Double versus single glazing. Part 2 – thermal comfort

In Part 1 we looked at the difference between double glazing and low-e glass in regard to heat gain; wanted or otherwise.

Clients will sometimes ask about how much ‘better’ double glazing will be than single glazing AND how much energy it will save. These are two separate issues.

Energy

The energy consumed in heating and cooling a home is used replacing lost heat (winter) or inserting coolth (summer) due to the undesirable flow of heat through the building envelope.

The better insulated your building envelope the less heat flow through it.

Let’s take a 8m long, 2.7m high wall with a 3.6m long, 2.1m door in it.

The wall has a total area of 21.6m2 of which the door is 7.56m2 or 35% of the area.

Let’s assume we are in coastal Sydney and have a winter design temperature of 3ºC and our indoor temperature is 20ºC.

We have heat loss through two elements; wall and door.

The wall has an area of 14.04m2 excluding the door. If we have a total R-value (insulation) of 2 we will experience the following heat loss at a temperature difference of 17ºC:Our single glazed door (same formula, U value of 6.2 taken from WERS) loses 797W. We used this door.

These numbers are instant heat loss in Watts, the amount of heat lost at that temperature difference at that moment in time. If the same conditions persisted for one hour then it would become W/h. As temperatures change this assessment is most useful to compare options rather than to calculate actual heating demand for a building; everything has limitations.

Our single glazed door in our R2 wall has a total instantaneous heat loss of 916W.

If we replace our single glazing with a clear double glazed unit with a U value of 4 then our heat loss through the door drops from 797W to 514W, a 35% decrease. The total heat loss through the wall/door drops from 916W to 633W, a 31% decrease.

These are large decreases however in a Sydney climate as the temperature differences are often not so great that actual energy savings can appear to be relatively small. This often discourages people from using double glazing and some in the industry will say “you don’t need that here, the climate is so benign”. We disagree……the energy matters but this matters more….

Thermal comfort

Our human experience of comfort is complex. One of the many factors is radiant asymmetry; sensing two differing temperatures from two surfaces.We can calculate the interior surface temperature of our two glass types:It is generally accepted that we sense temperature differences greater than 4 or 5ºC as discomfort so in our 20ºC room we still have an issue. We have two options, on a winters night draw the curtains or use a blind, this is the equivalent of R0.3. It would have the following impact:

• Single glazing with curtains   15.3ºC
• Double glazing with curtains  16ºC

The challenge with curtains is they rely on active users. An alternate approach is to use higher performance glass. CSR Viridian has a double glazed unit (Lightbridge) that has a U value of just 1.37. This would give an internal surface temperature of 17ºC with no curtains; guaranteed thermal comfort without curtains and no action required.

There are a multitude of imported windows that even greater levels of performance but for simplicity we are using only Australian products here.

These same principles work in summer too.

40ºC outside, 28ºC inside, internal surface temperature of glass:

• single glazing              37.2ºC
• double glazing            31.9ºC
• LightBridge                 30.1ºC

For the astute reader you’ll realise that LightBridge has other qualities such as a lower SGHC value which means other aspects of your design may need to change to accommodate that. Welcome to our world of fun!

Take Home message

Glazing and buildings are about so much more than just energy and while energy is important and can be easily valued, comfort is priceless!