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Underlayment for Vertical T&G redwood siding

PinneoConstruction | Posted in General Questions on

Getting ready to replace existing T&G redwood siding with new material (same stuff), and being from the East Coast, we don’t work with redwood very often. Due to existing clad windows and no trim, space is limited to do a WRB and rainscreen, so we are leaning towards Tyvek Drainwrap. Should we have any concern with the redwood directly against Tyvek? Are there better options out there for a rain screen, without the thickness of a Homeslickr?


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  1. gtmtx | | #1


    I am not sure if redwood is as corrosive as cedar, but suspect very similar so I would stay away from Tyvek in this application. Although drainwrap is advertised as a "rain screen", I have no faith it really delivers, particularly when siding is installed. There are a few similar products to homeslicker, cedarbreather for one, but since space is that limited (1/2" +/-) I suggest a few methods.

    Use a double layer system, such as a base layer of Tyvek drainwrap (or any other good housewrap, I like the non-perorated polyolefin such as Tyvek) with 30# felt over top so the redwood never touches the Tyvek. This gives you a "drainage" plane, small, but something is better than nothing. The felt has better wet/dry cycles then housewraps, but like the housewraps since you can tape/seal them better than felt. Although Tyvek meets the criteria, I prefer products that have a lower perm rating around 15 such as Dow WeatherPlus and Typar, whereas Tyvek's is much higher around 60. You may considering using the commercial grade product rather than drainwrap.

  2. jklingel | | #2

    Glen M: "...I prefer products that have a lower perm rating..." Curious. Why? thanks.

  3. gusfhb | | #3

    Owning a 40 YO house with the same siding, my first question is, why are you replacing it?

    Vertical siding is to some extent self draining. In an extensive renovation i saw no evidence of damage with t+g redwood over red rosin paper, except for some poorly flashed sliding doors and beam penetrations. It was, however, a poorly insulated drafty wall that also has 2 foot overhangs, so a perfect application for bad details.

    As a possible semi helpful note. All the window frames in my house were site built and it would seem a small miscalculation on some of them meant the channel in the sill which the siding was to slide up into was too narrow. As we deconstructed a wall section we realized that the top 1/2 inch of the siding had been milled too fit into the groove.

    My point is that if you think that a thicker underlayment is required, where there is a will there is a way, as long as it is flashed correctly.

  4. gtmtx | | #4

    In reality the difference is likely a non-issue, but the marketing at Tyvek about your wall breathing is way over-stated IMHO, which is their reason for the high perm rating. My feeling is why are you having to do all this drying? If it is "bulk" water, you have other issues than a high perm rating. Yes, I understand how moisture/vapor flows, but felt (around a perm rating of 15) has been used (and performed) for decades, but now we need this high perm rating. As such, your wall is going to breath whether the perm is 15 or 60, how much and does it really matter is another debate as well as the climate, wall assembly, etc.

    My general feeling is that limiting the vapor drive is a good thing. Some may argue you are trapping moisture on its way out, but I find that hard to believe, particularly when you see how most housewraps are installed. Also, I have not used/evaluated every possible housewrap, but the ones with lower perm ratings that I have experience will tend to perform better than Tyvek under wood siding, particularly those with tannins such as redwood. Why, I do not know.

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