Picture a window…. a picture window if you like, or any other kind of window. Imagine it is nice and toasty warm inside the room where you are and bitterly cold outside that room. Now imagine you can see the heat in the room leaving through the window. Does it go fast and evenly out at all locations on the window? Does it just escape around the window? Would you be surprised to know that more heat escapes through the frame than the glass itself? I was!
When I look at a window I see something fragile because glass is breakable. When I think of something fragile like glass I also fail to see it as having any robust qualities. When I see something I think should be robust, like a window frame, I often fail to see it can have fragile qualities as well.
Glass, being something you can see through, that light passes through, that feels cold to the touch can seem to be something that easily allows heat to escape – after all it is practically invisible. A window frame made of wood (they make buildings out of wood) or metal (they make bridges out of metal) or fiberglass (they make airplanes and boats out of fiberglass) or even vinyl (they make vinyl lawn furniture that supports a variety of backsides!) can seem to be strong, robust, even sturdy… and it is, but that doesn’t necessarily mean that it will conduct less heat than glass does.
Aha! Conduct – that’s the phrase we need to focus on, not strong, not fragile, not robust, not sturdy. It’s how that material allows or prohibits heat transfer that makes it energy efficient. You want strong for a frame to support the weight of the glass and the movement involved with opening it. You want invisible for being able to see out, neither of those properties tells you how energy efficient it operates. That is where my mind made the error – I was comparing apples and oranges. Strong and insulating are not the same thing. Would a block of ice melt faster on a hot day in a metal bucket or a Styrofoam cooler you can break apart with your hands? The metal bucket is stronger, the cooler does not transfer heat as readily. Apples, oranges. Metal conducts heat more readily than Styrofoam.
So if the frame conducts the heat more than the glass, then why don’t we make an all glass window or one with a Styrofoam frame? We still need it to be strong to attach hinges, locks, or other hardware to. If we still need a frame, then does that mean there is nothing we can do to make the window more efficient? Not at all. There are a variety of methods available to make windows efficient, still look like a window, operate well, AND help your Passive House to operate properly.
One thing we can do when looking at windows is to see where we can reduce the amount of frame. We can also compare the materials that the frames are being made of with other frame material options. If aluminum frames are cheap, strong, but transmit a lot of heat, and wood frames do not transmit as much perhaps wood is the way to go… until it gets eaten by bugs or rots from condensation if not designed properly. If vinyl transmits less heat than aluminum, is cheaper but doesn’t last as long, isn’t as strong as fiberglass, is made of more toxic materials then perhaps there are other features we need to consider beyond heat transfer and strength. And what about additional insulating materials within the frame such as cork or foam?
Suddenly the choices begin to spin around my head like satellites destined for destruction. How can I make a good choice with all the factors that need to be taken into consideration? Fortunately there is help. The fine folks at LBNL (Lawrence Berkeley National Labs) have created a program that I have mentioned before called THERM. It was designed to not only show how well different glass, gas, and film combinations would work for heat transmission, it also can be used to model window frames. It is this modeling that will allow you to make rock solid choices in windows without comparing apples to oranges.
THERM analyzes the window in a cross section and can show how heat will transfer through it. You can see where the weak points are, what works, and which material options allow you to have better performing windows. That modeling combined with the considerations the Passive House Planning Package uses to model a project allows your Passive House consultant the opportunity to plan the windows for your Passive House project to behave in a way that won’t leave you cold.
Up next – I’ll compare a poor frame with a well constructed window frame and show you how the THERM modeling tool allows your consultant to choose your Passive House windows wisely.