Every window in a showroom carries a white and black sticker with five numbers on it. Those numbers are the only objective, comparable information you will get about a window, and most homeowners are walked past them. Here is what each one means, which two matter most in a heating climate, and why the sticker still only tells you half of what you need to know.
What the NFRC label actually is
The National Fenestration Rating Council operates the certification and labelling programme that puts those numbers on the window. The ratings come from a standard test procedure, so a U-factor from one manufacturer means the same thing as a U-factor from another. ENERGY STAR does not run its own tests; its certification criteria are based on NFRC-certified ratings and vary by climate zone. Marketing language about “energy-saving glass” is unregulated. The five numbers on the label are not.
U-factor
The rate at which the window conducts non-solar heat. Rated from 0.10 to 2.00, lower being better. It describes how fast the warmth you paid for leaves the house on a February night, and in Michigan it is the number that matters most.
NFRC’s U-factor is a whole-window rating: glazing, frame and spacers together. DOE’s Building America measure guideline is explicit that the whole-window U-factor is higher — worse — than the centre-of-glass figure, because the frame and the edge of the glass conduct more heat than the middle of the sealed unit. If a salesperson quotes a strikingly low number, ask which one it is.
Solar heat gain coefficient
SHGC is the fraction of solar radiation admitted through the window, from 0.00 to 1.00. Lower means less solar heat gets in. In Phoenix that is the headline number. In Michigan it is not, and the reason is in the ENERGY STAR criteria themselves.
Visible transmittance
VT runs 0.00 to 1.00 and describes how much visible light comes through, higher meaning a brighter room. It is not an energy criterion in the ENERGY STAR window specification, but it is worth checking on north-facing rooms, because heavily coated glass can make a dim room dimmer through a grey Michigan winter.
Air leakage
Air leakage is measured in cubic feet per minute per square foot of window area, and lower means less air passes through the assembled unit. This is the one rating with a hard cutoff: above 0.3 a window is disqualified from ENERGY STAR altogether, and Version 7.0 sets the limit at 0.3 cfm/ft² for windows and sliding glass doors, 0.5 cfm/ft² for swinging doors. It is also the rating most often skipped in a sales conversation, which is odd in a state where wind-driven infiltration is what people actually complain about.
Condensation resistance
Rated 1 to 100, higher resisting condensation better. It is an optional rating rather than a required one, so it is not on every label. In a cold climate it is genuinely useful, because it tells you how well the window keeps its interior surfaces above the dew point of the air in your room.
The Michigan numbers: ENERGY STAR Northern zone
Michigan sits entirely in the ENERGY STAR Northern climate zone. Under Version 7.0, a window certifies there at a U-factor of 0.22 or lower with an SHGC of 0.17 or higher.
Read that second half again. The Northern zone sets a minimum solar heat gain coefficient, not a maximum. In a heating-dominated climate, winter sun through a south-facing window is free heat, and the specification is written to stop manufacturers chasing a low U-factor by piling on coatings that block it. The zone also allows equivalent energy performance alternatives, trading a slightly worse U-factor for more solar gain:
| Zone and pathway | U-factor | SHGC |
|---|---|---|
| Northern — prescriptive | 0.22 or lower | 0.17 or higher |
| Northern — equivalent performance | 0.23 | 0.35 or higher |
| Northern — equivalent performance | 0.24 | 0.35 or higher |
| Northern — equivalent performance | 0.25 | 0.40 or higher |
| Northern — equivalent performance | 0.26 | 0.40 or higher |
| North-Central (for comparison) | 0.25 or lower | 0.40 or lower |
The contrast in that last row is the clearest illustration of the principle. One zone south of us, SHGC is capped at 0.40 because unwanted summer heat starts to cost more than winter sun saves. Here, the specification pushes the other way.
Why U-factor outranks SHGC here
Michigan runs a long heating season and a short cooling one. Heat loss through the glass happens every hour of every night from October to April, in every orientation, whether or not the sun is out. Solar heat gain only happens in daylight, only on the elevations the sun reaches, and for much of the year it works in your favour.
The ranking for a Michigan house is therefore straightforward: get the U-factor down first, keep the air leakage rating low, and treat SHGC as an orientation question rather than a house-wide target. A low SHGC on a big west-facing living room window is a reasonable comfort decision. A low SHGC everywhere is throwing away free heat.
