For a Minnesota window, look for a soft-coat low-E coating on the inside face of the outer pane, argon fill, a warm-edge spacer, and a whole-window U-factor of 0.22 or lower if you want ENERGY STAR Version 7.0 performance. A good double-pane at 0.27–0.30 is still a big upgrade over old glass. The install decides whether any of it counts.
Every new window carries an NFRC sticker crowded with numbers: U-factor, SHGC, VT, air leakage. Brochures add names like LoE-366, SmartSun and Advanced Low-E. Behind the jargon are a few simple ideas that decide how warm a room feels in January and what it costs to heat. This is the plain-language version.
What does low-E glass do?
Low-emissivity, or low-E, coatings are layers of silver and metal oxide on the glass, far too thin to see. Glass on its own soaks up heat radiation and re-radiates it outward; a low-E surface reflects that radiation instead. In winter the coating bounces the furnace's heat back into the room. In summer it turns away part of the sun's heat before it gets in. Visible light passes through, so the room stays bright.
Two kinds exist. Hard-coat (pyrolytic) low-E is baked into the glass at the factory and is tough enough to face the room or the weather; it insulates less. Soft-coat (sputtered) low-E is applied in a vacuum chamber, performs far better, and has to live inside the sealed unit where nothing can touch it. Nearly every residential window sold in Minnesota uses soft-coat low-E on a protected surface, sometimes with a hard-coat layer added on the room side.
How are glass surfaces numbered, and where does the coating go?
Window people count glass surfaces from outside in. On a double-pane unit, surface 1 is the outside face of the outer pane, surface 2 is its inside face, surface 3 is the outside face of the inner pane and surface 4 is the face you touch from the room. A triple-pane has surfaces 1 through 6.
In a heating climate the main soft-coat low-E goes on surface 2. From there it reflects the sun's heat outward before it enters the gas space in summer, and in winter it reflects room heat that has crossed the gas space back inside. Cardinal, which makes the glass inside most major brands, specifies its solar-control coatings (LoE-366, LoE-272, LoE-270) for surface 2 in a double-pane and warns against putting them on surface 3, where the inner pane can overheat and crack from thermal stress. Its high-gain LoE-180 is the coating rated for surface 3 when a maker wants it there.
A second, hard-coat low-E can go on surface 4, the room side. Cardinal calls it LoE-i89; Andersen sells it as HeatLock and Marvin as ERS. It drops a double-pane's center-of-glass U-factor to about 0.20, which is how some double-panes reach the ENERGY STAR 7.0 Northern number. The catch is that reflecting room heat back into the room leaves the glass surface itself slightly cooler, so in a humid house it fogs sooner at the bottom edge on the coldest nights. Triple-pane units usually put low-E on surfaces 2 and 5, both facing gas, and get a warmer inside pane as a bonus.
Which low-E coating should a Minnesota window have?
Most brands buy their coated glass from Cardinal, and Cardinal's names are worth learning because they describe the coating. The first digit is the number of silver layers. The last two digits are the visible light transmitted through a double-pane unit, as a percentage. More silver means more solar heat blocked and a little less light; less silver means more free winter sun.
Manufacturers then wrap those coatings in their own names. Andersen's standard Low-E4 is its everyday cold-climate coating; SmartSun is its higher solar-control version, marketed as blocking about 95% of UV; PassiveSun is its high-gain option for houses that want winter sun; HeatLock is the surface-4 room-side layer. Pella's Advanced Low-E is its standard double-pane package with argon; NaturalSun is the high-gain option Pella recommends for far-northern climates; SunDefense is the strong solar-control option; AdvancedComfort sits between. Marvin lists Low E2 and Low E3 with an optional ERS room-side layer. In every case, ask which Cardinal coating or which NFRC numbers sit behind the name.
