Windows, storefronts, glazed doors, and skylights let in daylight and heat. They are also the thinnest part of an insulated envelope, so Section C402.4 limits how much glass a building may have, how well it must insulate, and how much sun it may admit. This lesson uses Climate Zone 2 numbers, because Houston and most of the GCHS service area sit in Zone 2A (Table C301.1 lists Harris, Montgomery, Galveston, Brazoria, Fort Bend, and the other Gulf Coast counties there). Local adoption governs which edition applies (Lesson 1).
The 30 percent cap is figured on the whole building. This clinic's storefront front is mostly glass, yet the building total is 486 of 2,400 square feet. The overhang section shows how a permanent shade raises the projection factor and loosens the SHGC limit.
What counts as fenestration
Fenestration is windows, glazed doors, glazed block, and skylights. The definitions split it by slope. Vertical fenestration is installed at 60 degrees or more from horizontal. A skylight is glazing at less than 60 degrees, including sloped glass in sunrooms and greenhouses. The split matters because each group has its own area cap and its own rows in Table C402.4. For the area cap, opaque doors and opaque spandrel panels do not count as glass, but glazed doors do. Opaque doors instead count as part of the gross wall area (C402.4.5).
Maximum area
Section C402.4.1 caps vertical fenestration at 30 percent of the gross above-grade wall area, and skylights at 3 percent of the gross roof area. The percentage is the building total: add up all the qualifying glass and divide by all the above-grade wall.
Worked example. A one-story clinic is 60 feet by 40 feet with 12 foot walls, so the gross wall area is 2 x (60 + 40) x 12 = 2,400 square feet. The front has four fixed storefront lites of 12 x 8 feet (96 square feet each, 384 total) and a glazed door pair of 6 x 7 feet (42 square feet). The side walls hold four operable windows of 3 x 5 feet (60 square feet total). Vertical fenestration is 384 + 42 + 60 = 486 square feet, and 486 / 2,400 = 0.203, or 20.3 percent. The cap would allow 0.30 x 2,400 = 720 square feet, so the project passes with 234 square feet to spare. The front elevation alone is 426 / 720, about 59 percent, and that is acceptable because the cap is measured on the whole building.
Trading area for daylight
Section C402.4.1.1 lets vertical fenestration rise to 40 percent of the wall area in Zones 0 through 6 if all three of these conditions are met. First, in a building of two stories or fewer above grade, at least 50 percent of the net floor area must lie in a daylight zone; in a building of three or more stories, at least 25 percent. Second, daylight responsive controls must be installed in the daylight zones. Third, the visible transmittance (VT) of the glass must be at least 1.1 times its SHGC. Skylights follow the same idea: up to 6 percent of the roof area if daylight responsive controls serve the toplit zones (C402.4.1.2). Daylight zones themselves are defined by the lighting section, C405.2.4 (Lesson 9).
Table C402.4 for Zone 2
Section C402.4.3 sends every product to Table C402.4. Lower U-factor means less heat conducted through the glass; lower SHGC means less solar heat admitted. In a cooling-dominated Zone 2 the SHGC limit does most of the work. Here are the Zone 2 rows, rebuilt with only the values this lesson teaches:
Product (Zone 2)
Max U-factor
Max SHGC, PF under 0.2
Max SHGC, PF 0.2 to under 0.5
Max SHGC, PF 0.5 or more
Fixed vertical fenestration
0.45
0.25
0.30
0.40
Operable vertical fenestration
0.60
0.23
0.28
0.37
Entrance doors (glazed)
0.77
The table lists a U-factor row only
Skylights
0.65
SHGC 0.30 (no projection factor)
Key numbers
Vertical fenestration: 30 percent of gross above-grade wall; skylights: 3 percent of gross roof (C402.4.1)
With daylight controls: 40 percent glass in Zones 0 to 6; 6 percent skylights (C402.4.1.1, C402.4.1.2)
Zone 2 SHGC, fixed: 0.25, 0.30, 0.40 as PF rises; operable: 0.23, 0.28, 0.37
Zone 2 skylights: U 0.65, SHGC 0.30
Projection factor PF = A / B (C402.4.3)
Sectional doors with one row of glass: U 0.440 or less when glass is 14 to 25 percent of the door (C402.4.5.2)
Projection factor and shading
A permanent overhang, eave, or attached shade lets a window take a higher SHGC. The projection factor is PF = A / B. A is the horizontal distance from the outer edge of the shade to the glass. B is the vertical distance from the bottom of the glass to the underside of the shade. Windows with different PF values are evaluated separately. In the figure, a 3 foot overhang sits 7 feet above the bottom of the glass, so PF = 3 / 7 = 0.43. That lands in the middle row, and fixed glass may have an SHGC up to 0.30 instead of the 0.25 that a bare wall requires. A PF of exactly 0.2 belongs to the middle row, and 0.5 belongs to the top row, because the table uses "under" at the upper end of each range.
