Triple Glazed Passive House Windows

Windows are typically the weakest thermal element in a building envelope, so it makes sense to invest in a high performance triple glazed window. Better windows let a designer shrink the heating system, improve comfort and take advantage of passive solar heating. All of that leads to the same result: lower energy use.

Why do quality windows matter?

Comfort. The quality of the windows directly affects thermal comfort. Anyone who has sat beside an old, frosted single pane window in winter knows it is not a comfortable place. A high quality window keeps occupants warm in winter and cooler in summer.

Energy. Choosing a quality window is the first step to a significantly better building envelope, and that means lower energy loss and lower energy bills.

Why triple glazed instead of double glazed?

Building codes require windows to meet CAN/CSA-A440, which in practice is roughly equivalent to a double glazed window with one Low-E coating. Compared with a high performance triple glazed window, that explains why most homes in Canada need a large mechanical system to heat and cool them: a lot of energy is lost through the windows and the rest of the envelope.

Windows that meet Passive House certification standards must reach a thermal transmittance of 0.80 W/(m²·K) or lower, counting every component: frame, glass, spacers and the gas between the panes. Direct and indirect solar gains come through the glazing as well, and in a well-designed house these gains can exceed the heat lost through the window, even in winter. It is critical for the designer to place windows correctly and to consider shading and summer and winter sun angles.

• Energy. A triple glazed window loses much less energy than a typical Low-E, argon-filled double pane window. Its glass is also warmer across its whole surface, not only at the edges where condensation usually forms.
• Comfort. The inside surface of a high performance triple glazed window stays much warmer than a standard one. That reduces cold radiation into the room and the cold air pocket that can form at the floor below a window.

Why does frame design matter?

The glazing unit, which is three panes of glass plus the spacers and gas between them, usually performs better than the frame. The frame therefore has to be optimized to meet the Passive House requirement of 0.80 W/(m²·K) or lower. A well-designed frame combined with an excellent glazing unit is what defines a Passive House quality window.

Certified windows are now available in wood, vinyl, aluminum, fibreglass and hybrids such as aluminum-clad wood. Each has its own thermal conductivity, dimensional stability, durability and appearance, but all must meet the low heat loss requirement.

• Thermal bridge free design. The spacer between the panes needs low thermal conductivity so it does not bridge heat from pane to pane and to the frame. How the window is installed in the rough opening matters just as much. Good practice places the window close to the centre of the insulation layer, over-insulates the frame, avoids extra framing and blocking at the opening and, where possible, uses a non-metal sill pan flashing.
• Airtightness. Passive House windows are triple gasketed to limit air leakage, and the frames are taped to the interior air and vapour layer and to the exterior weather barrier.
• Dimensional stability. Expansion and contraction of the frame material can affect the airtightness of the window assembly.
• Insulated frame and over-insulation. Because the frame performs worse than the glazing, manufacturers often insulate the frame core. During construction, adding insulation over the exterior or interior of the frame improves performance further.
• Durability. A Passive House window is designed so the interior glass and frame surfaces stay above the temperature at which condensation and mould form under design conditions.

What other components does a window need?

• Low conductivity spacer. Non-metallic spacers separate the panes and also act as a thermal break between the glass and the frame. The traditional hollow aluminum spacer conducts heat readily and is a significant source of heat loss.
• Low-E coatings. These are microscopically thin, nearly invisible metal or metal-oxide layers on the glass surface. Most heat transfer in multi-pane glazing is radiation from a warmer pane to a cooler one. A low-emittance coating facing the gap blocks much of that radiant heat transfer while staying transparent to visible light. Different coatings allow high, moderate or low solar gain.
• Argon or krypton fill. These gases conduct heat less than air and are heavier, so they move less between the panes and reduce convection losses.

Where do I find Passive House windows?

Certified products are listed in the Passive House Institute's Certified Component database. When we last surveyed the market, few of the certified manufacturers were Canadian, and the range of products has changed since. We look at current options with each project.

Windows are one of the six common sources of heat loss in a home. See Six Enemies of Heat Loss in Homes, and read how airtightness is measured in What Airtightness and Energy Numbers Actually Mean. For how we work, see the FAQ.