Single pane vs double pane in Arizona: what is it actually worth?

What the second pane actually does

A double-pane — properly, an insulated glass unit — is two panes held a fixed distance apart by a spacer, sealed at the edge, with the gap filled with an inert gas. Federal guidance describes it plainly: the space between glazing layers, "usually about 1/2", is filled with argon or krypton gas; both are inert, non-toxic, clear, and odorless."

That trapped gas is the insulation. It conducts heat far worse than a solid pane of glass does, and worse than plain air would. The second pane is mostly there to hold the gas in place.

So a single-pane window is not "a window with one less pane." It is a window with no insulating layer at all — a sheet of glass with outside on one side and your living room on the other.

And here is what it does not do

It does not stop sunlight coming through the glass and turning into heat inside your house. That is a separate mechanism, it is measured by a separate number, and in Phoenix it is the one that runs your air conditioning from April to October.

Two numbers describe every window, and both are printed on the NFRC label:

Pane count and gas fill move the U-factor. Coatings move the SHGC. Federal guidance for hot climates is one sentence and it is unambiguous: "Look for a low solar heat gain coefficient (SHGC)... Low SHGCs reduce heat gain in warm climates."

That is why "is double pane better?" is the wrong question here, and "what is the SHGC of the exact unit you are quoting me?" is the right one.

Low-e is the part doing the heavy lifting

A low-emissivity coating is a microscopically thin metallic layer on the glass that reflects heat while passing visible light. The DOE puts its effect at "reduce energy loss by as much as 30% to 50%", and notes that spectrally selective coatings "filter out 40% to 70% of the heat normally transmitted."

Read that carefully, because we are not going to let it be misread. Those are DOE's figures about low-e glazing in general, against uncoated glass. They are not a prediction about your power bill, they are not a Renmark promise, and anyone who converts them into a dollar saving for your address without seeing your insulation, your orientation, your ductwork and your habits is guessing.

Why the answer is different here than in Chicago

Almost everything written about single vs double pane online is written for a heating climate. In a heating climate the enemy is heat leaving, U-factor is king, and pane count is most of the story.

We have the opposite problem for most of the year. The enemy is heat arriving, through glass, in direct sun, at 86% of possible sunshine — the Arizona State Climate Office publishes Phoenix sunshine averaging "86 percent of possible" with a June maximum of 94 percent, and normal maximum temperatures of 103.9 °F in June, 106.6 °F in July and 104.5 °F in August. (Period of record 1971–2000 — a climate normal, not a current reading.)

So the ranking of what matters flips. Coating first, then the gas fill and pane count, then the frame. A national article that never mentions SHGC was not written about your house.

What the standard actually asks for

ENERGY STAR's current Version 7.0 specification, effective October 2023, sets windows in the hotter southern zones at U-factor ≤ 0.32 and SHGC ≤ 0.23.

A single pane cannot get near either number. A cheap double pane with clear glass will miss the SHGC badly. Both of those failures are invisible until you ask for the label — and "ENERGY STAR certified" on a brochure is not the label. Ask for the two numbers, for the exact product line and glass package being quoted, in writing.

One caution we apply to ourselves as well as everyone else: do not accept a manufacturer's restatement of the ENERGY STAR criteria as the criteria. At least one major manufacturer publishes a version of the v7.0 southern-zone thresholds that does not match the actual specification. Read them off energystar.gov yourself.

The third pane question, answered honestly

Triple pane comes up constantly and the honest answer is unglamorous.

Triple glazing is primarily a cold-climate and noise solution. Adding a third pane and a second gas cavity attacks the U-factor — the heat-conduction problem — which is the problem we have least of. It adds weight, it adds cost, and it does comparatively little about solar heat gain, which is our actual problem.

A well-specified double pane with a low SHGC will usually serve a Phoenix house better than a triple pane with a mediocre coating, and it will cost less. If someone is selling you triple glazing on energy grounds in this valley, ask them to show you the SHGC of both options side by side and let the numbers settle it.

Where triple pane genuinely earns its price here is noise — a flight path, a main road, a school. That is a real reason and it is a different reason.

If you have single panes right now

Then the pane count probably is not your only problem, and it may not even be the first one.

Across 472 Phoenix-area homeowners who told us what their window frames are made of, about 60% — 281 of them — have metal. Of those, 161 are twenty-plus years old and 75 are original to the house. Single-pane glass in this valley is usually sitting in an aluminum frame from the same era, and federal guidance on that frame is blunt: "metal or aluminum window frames conduct heat very rapidly, which makes metal a very poor insulating material", and such frames "should have a thermal break." Many here do not have one.

Putting a modern insulated unit into a bare metal frame is treating the symptom. How to tell what you have, in about two minutes, is on metal window frames in Arizona.

The other thing that happens to double panes here

There is a failure mode single panes simply do not have, and it is worth knowing before you assume double pane is the finished answer.

When the edge seal of an insulated unit fails, the gas leaves and moisture gets in — and you get permanent haze between the panes. Peer-reviewed testing found that edge sealants show six to eight times more permeability at 60 °C than at 20 °C, that "sunlight is regarded as the most important aging factor on IGUs," and that gas loss can raise a unit's U-factor by up to 32%. 83 of the homeowners in our data reported exactly that.

That is not an argument for single pane. It is an argument for asking what the spacer is made of — the same research found aluminum spacers leaked more gas than non-metallic warm-edge spacers over seventy weeks of testing. Full detail on why windows fog between the panes.

What to ask, in order

1. SHGC and U-factor for the exact product line and glass package — in writing, from the NFRC label. Not "ENERGY STAR certified."
2. What the coating is. Low-e, and which one? Spectrally selective?
3. What the gas fill is, and whether it is stated on the quote at all.
4. What the spacer is made of — non-metallic warm edge, or metal?
5. What the frame is, and if it is vinyl, what the frame is reinforced with. Most vinyl windows still contain aluminum reinforcement.
6. Retrofit or full-frame, stated explicitly.
7. Both warranties, as two documents — the manufacturer's, and the installer's labor warranty. They are almost never the same length, and the manufacturer's almost never covers labor.

A bidder who cannot produce the first item has not looked up the window he is selling you. That is worth knowing before you compare prices, not after.

Related guides

• Energy efficient windows in Phoenix
• When to replace your windows
• Why windows fog between the panes