Area-weighted U-factor
Section C402.4.3.4 lets a designer average U-factors by area within one product category. Fixed windows can be averaged with other fixed windows, but products from different table categories, such as fixed windows and entrance doors, may never be blended. Example: 200 square feet of fixed glass at U 0.40 and 100 square feet at U 0.55 gives (200 x 0.40 + 100 x 0.55) / 300 = 135 / 300 = 0.45. That equals the fixed limit, and because the table value is a maximum, it passes.
In the field
A strip-center rehab replaces an old storefront with stock aluminum glazing. The plans call for the Zone 2 limits, but the supplier ships glass with a higher SHGC because it was cheaper. The inspector reads the label on each lite, compares U-factor and SHGC with the window schedule, and checks whether a permanent canopy really provides the projection factor the designer counted. If the canopy is only 1 foot deep, the higher SHGC row is not earned. Catching it at delivery costs a phone call; catching it after installation costs new glass.
Skylights: minimums, limits, and haze
The code both caps and requires skylights. Section C402.4.2 requires them in large enclosed spaces: over 2,500 square feet, with at least 75 percent of the ceiling higher than 15 feet, and used as an office, retail store, warehouse, gymnasium, or a similar listed use. The toplit daylight zone must be at least half the floor area, and the skylight area, aperture, and VT must meet one of two options. Exceptions include Zones 6 through 8, spaces with lighting power density under 0.5 W per square foot, and storm shelters. Skylights over offices, storage, retail, and similar spaces need a diffusing material with haze over 90 percent (C402.4.2.2), and daylight responsive controls are required in toplit zones (C402.4.2.1). Where controls serve the zone below, skylights may have SHGC up to 0.60 and, in Zones 0 through 3, U-factor up to 0.9 (C402.4.3.1, C402.4.3.2).
Doors and dynamic glazing
Opaque swinging doors follow the opaque tables (Lesson 3). Glazed doors follow the fenestration rules above. Horizontally hinged sectional doors with one row of glass may instead meet a U-factor of 0.440 or less in Zones 0 through 6, provided the glass is 14 to 25 percent of the door area (C402.4.5.2). Dynamic glazing, such as electrochromic glass, may satisfy SHGC only if the ratio of its higher to lower labeled SHGC is at least 2.4 and it is automatically controlled in multiple steps, unless both of its labeled SHGC values already meet the table; it is judged separately and may not be averaged with ordinary glass (C402.4.3.3).
Practice questions
1. A Group B medical office in Climate Zone 2A has 9,000 square feet of gross above-grade wall area and no daylight responsive controls. What is the most vertical fenestration area allowed by IECC Section C402.4.1?
3,600 square feet, which is 40 percent of the wall area
1,800 square feet, which is 20 percent of the wall area
2,700 square feet, but only if all of the glass is fixed
2,700 square feet, which is 30 percent of the wall area
Answer: d. C402.4.1 caps vertical fenestration at 30 percent of the gross above-grade wall area: 0.30 x 9,000 = 2,700 square feet. The 40 percent figure needs daylight zones, controls, and VT of at least 1.1 times SHGC.
2. An inspector reviews a Group S-1 warehouse in Zone 2A with a gross roof area of 20,000 square feet. The plans show 900 square feet of skylights and no daylight responsive controls. What is the finding?
