This week we will start to look at windows for the Passive Bunkhouse. I say “start to look at” because picking windows for a Passive House project involves quite a bit of research including looking at all the ways windows can affect the building’s performance.
Since we are limited to the amount of energy we have to spend on our heating and our lighting in order to meet the Passive House requirements, it is best to take advantage of all the solar heat and natural light that we can while also making sure we are not creating a problem in the summertime with overheating.
Let’s start by looking at the parts of a window and what each does. Here is a diagram of a double pane window with the parts labeled as: 1: External surface (outside) of exterior pane 2: Inside of exterior pane 3: Outside of interior pane 4: Internal surface of interior pane 5: Window frame (Moulded) 6: Spacer 7: Seal 8: Reveal 9: Exterior Sill
A triple paned window would have one additional pane of glass or suspended film layer inside it as shown in the diagram of a typical Passive House window. Some manufacturers are using more than one suspended film layer.
What is important to note is that window science depends on how the light and heat are able to pass through or not pass through the glazing package depending on the application it is intended for. The surfaces of the window glass and films can be designed so that heat can pass through from the outside, but blocked from passing as easily back to the outside. This is done through films and coatings placed on specific surfaces of the window glass that have properties designed for that effect. The spacer is another part of the assembly. It allows for the cavity between the panes of glass or films to be filled with gas. The gasses that are used in the cavities enhance the performance of the window by protecting the films and coatings from decay, adding a measure of insulation, and in some cases contribute to sound reduction. Altering the thicknesses of the glass in windows can also contribute to sound reduction. You can read more about insulated glazing here.
The fine folks at Lawrence Berkeley National Labs in Berkeley, CA created a program to design windows and called it “Window”. You can create an enormous variety of windows from the products that are in their database and then import that window into their THERM 2d Heat Transfer modeling program to see how it will perform. There are almost 4,000 types of glass you can use, multiple gasses, frame materials, spacer materials. I can not even guess at how many combinations one could potentially come up with. Fortunately you don’t have to. But you could if you wanted to!
Passive House Institute lists certified Passive House components on its website that have already been designed to work with Passive House projects. The PHPP also has the latest certified components included in it at the time of its production. Finally, any window manufacturer can provide you with the information necessary to input their glazing package into the PHPP and you can test their products to see if their claimed values are suitable for your project. Should you decide to use a non-PHI certified window for your project and you are seeking certification, you may not be able to use the exact figures the manufacturer provides – the final decision rests with PHI.
So how does one actually pick a window for a Passive House project? First you need to know what your objective is. The Passive House Institute has set a recommendation that says:
“To meet the requirements of the Passivhaus standard, windows are manufactured with exceptionally high R-values (low U-values, typically 0.85 to 0.70 W/(m².K) for the entire window including the frame). These normally combine triple-pane insulated glazing (with a good solar heat-gain coefficient, low-emissivity coatings, sealed argon or krypton gas filled inter-pane voids, and 'warm edge' insulating glass spacers) with air-seals and specially developed thermally broken window frames.
In Central Europe and most of the United States, for unobstructed south-facing Passivhaus windows, the heat gains from the sun are, on average, greater than the heat losses, even in mid-winter.” [http://en.wikipedia.org/wiki/Passive_house]
You could then talk to a Passive House Window Guru like Bronwyn Barry at One Sky Homes. She is my go-to person when it comes to Passive House windows! You could attend the Passive House Northwest conference in Seattle March 15th, 2013 and talk to a vendor. You could try plugging in some windows that are already in the PHPP to see how they work with the design you are building. That's what I'm going to do. I'll be able to see how changing their size and location will affect the performance of the building. I'll also be able to compare how different window glazing assemblies and frames affect the performance. As I narrow in on what will work for my project from a building science perspective I will then be able to make decisions on price, appearance, or other considerations. I will also be asking a lot of questions along the way!
Up Next – let’s plug some windows into the Passive Bunkhouse.