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Community and Q&A

Explain capillary tubes and breather tubes in windows?

qofmiwok | Posted in Energy Efficiency and Durability on

I’m buying windows for 6000 feet and being told a variety of things from manufacturers about whether capillary tubes or breathing tubes or balloons are needed.  Some say yes, some say no.  Can someone explain to me exactly how these things work because I don’t quite understand how they equalize pressure without letting the gas out.  

For reference I’m talking about Passive House rated windows R6-R8 triple pane with argon. CZ 6B


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  1. Expert Member
    BILL WICHERS | | #1

    They do let the gas out, just very, very slowly. So slowly it's considered inconsequential. A capillary tube is really tiny, with a bore of maybe a few hair's worth of diameter, maybe a tenth of a mm or so. With a little length of that size tube, the gas exchange by diffusion will be VERY small -- but still allows for pressure equilization with altitude changes.

    Some windows also expect the capillary tube to be sealed at installation time, with the idea being that the pressure will have equilized by that point and the seal will stop future gas leakage. Check with your window manufacturer if that's what they want you to do.

    A good source of info for this might be Alpen since they manufacture in the Denver area, so they're starting out at pretty high altitude. Some manufactures will put the gas charge at a slightly lower pressure so that the windows will be correct when they get to the installation altitude too. There are different ways to accomplish the same goal, which is to not have the glass panes bowing and to not put excessive strain on the seals. Whichever window manufacturer you choose will be able to tell you which method they plan to use for your windows.


    1. oberon476 | | #2

      Cardinal's TSB on cap tubes and breather tubes

      Excellent article on high altitude windows written by Martin a few months ago

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