Low-E Glass Explained: What the Coating Does, Hard vs. Soft Coat, High-Gain vs. Low-Gain, Argon, Spacers, and Which Glass Goes on Which Wall

Windows & Doors

Low-E Glass: The Coating You Cannot See, the Two Kinds of It, and Why Your South Wall Wants a Different One Than Your West Wall

Every window quote says Low-E and almost none says which one. There are coatings that let the sun’s heat in and keep the room’s heat from leaving, and coatings that block the sun’s heat too. In Ontario the first kind belongs on the south and the second on the west, and most houses get one kind everywhere because nobody asked. Here is what the glass is actually doing.

Plain language, no physics beyond what you need to read a quote.

The numbers

~1/3less heat loss through the glass with one Low-E coat and argon, vs. clear double
0.25 to 0.55solar heat gain coefficient range you will see
$0to choose the right coating per wall; it is the same price
10%of the heat loss removed by argon over air

What the coating does

Heat leaves a room through glass mostly as long-wave radiation: the warm walls and furniture radiate at the cold glass, the glass absorbs it and radiates it outward. A low-emissivity coating is a few atoms of silver or tin oxide on the glass that reflect that long-wave radiation back into the room while letting visible light through. One coat on the inside face of the outer pane of a double unit turns an ordinary window into a much better one. It is why clear double glazing is no longer sold here.

Hard-coat (pyrolytic) Low-E is baked into the glass, tough, slightly less effective, and lets more solar heat through. Soft-coat (sputtered) is applied after, more effective at reflecting heat, has to be sealed inside the unit, and comes in versions that block more or less of the sun. Nearly all residential glass now is soft-coat; the question is which soft-coat.

High gain, low gain, and the compass

The second number on a window’s label is the solar heat gain coefficient: the fraction of the sun’s heat that gets through. A high-gain Low-E (SHGC around 0.5 or more) keeps room heat in and lets sun heat in: free heating from a south window in January. A low-gain Low-E (SHGC 0.25 to 0.35) keeps room heat in and blocks most of the sun: the right thing on a west window that cooks the family room at five o’clock in July.

  • South: high gain. In Ontario the winter sun is low and the overhang shades the summer sun. Take the free heat.
  • West: low gain. Low afternoon sun, no overhang helps, worst overheating in the house.
  • East: either; morning sun is weaker. Low gain in a bedroom you want cool.
  • North: does not matter for sun; pick the lowest U-factor. This is where triple earns its keep.
Ask for it by wall. Most Ontario manufacturers carry two or three coatings and will put a different one in each window for no extra charge if the schedule says so. The default, if nobody asks, is the one with the best Energy Rating on the sticker, which is usually a mid-gain coating everywhere.

Argon, krypton and the spacer

Argon in the gap between panes is denser than air and slows the convection that carries heat across; it is worth about 10% and costs almost nothing, so it is standard. Krypton is better in thin gaps, used in some triples, expensive. Both leak out slowly over 15 to 25 years, which is the sealed-unit clock.

The spacer is the strip around the edge that holds the panes apart. The old aluminum box spacer is a thermal bridge: the line of condensation along the bottom of your glass in January is the spacer. Warm-edge spacers (foam, stainless, composite) cut that. A good series has one; ask.

Reading the glass line on a quote

A proper glass spec looks something like: double glazed, 4 mm clear / 13 mm argon / 4 mm Low-E (name of coating), warm-edge spacer, tempered at W03 and W07. Or for triple: 4 / 12 argon / 4 Low-E / 12 argon / 4 Low-E. Then the certified whole-window numbers for the assembled unit in that frame. “Low-E argon” on its own is the start of a spec, not a spec. The ratings page explains what the certified numbers are and why centre-of-glass numbers in a brochure are not them.

Where homeowners go wrong

  1. Low-gain glass on the south side. Blocking free January heat to solve a July problem the overhang already solves.
  2. High-gain glass on a west family room. Then buying a bigger air conditioner.
  3. Judging tint. Good Low-E is nearly invisible; a slight green or grey cast is normal. If you can see a mirror effect from outside, that is a reflective film or a very low-gain coat, and it was probably the wrong choice for a house.
  4. Paying for triple and getting one coating. Triple with one Low-E is a heavy double. Two coats, or do not bother.

Questions people ask

Does Low-E glass make rooms darker?

A little: 5 to 15% less visible light depending on the coating, more on triple with two coats. The visible transmittance number on the label says exactly how much.

Is Low-E worth it?

It is not optional in practice; every window sold here has it. The worthwhile question is which coating on which wall.

Can Low-E glass damage siding on the neighbour’s house?

Very low-gain reflective glass on a concave or slightly bowed unit has melted vinyl siding on houses across narrow side yards. It is rare, real, and another reason to use high-gain glass where there is no overheating problem to solve.

Put the glass on the schedule

When you run the calculator and send the scope, add a note with the compass directions of the windows. The quote should come back with the coating named per wall.

Price my windowsU-factor and Energy Rating

Sources and further reading