Low-E is a microscopically thin metallic coating that reflects heat back toward its source. In a heating climate you want a soft coat low-E that reflects interior heat back inside while still letting reasonable solar heat through on south-facing walls.
Low-E is the single most consequential item on a glass spec and it is almost never explained. It is also not one thing: coatings vary in type, in the number of reflective layers, and in which surface of the glass they are applied to.
Here is what the variations do and which one suits a Capital Region house.
What the coating actually does
Low-E stands for low emissivity. The coating is a microscopically thin layer of metal oxide, usually containing silver, applied to a glass surface. It is essentially transparent to visible light but reflective to infrared, which is heat radiation. That means it lets daylight through while reflecting heat back toward whichever side it came from. On a winter night, heat radiating from your warm room toward the cold glass gets reflected back into the room instead of passing out through the window. In summer, solar heat gets partly reflected back outside. Which effect dominates depends on where in the assembly the coating sits and how many reflective layers it has.
Hard coat versus soft coat
| Hard coat (pyrolytic) | Soft coat (sputtered) | |
|---|---|---|
| How it is applied | Fused to hot glass during manufacture | Vacuum deposited after manufacture |
| Durability | Very durable, can face the room | Delicate, must be sealed inside the unit |
| Thermal performance | Good | Better |
| Solar heat transmission | Higher, more solar gain | Lower, tunable |
| Appearance | Slight haze or cast | Essentially clear |
| Best use here | Storm windows, single glazing | Sealed insulated units |
Silver layers and what they change
Soft coat low-E is described by how many layers of silver it contains: single, double or triple silver. More layers means more infrared reflection, which means a lower solar heat gain coefficient and generally a lower U-factor. In a cooling-dominated climate, triple silver is the goal because keeping solar heat out is the whole job. In the Capital Region, where heating dominates for roughly seven months, the calculus is different. Single or double silver is usually the better choice, because it still delivers strong insulation while permitting more solar gain through south-facing windows, which is free winter heating during our many clear cold days. Triple silver on a south wall in Albany is optimizing against a problem we have for two months at the cost of a benefit we get for seven.
Practical guidance for a Capital Region house
- Ask for a soft coat low-E in a sealed insulated unit. Hard coat belongs on storm windows, not in a modern replacement unit.
- Confirm ENERGY STAR certification for the Northern climate zone specifically, which is where the coating choice is already calibrated for heating.
- On south-facing walls with good winter sun, a moderate rather than very low solar heat gain coefficient is an advantage.
- On west-facing walls that overheat in July, a lower solar heat gain coefficient or exterior solar screens both help.
- North-facing walls get no direct sun, so prioritize the lowest U-factor you can afford there and ignore solar heat gain.
- Do not clean low-E glass with abrasives or a razor blade. Soft coat is sealed inside the unit, but some surfaces are coated and damage is not repairable.
Ranges reflect typical Capital Region projects and are for planning only. Every Sundog quote is written, itemized, and free.
Frequently asked questions
What does low-E glass actually do?
It carries a microscopically thin metallic coating that is transparent to visible light but reflective to infrared heat radiation. In winter it reflects heat from your warm room back inside instead of letting it radiate out through the glass. In summer it reflects some solar heat back outside.
Does low-E glass look tinted?
Modern soft coat low-E is essentially invisible from inside a room. Held edge on against an uncoated pane you might notice a faint difference in reflection. Older hard coat low-E had a more noticeable cast, which is where the reputation came from, and it is still used on storm windows where durability matters more.
Is triple silver low-E better?
For a cooling-dominated climate, yes. For the Capital Region, usually not. More silver layers block more solar heat, which is exactly what you do not want on a south-facing wall during seven months of heating season. Single or double silver typically suits a heating climate better.
Can low-E coating wear off?
Soft coat is sealed inside the insulated glass unit where nothing touches it, so it does not wear in normal service. What does happen is that when the seal fails and moisture enters the unit, the coating can corrode and stain, which is one reason a fogged unit should be replaced rather than vented.
Should every window in my house have the same low-E?
Not necessarily. North-facing windows benefit most from the lowest U-factor with solar gain irrelevant. South-facing windows benefit from moderate solar gain for free winter heat. West-facing windows that overheat in summer benefit from lower solar gain. Most houses use one specification for simplicity, but varying it by elevation is a legitimate optimization.
Services this relates to
Keep reading
Local pages
Questions about your own house?
A free measure and a written quote costs you nothing and gives you real numbers to work with.