Work out your glazing percentage and see whether your design stays on the simple Title 24 prescriptive path or needs a full performance energy model.
Every result is a planning estimate, not a quote or an engineering determination. Real numbers depend on site conditions, existing structure, engineering requirements, and your jurisdiction’s current fee schedule. For structural and engineering work we coordinate with a licensed engineer — we do not perform or stamp engineering ourselves. Get a project-specific estimate before committing to a budget.
Window-to-wall ratio is glazing area divided by gross exterior wall area. It is an architectural number. It tells you how much of your facade is glass, which matters for daylight, for structural shear wall length, and for how the elevation reads.
Glazing-to-floor-area ratio is glazing area divided by conditioned floor area. That is the number Title 24 cares about. The residential prescriptive path generally caps total glazing at roughly 20% of conditioned floor area, with a separate cap on west-facing glazing. Go past it and the prescriptive recipe no longer applies.
The two move independently. A single-story house with a big footprint and modest walls can have a low window-to-wall ratio and still blow past the floor-area cap. A tall two-story with lots of blank wall can look glassy and still comply. Run both.
Exceeding the cap is not a failure. It just moves you from the prescriptive path to the performance path, where the whole building is energy modeled and extra glazing is allowed as long as it is paid for somewhere else — better window specs, more insulation, tighter ducts, a heat pump, or added photovoltaics. Most modern Bay Area designs with large sliders and window walls end up there deliberately.
Use rough opening dimensions. Glazing area for Title 24 is the whole fenestration assembly — glass plus frame — not the visible glass. Measuring the daylight opening undercounts you by roughly 10 to 15% and can turn a passing number into a failing one at plan check.
Count glass doors. Sliders, French doors and any door more than about half glass count as fenestration. A pair of 12-foot sliders is close to 130 square feet on its own, which is most of the budget on a 1,400 square foot house.
Watch the west. West-facing glazing carries its own prescriptive limit because late-afternoon sun drives peak cooling load. In inland zones like Dublin and Pleasanton that penalty is severe. Shifting glass from west to north or adding an overhang often costs less than upgrading the whole envelope to pay for it.
Skylights are separate. They have their own limit and their own U-factor and SHGC targets, and they are frequently forgotten until the correction letter arrives.
If your number lands over the cap, we can model the building on the performance path as part of the Title 24 CF1R energy calculations and reflect the resulting specifications in the drawing set, so the window schedule and the energy report agree before anything is submitted.
No. It is the threshold in the residential prescriptive path, which is the simplified compliance recipe. Exceeding it does not make a design non-compliant — it means compliance has to be demonstrated by energy modeling the whole building on the performance path instead.
The whole fenestration assembly, glass plus frame, at the rough opening. Measuring only the visible glass undercounts the area by roughly 10 to 15% and is a common source of plan check corrections.
Late-afternoon western sun arrives at a low angle when the building is already at its warmest, so it drives the peak cooling load harder than any other orientation. Title 24 caps it separately, and in inland climate zones the modeling penalty for a lot of west glass is significant.
More energy modeling work and a more detailed CF1R, usually a few hundred dollars more than a simple prescriptive report and a few extra business days. In exchange you keep the glazing you designed and trade it against better windows, insulation, equipment or solar.
Tell us what you're building and we'll get back to you with next steps — usually within one business day.
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Yuna Drafting