For a Twin Cities house the middle coating (LoE-272/270 class, Andersen Low-E4, Pella Advanced Low-E) is the sensible default: low U-factor, moderate solar gain, clear appearance. A high-gain coating (LoE-180, PassiveSun, NaturalSun) suits south-facing glass with roof overhang, where winter sun is welcome and summer sun is shaded. A strong solar-control coating (LoE-366, SmartSun, SunDefense) suits big west-facing glass that overheats in July, or a room you want to protect from fading. Mixing coatings by wall is normal; the tint difference is slight.
| Coating | Silver layers | Visible light (VT) | Solar gain (SHGC) | Use in Minnesota |
|---|---|---|---|---|
| LoE-180 | 1 | About 0.79 | About 0.69 | South-facing, shaded in summer; passive solar |
| LoE-272 / LoE-270 | 2 | About 0.72 / 0.70 | About 0.41 / 0.37 | Default for most walls |
| LoE-366 | 3 | About 0.65 | About 0.27 | West glass that overheats; fade protection |
| LoE-i89 (surface 4 add-on) | Hard coat | Little change | Little change | Pushes a double-pane toward U-0.22; watch humidity |
What does argon fill do, and does it leak out?
The space between the panes is filled with gas. Argon is denser than air, invisible, inert and cheap, and it conducts less heat, so the window insulates better. It works best in a gap of roughly half an inch. It is a standard spec worth having in this climate. Krypton does better in narrower gaps, under about three-eighths of an inch, but costs far more, so it mostly shows up in triple-pane units built to fit a standard-thickness frame.
Argon does leave slowly. Units are filled to roughly 90–95% at the factory, and the industry standard tolerates about 1% loss a year through a sound dual seal; a well-made unit still holds a useful fill after 20 years. Fog between the panes is a different event: the seal has opened, the argon is gone within a season, humid air has been pumped in with every temperature swing and the desiccant in the spacer has saturated. That is a failed unit, not normal argon loss.
What is a warm-edge spacer, and why does it matter?
The spacer is the bar around the perimeter that holds the panes apart and carries the desiccant that keeps the gas space dry. For decades it was hollow aluminum, which conducts heat so well that the edge of the glass ran cold, frosted first, and flexed the seal with every temperature swing until it failed. That is the failure behind most foggy windows in Minnesota.
Warm-edge spacers fix that. The common types are thin stainless steel (Cardinal's XL Edge), silicone foam (Quanex Super Spacer) and thermoplastic (TPS), each far less conductive than aluminum. They pair with a dual seal: a butyl primary seal against gas loss and a silicone or polysulfide secondary seal for strength. The payoff is a warmer edge of glass, less condensation at the bottom of the pane, a slightly better whole-window U-factor and a seal that lasts. Ask what spacer a window uses; a plain aluminum box spacer is a reason to look at another line.
How do you read the NFRC label?
Every certified window carries a National Fenestration Rating Council label, and it is the only honest comparison between brands because the tests are the same for everyone. Four numbers matter in Minnesota.
- U-factor: heat loss through the whole window, frame and glass together. Lower is better. Double-pane low-E argon lands about 0.27–0.30; triple-pane and room-side coatings reach 0.17–0.23. This is the number to compare first.
- Solar Heat Gain Coefficient (SHGC): the share of the sun's heat that gets through, from 0 to 1. In Minnesota a moderate to higher SHGC is welcome on south glass for free winter heat; lower is better on big west glass. ENERGY STAR 7.0 requires at least 0.17 in the North.
- Visible Transmittance (VT): how much daylight comes through. Higher means a brighter room. A 366-class coating lands in the mid-0.60s for VT, a 180-class near 0.80.
- Air Leakage (AL): cubic feet of air per minute per square foot of window at test pressure. ENERGY STAR and the model energy code cap it at 0.3. Casements and fixed units test lowest. Some labels also show a Condensation Resistance figure; higher is better.
What does ENERGY STAR Version 7.0 require in Minnesota?
ENERGY STAR Version 7.0 took effect October 23, 2023 and put all of Minnesota in the Northern zone. A window qualifies with a U-factor of 0.22 or lower and an SHGC of at least 0.17, or through equivalent-performance paths that allow U-0.23 with SHGC 0.35 or higher, U-0.24 with 0.40, U-0.25 with 0.45 and U-0.26 with 0.50. The older Version 6 target of 0.27 is gone, which is why most plain double-pane windows sold in 2026 no longer carry the mark here.
ENERGY STAR is voluntary. Minnesota's residential energy code sets its own, looser ceiling for windows, and a good double-pane at 0.28 is legal and a real upgrade. Treat the label as shorthand for top-tier cold-climate glass: triple-pane, or double-pane with a room-side coating, in an efficient frame.
Does the frame matter as much as the glass?
