The Value of Solar Heat Gain

Double Glazing or Double Batts - Where to put your dollar. Do We Really Need Double Glazed Windows?

All existing and new homes have windows which gain energy through solar heat gain. In the cooler climates the homes are usually uncomfortably hot on the hot sunny days (and uncomfortably cold on cooler days). That's caused by solar heat gain, it's happening now and it's free... .

On how many days is this bonus delivered? Scientific study has shown that the sunny days tend to occur more often during the warmer weather, negating the need for it. The same study also shows that the solar heat gain does not happen at night when it's really needed. So what we have is the solar heat gain when we don't want it and a desperate need for it at other times. So how do we overcome this problem? We try to store the heat and we try to stop it from escaping. Thermal mass is a name we give to the quantity of materials in a building that stores the heat, it refers to concrete, bricks and anything else inside our home. And it's true, a thermal mass can assist with energy efficiency but it can only really help "if" we could receive, store and isolate (insulate) all that solar radiation for the whole year, and gradually let it out when we need it, but we can't. In traditional thermal mass, the thermal capacity has a half life of about 8 hours, so any heat stored is reduced by half every 8 hours or so. So what to do it the sun don't shine? Batteries maybe?.

So at best a thermal mass in a cold or hot climate only moderates the natural fluctuations of hourly and daily temperatures (but not weekly and certainly not seasonal fluctuations), it does not replace good insulation.

So what do we do to keep the heat out or in? We insulate our homes by using batts and double glazing. On the face of it, double glazing is more expensive compared to batts, but is it?

The efficiency of insulating batts is measured as R value, but the efficiency of windows is measured in U value, so what's the difference? U measures conductance (or how well windows conduct heat) and R measures resistance (or how well batts stop heat), meaning that U value is the reciprocal or opposite of R Value. So a U value of 2 is equal to an R value of 0.5. And an R value of 2 is equal to a U value of 0.5.

If you look at a typical brick veneer house with R1.5 batts in the walls then the combined R value for the wall is R1.89. The windows to match this would need to have a U Value of 1 1.89 = 0.53. Not economically feasible.

And as far as keeping the heat in is concerned single glazed windows are at best a poor insulator.

Example: Energy efficient home or standard home, we would be stupid not to put batts in your ceiling - right? Well mostly we fit R2.5. But if you made this R5.5 your total change in heat losses would be 0.22 W/m2. But if you changed your windows to double glazed then the difference would be 3.5 W/m2. That's saying that double glazing the windows would be almost 16 times more effective than upgrading your ceiling insulation. So if you have 100m2 of ceiling and only 40m2 of windows, you are still over 6 times better off doing windows.

Double glazing is really mandatory in the energy efficiency argument and the savings are calculable if we do our homework, average ambient temperatures, areas, desired inside temperature and "U" values. The result is kWh consumed by windows to maintain inside temperatures. So the payback is easy as we can predict the cost saving and we can also at the same time know the capital cost of the upgrade.

Double glazing will give us for more control over your home's internal comfort than thermal mass alone ever will.

Windows comprise the biggest single cause of heat loss (or gain) in a home and where thermal mass is useful, it's not the answer to inefficient glazing.

On a cool day, single glazed windows can consume 1.68 kWh or more per square metre per day which is over $0.20 per square metre per day. Most single storey homes have 30m2 and double storey 60m2 so that's $6 - $12 per day! So running costs related to windows are significant, whereas our ceiling insulation savings runs at just over 1 cent a day/m2.

You can't rely on thermal mass to replace the 40% of energy said to be lost through windows when an upgrade to double glazing will more than half it.

Make double glazing number 1 in my list of options, and that's before upgrading wall or ceiling insulation.