Over the 3 percent limit of 600 square feet; controls in the toplit zones would allow up to 6 percent
Complies, because the skylight limit is 6 percent of the roof area in every case, with or without controls
Over the limit, and nothing can raise it above 600 square feet, even with daylight responsive controls
Complies, because skylight area is limited only in Zones 5 through 8, where heating dominates
Answer: a. C402.4.1 limits skylights to 3 percent of the gross roof area: 0.03 x 20,000 = 600 square feet. C402.4.1.2 allows 6 percent (1,200 square feet) if daylight responsive controls serve the toplit daylight zones.
3. Fixed storefront glazing with no overhang is being installed in a Group M retail store in Climate Zone 2A. What is the maximum SHGC under Table C402.4?
0.30
0.25
0.40
0.23
Answer: b. With no permanent shading the projection factor is under 0.2, and Zone 2 fixed fenestration is limited to SHGC 0.25. 0.23 is the operable value for the same row; 0.30 and 0.40 need larger projection factors.
4. Operable windows in a Zone 2A fellowship hall sit under a permanent overhang that projects 4 feet. The distance from the bottom of the glass to the underside of the overhang is 10 feet. What is the maximum SHGC?
0.23, because PF is under 0.2
0.37, because PF is 0.5 or more
0.30, because operable windows use the fixed column
0.28, because PF is 4 / 10 = 0.4
Answer: d. PF = A / B = 4 / 10 = 0.4, which is in the 0.2 to under 0.5 row. The operable value in that row for Zone 2 is 0.28; the fixed value is 0.30.
5. A pharmacy in Zone 2A installs operable windows labeled U-factor 0.55. What is the result under Table C402.4?
Complies, because the maximum for operable windows in Zone 2 is 0.60
Fails, because the maximum for every kind of window in Zone 2 is 0.45, fixed or operable
Fails, because operable windows must meet the 0.77 limit used for entrance doors
Complies only if a permanent overhang gives a projection factor of at least 0.2
Answer: a. Zone 2 limits are 0.45 for fixed, 0.60 for operable, and 0.77 for entrance doors. A U-factor of 0.55 is below the 0.60 maximum, and the U-factor limit does not depend on shading.
6. A glazed entrance door on a Zone 2A clinic carries a label showing U-factor 0.80. Is it acceptable under Table C402.4?
Yes, because the entrance door maximum in Zone 2 is 0.83
Yes, because entrance doors are exempt from the U-factor limits
No, because the entrance door maximum in Zone 2 is 0.77
No, because entrance doors must meet 0.45 like fixed windows
Answer: c. Zone 2 allows entrance doors up to U 0.77. The value 0.83 belongs to Zones 0 and 1. A U-factor of 0.80 is higher (worse) than the limit.
7. A designer wants to average fixed windows at U 0.40 with glazed entrance doors at U 0.75 on a Zone 2A project. What does Section C402.4.3.4 say?
Products from different Table C402.4 categories may not be combined in one average
Any fenestration on the project may be averaged together by area, whatever its category
Area-weighted averaging is not permitted for any fenestration in commercial buildings
Averaging is allowed only when the entrance doors are opaque swinging doors
Answer: a. Area-weighted averaging is allowed within each product category, but fixed, operable, and entrance door products cannot be blended across categories.
8. A two-story Group E school in Zone 2A uses the 40 percent glass option of Section C402.4.1.1. What share of the net floor area must be in a daylight zone?
At least 25 percent, with controls installed only in the classrooms
At least 50 percent, with daylight responsive controls installed in those zones
At least 75 percent, with controls in every space
At least 40 percent, matching the glass percentage
Answer: b. For buildings of two stories or fewer above grade the daylight zone must be at least 50 percent of the net floor area; for three or more stories it is 25 percent. Controls and VT of at least 1.1 times SHGC are also required.
9. In the 40 percent glass option, fixed glazing in a Zone 2A library has SHGC 0.25. What is the lowest VT allowed?
0.25, the same as the SHGC
0.225, which is 0.9 times 0.25
0.30, the middle-row SHGC limit for Zone 2
0.275, which is 1.1 times 0.25
Answer: d. C402.4.1.1 requires VT of at least 1.1 times SHGC. 1.1 x 0.25 = 0.275. The point is that the glass must pass more light than heat.
