In this home we chose to install Pella 250 series windows with triple pane glass, argon gas, and low-E coating. The triple pane glass is about 50% more energy efficient than the double pane Energy Star rated windows we used to install (the new windows are still Energy star rated, just better). The U-factor of the glass in the windows we used is 0.20, compared to an Energy Star window with a U-factor of about 0.30 – 0.35. Since the U-factor is the inverse of the R-value (one divided by the U-factor), the windows in our home are R-5, or the equivalent of 1″ foam insulation.
The windows also have argon gas inserted between the panes of glass. Since argon is a very heavy gas (much heavier than air), it slows down the “convective looping” between the panes. This is heat that is lost in the winter when air or other gas picks up heat from the inside pane and then “loops” in the space and gives off the heat to the outside pane. The reverse is true in hot months, when the gas between the panes slows down the heat transfer from the outside in.
The low emissivity, or Low-E, coating on the glass acts to control solar heat gain into the home. The special coating reflects solar rays in the summer when the sun is high in the sky and at a sharp angle to the glass. In the winter, the angle of sunlight is lower, and the coating lets the rays in. This helps the home gain some heat in the winter and keep the heat out in the summer. Yet another example of the home saving energy without mechanical means.
One comment I often hear is that the materials we use, especially the windows, are expensive, and therefore drive up the cost of the home. The Pella triple pane windows on this home cost about $2700, whereas a set of double pane windows (which are 33% less efficient) would have been closer to $1900. Yes, we spent about $800 more for the better windows, but amortized over a 30 year mortgage, this will cost the homeowners an additional $2.22 per month, a small price to pay for superior performance and comfort. And windows are an integral part of the home that aren’t changed out every 10-15 years like a water heater or appliances. They should last 50 years or more, so doing it better from the start makes sense.
The last step in installing windows (and doors) is applying the flashing, or materials that keep water out of the building around the windows. This has to be done absolutely perfectly or there is the potential for water to leak into the building and cause major damage. This is a step often done improperly in homes even to this day,and something we take very seriously, since the consequences can be hundreds to thousands of dollars in damage down the road. Vinyl siding keeps most of the water away from the house wrap but not all, and changes in weather can also cause moisture to collect behind the siding. If this flashing isn’t done right, you won’t know until you start seeing evidence inside the home, and by that time the damage to the sheathing, framing, drywall, etc. is pretty severe. You can see in the photo above how we used Dow Weathermate flashing tape (blue) around the sides and top of the window, then overlapped the house wrap (light blue) to create a drainage surface that keeps water out. If you’ve done your flashings right, in theory, you could never install siding and water would still not get in. An added benefit to all of this flashing is that it helps air-seal the home, which is another way to prevent heat loss. The home works as a system, and all parts are related.