- windows -
Take another look at the infrared image shown in the previous discussion about
thermal bridging. Notice there is a component that’s an even bigger looser than
thermal bridges. It’s all those windows.
Windows are one of the most expressive and vital architectural features of our home.
They bathe the living space with natural light and invite the outdoors inside. They also provide safety, security and a sound buffer.
Yet windows do a terrible job insulating, losing 5 to 7 times more heat than a properly insulated wall. Although window efficiency has improved over the past decade, performance still falls woefully short of our needs.
You might say, “That’s just the way it is. Because most people are not going to
give up an attractive view to save $25-$30 a month in energy bills.”
We wholeheartedly agree. But the energy penalty is not the only drawback of
meagerly insulated, run-of- the-mill windows. Mediocre windows can also affect
Here’s the story.
Regardless of whose name is on them, windows in custom homes usually provide unexceptional insulation. Mediocre windows create not so mediocre comfort challenges. The more windows you have and the bigger they are, the greater their impact on comfort. Furthermore, if the floor plan dictates occupants will be sitting close to the windows, discomfort is also more likely.
Indoor comfort is dominated by four interrelated factors:
(1) Air temperature,
(3) Drafts or air movement and
(4) Indoor surface temperatures.
Each factor has a specific role in achieving indoor comfort. If one element falls to
fulfill its role, the comfort equation falls out of the balance and the owners’ are at
risk of discomfort. In other words, comfort requires symmetry.
The inside surface temperature of a window is determined by the insulation
qualities of the glass. In cold weather, the better insulated the window, the
warmer the inside surface temperature of the glass. The same is true in the
summer, just in reverse. This photo illustrates the principle.
Skin is our body’s largest organ. On the average it comprises about 20 sq. ft. The surface temperature of our skin is about 92°F. So when our skin sees any
surface temp lower than 92°F, we lose heat to that object. The closer the object
the more heat is lost. If it’s a large picture window the heat loss can be so
dramatic, it feels like a draft.
In the example above, the 7°F warmer triple glazed window offers a huge advantage in attaining comfort. But is it enough?
Humidity is also an element of thermal comfort. For ideal comfort and health,
winter humidity should be maintained at 40% to 45%. The chart below shows the
relationship between air temperature and humidity. Notice the trend. The higher
the humidity, the warmer you feel.
Proper humidity is critical to health as this chart demonstrates. Low humidity spawns a host of problems associated with dryness. Add to that such things as dry, itchy eyes, dry sinuses, nasal passages and nosebleeds. There is also the annoyance of getting shocked from static electricity when you touch a doorknob or metal object.
Too much humidity introduces dust mites and mold. Plus higher humidity increases the evaporation of chemicals called Volatile Organic Compounds (VOC’s) into the air we breathe.
Observe the delicate balance between too little and too much humidity.
Our goal was to maintain was 45% relative humidity in the winter and 50% in the summer. Unfortunately, poorly insulating windows are a limiting factor in maintaining our humidity goal. That’s because the inside surface temperature of the glass determines the indoor humidity in the winter. If indoor humidity gets too high, the glass will condensate, leaving us with a mess.
If we used typical double pane glass windows, the inside surface temperature limits our indoor humidity to about 35%. If humidity gets much higher condensation will start to form around the edges of the glass.
Our challenge was to develop an appropriate balance between daylighting that would also support health and wellness without runaway energy bills.
At 340 Fairfax, we chose Marvin Infinity triple glazed units with warm edge spacers that provide stellar performance.
The frames are made from a fiberglass material called Ultrex. Fiberglass is hands down the best window frame material available and is virtually indestructible. It’s strong, provides decent insulation value, doesn’t rot and is dimensionally stable.
Dimensional stability is important to maintain a tight seal for the inert gas between the glass panes that enhance insulation value. When a window frame expands and contracts with changing temperatures, it can stress the seal and the window can prematurely fail. Vinyl frames are known for excessive movement. With fiberglass frames we will not likely be installing “replacement windows 10-20 years in the future. This is a better fit for our sustainability goals.
The space between the glass is filled with an inert gas called Argon. Argon reduces heat transfer dramatically better than air, commonly used in standard windows.
Overall, with a 50% improvement in insulation value we hope to enjoy warmer inside surface temperatures, better humidity control and improved comfort and wellness.