It’s one of the most common questions homeowners type into a search bar: what are the best energy efficient windows? And it usually gets asked right after opening a summer utility bill, or after walking past a sunny window and feeling the heat radiating off the glass like an open oven door.
A powerful HVAC system can only do so much. If your windows are leaking conditioned air and letting solar heat pour straight through the glass, your air conditioner is fighting a battle it can’t win — and you’re paying for every round.
At Great Glass, we fabricate, temper, and install glass every single day, which means we spend a lot of time on the technical side of what actually makes a window perform. Here’s what we’ve learned, and what you should look for when you’re ready to upgrade.
Energy Efficiency Starts with the Glass — Not the Frame
Most window shopping conversations start with frame material. Vinyl or aluminum? Wood or fiberglass? Those are fair questions, but they miss the bigger picture. In a typical window, the glass makes up roughly 80% of the total surface area. That’s where the heat gain, the heat loss, and the majority of your energy performance lives.
If you want a genuinely efficient window, the glass package is where you should focus first.
The Building Blocks of High-Performance Glass
Insulated Glass Units (IGUs)
A single pane of glass is essentially a hole in your wall with a view. An insulated glass unit — two or three panes separated by a sealed air space — creates a thermal buffer between the outside temperature and your interior.
Double-pane IGUs are the modern standard and deliver a dramatic improvement over single-pane glass. Triple-pane units push performance further, though the returns are most noticeable in extreme climates. For most homes and commercial buildings, a well-built double-pane unit with the right coatings and gas fill hits the sweet spot between performance and cost.
Low-E Coatings
Low-emissivity (Low-E) coating is a microscopically thin, virtually invisible metallic oxide layer applied to the glass surface. It reflects infrared energy — heat — while still allowing visible light to pass through.
In practical terms, a Low-E coating keeps summer heat outside and winter heat inside, all while protecting your flooring, furniture, and artwork from UV fading. There are different Low-E formulations tuned for different climates and orientations, and choosing the right one matters. A coating optimized for a cold northern climate isn’t the right call for a south-facing wall in a hot, sun-drenched region.
This is one of the areas where working with a fabricator pays off. We can help you match the coating to your building’s exposure, rather than defaulting to whatever’s sitting in a warehouse.
Argon and Krypton Gas Fills
The space between panes doesn’t have to be filled with ordinary air. Argon is denser than air and conducts heat more slowly, which improves insulating performance at very little added cost. Krypton is denser still and is typically used in narrower cavities, such as the tight spaces found in triple-pane units.
Gas fills are one of the most cost-effective upgrades available in an IGU. If you’re specifying new glass, there’s rarely a good reason to skip them.
Warm-Edge Spacers
The spacer is the component that separates and seals the panes around the perimeter of the unit. Traditional aluminum spacers are highly conductive, creating a thermal bridge right at the edge of the glass — which is exactly where you’ll see condensation form first.
Warm-edge spacers made from structural foam, thermoplastic, or stainless steel dramatically reduce that edge conduction. They improve overall U-factor, cut down on condensation, and extend the service life of the seal itself.
Learning to Read the Numbers
Every certified window carries a performance label. Once you know what the ratings mean, comparison shopping becomes far more straightforward.
U-Factor measures how well the window resists heat transfer. Lower is better. Values typically range from about 0.20 to 1.20. In cold climates especially, a low U-factor is your priority.
Solar Heat Gain Coefficient (SHGC) measures how much solar radiation passes through the glass. It’s expressed from 0 to 1, and lower means less heat gain. In hot, sunny climates, SHGC is arguably the single most important number on the label.
Visible Transmittance (VT) tells you how much daylight comes through. Higher numbers mean brighter interiors. The engineering challenge is achieving a low SHGC while maintaining a high VT — you want the light without the heat. Advanced spectrally selective Low-E coatings are designed to do exactly that.
Air Leakage (AL) measures how much air passes through the assembly. Lower is better, and anything at or below 0.30 is considered solid performance.
Condensation Resistance is rated from 1 to 100, with higher numbers indicating better resistance to moisture buildup on the interior surface.
Don’t Overlook Safety and Code Requirements
Energy efficiency is only part of the equation. Depending on where a window is located, building codes may require tempered or laminated safety glazing — near doors, in bathrooms, close to floor level, in large panels, and in stairwells, among other locations.
Tempered glass is heat-treated to be roughly four to five times stronger than annealed glass, and when it does break, it fragments into small, relatively harmless pieces rather than dangerous shards. We temper glass in-house at Great Glass, which means safety glazing requirements never have to compromise your energy performance goals — or stretch out your project timeline while you wait on an outside vendor.
Laminated glass, built with an interlayer bonded between panes, adds impact resistance, sound dampening, and additional UV blocking. In coastal and high-wind regions, impact-rated laminated glazing is often required outright.
Installation Is the Difference Between Good and Great
Here’s the part that gets overlooked far too often: the most advanced glass package on the market will underperform if it’s installed poorly.
Gaps in the rough opening, inadequate insulation around the frame, missing flashing, improper shimming, or failed sealant joints will undo the engineering you paid for. Air infiltration around a badly installed window can erase the benefit of upgrading from single-pane to high-performance IGUs.
Precise measurement, square and plumb installation, proper flashing and weather sealing, and full perimeter insulation aren’t optional details — they’re the foundation of real-world performance. It’s why we treat installation with the same rigor as fabrication.
So — What Are the Best Energy Efficient Windows?
The honest answer is that there’s no single winner, because the right window depends on your climate, your building’s orientation, your budget, and how you use the space.
That said, here’s a reliable framework:
For hot, sunny climates: Prioritize a low SHGC with a spectrally selective Low-E coating. Double-pane, argon-filled units with warm-edge spacers will handle the majority of applications beautifully. Add laminated impact glazing where wind loads or coastal exposure demand it.
For cold climates: Prioritize a low U-factor. Triple-pane construction, krypton or argon fills, and multiple Low-E surfaces deliver the strongest results.
For mixed climates: Balance both metrics, and consider varying the glass package by elevation — a different specification for the sun-blasted south and west faces than for the shaded north side.
For every climate: Insist on quality fabrication, verified performance ratings, and professional installation. Those three things determine whether you actually get the savings you were promised.
Custom Glass, Fabricated and Installed by Great Glass
At Great Glass, we don’t just resell whatever’s on the shelf. We fabricate insulated glass units to your exact specifications, temper glass in-house for safety and strength, and install with the precision that high-performance glazing requires.
Whether you’re replacing failed IGUs in an existing home, specifying glass for a new commercial build, or finally doing something about that west-facing wall that turns your living room into a greenhouse every afternoon, we can help you select and build the right glass package for the job.
Reach out to Great Glass for a consultation. We’ll walk you through the options, explain the numbers in plain language, and fabricate glass that performs exactly the way it should — for decades.