Image 1: Metal panel or curtain wall system attached to a structure frame (Source: Berkeley Lab)


Windows and skylights are integral components of a building's fenestration system, crucial for sustaining a well-insulated building envelope. The fenestration system, outlining the arrangement of openings in a building envelope, dictates the HVAC needs and daylighting of a structure. Strategically positioning and choosing energy-efficient or high-performance windows optimize the influx of natural light, reduce glare, and minimize heat transfer, thereby reducing reliance on electric lighting and HVAC systems.
 High-performance glass encompasses various mature static technologies that effectively decrease heat flow across the glass. These technologies include multiple layers, low-emissivity glass, tinted glass, and vacuum glazing. Different technologies are suited to different applications based on their design and cost considerations.


Recommendations provided: 

  1. Upgrade windows 
  2. Upgrade skylights 
  3. Add Retrofit Film


Asset Score Report Recommendation: 

1. Upgrade to high-performance Windows 

Cost: $$ - $$$, based on glass and frame type


Window upgrades include a range of options, such as transitioning from single-pane to double-pane windows and replacing existing double pane windows with those having higher thermal performance. Additional strategies involve introducing shading, storm windows, or specialized films to mitigate solar heat gain. Compared to regular double pane windows, high-performance double pane windows can incorporate combinations of low- emissivity film, higher performance frames, and gas fill.

For north and south-facing windows, consider windows with low solar heat gain coefficient (SHGC) and an appropriate visible transmittance (VT). Certain window coatings, called selective low-e, transmit the visible portions of the solar spectrum selectively, rejecting the nonvisible infrared sections. These glass and coating selections can provide a balance between VT and solar heat gain. Higher SHGCs are allowed in colder regions, but installing continuous horizontal overhangs is still advantageous in that it blocks the high summer sun angles. Table 1 provides general guidelines for upgrading windows in different climate zones. Note that Table 1 only shows examples of combinations of glass, frame, and fill characteristics; other options are available and should be evaluated when considering a window upgrade.



Table 1 Vertical Fenestration Description


For buildings with operable windows, the mechanical systems should use interlocks to ensure that the HVAC system shuts down in the affected zone while the windows are open. A high level of integration between envelope and HVAC system design helps to maximize energy efficiency.


2. Upgrade to high-performance Skylights

Cost: $$


As skylights are positioned on the roof, they may lead to undesired heat gain during summer and heat loss from the interior of the home in winter. Considering to upgrade skylights can lead to energy efficiency and cost savings.

Upgrading to high-performance skylights involves a comprehensive approach similar to that for windows. This encompasses various options, such as transitioning from standard skylights to those with advanced thermal properties. Additional strategies include heat-absorbing tints, low-emissivity (low-e) coatings, and translucent insulation material inserted between multiple glazing layers. 


3. Add Retrofit Film 

Cost: $


If the expense associated with window replacement or upgrades proves to expensive as compared to the project's budget, an alternative consideration is the application of a low-e or sun control film to the fenestration to enhance the building's performance. A film is a flexible product typically composed of multiple layers of coated or chemically treated polyester, often with a lightly metalized layer for better solar properties that can be installed on glass. Films range in thickness from 1-mil to 15-mil. It's important to note, however, that although this measure is viable, it is not currently implemented by Asset Score.