How does Low E work?

Where should the low-e coating be installed?

To be most effective in climates where you want to block heat loss but allow beneficial solar gain to enter, the low-e coating should be located on the outer surface of the inner pane of glass–in the window industry, this is known as the #3 surface (in denoting surfaces, you always start with the outermost surface). In warmer climates where you’re more concerned with keeping unwanted heat out, the preferred location for the low-e coating is the #2 surface (the inner face of the outer pane of glass). I remember seeing a German window one time with a sash designed to be flipped seasonally to optimize the low-e coating location (those Germans!), illustrating that point.

While those low-e placements (#2 or #3 surface) are preferred, having the coating on the other surface isn’t the end of the world. The difference between the overall energy performance of the window with the #2 vs. #3 surface in any climate is far less significant than the the difference between having a low-e coating and not having one. Some manufacturers only put the low-e on the #2 surface, citing concern about seal failure when the coating is on the #3 surface. That’s not a huge problem even in a cold climate.

With low-e coatings directly deposited onto the glass there are two broad categories: soft-coat and hard-coat. With soft-coat low-e, a thin layer of silver is deposited onto the glass through a sputtering process after the glass has been manufactured. While the earliest soft-coat low-e had a single layer of silver, coatings with two layers (low-e squared) and three layers (low-e cubed) came along since that have even lower emissivity and lower heat loss. These sputtered coatings have been referred to as “soft-coat” because the coatings remain fairly delicate and have to be protected within the insulated glass unit (facing the air space)–though that might be changing, as described below.

With traditional hard-coat low-e, a low-emissivity layer of indium tin oxide is applied when the glass is still molten and just beginning to harden in the float-glass “lehr” where it is produced (see last week’s blog for a description of float-glass manufacturing). Denoting the high-temperature production, these coatings are also referred to as “pyrolytic” low-e. The indium tin oxide becomes part of the glass and, as a result, the low-e coatings becomes more durable. That’s why hard-coat low-e is the type of low-e preferred for storm windows where the coating has to withstand washing and other abrasive actions.

While hard-coat low-e is more durable than soft-coat, the emissivity isn’t as low, so these glazings don’t achieve as low a U-factor. On the other hand, they allow more sunlight to pass through, so they are usually better for houses that are relying on passive solar heating.

What’s new with low-e?

The distinction between soft-coat and hard-coat low-e is getting muddier. Cardinal Glass recently introduced an ultra-clear sputtered coating for glass (LoE-i89) that can be installed on the #4 surface of an insulated glass unit–the surface of the glass facing the room. According to Jim Larsen of Cardinal, this glass provides a remarkably high visible transmittance of 89%–significantly higher than standard pyrolytic hard-coat low-e glass. He described this to me as a “sputtered hard-coat,” with the “i” standing for indium. The coating is durable and it doesn’t have the bluish tint that some people object to with pyrolytic hard-coat low-e.

Glass with this coating can be combined with a low-e-squared or low-e-cubed glass to achieve a center-of-glass U-factor as low as 0.20 (R-5) with a double-glazed window. Previously, we needed triple glazing to reach this level of energy performance.

Cardinal also recently introduced a new high-solar-transmission soft-coat LoE-180 (80% visible light transmission), which is a single-layer soft-coat low-e. This provides significantly greater solar gain than the original single-layer soft-coat low-e glazings.