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Community and Q&A

Autoclaved Aerated Concrete and Window Shattering

HeatherCFS | Posted in General Questions on

Our Autoclaved Aerated Concrete (AAC) home is 18 years old and we are in the American southeast (3a but we can see 4a from here).

We have just had our second (double pane) window spontaneously shatter in the last 18 months.  The first was on the east side, this one on the south.

No obvious impact or weather. Any ideas?

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  1. Expert Member
    NICK KEENAN | | #1

    What's an AAC home?

    1. HeatherCFS | | #6

      Aerated autoclave concrete (hebel block)

  2. walta100 | | #2

    Can we assume the windows that shattered were tempered glass?

    1. HeatherCFS | | #5


    2. HeatherCFS | | #7


  3. user-2310254 | | #3

    Autoclaved Aerated Concrete, correct? Are these vinyl windows on a west or southern exposer?

  4. onslow | | #4


    Curiously, I recently delivered a window to my next door neighbor whose AAC home has had the seals go on several of his windows. Not shattered, just the seals giving out and condensation forming between the panes. We are at high elevation and also have strong thermal cycling conditions, so I had not considered his autoclaved aerated concrete block to be a risk factor.

    You haven't mentioned the window size, so I would first guess the offenders are in larger openings. The real puzzler is the east and south locations. My first thought was a change in sun load due to tree removal or an newly built adjacent home causing reflected hot spots. It seems a bit unlikely two different faces of you home could suddenly be subject to extreme sun load. I am ruling out window washer accidents, but maybe if you have really big misguided geese or owls? Were either events witnessed?

    I also can't tell from your description if the glass is tempered or not. Each responds to stresses and scratches differently.

    I would check for signs of movement of the blocks above the windows first then below the windows. If the core and beams were not done well, it is possible that the wall weight above the window has forced down a block enough to distort the frame to the point where the window stress causes glass failure. It should be pretty clear where that is happening, especially if you have stucco overcoat.

    Another possible problem could be swelling of the soils under your home lifting the wall blocks under the windows. You many not have officially expansive soils, but the extensive rains occurring in the southeast may have super saturated any clay in your soil which can make it expand more than normal. If the windows are first floor ones, the blocks under the window may not have a poured beam, which would allow the blocks a bit more movement than those above. The amount of lift it would take to distort a window frame to failure should, again, be pretty easy to see.

    I had the exact reverse soil conditions occur in my 70+ year old Illinois home when we had a very severe drought. The clay soil dried so much that my precast front stoop broke free of the house foundation and tilted downward. Sadly, it didn't rise back up once the drought ended. Hopefully, your foundation will settle back once it gets dry again.

    Other possible answers. If you live in a golfing development, do the two sides face any drive lines? A golf ball might end up further away from the shattered window than one expects. If not in a golf zone, are you in an area with armed neighbors? Do you front onto wood areas that allow hunting? Errant projectiles from target practice killed a picnic guest in one case (again in Illinois) The target pit was nearly a 1/4 mile away. One round cleared the dirt bank and an innocent person died.

    Hopefully, a malignant act is not the source of your breakage, but the wackdoodle level in the country is very high now.

    1. HeatherCFS | | #9

      In looking at it in the sunlight. We have multiple stucco cracks that mirror the block underneath- but just on the corner of the house where the window broke last night. Phooey.

  5. HeatherCFS | | #8

    Thank you for the robust response. We’ve had some curious drainage/water so I know what I’m looking at when the sun comes up.

  6. Expert Member
    BILL WICHERS | | #10

    If you're having thermal cycling issues, you might need to shrink the size of the window a bit and frame around it using something like EPS to provide some padding to absorb the forces. If too much force is transfered to the glass, the glass can fail.

    There must be some pretty significant thermal exapansion going on if that's what broke the window. I haven't seen this happen before.


  7. user-723121 | | #11

    As Roger pointed out it could be stress within the wall acting on the glass. Some casement windows have a tight operating margin and just a bit of building movement can put stress on the window sash. Does the broken window operate freely in the opening?

  8. onslow | | #12


    Well, at least the description you gave doesn't point to duffers or lunatic neighbors. Sadly, it does sound like you have soil expansion under the footings that is driving a wracking of the window opening. Your description of the cracking echoing the block pattern is suggestive. I would guess that the window opening is near the corner rather than mid wall.

    While thermal expansion of certain window materials can be large, the fact that you made it 18 years without issue puts a damper on that line of investigation. Also the occurrences on two different sides of the house. However, if trees have been removed, new sun load may be an aggravating factor

    The vinyl windows I have dealt with, more frequently have the IGU seals fail rather than break the glass. That and the vinyl frames simply degrade and get brittle. The exterior pane, which is glued to one side of the spacer, cycles "bigger-smaller" more aggressively than the interior pane, which is closer to room temperature more of the time. Rigid spacer materials will also cycle at their own rates. I would guess that aluminum or steel spacers expand more than the newest super spacers. If the windows are also 18 years old then the spacers are most likely metal.

    Tempered glass is quite tough against impact compared to standard glass. It does have an Achilles heel, small point loads on the edge or scratches on the face can initiate fatal cracking that instantly propagates across the whole pane. It might be a small burr on the metal spacer shifting into contact or perhaps a screw edge from long ago being shifted as the blocks wiggle. Pellet guns are a less enjoyable prospect that occurred to me.

    As Zephyr and Doug note, any new windows will need to be set in with some form of buffer space. The awkward part is having to replace the window before you have determined whether the cracking in the block is permanently up or if the corner will go back down once the drainage issues are resolved. Depending on the soil and future weather, it could be a while before things settle down.

    A window installer that is familiar with setting windows in log homes would be a good bet for already knowing how to deal with drifty window openings. A log home in my former neighborhood needed to account for nearly 3" of height shrinkage expected to occur in following years after construction. This despite being cut and set to dry for a full year before final fitting of the logs. Your maximum movement will hopefully be in the 1/4 to 1/2" range.

    Do consider checking on water management into the block and further into the wall. The cracks in the stucco may be allowing the blocks to pickup more water than is healthy for interior framing and finishes.

  9. user-2310254 | | #13


    Was the AAC sourced from the plant in Ringgold? My impression of that manufacturer was that it attracted lots of DIY home builders. DIY can be fine, but it is easy for an inexperienced person to make mistakes that have long-term consequences (such as a building that settles and moves in ways that compromise openings). It might be helpful if you could post some pictures.

  10. walta100 | | #14

    Please post photos of the cracks in the walls long shots and close-ups with a ruler. You need to understand if the cracks are changing over time.

    Tempered glass is strange stuff when I need to break it I can pound on it with a hammer and it bounces off. When I am trying to keep it if I look at an edge it will explode.


    1. Expert Member
      BILL WICHERS | | #15

      The easiest way to look for crack propagation is to mark the gracks with a grease pencil. Mark a few places across the crack, and mark the ends. If the crack grows over time, the crack will pass the end marks. If the crack "slides" (the two sides moving in opposite directions from each other), the marks across the crack will open up. Either way, you can easily see if the crack is changing over time this way. I've had people mark foundation cracks like this many times to see if they are serious problems or not.


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