This month's Green Building Tip examines three retrofit strategies for reducing heating and cooling losses through windows: storm windows, window films, and insulating blinds.

March marked the second Harvard FAS Operations and Maintenance Forum, pictured above, where we discussed fenestration and means of improving their environmental performance. According to the Whole Buidling Design Guide, "unwanted heat losses and gains through windows... cost the United States $20 billion" each year, or roughly 25% of all the energy used for space conditioning.

Window films are relatively inexpensive, quick to install, and can significantly reduce problems related to solar heat gain and glare, particularly on facades facing South, East and West.

Brand names typically include a number that represents the amount of visible light the film will allow to pass through it (e.g. the NV25 film from 3M allows ~25% of the visible light from one side to pass through to the other), but there are other factors that need to be considered to determine the full performance impact a window film may have. Some specialty films feature low-E coatings that improve the emissivity rating of the window to prevent energy losses from thermal conduction in addition to reductions in solar heat gain found in most films.  

Issues to Consider

• Historic buildings may need additional approval to change the appearance of facades. 
• Installation during unoccupied periods may result in building occupants not even noticing the switch.
• Highly absorptive films may cause thermal stress on glazing that has the potential to crack or shatter individual panes.
• Should the film need to be removed, it is strongly recommended to hire the installing contractor for removal rather than attempt to pull them down yourself.

Storm windows, either on the exterior or interior side of existing glazing, have the potential to dramatically improve the thermal performance of existing single pane fenestration, especially if the existing windows are poorly sealed and allow significant air intrusion. 

A 2010 study examining field performance of storm windows on six homes determined found heating load reductions from 13% (clear glass storms) to 21% (low-e glass storms) compared to homes with single panes only.  Though initially more expensive, the retrofits featuring low-e coatings had simple payback periods that were less than half as long as the clear glass storm windows tested.  

Issues to Consider

• Historic buildings may want to consider storm windows designed for interior installation to reduce changes to the facade.  
• Designs will need to consider and address potential condensation issues, specifically working to seal windows well to prevent warm, humid air from contacting the cold side of the window.
• If installing storms on operable windows, consider how they will be used in swing seasons.
• It's possible to purchase storm windows with window films or low-E coatings mentioned above.

According to the Department of Energy, the installation of new or better management of existing blinds, curtains, or shades can potentially reduce heat transfer through windows from 10-33%. Shades can be purchased that provide an R-value of 4. 

Issues to Consider

• In winter months, encourage occupants to open shades while the window is exposed to sunlight, and close them when dark. Ideal shades for winter would be dark and absorptive.  
• In summer months curtains can be drawn all day long to reflect heat when exposed to sunlight and prevent thermal conductance when no daylight is present. Ideal shades for the summer are bright and reflective.
• Magnetic or velcro tapes can help to seal the edges of drapes to the perimeter of the window to increase their effectiveness. 

All Harvard community members are encouraged to join us Thursday, April 12th from 3:30-5:30 in Sanders Theater as we celebrate environmental efforts from Harvard teams and individuals over the past year.  Lester Brown, Founder of the Earth Policy Institute, HKS '62 will be sharing his experiences as the keynote speaker in addition to other entertainment, awards, and student performances.  Reception to follow in the Queen's Head Pub.

Click here for more details including a full list of presenters, performers, and nominees!