10. A plans reviewer checks a new 40,000 square foot Group S-1 nonrefrigerated warehouse in Zone 2A with 24 foot ceilings over the whole floor. Which skylight rule applies?
None, because skylights are only ever optional in Zone 2
C402.4.2 requires skylights over 10 percent of the roof with no exceptions
C402.4.2 requires skylights with a toplit daylight zone of at least half the floor area, unless an exception applies
Skylights are required only in Zones 6 through 8
Answer: c. The space is over 2,500 square feet, has ceilings over 15 feet, and is a listed use. C402.4.2 applies, with exceptions such as low lighting power density. Zones 6 through 8 are excepted from the skylight requirement; they are not the only zones that must meet it.
11. A sunroom addition at a community building has a glass roof sloped 45 degrees from horizontal. How does the energy code classify that glass?
As vertical fenestration, because it is glass that is part of an exterior wall assembly
As a skylight, because it is at a slope of less than 60 degrees from horizontal
As vertical fenestration, because the slope is more than 30 degrees from horizontal
As a skylight, because it is above the second story of the building
Answer: b. Vertical fenestration is at 60 degrees or more from horizontal. Sloped glazing below that, including sunroom and greenhouse glass, is a skylight and counts against the roof-area cap.
12. A Zone 2A fire station has horizontally hinged sectional doors with a single row of glass that is 20 percent of each door. Under Section C402.4.5.2, what maximum assembly U-factor applies?
0.440, because the glass is between 14 and 25 percent of the door
0.360, the value that applies to sectional doors in every climate zone
0.77, the Zone 2 entrance door value used for glazed swinging doors
0.45, the Zone 2 fixed fenestration value that all glazed doors must meet
Answer: a. Sectional doors with one row of glass may use U 0.440 or less in Zones 0 through 6 when the glass is 14 to 25 percent of the door. 0.360 applies only in Zones 7 and 8.
13. A Group E school in Zone 2A has 4,800 square feet of gross above-grade wall. It has 1,300 square feet of windows, a 150 square foot opaque steel door, and 80 square feet of opaque spandrel panels. What is the fenestration percentage used for the cap, and does it comply?
31.9 percent, which fails because the door and spandrel panels count as glass
27.1 percent, which complies with the 30 percent cap
27.1 percent, which fails because Zone 2 limits glass to 25 percent
24.0 percent, which complies because the door counts as wall only
Answer: b. Opaque doors and opaque spandrel panels are not counted as fenestration. 1,300 / 4,800 = 0.271. That is under 30 percent.
14. A designer wants dynamic glazing to satisfy the SHGC limit on a Zone 2A office. Which requirement applies under Section C402.4.3.3?
The glass may be averaged with ordinary glass by area
The higher labeled SHGC must be under 0.25 and manual control is enough
The higher labeled SHGC divided by the lower must be at least 2.4, with automatic multi-step control
The ratio must be at least 1.1 and the glass must have a skylight rating
Answer: c. Dynamic glazing is judged separately, needs a labeled SHGC ratio of at least 2.4, and must be controlled automatically in multiple steps. It may not be area-averaged with non-dynamic glass.
Glossary
Fenestration
Windows, glazed doors, glazed block, and skylights: the glass parts of the envelope.
Vertical fenestration
Glazing installed at a slope of 60 degrees or more from horizontal, including glazed doors.
Skylight
Glazing installed at a slope of less than 60 degrees from horizontal, including sunroom and greenhouse glass.
U-factor
The rate at which heat passes through an assembly. Lower is better, and Table C402.4 sets maximums.
SHGC
Solar heat gain coefficient: the fraction of sunlight that becomes heat inside. Lower admits less solar heat.
Projection factor (PF)
The overhang projection A divided by the vertical distance B from the glass bottom to the overhang underside.
Visible transmittance (VT)
The fraction of visible light that passes through the glass, used in the 40 percent glass option.
Daylight zone
The floor area near windows or under skylights where daylight can replace electric light, defined in Section C405.2.4.