Mid-century modern window replacement is rarely a simple swap. In a post-and-beam house the glass is part of the architecture. The National Register nomination for Palo Alto's Greenmeadow tract describes floor-to-ceiling, wall-to-wall plate glass along the back wall, clerestory windows on the front and almost no windows facing the street. Change the glass and you change how every room feels.
The usual goals are less heat loss in winter, less heat gain in summer and fewer drafts, without bulky frames. This guide covers original windows, repair and retrofit options, window film, the 2025 Energy Code's rules for replacements, and how to keep the look.
What mid-century windows were made of
Metal frames defined the era. National Park Service guidance on historic steel windows explains that rolled steel windows stayed in wide use until after World War II, when cheaper, non-corroding aluminum windows became increasingly popular. The same guidance notes that postwar modernism depended on the slim profiles and streamlined look of metal windows for much of its impact.
In Greenmeadow, the nomination describes the tract's community building, which shares the houses' design language, as having steel-framed windows: horizontal-sliding in large sizes and vertical-sliding in small ones. For the houses, it describes a typical side window of roughly four by four feet on the bedroom side, floor-to-ceiling glass on the living room side, and clerestory glass set in the gaps between roof levels. Windows may have been replaced over the decades, so start with what your own house has:
Frame material. A magnet will stick to a steel frame but not to aluminum.
Single or dual glazing. Look at the edge of the glass for a spacer between two panes.
Which panels are fixed and which open, and how they open.
Any labels or etched marks in the glass corners or on the frame.
Rust, flaking paint, fogging between panes or failed glazing compound.
Repair and retrofit before replacement
Preservation guidance starts with the windows you have. The National Park Service's brief on steel windows says that in many cases repair and retrofit are more economical than wholesale replacement, and that replacement units are often unlike the originals in design and appearance. It identifies corrosion as the controlling factor in deciding whether a window can be repaired, and it warns that steel windows have historically been coated with lead paint.
The brief describes three ways to add a layer of glazing. Each gives roughly the same energy benefit, about double the original insulating value of the window, so cost and appearance usually decide:
A clear panel over the window. Rigid acrylic or glass is fixed over the original. It's the least expensive option, but a panel secured over the whole window makes the sash inoperable, so windows that need to open get separate panels attached to the sash.
A separate storm window, inside or outside, laid out to match the original sash configuration. Exterior storms add protection but partially obscure the original window.
Thermal glass in the existing sash. Replacing single glass with insulating glass keeps the window fully operational and preserves its appearance, but the brief calls it the costliest of the three. Steel sash sections are generally 1 to 1 1/2 inches thick and can normally take insulating glass 3/8 to 5/8 inch thick, if the hinges can carry the extra weight.
For scale, the brief cites a 1977 engineering handbook: single-glazed rolled steel sash has a U-value of 1.3, and adding storm windows or reglazing with insulating glass brings it to 0.69. Lower numbers mean less heat flow. The same brief also describes several types of weatherstripping for metal windows to cut air leakage.
Where window film fits
Window film is the lightest-touch option. California's Energy Code defines it as a fenestration attachment: a flexible, adhesive-backed polymer film applied to the inside or outside surface of existing glass. The U.S. Department of Energy suggests low-e film on the inside of windows to keep heat from radiating out, and notes that films are rated by the National Fenestration Rating Council (NFRC). The Energy Code says windows with film that isn't NFRC-rated must use the code's default values, so if film is part of a permitted energy project, ask for a rated product.
Before you choose a film, ask the installer:
Is the film NFRC-rated, and what are its U-factor and solar heat gain numbers?
Is it compatible with your glass, including dual-pane, tempered or laminated panels, and how does it affect any glass warranty?
How does it look from inside and outside? A sample on one pane, viewed in daylight, settles a lot of questions.
On older single glass, should the frame and glazing compound be repaired first?
What California's Energy Code requires of replacement windows
The 2025 Energy Code applies to permit applications filed on or after January 1, 2026. Its rules for existing homes draw a line between repair, replacement and added glass:
Glass-only work can be a repair. Glass replaced in an existing sash and frame, or sashes replaced in an existing frame, are considered repairs as long as the replacement is at least equivalent to the original in performance.
New windows must meet the prescriptive limits. New manufactured windows that replace existing ones of the same total area must meet the U-factor and solar heat gain coefficient (SHGC) values in the code's Table 150.1-A. In Climate Zones 3 and 4, the maximum U-factor is 0.27. Zone 4 adds a maximum SHGC of 0.23; Zone 3 has no SHGC limit.
