Passive Solar and “Tilt and Turn” German Windows

7.27.2011

Why did we haul windows all the way from Germany for TerraHaus? First, achieving the Passive House standard of 90% less energy use than a code-compliant home of the same size necessitates the use of the best technology available. Second, we hope to promote some of the features of these windows to influence North American window manufacture and even energy policy. GO Logic’s Alan Gibson and Matt O’Malia came across these windows, made by Kneer Sudfenster, at a green building trade show in Germany, home of the Passivhaus standard.

Windows can capture solar energy to warm a home, but they also lose energy, partly through radiant losses (reduced through the use of low-E glass), partly through conduction due to the low R-value of glass and many types of window frames, and partly through convective losses due to air leakage.  The goal in a passive solar home, therefore, is to gain more heat energy in the form of solar radiation than is lost through the same window.

Many Americans have some understanding of R-value, the insulation value of building materials, and those with leaky windows are sensitive to how they let in the cold, but few probably understand a relatively new measurement of window performance known as the Solar Heat Gain Coefficient (SHGC). The SHGC is a measure of how much of the sunlight hitting a window passes through the glass. While it may seem like all the incident light should make it through to the house interior, for most glazing only 30-40% of the radiation may be transmitted. Triple pane, low E windows, used for their insulative quality, generally block high amounts of incident solar radiation.  If we want to use solar energy to heat our homes we need south-facing windows that transmit 60% or more of the radiation hitting them to make up for the heat loss from the same windows for a net energy gain.

South Facing Window

With a SHGC of 0.5 to 0.6 (meaning 50%-60% of incident solar radiation penetrates the glass) our south-facing TerraHaus windows can pick up a lot of solar radiation. Remarkably, these windows also have a very high R value (9.46 for the panes) and low level of radiant heat loss. Somehow those clever Germans have managed to embed low-E material into their glass in a way that doesn’t block the short wave radiation from the sun. And those of you fortunate to take a tour of the TerraHaus this fall will want to experience how positively tight they close, latching in multiple places, keeping warm air in and cold air out. Finally, these windows achieve their very high R value due to their thick wooden frames equipped with sealed air channels for extra insulation and aluminum cladding on the outside that is attached in a way that does not form a thermal bridge to the inside.

In addition, TerraHaus will demonstrate increasingly popular European-style “tilt and turn” windows on the north, east and west. These windows close much tighter than double hung windows, and they are much easier to clean than standard casements. In their tilt position, they offer summer ventilation while blocking drafts and rain. These windows also offer extra security and safety: In the turn position they offer easy egress, and in the tilt position it would be virtually impossible for an intruder to enter or a young child (or college student?) to fall out.

Tilt Position

"Turn" Position

How do we hope to influence energy policy through use of these windows? While you will frequently see Unity College promoting Energy Star products, the Energy Star program and the other programs that rely on Energy Star ratings have a flaw when it comes to windows. The very windows we most need in the Northeast—high SHGC windows for southern exposures—are not Energy Star rated and do not qualify for federal and state incentives! The Energy Star program promotes low SHGC windows, which makes sense for homeowners in the South where these windows help keep homes cooler.

Doug Fox, Director, Center for Sustainability and Global Change, Unity College

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