Apartment Window Replacement Under Title 24

Replacing windows in an existing multifamily building is a fenestration alteration under Section 180.2(b)1C. Like-for-like replacements meet the maximum U-factor and RSHGC of Table 180.2-B for the climate zone, or an area-weighted equivalent from Table 170.2-A, and where 150 square feet or less of the building's vertical glazing is replaced, only the U-factor applies.

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Repair or alteration

The code draws the line at the frame. A note to Section 180.2(b)1C treats new glass in an existing sash and frame, or new sashes in an existing frame, as repairs, which must be at least equivalent in performance to the original. Replacing the whole window is a fenestration alteration. The Multifamily Compliance Manual adds that replacement fenestration need not go in the same openings, as long as it replaces an equal or smaller area in the same existing wall or roof; any extra area counts as new.

Duplexes and townhouses are single-family buildings under Section 100.1, so their windows follow the single-family rules on windows and Title 24; see also townhome Title 24.

Like-for-like replacement

Windows replacing the same total area meet either the per-window values of Table 180.2-B or an area-weighted U-factor and RSHGC from Table 170.2-A across all the replaced windows. The table has rows for curtainwall and storefront, for NAFS Performance Class AW fixed and operable windows, for skylights, and for all other windows and glazed doors, where most ordinary apartment windows fall. For that row:

  • Maximum U-factor 0.28 in climate zones 1, 3, 4, 5, 11, 13, 14 and 16.
  • 0.30 in zones 2, 6, 8, 9, 10, 12 and 15.
  • 0.34 in zone 7.
  • Maximum RSHGC 0.23 everywhere except zones 1, 3, 5 and 16, which have no RSHGC requirement.

Where 150 square feet or less of the entire building's vertical fenestration is replaced, the RSHGC and VT requirements fall away and only the U-factor remains. Minimum VT applies only to the curtainwall, storefront and AW rows and to skylights serving common areas, and the window VT minimums do not apply to buildings of three habitable stories or fewer.

Adding window area

New vertical glazing or skylight area meets the total fenestration area limits of Section 170.2(a)3 and the Table 180.2-B values. Adding up to 50 square feet of vertical fenestration is exempt from the area limits and from RSHGC and VT, though not from the U-factor. Up to 16 square feet of new skylight per dwelling unit, at U-0.55 and RSHGC 0.30 or better, is exempt from the area limits.

Limits that apply to every window

Section 160.1(e) caps fenestration at a U-factor of 0.58, or an area-weighted average of 0.58, except in buildings built to Part 7 of the Building Code in fire hazard severity zones or wildland-urban interface areas. Under Section 110.6(a), each product must be certified, leak no more than 0.3 cfm per square foot, and carry an NFRC-rated U-factor and SHGC or the default values of Tables 110.6-A and 110.6-B.

Performance route and paperwork

On the performance path of Section 180.2(c), each replaced window's standard design is the more efficient of the existing window and the Section 180.2(b) value, so a better window elsewhere can offset one that misses. A prescriptive replacement is documented on LMCC-ENV-01-E up to three habitable stories and NRCC-ENV-E from four; the full set is on multi-family Title 24 forms, and wider scopes on multifamily remodel Title 24.

Common Title 24 Questions

  1. Do apartments need Title 24?
  2. Does a duplex need Title 24?
  3. Do triplexes and fourplexes need Title 24?
  4. Which Title 24 forms does a multi-family project use?
  5. What is an LMCC certificate?
  6. How much does a multi-family Title 24 report cost?

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