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

 

  1. All windows are bad
  2. There are no HIGH performance windows, it's an oxymoron, refer back to item #1
  3. Windows are not drafty
  4. Low E windows don’t save on heating
  5. Occupants don’t want windows
  6. Windows are NOT an energy measure

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.

  1. Use as little glass as possible
  2. Only buy Low E in cooling climates
  3. No metal frames thank you
  4. Thermal pane and argon filled will do
  5. Use fixed windows whenever possible
  6. If the window has to operate buy casements
  7. Don’t replace windows just for energy savings
  8. Only replace windows when they no longer work

Joe Novella is the host of Home Energy Radio and the owner of Green Star Energy Solutions.

GoGreenStar.com  HomeEnergyRadio.com

 

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Comment by Bob Blanchette on January 14, 2012 at 9:14pm

Having operable windows can sometimes prevent the homeowner from turning on the A/C unit in mild weather. South facing windows can help with solar gain in winter without being too painful in summer. I like the natural light in the daytime, but to each their own I guess.

Replacing windows can be cost effective from an energy prospective IF the old windows are in really bad shape (rotted wood, etc) and the new windows are installed by the homeowner. I had this on my last house, made a big difference for not much investment. New construction vinyl windows fit the openings with little modification and only cost $80 each. Replacement wood for each opening about $20 per window. 8 windows for $800 installed. If the bill was the more typical $4,000 installed, no it wouldn't have been cost effective.

The real stinker is when new windows are typically installed by "window companies" they only replace the sashes with a custom fit "window unit". Nothing is done about the half rotted window sill or the huge gap between the old window frame and the wall studs.

Comment by Gary Kahanak on October 6, 2011 at 4:20pm
I think you hit a nerve, Joe.  Public opinion about windows is a tribute to the incredible success of the window industry marketing campaign.  Although, architects designing sexy glass boxes and calling them houses hasn't helped; nor have all of the LEED-certified commercial buildings with transparent R-2 walls.
Comment by Robert H on September 28, 2011 at 7:34am

I have to agree with you on windows.  Numbers 7 & 8 hit it on the head and is what I tell people all of the time.

 

There is a lot of push out there to install windows cloaked in energy savings. Think how much better off an owner would be from the comfort and lower bills standpoint if they started with air sealing and increased energy efficiency.  How many homes have new windows and then you go to the attic an see insualtion levels below an R20 and no air sealing.

 

With the talk of "leaky drafty windows". When are these most noticable? On cold days when the stack effect is exerting its most influence.  If you stop the air from flowing out the top of the house you will cut down on the drafts from the windows. Low cost, simple weather stripping will complete the job.

Sexy things like windows, solar, high eff furnaces, heat pumps, ground source etc get way to much attention. Nothing else should be considered until a home has been air sealed and well insulated but that is not the message that is being put out.  I do pt air sealing first on the list too. 

 

 

 

 

 

 

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