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Community and Q&A

Dual pane window with 1 tempered pane – should it be inside or outside?

RobInNorCal | Posted in General Questions on

I’m placing the window order for my ‘good enough’ house and have been through several iterations of colors/wood/Low-e variations, etc. Through 3 iterations, the glazing was always specified as “Tempered Inside Pane only/Fire Glass” but the most recent and (almost) final quote has all windows listed “Tempered is Outside Pane”. (The house is in a WUI (woodlands-urban interface) area, hence the ‘one tempered pane’ requirement). Do I care whether the tempered pane is inside or outside? I searched both this site and the web for recommendations or requirements and came up empty… The only guess I came up with was that there might be a small safety benefit to having the tempered glass on the inside.
Thanks, Rob

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  1. Expert Member
    Michael Maines | | #1

    Rob, the building code that requires safety glazing in your situation should state what panes need to be tempered. It's not a requirement in the IRC. The IRC requires the interior pane to be safety glazing in certain situations, but a WUI is not one of them. I would assume that the perceived danger is from wildfire, so the outer pane would be the one you want tempered, but the code should specify, or your building official should be able to tell you.

  2. RobInNorCal | | #2

    Thanks, Michael. California code was first thing I checked, it was not helpful:
    704A.3.2.2 Exterior glazing and window walls. Exterior
    windows, window walls, glazed doors, and glazed open-
    ings within exterior doors shall be insulating-glass units
    with a minimum of one tempered pane, or glass block
    units, or have a fire-resistance rating of not less than 20
    minutes, when tested according to ASTM E 2010, or con-
    form to the performance requirements of SFM 12-7A-2

    Or, another way to view is, I'm legal either way... But still wondering if there's a right choice here?

  3. Expert Member


    I suspect that the reason it isn't specified is because it doesn't matter. They want a 20 minute rated window, and how that is achieved isn't important. Practically, while the rating may save your house, I doubt whether the location of the tempered pane will make much of a difference saving the window seals from exposure to the fire. The units would probably have to be replaced anyway.

  4. Expert Member
    Michael Maines | | #4

    I agree with Malcolm; it reads to me like it's specifically your choice. If you need tempered glazing at some interior locations, you may want to keep all of the windows the same, with the interior pane tempered.

  5. RobInNorCal | | #5

    Agree that from a fire protection standpoint it's probably irrelevant, Malcolm - California will regulate anything for almost any reason, so there must not be any substantiating data one way or the other - so we're back to the common sense question: given that the windows will have one tempered pane, and that I can choose to put it on the inside or the outside at no cost penalty (price is the same either way), which one would you/should I choose?

    Agree it isn't the biggest question of the day, let alone all time, but it's sunday morning and i'm waiting for the rain to stop in the North Bay so we can start digging...

    Added: to explore Michael's answer - why would having tempered glass on the outside during a fire make more sense? As I understand it, imploding windows during a flashover are the third most likely cause of catastrophic results from wildfire (behind embers getting trapped in venting and combustibles too close to the building) ; is tempered glass less likely to implode?

  6. Expert Member


    The short answer is I don't know. The tempered glass can resist heat longer, so if it was on the outside it might protect the inner-pane. Conversely, perhaps having it on the inside would allow the regular glazing to act as a sacrificial surface, protecting the tempered glass or some initial period of exposure.

    The tests will have been done with complete assemblies and probably don't parse out small differences like that. A 20 minute rating, while perhaps appropriate for the circumstances, is really not much of anything. Other 20 minute rated things, like doors, aren't much different from unrated ones.

  7. RobInNorCal | | #7

    I think I'll make them revert to "tempered inside". My reasoning is that if there were a significant fire safety reason to put it outside, Cal Fire would require it; and the chances of someone breaking a window is slightly higher inside than outside the house.

    Put this on the long list of decisions no one knows they're making when they set out to build a home...

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