Energy Audit-Really Good Windows

Occasionally, when conducting energy audits and assessments, a customer will make a statement to the effect that they believe their windows are the main issue with why they are experiencing high energy costs.  They are hoping to replace all their windows to lower the heating (or cooling) bills. Window replacement rarely makes economic sense because of the very high cost to remove the old windows, purchase and install the new ones.  The energy cost savings will take decades to pay for the replacement, but window replacement does happen, how do you know you are replacing your existing windows with something better? 

A really good window will not only have an effect on energy costs, but more importantly, a really good window will add more comfortable to the home.  A really good window will almost never have condensation on them, even during very cold periods when temperatures in my area can reach -40°F.  A really good window will have a very good seal so that the outside environment is kept outside.  And a really good window is one that is installed correctly.  Before we get into the details of what makes a window really good, lets look at a few I’ve come across over the years that weren’t so good.

This is a window at a good friend’s house, it’s about a 20 year old double pane manufactured by a quality window company.  The argon gas fill between the window has escaped which allows the two panes of glass to come closer together, lowering the temperature of the center of the glass during cold weather.  This area of the glass then becomes a dehumidifier, allowing the moisture from the air interior air to condense on the cold surface.  He has since replaced the window.

This is a thermal image of a window that has also lost it’s argon gas fill.  The panes of glass are closer together which becomes evident with the dark area of the center of the glass.  The center of glass is cooler than the surrounding glass.

This window is not sealing correctly where the frame meets the sash.  It could indicate a problem with the window itself or in the installation of the window not being installed plumb, level and square.  This photo was taken during a blower door test.

This window was leaking air during a blower door test between the window frame and trim, most likely caused by not proper air sealing after installation.  This would be an installation error, failure to properly make the air control layer continuous, probably not an issue with the window.

Window condensation is common in my climate when temperatures drop to -20°F, even in good double pane units.  This photo was taken during a cold spell in a new home that had a higher humidity level due to the new construction.  Operating the HRV or heat recovery ventilator reduced the humidity levels inside the home after just a few days, eliminating this problem.

So, what makes a window good?  In my very cold climate, I’m looking for a window that has certain qualities.  First is a lower U-value.  A code minimum window in the state of Minnesota will have a U-.32 which will have an R-value of 3.125.  Not very good when our walls are required to be insulated to a minimum R-21.  The photo below shows a local window manufactured that I like to install in homes with a lower budget.

The window is U-.24 or R-4.16.  Not great, but better than the code minimum.  The lower U-value in this double pane window means there has been at least on coating applied to one of the glass surfaces to make it a better insulator.  This coating will also lower the visible light transmittance through the glass, giving it a dark tint as you look through the window.  This is indicated by the visible transmittance rating of .42, higher numbers are more desirable.  The window above also happens to be a casement window, which will seal more like a door, making it less likely to leak air around the window seal.  Single and double hung along with sliders tend to have more air leakage around the seals.  Casement, awing and tilt and turn windows tend to have the better air sealing.  The best, a fixed glass window.

What if I want an even better window, first thing, you’ll need to move to a tri-pane or triple pane unit.

This is the window sticker from the windows used on the concreteless slab on grade project from a couple years ago, they are Marvin triple pane, U-.20 or R-5 windows.  The reason we selected Marvin, they are local to the project, Marvin’s manufacturing plant is only three hours away.  The homeowner wanted a wood interior and both the contractor and myself have had decent luck with these windows.  So why move to triple pane?  Number one reason is comfort.  A comfortable house has two things, a consistent temperature throughout and a consistent humidity level, between 30% and 40%.  Maintaining a humidity level above 30% in my very cold climate is difficult.  During this past years polar vortex, the humidity level in my personal home had dropped to 8% for a short time.  No condensation on any of my windows, but at that low of humidity level, even 68°F air temperature has a little chill to it.  A homeowner with triple pane windows is able to maintain a higher humidity level without the concern of frost or moisture forming during the majority of the heating season.

The photo above is a newer window technology, triple pane using a thinner piece of glass for the center pane.  Not widely used, yet, I’m hoping this detail to have a good triple pane window with the weight closer to the double pane windows we are more accustom to catches on.  Time will tell.

Can we get an even better window? Definitely, there are a few manufacturers who have windows with U-.10 or R-10 insulation values.  Many European companies and a few in North America manufacture these super insulated windows.

This is Jake Bruton’s personal home in Columbia, Missouri.  The windows he chose are from Europe, Schüco Windows which are tilt/turn triple pane with U-.158 or R-6.3 and a very high visible light number of .71 which is much higher than the double pane windows I use for projects with lower budgets.  His very large sliding patio door as an even lower U-value, U-.134 or R-7.46.  These were installed and sealed to the air control layer in such a way he was able to achieve a .3 ACH50, very tight construction.

Lets talk a little about the installation of windows.  Windows that are not properly installed (plumb, level and square) in accordance to the manufacturer’s instructions or air sealed correctly will effect the comfort, cost to operate and possibly the durability of the home.  Number one concern with windows is that they have a great connection to the water control layer.  The water resistive barrier (WRB) used in the build will dictate how the window is connected to the WRB.  This connection must be made to shed water at the exterior, and there must be a way to allow any moisture that should find it’s way past the WRB and into the window rough opening, a way to drain out.

Next we want to connect the window to the air control layer.  There are many ways to do this, canned spray foam, backer rod and caulk and there are even some tapes that can be used on the interior for this connection.  The photo below shows a tape from Siga, called Fentrim 20, designed for interior window air sealing.

The tape, in this installation is connected to either the window rough opening wood framing or the the WRB, which is this photo is the self-adhered Henry Blueskin, and the window itself.  This is the construction detail I will be using with projects that have a higher budget, backer rod (used as insulation between the window and framing) and Fentrim 20 (used as the air seal).

So, what makes a really good window?  A really good window is one that has been well built, that has been installed correctly, and has the best possibility to last the life of the home, hopefully a century or more.  Want more info about windows?  Click here, to learn about the window stickers, here to see why I ask about window condensation during energy audits, and here to see how I installed a window in my personal home.

Leave a Reply

Your email address will not be published. Required fields are marked *