The gap available on U-factor is large. DOE’s Building America guideline gives representative whole-window U-factors of about 1.29 for clear single glazing, 0.52 to 0.83 for clear double glazing, 0.27 to 0.65 for double low-e depending on coating and frame, and 0.19 to 0.20 for triple low-e. Note the width of that double low-e band — a poor frame can put a low-e double unit well outside Northern zone territory while a good one lands inside it.
What the glass package and the spacer actually do
Low-e coatings
A low-emissivity coating is a microscopically thin metallic layer on one of the glass surfaces inside the sealed unit. It reflects long-wave heat back toward its source, which in winter means back into the room. Coatings are tuned differently: some hold heat in while still letting solar gain through, which is exactly the behaviour the Northern zone criteria reward. Which surface the coating sits on changes the result, which is why two windows both described as “low-e” can land far apart on the label.
Gas fills
The cavity between panes is usually filled with argon, sometimes krypton in narrower cavities. Both conduct heat more slowly than air. DOE notes that a well-designed and well-fabricated unit loses less than 0.5 percent of its fill per year. The important word in that sentence is “well-fabricated” — the gas is only doing its job as long as the seal is intact, which is the same seal whose failure produces fog between the panes.
Spacers
The spacer is the strip that holds the two panes apart at the perimeter and carries the desiccant. Traditional aluminium spacers conduct heat efficiently, which is precisely what you do not want at the coldest edge of the assembly. DOE’s guideline puts the improvement from moving to a good-quality warm edge spacer at approximately 0.02 Btu/h·ft²·°F off the overall window U-factor, plus a meaningful gain in condensation resistance.
That 0.02 sounds small until you look at the Northern zone table above, where the entire spread between the prescriptive pathway and the last alternative is 0.04. Spacer choice is a real fraction of whether a window certifies.
Why the sticker is only half the story
Every number discussed so far was measured on a window in a laboratory frame. None of them describes the hole in your wall.
The NFRC air leakage rating tests the window unit itself — sash against frame. It says nothing about the gap between the frame and the rough opening, and that gap is where most real infiltration happens in a retrofit. ENERGY STAR estimates air leakage across the whole building envelope at 25 to 40 percent of the energy used for heating and cooling in a typical home. A 0.3 cfm/ft² window set into an opening that was shimmed carelessly, foamed badly and trimmed over will underperform a mediocre window installed properly.
The details that decide it are unglamorous and invisible once the trim is on: a sloped, sealed sill pan so any water that gets past the exterior finish drains out rather than into the framing; head flashing lapped over the window and under the weather barrier above it; low-expansion foam or backer rod and sealant in the perimeter gap rather than nothing at all or expanding foam that bows the jambs; and shimming that leaves the frame square so the sash seals evenly along its whole length.
The same logic runs through the rest of the wall. If the siding and trim above an opening are not shedding water clear of the head, the best-flashed window in Michigan will eventually rot its own sill. When we quote window replacement we look at what is happening above the opening as well as in it, because those two failures arrive together often enough to be predictable. Getting the label right captures part of the 25 to 30 percent DOE attributes to windows. Getting the installation right is the rest, and it is the half nobody prints on a sticker.
Questions about window ratings
What U-factor should I ask for on a Michigan house?
0.22 or lower with an SHGC of 0.17 or higher meets the ENERGY STAR Northern zone prescriptive criteria, which is the right baseline here. The specification also allows equivalent pathways up to a 0.26 U-factor if the window admits more solar heat, at 0.40 SHGC or above. Make sure any number you are quoted is the NFRC whole-window rating, not a centre-of-glass figure.
Is triple glazing worth it in Michigan?
On pure heat loss it wins — DOE’s representative figures put triple low-e at about 0.19 to 0.20 against 0.27 to 0.65 for double low-e. The trade-offs are weight, cost, and the fact that a well-specified double low-e unit with a warm edge spacer and a good frame already clears the Northern zone threshold. Price it on exposed elevations and rooms that never feel warm rather than assuming it house-wide.
Why do two windows with the same low-e glass have different ratings?
Because U-factor is a whole-window number. The frame material, the spacer, the gas fill and which glass surface carries the coating all move it. DOE puts the spacer alone at roughly 0.02 on the overall U-factor when you go from standard aluminium to a good warm edge product, and that is a large share of the range separating the ENERGY STAR pathways.
Should I care about the air leakage number?
Yes, and more than most people are told. A rating above 0.3 cfm/ft² disqualifies a window from ENERGY STAR entirely, so it functions as a floor for product quality. Just remember it rates the unit, not the opening. Ask any contractor how they will flash and seal the perimeter, because that is where the leakage you will actually feel comes from.