Acelab Insights

Window Selection Part 2 - Single-Hung vs Double-Hung


Single-Hung vs Double-Hung?

This question is a case study in how form forgot function. Put simply, in a double-hung window, cool air comes in through the lower operable pane, and hot air escapes through the upper operable pane. Whoosh. Ventilation as no other window type can deliver.

A single-hung window, which only opens on the bottom, requires a second window on a different exposure of the building to work the same way. Single hung windows are good for other reasons, but when looking at ventilation, you have to consider this factor.

A variation of the double hung window, the horizontal pivot, offers the same ventilation benefit but in a larger footprint.

Single-Hung Windows

Single-Hung Windows are built with two sashes. A lower sash, set toward the interior, slides up to open using a variety of track technologies to compensate for the weight of the sash. A second, upper, fixed sash is set toward the outside. The fixed upper sash improves the rigidity and air tightness of the overall window. The operable lower sash provides ventilation by sliding upward so there are no pieces which swing in or out from the plane of the window. Characterized by its simplicity, limited moving parts, and lack of complicated hardware, single-hung windows are a great design choice in many situations

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Double-Hung Windows

Double-Hung windows are built with two sashes, both operable. The lower sash, set toward the interior, slides upward. The upper sash, set toward the outside, mirrors the lower sash but slides downward. Both use engineered tracks to compensate for the weight allowing the open windows to stay fixed in any position. In the open position, the two panes overlap in the middle of the frame, with not no projection into the space. Double-hung windows are a great design choice in buildings looking to provide natural ventilation and especially effective for smaller spaces.

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Horizontal Pivot Windows

A less common variation of the double-hung window is the horizontal pivot window. In a horizontal pivot window, a single sash pivots on a horizontal axis bringing the upper portion into the building and the lower portion out of the building. The effective ventilation area is generally larger than any other window type. If the space is available to accommodate the sweep of the sash, than horizontal pivot windows can be great design option for natural ventilation in rooms which don’t get a cross breeze.

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What to look out for

In single hung windows, the lack of projecting elements can make the window safer in situations where people may be walking close to the window, where furniture or other elements want to sit next to the window or where there is a risk of wind catching a sash projecting outward.

The same is true for double hung windows staying safely out of the way. Plus, the combination of two separate openings and lack or projecting elements makes the double-hung window unique in its ability to provide natural ventilation.

The advantage of pivot windows over a traditional double-hungs is that there are two much larger openings for air to move through while maintaining the functionality of hot air rising through the top opening and cool air entering through the lower opening. The disadvantage is that the open window projects both into and out of the building requiring significant clearance to function effectively. Consider that if planting outside of furniture inside sit too close, the window may become blocked from functioning.

Deep Dive

The Monadnock Building in Chicago, famous for its load bearing brick construction is also popular for having offices with operable windows.

Double Hung windows were ubiquitous in pre-war architecture, not because they were fashionable but because they were functional. After the introduction of air conitioning, we've mostly reduced double hung windows to a visual style. We have collectively forgotten their usefulness! A web search on the difference between a single hung and double hung mainly gets you results saying that a double hung has twice as many moving parts as a single hung. A better answer is that double hung windows were designed to accommodate natural ventilation in buildings which didn’t have air conditioning and didn’t have a layout which accommodated a cross breeze.

Projects with design objectives which align with the double hung aesthetic - whether to please design review boards, match a historic neighborhood or simply to seek out the classic look that double hung windows provide - sometimes have the option to choose the cost savings which single hung windows provide. Those projects may benefit from cross ventilation options and not need the ventilation advantage of double hung windows over single hung windows.

As architects expand their toolkit of passive energy saving options, remember the double hung’s ability to open the windows both high and low. In spaces without cross ventilation pulling air through a space, natural convection occurs because hot air rises. In most windows, which have a single opening, turbulence created by air entering and exiting in the same place significantly limit the ventilation potential of an opening. Adding additional windows side by side multiplies the problem, it doesn’t solve it. By contrast, in a double hung window the two openings are separated from each other vertically so that each provides one way flow for different temperatures of air.

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