Very low VT windows (below 0.40) should be reserved for specific applications where glare or solar overheating is a major concern.
To see VT values for your options, look at the full product specification sheet (not just the headline marketing figures) and compare VT alongside U-factor, SHGC, and LSG ratio before making a final choice.
When you’re ready to go through the glazing options for each room in your home, book a free in-home consultation and a TotalSeal adviser will walk through the specifications that suit each exposure.
Frequently Asked Questions
What is a good visible transmittance for windows?
For most residential rooms, a VT of 0.50 to 0.65 is a good target. It balances natural daylight with the Low-E coating performance needed to achieve a good U-factor. North-facing rooms benefit from higher VT (up to 0.70) to maximise light from a low-solar-gain exposure.
Do Low-E windows reduce visible light?
Yes, but only modestly. A standard double-pane Low-E window typically has a VT of 0.55 to 0.70, compared to about 0.80 for the same unit without a coating. The slight tint is usually unnoticeable inside the room unless you compare it directly to uncoated glass.
Does triple pane have lower visible transmittance than double pane?
Generally yes. Adding a third glass layer and additional coatings reduces VT by a further 10 to 20% compared to an equivalent double-pane configuration. A typical triple-pane window has a VT of 0.40 to 0.55.
What is the difference between VT and SHGC?
VT measures visible light only. SHGC (solar heat gain coefficient) measures all solar energy entering the room: visible light, infrared, and the heat produced by the glass warming. A high VT with a low SHGC (achieved with spectrally selective coatings) is ideal for rooms that need daylight without solar overheating.
Can visible transmittance affect how dark a room looks?
Yes. A window with VT of 0.40 will make a room noticeably darker than the same window with VT of 0.65, particularly on overcast days and in north-facing rooms. If natural daylight is a priority, confirm the VT specification before ordering.
Do windows with higher VT fade furniture faster?
Not necessarily. Visible light is less damaging to fabrics and finishes than UV light, and most Low-E coatings block 85 to 95% of UV regardless of VT rating. Ask for the UV transmittance spec separately if fading of floors or textiles is a concern.
