Windows, glazed doors, and skylights do more than connect indoors and outdoors: they affect natural light, views, and a building’s heating and cooling loads. Although they do not consume energy themselves, they can increase the energy a heating and cooling system needs to maintain indoor conditions. Choosing an efficient opening is therefore not simply a matter of looking for the lowest label value: what matters is how the product performs in its climate and in the way the space is used.
Two measures for understanding thermal performance
The U-factor indicates how quickly heat passes through a glazing system. A lower value means less heat transfer. The solar heat gain coefficient (SHGC) indicates how much solar heat passes through the glazing: the lower the value, the less solar heat is transmitted.
These measures are related, but they are not interchangeable. U-factor concerns heat transfer between indoors and outdoors; SHGC concerns incoming solar energy. The right choice depends on the climate zone where the product will be installed. A strategy that is useful in a hot climate may offer little benefit in a cold one, where solar heat gain can play a different role.
The project’s orientation can also prompt consideration of light and heat as separate design decisions. Visible transmittance (VT) describes how much visible light passes through the glazing, while SHGC concerns transmitted solar heat. So a discussion of how much daylight to bring into a room cannot be settled by a solar heat gain figure alone. Product values help compare performance, but should be considered alongside the needs of each space.
Glazing composition shapes the experience
Construction features can affect U-factor. These include the number of glass panes, low-emissivity coatings, and the gas between the panes. Frames and sashes matter, too: less conductive materials such as wood, vinyl, or fiberglass can limit heat transfer; where metal predominates, the purchasing guidance emphasizes integrating thermal breaks.
The design of these components is not only a technical matter. Adding layers and coatings can change the weight and composition of the assembly. Glass tint also involves a trade-off: tinting can reduce solar heat gain, but it makes it harder to see clearly through the glass. For that reason, the federal guidance does not recommend tinted glass for residential buildings.
Controlling solar gain does not depend on glass alone. Overhangs or exterior sunscreens, as well as interior blinds or shades, can also reduce unwanted solar heat gain. Indoors, these elements also help regulate light and views; their effect depends on the space and how it is used.
Climate changes what “efficient” means
ENERGY STAR specifications for residential windows, doors, and skylights take climate zones into account. The purchasing guidance’s general recommendation is to identify the installation zone first, then compare the product’s U-factor and SHGC with the levels applicable to that zone. An efficiency label does not replace this contextual assessment.
The ENERGY STAR program updated its residential specifications to Version 7.0, announced in 2022. The update raised performance benchmarks and drew, among other work, on analyses by the U.S. Department of Energy and Lawrence Berkeley National Laboratory. One product type highlighted in this context is the triple-pane window, which has a third pane and two insulating cavities. The Department of Energy source also describes thin triple-pane options developed with support from public research and industry collaboration.
That evolution does not mean that the same solution is right for every home. The differences between options depend on climate, product characteristics, and what is prioritized in each room: thermal control, light, views, or reduced exterior noise. The performance of a particular model should be checked in its certification information, rather than inferred solely from having more panes.
Choosing also means deciding what to prioritize
Energy efficiency is part of the decision, but not the only part. Appearance, durability, maintenance, sound control, ventilation, thermal comfort, glare, and material fading are all relevant when selecting openings. A façade may call for a different approach from a room intended to receive daylight; a skylight and a glazed door do not necessarily serve the same spatial role, either.
A sequence of observations can help organize the questions, without replacing product evaluation or project design:
- Location and climate: identify the installation climate zone.
- Performance: check U-factor and SHGC; also consider visible transmittance when daylight is important.
- Room use: consider heating, cooling, light, views, and solar control.
- Construction assembly: look at glazing, frame, sash, and exterior or interior shading options.
- Other criteria: compare appearance, maintenance, durability, ventilation, and noise control.
The federal guidance concerns purchases of residential windows, doors, and skylights that meet the applicable product definitions. It does not cover every system assembled on site or products intended for nonresidential buildings. For covered commercial products, certification and labeling by the National Fenestration Rating Council (NFRC) provide access to performance data; applicable directories and requirements can vary by category and climate zone.
In a design project, the aim is not to choose an isolated number, but to understand what each opening does in context: how much heat it lets through, how much light it brings in, and how it contributes to the façade and everyday life of the space.
At a glance
Scope
Editorial overview; not a site visit or technical assessment.
Sources and references
Purchasing Energy-Efficient Residential Windows, Doors, and Skylights ↗23/10/2026 U.S. Department of Energy, “Purchasing Energy-Efficient Residential Windows, Doors, and Skylights.”
EPA’s New ENERGY STAR® Specifications for Windows, Doors, and Skylights Made Possible through DOE Investments and Analyses ↗23/10/2026 U.S. Department of Energy, “EPA’s New ENERGY STAR® Specifications for Windows, Doors, and Skylights Made Possible through DOE Investments and Analyses.”
Purchasing Energy-Efficient Residential Storm Windows ↗25/10/2026 U.S. Department of Energy, “Purchasing Energy-Efficient Residential Storm Windows.”