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Painting Fiber Cement Siding

wYw2rzZcWh | Posted in General Questions on

I’m currently replacing my old cedar siding with CertanTeed fiber cement lap siding. In the process I am replacing the R-11 insulation with an inch of spray foam followed by R-15 Batt. I will then add furring strips to the studs to add depth to make up for the increased thickness of the insulation since I have 2 x 4 walls.

The furring strips will then be followed with 7 x 16 inch OSB panels. On top of the OSB I will attach 1/2″ R-Max foam board which I will tape. Next, I will add a layer of Typar over the foam followed by 1 x 4 furring strips that will be attached to the studs using Fasten Master Wafer Head Timber screws. I will use the SV-3 Core-A-Vent product at the top and bottom of the wall to allow air to circulate for a rain screen.

I have a tendancy to over engineer things when I start working on a project and thought it would be a good idea to paint both sides of the fiber cement siding before I places it on the house. I am using Valspar Duramax satin paint which seems to be doing a good job. I noticed that Certain Teed says that the back of the siding should not be painted or primed. That doesn’t seem logical to me. I thought that if you want to protect the siding from moisture it would be best to paint all sides of the siding. Does anyone have any suggestions?

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  1. wYw2rzZcWh | | #1

    The house is located in Marietta, Ga.

  2. SLSTech | | #2

    It appears that they are worried about moisture being trapped in the boards from improper handling and storage - if moisture is trapped it could possibly cause bubbling on the front. Here is a copy of their exact directions...
    "All WeatherBoards™ Fiber Cement Siding, Soffit, and 7/16" Trim are sealed with CertainTeed’s FiberTect® Sealer/Primer. Any field cuts made to fiber cement siding must be coated with a high-quality 100% acrylic latex paint or primer, or caulked. WeatherBoards™ Fiber Cement Siding, Soffit and 7/16" Trim must be allowed to breathe. Do not prime, paint, or stain the back side. Some “spill-over” from primer, paint, or stain on the back is normal."...
    As for a wood based product - I recomend & seal all 6 sides... The Johns Mansville FC product I used is already pre-primed & I have yet to paint the back or worry about it due to the makeup of the material - I may have to pull out the specs on their product, because I don't recall seeing them ever saying it is a bad idea, it generall gets into being a budget issue

  3. Riversong | | #3

    With a fully-vented rainscreen, it is counterproductive to seal the back of the siding as that will inhibit drying to the rear.

    The fact that Certainteed prohibits back-sealing indicates the vulnerability of their product to moisture accumulation, which suggests that it simply cannot compete for durability with real wood siding.

    You may be better served by salvaging your cedar siding and reusing it.

  4. user-869687 | | #4

    Chris, there are some issues with your wall design. First, if you're planning to spray closed-cell polyurethane foam at the interior face of OSB sheathing, and then add foil-faced foam board on the exterior, this is a bad idea. That would be sealing in the sheathing between two impermeable materials, which is a time bomb.

    Also, if you mean to add the furring at the exterior face of framing, then attach sheathing over that, this may not give the wall sufficient lateral stability. When sheathing gives the frame lateral support it must be attached to the studs directly. It may be better to attach furring at the interior for that reason, and to run it perpendicular to the studs to reduce thermal bridging. In this case it would be necessary to use blown fiber insulation rather than batts.

  5. wYw2rzZcWh | | #5

    Thanks Thomas,

    It sounds like I should not use the spray foam so the wall can dry to the interior. By not adding spray foam I would still be able to use the R-15 Batts without having to add furring strips on the 2 X 4s to increase the depth of the wall. I can then attach the OSB directly to the studs and add the R-Max on top of the OSB. Will that work?

  6. user-869687 | | #6

    Chris, here are a few brief points about your questions:

    1) Polyurethane foam (and especially closed-cell foam) has a history of problems with chemical off-gassing. There are numerous reports of this on this website. It's a material that solves some problems but creates new ones.

    2) The R-15 rating of a 3.5" fiberglass batt is an exaggeration. The installed whole-wall R-value will be considerably less than that. Fiberglass is inexpensive but less effective than some other choices. Generally the preferred cavity insulation for green building is dense-packed cellulose.

    3) If you do add horizontal 2x2 furring at the interior face of the studs, it will both increase the insulation depth and reduce thermal bridging. In combination with the cellulose, this is a much better wall than one insulated with batts.

    4) Foil-faced foam boards have zero permeability, which means any water that finds itself behind the foam must dry to the interior of the house. There will be plenty of nails going through the foam board when you install the siding. This is a source of risk. And when the foam is thin it may not keep the sheathing warm enough to avoid condensation from the interior of the wall. It would be safer to use a greater thickness of foam board to address the condensation. As for trapping water from small leaks (nails and joints) some of us prefer to avoid impermeable foams altogether.

  7. Sheriff Construction | | #7

    We actually ran into this exact situation with recently recieving counterfiet Hardi Color Plus siding on our current building project. The outfit in Montana that sprayed it used the correct 100% acrylic latex paint, however, the true Color Plus can only be manufactured at their plants through a detailed process of heating and sealing the paint to the siding surface, we have since learned.
    ***NO Color Plus siding is outsourced *** for those of you who might be told this!
    Hardi Plank ( basicly the same product, only primed) CAN be field painted, and commonly is, but this product was sold to us as the real deal. The first thing we noticed was that the siding had a different packaging ( foam instead of thermal plastic) but the biggest difference was that the outfit had completely coated BOTH sides. Otherwise it had a uniform finish, and the lumberyard was telling us it was the new thing. After a little investigation, we knew it was bogus. So we immedietly contacted James Hardi, who told us that the paint will fail, and possibly the substrate because of this method.
    Why? They said that sealing a concrete surface, whether it be their plank siding, a block wall, etc. Locks the salt and alkaloyd properties inside, and when subjected to heat, or extreme cold, that salt will naturally come to the surface. So.... if you have a painted back side, with house wrap and a building behind it, where do you think that is going to go? You got it, RIGHT OUT THE PAINTED FACE, and it did, within 2 days of the only wall we installed it on, it was 10 degree's here, and all of the edges of the lap began hazing with a white edge, and there were whole area's of the laps you could see it clearly.
    So, it took me 2 hours to find this thread, but I at least learned that certanteed's installation actually states not painting the back side, which will go a ways in helping us recover our installation, and downtime after recieving the counterfiet material.
    I hope this helps someone from making a big mistake. According to DP Coating's website, they paint cement siding products for large scale products, and have been for awhile, which is surprising, considering the almost immediate problem we saw. Let me clarify that DP Coatings did NOT sell us counterfiet product, they were only subcontracted to apply a finish, it was another, larger distributor in Spokane that sold an off site finished material as a factory finished one. It will be interesting to see what happens in our court case.

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