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Community and Q&A

Rain screen with lap siding

arnoldk | Posted in General Questions on


I looked around on this site and Google but could not find a clear answer. 

Currently my double stud wall assembly has the exterior 2×6 at 24″ OC followed by 2″ on ConfortBoard and by a rainscreen. We plan on installing LP Smartsiding Lap siding which requires 1-1/2″ of nail penetration for both their 3/8 (16″ OC) and 7/16 (24″ OC). I have been told their 7/16 is difficult to source which is leading me to likely go with 16″ OC for that exterior siding. 

Now my question, if the siding requires 1-1/2 nail penetration, I am forced to use 2×3 or 2×4 with 5 inches fastener. Will such a big rainscreen gap (1-1/2) introduce other issues down the road or is that fine?

Thank you,

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  1. Expert Member


    The only disadvantages of a deep rain-screen are the complications it may cause detailing openings, the necessity to use longer and heavier fasteners, and protecting the bottom. Once built the added depth aids all the benefits it is designed to provide.

    My one reservation is that a cavity that size may aid fire spread. It should definitely not be continuous with the roof above.

    1. arnoldk | | #2

      Hi Malcolm,

      Do you know of any wood lap siding that only requires 3/4" nail penetration?

      As you suggested, I thought about removing the exterior Comfortboard insulation but my wife really wants it because it provide thermal break at the rim joist and the sill plate. I may reduce the insulation from 2" to 1-1/4" but that still doesn't address the lap siding installation requirement.

      Fire (house or forest) is something we are keeping in mind with this build but they are not a concern in my area of Ottawa, Canada.
      Would putting some type of spark arrestor such as Cor-a-Vent or mineral wool at the top of the rainscreen reduce that risk without affect air flow?

      Thank you,

      1. AlexPoi | | #6

        A full 1 1/2 inch drain cavity with no fireblock is not legal in Quebec from a fire standpoint. You should check if the same rule applies in Ontario. 1 inch is the maximum thickness you are allowed here.

        1. Expert Member
          MALCOLM TAYLOR | | #10


          Do you mean fire-blocking a the top of the cavity, or is it at some prescribed intervals? We can't join the cavity to the roof space above here in BC in any depth rain-screen.

          1. AlexPoi | | #11

            No idea. I was having the same problem with nail penetrations so I was planning to install 2x3 instead of furring strips and my draftsman told me I couldn't according to the fire code.

            Here's the article :
   Construction of Exposing Building Face and Walls above Exposing
            Building Face
            3) Except as provided in Sentences (4) to (8) and permitted by Sentence (9), cladding on exposing building faces and on exterior walls located above exposing building faces of buildings or fire compartments where the maximum permitted area of unprotected openings is more than 25% but not more than 50% of the exposing building face need not
            be noncombustible, where :
            the cladding
            i) conforms to Subsections 9.27.6., 9.27.7., 9.27.8. or 9.27.9.,
            ii) is installed without furring members, or on furring not
            more than 25 mm thick, over gypsum sheathing at least
            12.7 mm thick or over masonry

            It's seem to come directly from the Canadian code so it's probably the same in Ontario and BC unless it was amended.

          2. Expert Member
            MALCOLM TAYLOR | | #12


            Section 9.14 is about fire protection of building faces close to other building or lot lines. It has a lot of requirements specific to those situations. There isn't a blanket prohibition on using a 2"x cavity for a rain-screen in the code.

          3. AlexPoi | | #22

            Weird. I will ask again then. Thanks!

  2. arnoldk | | #3

    I am still trying to figure out how others are installing wood (lap) siding with exterior insulation. From what I have read, most manufactures require 1-1/2 inches nail penetration which forces you to installed a 2x3+ rainscreen but all of the examples I have found simply show a 1x4 furring strip.


    1. Expert Member
      MALCOLM TAYLOR | | #4


      There are really two issues: the manufacturer's installation instructions, and practically whether wood siding needs thicker furring.

      Unlike composites and other manufactured cladding, I've never bought wood siding from a mill or supplier that cared how it was installed, so I really can't give much useful advice how to proceed if they do.

      From a practical perspective 1"x furring is sufficient to fasten wood lap siding to. That's why you see the detail so frequently.

      You may find this useful:

      1. Expert Member
        Michael Maines | | #5

        My experience is similar to Malcolm's--I typically specify real wood siding and install it in a way that makes practical sense. If you want to go with a manufactured product and you want their warranty, you're at the mercy of their specifications.

    2. AlexPoi | | #7

      I think it's your furring that must be fastened to the structure with a minimum penetration of 1-1/2'' when you have exterior insulation.

      At least according to this guide (section 8):
      (Maibec Resistech Siding is built on LP SmartSide)

      1. Expert Member
        Michael Maines | | #13

        Alex, LP requires 1 1/4" penetration for their siding fasteners.

        1. arnoldk | | #14

          I was told 1-1/2" by the LP sales rep.


          1. Expert Member
            Michael Maines | | #17

            Arnold, you are correct: "Penetrate structural framing or wood structural panels and structural framing a minimum of 1-1/2 inches (38 mm)." My point was about furring attachment vs. siding attachment but I should have double checked the depth required.


          2. Expert Member
            Michael Maines | | #18

            From the same LP document: "For rigid foam insulation sheathing up to 1 inch (25 mm) thick, siding may be nailed directly to the foam sheathing unless a drainage plane is required by the local building code. Nail length must be increased to ensure a minimum 1-1/2 inch (38 mm) fastener penetration into the structural framing.

