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Fastening vertical pine lap siding over rainscreen — horizontal nailers?

ovolo | Posted in GBA Pro Help on

I am having a hard time satisfying all our intentions in this scenario and would appreciate some timely advice.(ASAP 🙂 If there are some available drawn details to point me to that would be much appreciated. We are are in climate zone 5A.

2×6 Wall
5.5″ of Icynene insulation, will be sprayed in.
7/16 Zip wall sheathing
Tyvek – (taped with the small folds. We think its useless and have nicknamed it ‘wrinklewrap’)
Marvin aluminum clad windows.

Assuming we want the windows nailing fin on-top of but taped/lapped to the WRB.
How do I do the window flashing at bottom of the window sill and how do I not penetrate all the WRB with nails on the horizontal runs without doing first a vertical 1″ and then a horizontal 1.5″ to hold off the nails?

I was going to do 4′ and 32″ lengths of 3/4″ horizontal furring leaving spaces for airflow going up the wall. Space at top and bottom (with bug screen).

Lastly I was originally hoping to add a layer of 1″ ridged insulation, a thermal break and then do the furring but now I think that is just adding to this problem.

Suggestions welcome.


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  1. Expert Member
    Dana Dorsett | | #1

    Adding an inch of rigid foam adds considerably to the performance. As-described your stackup would come in at around R14 whole-wall after thermal bridging, (assuming 16" o.c. spacing, 25% framing fraction typical.) Adding an inch of unfaced type-II EPS (1.5lbs nominal density) would bring that up to a bit better than R18 (a 22% reduction in heat loss) and would still allow adequate drying rates into the rainscreen gap.

    Don't put foil faced goods or XPS unless you have at least R7.5. XPS runs about 1 perm at 1" which would restrict outward drying too much, and foil facers are about 0.05 perms, for zero outward drying. With slow/no exterior drying you need at least R7.5 (R10 is better) outside the sheathing in climate zone 5 to sufficiently limit the wintertime moisture uptake in the ZIP sheathing from interior side moisture drives.

    But unfaced Type II EPS runs between 3-4 perms, significantly more permeable than the OSB itself, and about the same permeance as the interior side latex paint. That allows it to pass wintertime moisture into the rainscreen gap as quickly as it can enter from the interior side- it can dry roughly equally in either direction, and by keeping the sheathing warmer, the sheathing doesn't NEED as much drying capacity.

  2. GBA Editor
    Martin Holladay | | #2

    Your post raises several questions. It seems like you are asking:

    Q. "Should I include rigid foam?"

    A. The answer to this question, as Dana suggests, is Yes. Here are links to 2 articles to help you:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    How to Install Rigid Foam Sheathing

    Q. "How should I orient my furring strips?"

    A. I think that it is perfectly OK to install horizontal furring strips to support vertical board siding. The amount of liquid water that gets past your siding is small, so it doesn't really have to drain. It will evaporate. If you are still worried, you can buy notched furring strips that allow drainage from a roofing supply company.

    Q. "Will the fact that my housewrap has fastener holes cause any problems?"

    A. No, as long as you have an air space between the housewrap and the back of the siding. For more information on this issue, see All About Rainscreens.

    Q. "How do I flash my window rough openings and how do I integrate my window flashing with my housewrap?"

    A. This is a big question, and one that I hope you aren't tackling at the last minute. If you aren't confident of your understanding of flashing issues, it may be time to hire a contractor who understands these issues, because the integrity of your wall can be at risk if you get your flashing details wrong. Here are the basic principles:

    Flash the window rough opening, not the window.

    Direct all water that reaches the rough opening to the outside surface of your water-resistive barrier (WRB) with a sill pan that is installed under your window.

    Flash the window jambs so that the jamb flashing overlaps your sill pan.

    Flash the head so that your head flashing overlaps your jamb flashing.

  3. wjrobinson | | #3

    Windows are shipped with installation instructions.

  4. ovolo | | #4

    Dana and Martin, thank you very much for your response and answers. Always impressed by Dana's math and then Martin's summaries. The way I wrote my post, I did raise a number of questions. Its amazing how when writing and talking about these things in this limited context it can raise more questions then answer and I admire how you continue to tackle and respond anyway. (I was hoping that there is a detail drawing out there for this scenario but I can't seem to find one. )

    It was good to have confirmation that the layer of ridged foam will add so much effect. (We are however about to install polyiso IKO Enerfoil CCMC-13188-L. is that a problem?)

    Penetrating the WRB with multiple nails through a horizontal nailer was my largest concern.

    Having built homes for more then 20 years I am familiar with window flashing and we do a lot of unusual building but this is still really only our third 'rainscreen’ project and previous rainscreen assemblies were always for a horizontal siding. The recent change of design from the client, led me to these questions. The article 'All About Rainscreens' is very good and I had read that.

    2x6 construction, 16” oc
    7/16” zip wall (untaped)
    1” -Polyiso IKO Enerfoil CCMC-13188-L. (is it better un-taped?)
    1” thick by 1.5” wide nailing flange around window RO's for window nailing fin to be on same plane as insulation.
    Tyvek (‘Wrinklewrap”) , taped and tucked on-top of the 1” foam as our WRB. (taped over the window flashing tape but not on bottom)
    3/4” thick horizontal nailing strips, applied to the foam. every 2’ oc ; 3 and 4 feet long with 16” spaces
    random width 1x8-10-12 shiplap vertically applied pine, stained all edges.

    As you realize, in this configuration the Alumin Clad window profile edge is now back from the siding face a +/- 1/4" then. The window trim detail on the vertical sides is a ripped piece of siding returned to the taped nailing fin of the window. The top I’ve resolved. The bottom seems like it will need a piece of drip cap ontop of the exposed siding maybe bent and going back to nail flange etc.

    Is this all worth it or am I totally on the wrong track?


  5. Expert Member
    Dana Dorsett | | #5

    The foil facer on the mere-inch of the iso IS a problem. DO, tape the iso- you want it air- tight. Air leakage at the seams would not allow anywhere NEAR enough drying capacity into the rainscreen gap, but it could allow liquid moisture in, and is a heat leak to boot.

    With less than R7.5 on the exterior it means you will now need to install a "smart" vapor retarder under the interior gypsum to avoid mold risk at the sheathing. Any chance you can bump that to 1.5"?

    Alternatively, a layer of 3/8" or 1/4" fan-fold XPS (sold as siding underlayment) between the iso & ZIP would nudge you into the ~R8 range, and it would meet code without an interior side vapor retarder.

  6. ovolo | | #6

    Dana are you keeping in mind that I have 5..5" of open cell icynene in the bay?

    in spite of the extra cost, should I switch to 3" of closed cell?

    Ok, if I'm understanding this correctly, even though my 1" of foam is not doing as much as we would like but do not want the added profile of more thickness, it would still be better to switch to Type 2 EPS and I should tape the seams?

  7. skibumo1 | | #7

    Arthur, i know it was quite a while ago on this project, but it there any chance you have some pictures of the final project? i was looking to do vertical board siding and i like the thought of the random widths. I did that on the ceiling with nickel gap shiplap, looks awesome, might be kind of interesting on the exterior and a little interest to the exterior of my big old box, 46' x50' relocated barn, slowly converting into a home.

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