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Community and Q&A

Hydro Gap over existing T1-11

DB_Seattle | Posted in General Questions on

Hello there,

I previously posted a question re: installing fiber cement siding over existing T1-11 plywood sheathing/ siding of our house. Since then, I’ve received 3 proposals, and all of them recommend that I use Hydro Gap as WRB over the T1-11 as opposed to a standard rainscreen, since using a standard rain screen would be costly to install (plus our existing windows would have to be moved forward to be level with the new siding).

Has anyone have any experience with Hydro Gap and its performance/ longevity? Any downsides or concerns over this product? We are in the greater Seattle area (Zone 4 Marine), and our house does get a fair amount of rain, and we don’t have big roof overhang protecting our walls.


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  1. RussMill | | #1

    Hydrogap , in my mind, is doing the job halfway.

    It is a good product. However, a TRUE RAINSCREEN is unbeatable! I would venture to say that even a 1/4" rainscreen gap would beat hydrogap due to air movent causing drying potential to be excellent in a true rainscreen.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Dan,

    I agree with Russell. Hydro Gap is a good product and Benjamin Obdyke does a good job of using the latest in building science to develop their products. That said, a drainable housewrap does not provide a true rainscreen. I would think that in your area and with a reservoir siding like fiber cement, a built rainscreen assembly with a more robust ventilation channel would be worth the added time and expense. This FHB article should help you understand the issues and make a decision: Are Drainable Housewraps Enough?

  3. Jon_R | | #3

    Also see this for "...or cement siding, then a drainable house wrap is often considered good enough." Or here which puts the dividing line at 40 inches rain/year (I believe Seattle is 38, so marginal).

    Perhaps you can find enough local examples of fiber cement over drainable house wrap to get a better idea of "it is good enough in your situation".

  4. mikeolder | | #4

    Do your windows stand proud enough for the additional thickness?

  5. DB_Seattle | | #5

    Thank you all! Mike -- good question about the window - that was my concern as well. From the outside, it looks like there is enough part of the window that sticks out past the existing sheathing to accommodate a small rain-screen, but I'm not positive (will need to measure later today). Is it feasible to still use Hydro gap as the WRB and install thinner furring strips (perhaps 1/4" as suggested by Russell above) on top of it? I asked the estimator of one of siding contractors I've been talking with, and he said he's never heard of it.

    1. RussMill | | #6

      SURE YOU CAN! Hydrogap will cost a little more but, its a much HEAVIER wrb.

    2. RussMill | | #7

      In fact it will allow "some" horizontal ventilation of the 1/4" furring!

  6. DB_Seattle | | #8

    Thanks Russell. What are your thoughts on Slickermax? It looks like it would provide airgap (6mm) but it may be easier to install since it is like rolled like WRBs.

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