Window retrofits

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I have heard recommendations that windows be double paned, but in some cases a single tempered pane is required and in others 2 tempered panes. Does anyone know why there is a difference?

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Thank you very much for your question!

According to the IBHS:

  • A double-paned window doubles resistance to fire compared a a single-paned one.
  • Double tempered double-paned windows double again their resistance compared to double-paned windows with no tempering.

According to the latest version of the California Residential Code:
"Exterior windows, skylights and exterior glazed door assemblies shall comply with one of the following requirements:

  1. Be constructed of multipane glazing with a minimum of one tempered pane meeting the requirements of Section R308 Safety Glazing, or

  2. Be constructed of glass block units, or

  3. Have a fire-resistance rating of not less than 20 minutes when tested according to NFPA 257, or

  4. Be tested to meet the performance requirements of SFM Standard 12-7A-2."

[source: 2022 California Residential Code, Title 24, Part 2.5 with July 2024 Supplement - CHAPTER 3 BUILDING PLANNING - R337.8.2.1 Exterior windows, skylights and exterior glazed door assembly requirements.]

However, the new IBHS certification standards call for a higher standard for their Wildfire Prepared Home Plus Designation (their highest certification standard), which some insurers (such as CSAA) now require as a condition to re-acquiring home insurance coverage after a non-renewal. Home Plus certification requires:

Windows
☐ Replace all exterior windows with tempered, multipaned glass (at least 2 panes tempered) or fire-resistant glass blocks.
☐ Replace domed, plastic skylights with flat, multipaned, tempered glass skylights.

Doors
☐ Upgrade to solid exterior doors that have a metal threshold and are constructed with a noncombustible or ignition-resistant material such as metal, fiberglass, or solid hardwood.
☐ If you choose a door that includes glass, make sure it is made with tempered, dual pane glass.
☐ Alternatively, install a noncombustible storm door to cover the existing door. These are fire-resistant.

[From the IBHS Self-Inspection checklist (PDF)]

Nobody knows how and when California will enforce tougher standards than today. It is now clear, however, that enforcement of tougher requirements will come much faster through our insurance companies. It would be short-sighted, today, for homes in the wildland urban interface to install anything else than windows, skylights and glass doors with double-paned double-tempered glass.