Buy the number on the label, not the adjectives in the brochure.

Double pane or triple pane in Minnesota?

Ask the question a different way: what U-factor are you being offered? Pane count is one of several ways to get there, and the label reports where a particular window actually landed rather than what category it belongs to.

A third pane adds another insulating air space and another low-E surface, so triple-glazed units generally reach lower U-factors than double. But a good double-pane unit with a decent coating and gas fill can beat a cheap triple, and the label is how you find out. Comparing a 0.27 against a 0.30 is a real comparison; comparing “triple pane” against “double pane” is not.

The honest trade-offs with triple: it is heavier, which matters for large operating sashes and for the frame; it costs more; and the extra pane slightly reduces visible light. In a Minnesota heating climate the insulating gain is usually worth it on the north and east. Whether it pays back against a decent double unit is a question your actual numbers answer and a brochure does not.

What do the numbers on the sticker actually mean?

The sticker is an NFRC label, and the ratings on it are independently tested rather than claimed by the manufacturer. There are five, and in Minnesota they do not all matter equally.

U-factor is the one to lead with here: it measures how fast heat escapes, and lower is better. SHGC is how much solar heat gets through — in a heating-dominated climate a higher SHGC on south-facing glass is free winter heat, which is the opposite of the advice someone in Arizona gets. Air leakage is draughts through the assembly. Visible transmittance is how much daylight you get. Condensation resistance scores how well it resists moisture forming on the inside surface — worth a look given how Minnesota winters go.

One useful detail about NFRC itself: it does not grade windows. It tests and reports. It sets no minimum and says nothing about whether a given number is good for your house.

ENERGY STAR: “U-Factor measures the rate of heat transfer and tells you how well the window insulates […] The lower the U-factor, the better the window insulates. […] Air Leakage (AL) measures the rate at which air passes through joints in the window […] Condensation Resistance measures how well the window resists water build-up […] scored on a scale from 0 to 100.” And on NFRC: it “does not distinguish between ‘good’ and ‘bad’ windows, set minimum performance standards, or mandate performance levels.” Source: ENERGY STAR, Independently Tested and Certified Energy Performance

Does the ENERGY STAR sticker mean the window is right for Minnesota?

It means it meets EPA criteria for your climate zone, and Minnesota is in the Northern zone, where the criteria are the most demanding in the country. That is a genuinely useful filter.

What it does not mean is that every number on the label has been vetted. ENERGY STAR qualification is based on U-factor and solar heat gain coefficient only. Air leakage, visible transmittance and condensation resistance are reported on the label but are not what the certification turns on.

So if draughts or interior condensation are what you are trying to solve, the ENERGY STAR logo alone does not answer you. Read the air leakage and condensation resistance figures yourself.

ENERGY STAR: “ENERGY STAR qualification is based on U-factor and SHGC ratings only.” Performance criteria “are based on ratings certified by the National Fenestration Rating Council (NFRC), and vary for each of the climate zones.” Source: ENERGY STAR, Independently Tested and Certified Energy Performance

What is low-E glass, and is argon fill worth paying for?

Low-E is a microscopically thin metallic coating on a glass surface that reflects long-wave heat back the way it came — in winter, back into your room. Argon, and occasionally krypton, replaces the air between panes with a denser gas that conducts heat more slowly.

Both are real and both work. But here is the thing worth understanding: they are inputs, and the U-factor on the label is the output. A window with low-E and argon has already had that credit counted in its tested number. You do not add them on top.

That matters when you are being sold. “Low-E and argon” on a quote tells you how the window was built. The U-factor tells you how well it worked. If two quotes both say low-E and argon and their U-factors differ, the U-factors are the fact and the feature list is the marketing.

Will new windows stop my floors and furniture fading?

They will reduce it. They will not stop it, and the number used to sell you on it is one that a national laboratory says is close to meaningless.

Here is the problem. Manufacturers advertise a UV-blocked percentage — often 95% or 99%. That figure is Tuv, the plain unweighted transmittance across the ultraviolet band. Lawrence Berkeley National Laboratory, which develops the software the industry models glass with, is blunt about what it is worth: it carries no correction for how sensitive materials actually are to damage, it does not even cover the full range of wavelengths in which fading happens, and it is retained for historical comparison because manufacturers used it before better measures existed.

The reason is that fading is not caused by ultraviolet alone. Visible light fades fabric and wood too, and a coating can block essentially all the UV while still passing plenty of the visible light that does the damage. A 99% UV figure and a badly faded rug are completely compatible.

Lawrence Berkeley National Laboratory on Tuv, the UV number manufacturers quote: “Because it is unweighted by a ‘detector’ function, TUV has no correction for the spectral sensitivity of materials to radiation damage. It doesn’t even cover the spectral range in which damage occurs, just the ‘uv’ meaning below the visible. This function is included for historical comparison purposes only, since many manufacturers used this function before corrected functions were available. It is believed to have little meaning.” Source: Lawrence Berkeley National Laboratory, UV Transmittance Values in WINDOW

So what should I ask for if fading actually matters to me?

Ask for the damage-weighted transmittance, Tdw-ISO. It weights each wavelength — ultraviolet and visible — by how much it actually contributes to fading, which is the question you are asking. Berkeley Lab notes that the NFRC standard was harmonised with the CIE function behind it at the last update.

It will not be on the window sticker, because the label’s five ratings do not include it. It is in the glass manufacturer’s technical data, and a supplier who can produce it is telling you something about how they sell.

The honest answer for most homeowners is that this is a real but secondary consideration. Buy on U-factor, air leakage and condensation resistance, which are on the label and which you will feel every winter. Treat the UV percentage as marketing, because the laboratory that models this for a living says it is.

Lawrence Berkeley National Laboratory: “Tdw-ISO […] is weighted using a function recommended in the CIE standard […] considered to have more general validity. We had the opportunity to harmonize with CIE when the NFRC standard was last updated and we took it.” Source: Lawrence Berkeley National Laboratory, UV Transmittance Values in WINDOW

Will better glass fix the condensation on my windows?

Partly, and it is worth knowing why only partly. A warmer interior glass surface condenses less, which is what the condensation resistance rating is reporting, so better glass genuinely helps.

But the water is coming out of your indoor air, and new windows do not remove moisture from the house. If the humidity is high, it will find the next coldest surface — often an exterior wall corner or a closet, where it grows mould instead of running down glass where you can see it.

Deal with the humidity first, then buy windows on their merits. We wrote that up separately, with the University of Minnesota’s guidance on where the level should sit in a Minnesota winter.