Double-glazing versus low-e glass

In specifying windows for single residential projects when seeking quotes for window supply we often come across window manufacturers proposing to change the glazing specifications.

In the latest occurrence we were told “low-e glass is better than double glazing”. Is that the case?

In short, no. In reality, it depends…..

In order to answer the question it is necessary to begin with the question, what is the glass trying to do? We can assume that all windows are trying to keep out water so the issue becomes its response to solar gain.

In my example above the manufacturers statement was based on an incorrect assumption. Possibly through his experiences of the buildings he has been involved in, he had assumed that the windows were needing to reject heat i.e. that the building had a tendency to overheating.

What is low-e glass?

The “e” in low-e stands for emissivity. The low emissivity coating (low-e) means that less heat passes through the glass than would usually be the case (compared to standard float glass).

What is double glazing?

Double-glazing is also known as an IGU, an insulated glazing unit. It is two sheets of glass that come as a sealed unit with a small air gap in between. The air gap acts as a buffer between the two materials that reduces the flow of heat energy through the IGU. The air gap is often filled with an inert gas, argon, as it further reduces the flow of heat. Double-glazing is very good at reducing heat flow, this works to keep heat inside a building in winter but also to reduce heat flow into a building in summer.

Which is best?

If a building needs to reject heat then low-e glass can be a useful tool. As the sun hits your window less of that heat will transfer into your room; a good thing in summer.

However, if you have designed you building using passive solar principles and are relying on those great north facing windows to keep you warm all winter then you will be disappointed and in need of another jumper or heater.

The low-e coating will always reduce heat flow through the glass, even when you do want it.

The double glazing will do a good job of keeping heat inside the building but it will also allow greater heat gain during winter as the glass tends to have a higher SGHC value (see below)

These are the glazing specification from Viridian, Australia’s only glass manufacturer, for both low-e (ComfortPlus) and double-glazing (4+12+4 is quite standard).

The two numbers that most important here are:

Visible Transmission

The amount of light that gets through the glass. The higher the number the more light gets through, this is not the same as the amount of heat that gets through (see SGHC later). Looking at the two numbers above (first line of table one, second line of table two), the double-glazing lets in more daylight (80 vs 59) than the low-e laminated glass; that’s a 35% increase in daylight.

SGHC

Solar Heat Gain Coefficient. The proportion of total radiation transmitted through the glass, this is the stuff that will keep you warm in winter but also overheat you in summer The SGHC is 0.24 higher for double glazing; that’s a 47% increase in solar heat gain which is very useful in winter.

What does this all mean?

If your home has been well designed and has appropriate shading then it is quite possible that low-e glass will do you more harm than good. However, if you are unable to provide adequate shading in summer it is possible that low-e glass will, on balance, be the best option i.e. if you summer cooling load is going to be greater than your winter heating load.

Needing low-e glass is not a sign of a failed design nor a poor designer. The many variables on a site often conspire against good passive design but do be mindful of reverting to technological fixes when a design change could be more effective and more cost effective.

While low-e glass is generally cheaper than double-glazing it will also reduce you winter heat gain leading to a need for either (a) bigger windows (b) more mechanical heating. The bigger windows solution can then create additional heat loss issues leading to a downward spiral in performance coupled with an increase in costs.

In summary

Providing adequate shade to windows should eliminate the need for coatings to glass.

As always, good passive design should be the first stoop in the hierarchy of design decisions, technological fixes should only come once the passive (i.e. cheap) options are exhausted.

Low-e is a great product in the right situation; unfortunately many in the industry have resorted to easy fixes to solve issues caused by poor design. Great design does not need cost more; it takes considered thought and a knowledgeable assessment of the task at hand.

Be mindful of those with such binary statements such as “x is the best”. It is rarely that simple.

Many of you will note that no mention was made of window frames in this post. We will visit that later but in the mean time f you need to know more please get in touch…