Specifying Passive House Windows: A Pane in the Glass?

Few components affect the character of a home more than windows.  They influence the appearance, lighting, ambiance, energy use (and gain) and ventilation to name a few.  In a standard code-built home little thought is given to windows, but in a Passive House they are critical to it’s success.  Aesthetics are important of course, and we’d like natural light in as many rooms as possible, but this is just the beginning.  Over-sizing a North facing window or specifying the wrong low-E coating for a given location can be the difference between pass and fail.  It is a delicate balance; a game of give and take.

The neighborhood is established, and being relatively flat (and inland) we aren’t tasked with showcasing a particular view.  The main challenge comes from the lot orientation.  There isn’t enough room on the city lot to position the house for optimal solar gains.  The front of the house faces North-West, but it is a corner lot so we have some decent exposure on the South-West side.  There is also an existing house next door (to the South-East) which we felt the need to model for shading.  We created a 3D rendering in SketchUp; geo-locating the house and rendering the neighbors house allowed us to see the shading characteristics at any day/time during the year.

The SketchUp shading animations helped us place and size most of the windows on the house, along with proportions for the roof overhang.  Lastly, they assisted us with modeling the shading characteristics of the neighbors house in the PHPP (Passive House Planning Package) software. PHPP is quite limited in terms of defining different shapes of objects in order to calculate their shading effects on each window.

If this build were in Europe, we would have many types and manufacturers of Passive House Certified windows to choose from.  Windows having passed the scrutiny of the International Passive House Institute are listed in the PHPP software along with their dimensions and performance factors.  Using a certified window frame and glazing combination does make the energy modelling easier, but you pay a premium for imported windows and we hope to showcase Canadian suppliers where possible.  We chose fiberglass for its strength, thermal performance and limited expansion/contraction through temperature extremes.  It is also low maintenance and reasonably priced.

Picking a frame and sash material is a good start.  The South-Eastern Ontario climate necessitates a triple-pane glazing assembly.  Beyond these basics, we chose casement and awning type windows for their air sealing properties. Sliding windows of any kind could never achieve the air-tightness requirements of a Passive House.  A little investigation turned up four Canadian makers of windows that met our basic criteria. Of these four, only a couple offered a closed-back frame; an open-backed frame is slightly better in terms of an installation thermal-bridging coefficient, but the frames are weak and require much greater care during installation to avoid twisting or bowing the frame (which affects thermal performance and air-tightness).  Both the frame and sash should be insulated in a fiberglass window; they do not typically have multiple isolating chambers like an extruded PVC window frame does. Insulating the frame and sash helps mitigate convection currents within the cavities.  Finally, there should be a triple-seal between the sash and the frame; the three seals combine to create 2 distinct air spaces between you and the outdoors.  More air spaces, more insulating value.

The glazing assembly (also called a “Thermal Pane” or “Insulated Glazed Unit”) is where the magic happens.  The frame and sash are functional, but contribute only to energy loss.  This is not the case for the glazing assembly, which can contribute to heat gain, and as such needs to be very carefully planned.  Not all windows can be heat generators; there is very little heat energy to be harnessed from indirect sun-light, so North-facing or heavily shaded windows should be fine-tuned for heat retention.  Windows with Southern exposure are another matter; in this climate it makes perfect sense to adjust the low-emissivity coatings to strike a balance between gains and losses.

There are two general types of coatings: hard and soft.  The hard coatings (also referred to as pyrolytic) are built into the glass as part of the manufacturing process; they are a better choice for South-facing windows, but have a poor U-value.  Soft Low-E coatings (also called Sputter coatings) do a better job of blocking infrared solar heat and as such, have a superior U-value but a poor SHGC (Solar Heat Gain Coefficient).  There are of course many glass manufacturers and they each have a wide selection of Low-E coatings to choose from.  Simply put, use a hard coating where you want to take advantage of solar gains, and use soft coatings everywhere else.  It is also important to note that location matters too; applying a coating to glazing surfaces 2 and 4 will have a different affect on thermal performance (U-value) and the SHGC, than applying the same coating to surfaces 3 and 5 will (each surface of each pane of glass in the assembly is assigned a number).

We also need to look closely at the spaces in between the glass panes; they can be of varying widths and will normally contain an air, argon or krypton fill – listed here in order of increasing performance… and cost.   As if all these choices and specifications aren’t enough, there is one more component which ties it all together: the spacer.  The type of material used for spacers is extremely important; they have to keep the glass panes together, sequester the gas filling and do their best not to act as a thermal bridge in the assembly.  Aluminum spacers (with sealants) are the poorest performers.  Stainless steel (also called a warm-edge spacer) is a better choice because it is much thinner.  The top performer is called a Super-Spacer; it contains no metal, minimizing the thermal bridging effects.

From here, it’s all a numbers game;  every dimension, configuration and performance rating is entered into the PHPP to ensure you have chosen the right products/specs for the right locations.  In the interest of limiting this rather lengthy post (sorry), I’ll dive into that next time!