Smaller replacements get more room. Replacing up to 75 square feet of vertical glass can use products with a U-factor up to 0.40, and an SHGC up to 0.35 in Climate Zones 2, 4 and 6 through 15.
Adding glass has its own rules. New window area must meet the U-factor and SHGC limits and the code's caps on total and west-facing window area, though adding up to 16 square feet is exempt from those area caps.
Climate zone matters. Pacific Gas and Electric's Climate Zone 4 sheet uses San Jose as its reference city and lists Sunnyvale, and the California Energy Commission's climate zone search tool confirms any address. When you compare products, read the NFRC label and use whole-unit U-factor and SHGC numbers, which describe the entire window, rather than center-of-glass values. Your building department applies the code to your project; this summary is for planning, not legal advice.
Other rules that come with a window project
Glass near doors and floors. The California Residential Code treats some window glass as a hazardous location, the category that calls for tempered, laminated or other approved glazing. One test covers a single pane larger than 9 square feet, with its bottom edge less than 18 inches above the floor, its top edge more than 36 inches above the floor and a walking surface within 36 inches. Floor-to-ceiling glass can fall into that group.
Bedroom escape openings. Under the 2025 California Residential Code, sleeping rooms need an emergency escape and rescue opening. City building handouts that quote the code list the minimums: a net clear opening of 5.7 square feet (5.0 for grade-floor openings), at least 24 inches high and 20 inches wide, with the bottom of the opening no more than 44 inches above the floor. A high clerestory can't meet that sill limit. The code exempts a replacement window from those size and sill rules when it is the manufacturer's largest standard size that fits the existing frame or rough opening, is the same operating style or one with an equal or larger opening, and isn't part of a change of occupancy. Ask your building department before you order.
Lead paint. In homes built before 1978, the U.S. EPA's lead renovation rule treats replacing a window of any size as a renovation, not minor repair, when the work is done for pay. Replacing a pane of glass can count as minor repair if the job stays within the rule's limits. Painted steel frames deserve particular care.
Permits and licenses. Ask your city whether your project needs a permit before you order. California licenses glazing contractors under the C-17 classification.
Tax credits. The IRS says the federal Energy Efficient Home Improvement Credit, which covered qualifying exterior windows and skylights, applied to improvements made through December 31, 2025. Ask a tax advisor about any incentive for a current project.
Keeping the look
Windows set the proportions of a mid-century house. The Greenmeadow nomination describes contributing houses as solid planar surfaces contrasting with large panes of glass. Its evaluation criteria count added street-facing windows that don't fit the original style, and window trim that clashes with the design, against a house's integrity. When you plan new windows:
Compare frame widths. A replacement frame wider than the original turns a glass wall into a grid.
Keep the same panel divisions and the same way of opening where you can.
Keep clear glass clear. Look at any tinted or coated glass in daylight before you order.
Skip decorative trim the original design didn't have.
Our page on mid-century modern design guidelines and our Eichler renovation guide show how window work fits into a larger remodel, and materials and construction covers the original building materials.
Frequently asked questions
Can I put dual-pane glass in my original frames?
Sometimes. Preservation guidance notes that steel sash 1 to 1 1/2 inches thick can normally take insulating glass 3/8 to 5/8 inch thick, if the hinges can carry the weight. Under the Energy Code, glass replaced in an existing sash and frame counts as a repair if it performs at least as well as the original. A glazing contractor can check your frames.
How does the Energy Code treat window film?
It treats film as a fenestration attachment, and windows with film that isn't NFRC-rated must use the code's default values. For a permitted energy project, choose a rated product and confirm the approach with your building department.
What U-factor do replacement windows need in San Jose?
PG&E's Climate Zone 4 sheet uses San Jose as its reference city. In Zone 4, full replacements must meet a maximum U-factor of 0.27 and a maximum SHGC of 0.23, while replacements of up to 75 square feet can use units up to U-0.40 and SHGC 0.35. Confirm with your building department before ordering.
Are storm windows worth considering?
Preservation guidance puts storm windows, clear panels and new thermal glass at roughly the same energy benefit, so cost and appearance usually decide. Exterior storms partially obscure the original window, which matters on a visible elevation.
Related reading
Mid-Century Sliding Glass Doors: Repair, Replace and Keep the Look
Eichler Skylights: Leaks, Replacement Options and Daylight Rules
Mid-Century Clerestory Windows and Window Walls: How They Work
Planning window work on a mid-century home, or weighing a house whose glass needs attention? Call or text the Boyenga Team at (408) 373-1660 or (650) 383-8606, or contact the Boyenga Team and we'll help you think through the options.
