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How much gap for first course of siding?

airfix | Posted in General Questions on

Climate zone 6b.  3″ exterior polyiso foam, protected by tyvek on the outside of the foam, then a rain screen gap to the boral shiplap siding.

I’m wondering how much gap I need to leave at the top and the bottom course of siding to allow enough air passage for drying?

I was thinking 1/2″?

See the picture attached.



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  1. Expert Member


    - At the bottom presumably the gap is the same depth as your cavity, and is protected by a perforated flashing or Cor-A-Vent strip.
    - At the top 1/2" is fine.
    - At windows and other openings 1/2" will do too. Remember to bed the head-flashing in caulking and provide end-dams. Another approach is to stagger the battens at the sill and head so that air can move horizontally and eliminate the venting above the windows. Then the trim at the head is just kept 3/32" above the flashing to allow any bulk water to drain.

  2. airfix | | #2

    Thanks Malcolm.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      One other small thing. The closer you can locate the Cor-A-Vent to the openings, the less space there is for pests to set up shop and block the cavity.

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