Windows and Whole Wall Performance

Understanding Windows and Whole Wall R-value

When it comes to the energy performance of a building’s exterior walls, performance is really only as good as the weakest link – typically our windows.

Windows are a significant part of any new home build, serving many functions:

  • Fundamental to home design & aesthetics
  • Provision of natural light
  • Can provide fresh air and ventilation
  • Insulate us from outside sound
  • Reduce heat flow from indoors to outdoors in winter, and outdoors to indoors in the summer
  • Provide a “free” source of heat in the winter.

As none of us wish to compromise any of the above substantial positives in our home, we work towards a balance of these items which are being offset by thermal performance (energy efficiency) of our building envelope.  This is where knowledge, choices and advice of an Energy Advisor becomes invaluable, EAs offering the expertise in finding the perfect balance for your home.

Wall Performance Factor 1 – Window to Wall Ratio

The ratio between a wall surface area and window surface area is what is termed the Window to Wall Ratio (WWR) and this ratio has a dramatic effect on energy efficiency of your home.  As the weakest link in our home’s performance, to over simplify, as we increase the number of windows and/or the window sizes, our energy efficiency drops dramatically.

In the following table, we compare a windowless home (essentially a box) with varying amounts of glazing to two other houses with different WWR ratios.

As the table illustrates, by simply adding building code approved windows covering 15% of a wall, the efficiency of the wall drops to almost in half to 60%!  Double your window surface area, and efficiency plummets further to 42% of the initial wall assembly – imagine throwing half of your heating costs, literally out the window!

Conclusion 1: More windows = Lower thermal performance

Wall Performance Factor 2 – Thermal performance of insulation

Knowing we do not want to give away 40 to 60% of our hard earned money which heats our home, we start looking into solutions to re-establish a balance.  The first reaction by many is “Let’s just add more insulation to the wall!” to offset the losses.  Not a bad idea at face value, so we look into the result of increasing our walls from 2×8 or even doubling the thickness of the wall to 2×12 – assuming that surely that will take care of it!

Notice that even if we make a substantial investment in our envelope by adding 100% more insulation to the wall, we have only improved our envelope performance by just over 30%.  Doing some quick napkin math, increasing our investment in wall insulation gains a disproportionately small benefit.

Conclusion 2: More insulation in wall does not offer a proportional benefit to an envelope with code minimum windows.

Wall Performance Factor 3 – Strengthening the weakest link

Now, with the two conclusions we have come to, explore other ideas.  Since the windows are the weakest link, surely there are better windows available.  So what happens when we strengthen the weak link, all other things being equal?

As consumers, knowing that our window and door package can cost between 15 to 35% of our total build costs, we are reluctant to pay the additional costs to increase the window performance by 50 or even 200%, assuming that this will increase our window package by the same percentage.

The reality is, glazing is a proportionally small cost of our window package, we intuitively do not acknowledge the other costs of a window – largest being the frame and hardware – glazing, being what we see, is just but a slice of the pie.  Fortunately, frame and hardware costs do not change much, if at all, when upgrading to higher quality glazing.  Given that, if the price of the glazing was to double, it is likely that the price of our window package would increase by as little as 5 to 10%.  Noting this, look at the effects the above table illustrates.  If nothing other than specifying the manufacturer to include high quality glazing, a proportionally small investment, our wall efficiency can increase by 48%!

Conclusion 3: Upgrading window glazing offers benefits which are proportionally high.

In summary, the improvement in windows resulting from better glazing selection can be as little as 5% yet the energy savings, which you will see on your annual heating bill, can more than offset the cost of the lower performing windows.