On a small window the frame and sash are a quarter or more of the area, and the NFRC U-factor already blends them in. The same glass package rates differently in a hollow vinyl frame, a multi-chamber or foam-filled vinyl frame, a foam-filled fiberglass frame and a wood frame. Compression-seal styles (casement, awning) and heavier frames also post lower air leakage than sliders and double-hungs. Weatherstripping is part of the frame story: bulb seals on casements, pile or fin seals on sliding sashes, and they wear faster in a frame that moves a lot with temperature. Buy the whole-window number, not the center-of-glass number.
Why does the label only count if the install is right?
A U-0.22 window set in an opening that was not insulated and sealed performs like a much worse window. The gap between the frame and the rough opening, usually a quarter to three-eighths of an inch all the way around, is where efficiency is won or lost. Left empty or stuffed with fiberglass, it is a direct air path from the wall cavity to the room, and the reason frost shows up at the casing in January.
The right sequence on a full-frame job: a sill pan with a back dam so any water drains out, flashing tape at the jambs and head lapped shingle-style over the housewrap, the window set level and plumb on shims, low-expansion window-and-door foam in the gap (high-expansion foam bows the jambs and binds the sash), backer rod and sealant on the interior where the gap is wide so the air seal is continuous from drywall to frame, then drip cap, exterior sealant and trim. On an insert, the joint between the new frame and the old gets sealed on both faces and the old exterior sill gets a sloped cap. None of that is on the NFRC label, and all of it decides whether the label was worth paying for.
What JMB Builders does
JMB Builders is a licensed Minnesota residential building contractor (BC809005) based in Zimmerman, working across the Twin Cities metro. Whether we order the window or you bring it home, the opening gets flashed, foamed and sealed the same way. We install every brand and are a dealer for none. Call 763-204-2099 (text) or send photos through the site.
- Labor-only installation of windows you bought at a store or online — jmbbuildersmn.com/services/labor-only
- Insert and full-frame replacement windows, measured and ordered by us — jmbbuildersmn.com/services/full-service
- Which window brands you can buy without a dealer, and what to check first — jmbbuildersmn.com/services/labor-only/which-window-brand-to-buy-for-labor-only-install
- Trim-out after another company's window installation, including foaming the gap they left — jmbbuildersmn.com/services/window-trim/trim-out-replacement-windows-left-bare
- Send three photos and we reply within one business day.
Frequently asked questions
What is a good U-factor for windows in Minnesota?
0.22 or lower meets ENERGY STAR Version 7.0 for the Northern zone, which covers all of Minnesota. A good double-pane low-E argon window lands around 0.27–0.30, which is still a large upgrade over old glass and fine for most houses. Triple-pane units and double-panes with a room-side low-E coating reach 0.17–0.23 if you want the warmest glass.
What is the difference between LoE-366, LoE-270 and LoE-180?
They are Cardinal coatings with different amounts of silver. LoE-180 has one silver layer, lets in about 79% of visible light and most of the sun's heat, so it suits south-facing glass in a cold climate. LoE-272 and LoE-270 have two layers and moderate solar gain; they are the everyday choice. LoE-366 has three layers, blocks the most solar heat, and suits west glass that overheats or rooms with fading concerns. All three go on surface 2 in a double-pane.
Is argon gas in windows worth it?
Yes. Argon conducts less heat than air, costs little, and is standard on most low-E windows sold in Minnesota. Do not pay extra for a window without it. A sound dual-seal unit loses only about 1% a year. Krypton performs better in narrow gaps but the price jump is large, so it mainly appears in triple-pane.
Does low-E glass make rooms darker?
Slightly, and most people never notice. Low-E coatings block heat radiation, not visible light, so the VT number on the label stays high: around 0.70 for a two-silver coating and the mid-0.60s for a three-silver coating. Some coatings show a faint tint in certain light. If daylight is the priority, compare VT numbers between the options.
What is the difference between U-factor and R-value?
They measure the same thing from opposite ends. R-value is resistance to heat flow, used for insulation, and higher is better. U-factor is the rate of heat flow, used for windows, and lower is better. U-factor is 1 divided by R-value, so a U-0.25 window is about R-4 and a U-0.20 window about R-5.
Should I keep the NFRC label on a window I bought?
Yes, until the install is done and inspected. The label is your proof of U-factor and SHGC for any utility program that still requires it, and it confirms the window matches what you ordered. The federal window tax credit expired for installations after December 31, 2025, but the label still matters for the inspection and for your records. Photograph it before it is peeled off.
