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Undersize replacement IGU's? Do or Don't?

Posted by xlr8tion (My Page) on
Mon, Jul 11, 11 at 19:40

New to the board and want to thank everyone for their advice in advance.

I am replacing 18 IG's in my 10 yr old Simonton Dimension One PVC clad Casement and picture windows.

All IG's are 3/4" and all are inside mount with pvc stops.

Spoke to a Simonton tech and they suggested undersizing the replacement IG's 1/8" on the width(leaving 1/16" on the left and right) and 1/16" on the height due to their tight mfg tolerances and that the frames sashs are 10 yrs old.

Rep said better to be slightly "too small" (1/16") than too large due to over shimming and PVC expansion.

My glazier is not worried about the casements but said the picture windows(transoms that are 18.5H x 66"W may be a problem if the original installer over shimmed.

Long to short....which is more dangerous? Going with original mfg IG sizing or baking off am 1/8" on the width and 1/16" on the height?

BTW the replacement IG's are direct from Cardinal IG...3.1mm Low E 366/Argon/XL Edge Spacer......

Any opinions greatly appreciated as I have to order the IG's tomorrow


Follow-Up Postings:

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RE: Undersize replacement IGU's? Do or Don't?

Go under you can use glass shims to take up the space if necessary. If something had happened and the the opening size was compromised you may end up buying them again. The stops will more than cover the 1/8" gap


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RE: Undersize replacement IGU's? Do or Don't?

Why are you replacing the IG's? I just ordered some Simonton, and am wondering.


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RE: Undersize replacement IGU's? Do or Don't?

I would go with what the Simonton Rep says. That sounds about spot on as you can always lay down a slightly thicker bead of silicon to re-seal the IGU to the sash prior to re-installation of the glazing bead.

There should be more than enough width behind the interior stop on the glazing bead to hide the 1/8th of an additional inch behind the stops.


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