It's amazing to me how we keep chasing windows in the Building Performance industry as if they were an important energy component. They are not (except when they are, I'll explain). Let’s look at some basic realities.
Joe’s Crazy Window Facts
Our attitudes and perceptions of windows within the context of home performance have been greatly distorted by decades of very effective marketing and propaganda, not only by window manufacturers but by the entire home building industry. One of my favorite home performance mantras is “beware of sexy products”. Windows are sexy, which means we irrationally want them, a lot of them, and we are willing to be punished for having them. Let’s step back and take a fresh look.
Old single pane windows are about R-1.5, new “high performance” windows are R-3.5, the best windows ever fabricated are R-6.5. If I was talking about any other energy component you would be aghast. They are all horrible. The upside is that in a reasonably designed building they are only 15-20% of the building enclosure. Which means that even if windows were R-Infinity,no conductive heat loss, it still wouldn’t make that much difference. Bottom line is that windows don’t really matter in the big picture except when they do. They matter when the entire building enclosure is made out of glass. ARE WE INSANE? Even the Three Little Piggy’s didn’t build a glass house. Anyone wanting an all glass building is dumber than a pig.
The idea that windows are drafty was pounded into our subconscious by the persistent use of the term “drafty windows” in advertising. The reality is that windows, typically, are not that drafty. What are the facts? Most uncontrolled air flow is from thermal bypasses, the frame, door weather stripping, chimneys, ductwork, chase ways, and plumbing stacks, but not the windows. As a matter of fact, in many older homes the windows have been painted shut numerous times over and could not let in outside air even if the owner wanted them to.
From a building science perspective windows create moving chilled air that mimics a cold draft. Warm inside air is typically distributed directly under a window in the form of baseboard heat, steam caste iron radiation, or a ducted supply grill. The warm air rises directly into the window and is cooled by the interior window surface. The cooler air then drops away from the window and back down creating a “convective loop”. This is a real draft but the air is not coming from outside.
Another factor that strongly reinforces the perception of a draft is that much of what we perceive as indoor warmth is actually the surrounding surfaces radiating heat at our bodies. A window is a hole in the radiative enclosure. When we approach a window that hole gets bigger and we feel colder.
People don’t want windows, they think they want windows, but they are wrong. Of course all buildings with occupants need some fenestration but not what architects and designers want. Windows are an order of magnitude more expensive than the surrounding wall assembly and they conduct heat an order of magnitude faster. They also let in sunlight, sounds good but most people will not sit in direct sunlight. If you look at any building with a lot of glass, invariably the windows will be covered to the level of greatest occupant comfort, which will be most of the windows. This is a fantastic metric. A building performance factor that is variable, directly controlled by the occupant and obvious to the rest of the world. So based on my casual observations and a little calculus I have determined that people don’t want windows they actually want wall space, privacy and comfort.
Windows are not an energy measure. Window upgrades will save 3-5% on total energy costs for most homes and have a 50-60 year payback, which is longer than the life of the windows. This absolutely disqualifies windows as an energy measure for most state energy programs. Windows do affect home energy consumption but so does siding color but it is not an energy measure. Three cans of foam and a box of weather stripping for less than $100 can save you more than new windows. One CFL can save more than one window. Let’s just agree to abandon windows as a building performance energy measure.
What about Low E coatings. I’ll keep it simple; Low E windows reduce the solar load. The solar load is good in a heating climate. Only buy Low E windows in a cooling climate. Why hasn’t this message reached the window marketing department?
You will never see my rules of thumb for buying windows in any trade publication so you better keep this handy for future reference.
Joe Novella is the host of Home Energy Radio and the owner of Green Star Energy Solutions.