Low-E (low-emissivity) glass carries a microscopically thin metallic coating — layers of silver measured in atoms — that reflects infrared heat while letting visible light pass through. In winter it bounces your furnace heat back into the room; in summer it rejects solar heat before it gets inside. The coating is invisible in normal use and comes standard on every quality window sold in Idaho.
The Two Numbers That Matter
Low-E performance shows up on the NFRC label as two ratings. U-factor measures how fast heat escapes — lower is better, and it's the number that matters most in Boise winters. SHGC (solar heat gain coefficient) measures how much of the sun's heat comes through — lower means cooler in July, higher means free heat in January. Every certified window's ratings are public in the NFRC directory, so you can compare any two units on equal footing.
Passive vs. Solar-Control Low-E
| Type | What it does | Best for |
|---|---|---|
| Passive (high-solar-gain) | Holds heat in, admits winter sun | Cold climates, shaded or north-facing glass |
| Solar-control (low-SHGC) | Holds heat in and rejects summer sun | Hot-afternoon exposures, big glass areas |
Both types insulate well; the difference is how much solar heat they invite in.
Which Elevations Get Which in Idaho
Treasure Valley summers hit triple digits, so we generally spec solar-control Low-E on west- and south-facing glass, where afternoon sun turns rooms into ovens. On north-facing elevations — and often east — a higher-gain coating is perfectly acceptable and lets you keep a touch of passive winter warmth. Mixing coatings by elevation on one house is routine for us and costs nothing extra beyond ordering it correctly.
Manufacturers brand their packages differently: Andersen SmartSun, Milgard SunCoat Max, and Marvin Low-E3 are all solar-control coatings we order regularly. The label differs; the physics is the same.
Wood Windows Inc has specified glass packages for Treasure Valley homes for 46 years, and our in-house installers see how each coating performs across thousands of local installs. Call (208) 376-2200 for a free in-home measure and an elevation-by-elevation glass recommendation.
Frequently Asked Questions
Does Low-E glass make rooms darker?
Barely. Modern Low-E coatings block infrared and UV while passing most visible light — typically you give up around 10 percent of visible transmittance versus clear glass. Side by side you can spot a faint tint; in a finished room, almost nobody can.
Does Low-E glass stop furniture and floors from fading?
It helps a great deal. Low-E coatings block the large majority of UV radiation, which is the biggest driver of fading in hardwood, fabric, and art. It won't stop fading entirely — visible light causes some too — but it slows it dramatically.
Is Low-E glass worth the extra cost?
In Idaho, it isn't really optional. ENERGY STAR certification in our Northern climate zone effectively requires it, and every quality window we sell includes it as standard. The energy savings over the life of the window far outrun the modest cost difference.
Sources & Testing Data
Last reviewed 2026-07-05 by Wood Windows Inc — (208) 376-2200