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How to Treat Shiplap Siding Over Rainscreen

Rob Wotzak | Posted in General Questions on

It’s common knowledge among smart builders that back priming painted wood siding will help it hold the paint longer, minimize cupping of the boards, and reduce the chance of mold ant rot damaging the wood…but does the same hold true if you are using a translucent or semi-transparent stain?

I’m installing 1×8 pine shiplap siding over a rainscreen of 1×4 furring strips over 1-1/2-in. Rockwool. I plan to use a semi-transparent water-based deck stain on the siding, and it will be fairly well protected from rain because the wall is under a 6-ft. deep porch roof.

Is it worth the effort to seal or stain the back of the boards?

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Replies

  1. Tyler Keniston | | #1

    Rob,
    Some musings:
    I think this might be a classic case of: 'better to, but is it worth it?'
    I can't say from any sort of empirical standpoint what the difference would be. I doubt many could (how many people have both back-stained and not back-stained, then witnessed the difference in performance?).

    In theory, the stain will slow moisture movement (in and out of the wood) and may provide a more even moisture content between front and back (cupping implications). If/when water gets behind the siding, if it's stained, it may shed the bulk water with less moisture uptake.
    On the other hand, most stain passes moisture without blow-off better than paint. And with an open air space behind the siding, water won't be held in capillarity against the siding.

    FWIW, I recently installed some hemlock board and batten over a rainscreen, and couldn't resist the urge to stain all 6 sides. Was it worth it? I have no idea.

  2. GBA Editor
    Rob Wotzak | | #2

    Thanks, Tyler. I like the "better to, but is it worth it?" perspective. It's pretty much what I was thinking, but I still wanted to see what others thought about it. I believe I'm going to skip staining the back in this case, because I'm willing to take my chances and avoid the extra hassle and mess.

  3. Jason S. | | #3

    Low exposure, low risk. If you get meaningful snow accumulation where you are, I'd still back-stain/back-prime the bottom few boards and the first course over openings.

  4. Roger Berry | | #4

    Rob,

    Tyler covered the essential thinking about whether to back stain. In your case, consider your local dew points and humidity profiles. If you have experienced morning/evening dew on the front of your siding, then it is a pretty good bet that the back side is also experiencing the same conditions. It doesn't take bulk water to get a board damp.

    While stain is better than paint at letting moisture pass, the differential of transport of moisture between a stained side and an unstained side could still drive the cupping effect noted. By choosing a relatively wide board you have upped the risk. Especially with current lumber. Everything seems a lot more prone to twisting compared to "the olden days".

    Your rain gap might actually slightly enhance the risk of humid conditions on the back side of the siding. Despite protection from the elements, a frequent cycling across dew point conditions might slowly pump moisture load into the gap behind the siding. Air exchanges behind the siding will be very slow. The wind washed front side will have a distinct drying advantage.

    When I have installed bead edged board ceilings on porches, I always have back sealed the boards to minimize (note I won't assert "eliminate") cupping. Using 3" wide stock also keeps things under control thanks to a much better width to thickness ratio.

    I am of the belt and suspenders school. I don't like thinking "I should have, could have...". I would vote for back coating unless you live in a very low humidity environment and using cedar. In that case I can attest to no discernible value to back coating thanks to the 25 year old board and batten siding I am recycling. Living in a near desert does have a few perks.

  5. GBA Editor
    Rob Wotzak | | #5

    Thanks, Roger. When you put it that way, I would be pretty frustrated if that skipped step led to issues, especially since I know it’s a risk going into the job. Maybe just a clear sealer on the back? I might have an old can of deck sealer lying around. My main concern is not the time it would take to stain it but the mess it will make if the stain drips into the front as I satin the back first (an issue I’ve had with this stain before). That wouldn’t be a problem with a clear sealer.

  6. CarsonB | | #6

    I'm planning on doing this to my cedar and T&G siding (and debating between hand dipping and spraying). I have repeatedly heard from every professional painter I've talked to, as well as the shingle and stain manufacturers, that this may help avoid issues and reduce time between re-coats, reducing maintenance. None have provided any sort of empirical test and I'm a bit skeptical of increased time to re-coat if the main culprit is UV, but for a home I'm living in and having to re-coat over the years if I can do anything now to decrease issues I'm all for it.

    Your idea of a clear sealer first is a good one but may in the end have the same effect if it blocks your colored stain from penetrating on the other side. One idea would be to spray both sides on the ground before hanging so you can get the front first if it's even an issue. Another that comes to mind would be locking them together when you spray the backs so they are less likely to run down to the other side.

  7. Roger Berry | | #7

    Rob,

    Got delayed responding by balky computer. FWIW, I use a pad type applicator if I want to put on very low viscosity finishes like a clear sealer. Rollers always seem to give up the product in runs no matter the nap depth or material. If you do go with a sprayer there is some risk that the overspray will get to the opposite side of the board no matter. Putting on the stain coat first and then using a compatible clear coat for the back will at least avoid risking uneven stain penetration. Use a compatible clear coat that won't alter the sheen of the stain if you do spray. You will likely find that pine boards by their nature will take up stain unevenly due to the wood's growth density variations and knots. The more transparent the stain, the more it shows typically. Pitch spots will make for drools.

    If you are planning on an eco-friendly water based stain, be warned about keeping your "wet edge" moving. I ran into a nightmare deck coating situation using a water based stain. The old deck was all prepped and dry which I thought would make the coating even. Instead the old wood sucked up the stain so fast that I could not keep the limit of the finish from going dry before reloading the applicator. The stain which touted it's water repellency lived up to the claim becoming so water repellent in seconds that the stain barely soaked in at the now not wet edge.

    The problem continued to frustrate me as it rendered touch up of edges and hard knot areas almost impossible. At least it was my own deck, not a clients. Still, after that I always used oil based stains. Edges and re-coating for depth of color is much simpler.

    To minimize drips of whatever you choose for back coating, place the boards at an angle like shutter slats while you apply finish. The lap edge that makes a little seat goes at the top. It will all take time and seem thankless until you realize 10 years on that the cladding still looks good.

    1. GBA Editor
      Rob Wotzak | | #8

      Thanks for all of the tips, Roger. I ended up going with a Behr water-based siding stain that I had experience with before. I too have experience with trying to keep up with the fast-drying stain, so I ended up using a 6-inch roller and was able to keep my edges clean. I'm still debating whether or not to do the second coat after installation, but I likely will. I'll also roll some clear deck sealer on the back of the boards that I had lying around. Now that I know I can be neat with the roller, I',m not worried about dripping onto the already-stained face of the boards. I've attached some pics of the project.

      BTW, on the house you can see some of the new Benjamin Obdyke HydroGap SA house wrap which was just recently released. I've loved working with the stuff so far--it just makes for such a tidy installation compared to mechanically-fastened house wraps.

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