            "For rigid foam insulation sheathing greater than 1 inch (25 mm), a minimum 1-1/2 inches (38 mm) thick by 3-1/2 inches (89 mm) wide vertical strap or furring strip must be installed over the sheathing to provide a solid level nailing base for the siding. The strapping must be securely fastened to structural framing spaced no greater than 16 inches (406 mm) o.c. with a minimum nail penetration of 1-1/2 inches (38 mm) and a maximum nail spacing no greater than the width of the siding."

        2. AlexPoi | | #21

          Thanks Michael. So 2x3 seems to be the only solution.

          1. Expert Member
            Michael Maines | | #23

            Alex, they actually specify 2x4. But I would call their technical department to see if they have more information. I did a few weeks ago and got a call right back.

  3. Expert Member
    Akos | | #8


    The exterior mineral wool is buying you almost nothing in terms of energy use.

    If you want insulation around your rim joists, you can usually offset the 2x6 wall and cantilever it a bit(check your local code), lot of times it only need 3.5" of bearing. This will give you plenty of space for a layer of insulation around the rim joist..

    You can always bump up the thickness of the double stud to compensate for the R value loss of the mineral wool. With a dense packed wall, this adds almost no extra cost but you save a LOT by not having to buy the mineral wool and more importantly, having to deal with trying to get strapping, thus siding, flat over it.

    Bonus, your nail issue is also gone.

    1. arnoldk | | #15

      Hi Akos,

      I have been trying to convince my wife to forgo the exterior insulation and move the interior 2x4" wall by 2 inches which will accommodate 2x6 bat insulation versus the 2x4 we currently has prescribed in our plans.
      With an R-60 wall, I would even be ready to drop it all together but my wife really wants the thermal break at the sill plate (over lapping by 6-8") and at the rim joist. She wants to get an energy model done to determine to penalty of not having the exterior insulation.


      1. Expert Member
        Akos | | #28


        See attached a rough sketch of what I mean. If your code allows for a 2x4 load bearing wall, you only need 3.5" of bearing for your 2x6 wall. You can now cantilever this wall past the rim joist to make space for extra insulation. There is still an ever so slight thermal bridge across the subfloor but that is noise in terms of overall energy use. This gets you continuous insulation across almost all of the building envelope.

        Rigid mineral wool is not the greenest material. The 2" panel is an 8" batt squished down, so you are getting an R8 of insulation from R32 of material. It takes a lot of pretty energy intensive material to make it. In your case with a double stud wall, the carbon footprint of the rigid mineral will never be made up in energy savings. Never mind the fact that it will barely budge your building's overall energy consumption.

  4. ohioandy | | #9

    Arnold, you'll be fine with 3/4" furring--just be sure to use ring shank nails to secure the lap siding. It may technically be out of compliance with LP's instructions, but as long as you don't use the softest and cheapest pine for furring, this is a VERY robust installation.

  5. Patrick_OSullivan | | #16

    Just to give a data point... I'm currently installing LP SmartSide over furring. I have 1.5" of Zip R-sheathing as well. I went with 3/8" CDX for the rain screen gap as it kept the trim details a bit more manageable. Fasteners are 3 1/4" 0.120" ring-shank nails (i.e. a framing nail) which are hitting studs with about 1 1/8" penetration. While this might technically be insufficient per LP's specs, I'm pretty confident that this is a very robust install, at least where I live. The strength to weight ratio of SmartSide is impressive and it is held in place very firmly.

    P.S. if you're wondering why I have a WRB over Zip, it's because the Zip passed the 180 day exposure limit and this is cheap insurance that helps me sleep better at night.

    1. Expert Member
      MALCOLM TAYLOR | | #19

      Looks good Patrick!

    2. arnoldk | | #20

      Thanks for that Patrick. I suspect I'll reduce the exterior rigid mineral wool from 2" down to 1-1/4" and use 1x4" furring strip which will let me get away using 3.5 inches nails. With 3/8" lap siding, I should be pretty close to the LP 1-1/2" nail penetration.

      Does anyone know if the 1-1/2" nail penetration needs to be continuous?
      My above plan would have the nail penetrate the stud by 1-1/8" but it will also go through 3/4 furring strip for a total of 1-7/8" nail penetrate.


      1. Expert Member
        Michael Maines | | #24

        Arnold, I'd encourage you to consult their technical department--I'm sure they get asked questions like this a lot, and may be willing to reduce their specs on a case-by-case basis.

        1. arnoldk | | #25

          Hi Michael,

          I have reached out to them after speaking with their Service Rep and I still have not heard anything back. I followed up with them yesterday morning hoping they can come up with a solution for me.
          I'll report back if I hear from them.


  6. Expert Member
    Michael Maines | | #26

    I just talked with a technical rep at LP and got some great information on this topic.

    With lap siding they have a few options. Their 38-series can only be installed at a maximum 16" o.c. but their 76 series can go over 24" o.c. furring. They have discontinued their fiber-based line and are now only making their strand-based line. They are in the process of updating some documents so the information below is more current than what you'll find online.

    For exterior foam insulation up to 1" thick you can simply extend the fastener length to penetrate through the foam and into the framing 1 1/2".

    For insulation more than 1" thick, they allow 1x4 furring strips, as noted in this document: They are removing the requirement for nail-head size but the shank size is still important, and it needs to be ring-shank, two nails at each location.

    For furring strip fastening, they directed me to this table: They said LP Smartside is 1.5-2.0 pcf so we would use the 3.0 pcf columns. Note that the thickest exterior foam allowed is 4.0 inches at 16" o.c. and 3" at 24" o.c..

    They noted that this is only for their lap siding, not their panel siding.

    I forgot to ask about mineral wool or wood fiber insulation so I have followed up and will post back with any information.

    1. arnoldk | | #27

      That is awesome information. Thank you for that.


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