Calculating sealant thickness for acrylic windows

Discussion in 'Boatbuilding' started by Herman, Apr 11, 2020.

  1. Herman
    Joined: Oct 2004
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    Location: The Netherlands

    Herman Senior Member

    I am wondering if there are fixed guidelines or manufacturer recommendations for calculating the thickness of sealant for installing windows (glass or acrylic)

    As Acrylic windows expand and contract a fair amount more than the underlying building material (usually GRP or metal), you need a certain amount of sealant to compensate.

    How to calculate this? Calculating the difference in expansion/contraction is not difficult, but how to relate a certain amount of mm of expansion to the needed thickness of the sealant?
  2. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    I use 25% of glass thickness.
  3. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Lotsa people prefer butyl. If it leaks; you add more.
  4. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    I used Sika bedding compound and rubber spacers across the bottom.
    I made my windows out of yellow cedar so simply allowed 1/8" all round.
    1/4" tempered glass.

  5. rxcomposite
    Joined: Jan 2005
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    Location: Philippines

    rxcomposite Senior Member

    I agree with BlueBell. This is a rule of thumb spacing required. That is the glass (material 1) should not touch the substrate (material 2) but will be separated by the adhesive. Material 1 or 2 should be allowed to expand all around in the operating temp it will be subjected to. I give a minimum of 1/8" from glass to frame and 1/8" per side for expansion.

    If you want to really be technical, this is an "adhesive bonded lap joint" and you compute the thermal expansion of both material and the differential force that will be induced due to difference in expansion/contraction. This is the shear strength of the adhesive. Try searching adhesive design + Huntsman. I don't know if they are still maintaning the web site.
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