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Community and Q&A

Installing LP Smartside over foam and 24 inch centers

William Costello | Posted in Green Building Techniques on

We’re building a new home (SW Virginia) with 2×6 studs on 24 inch centers. We had hoped to use 1.5 to 2 inches of EPS rigid foam to the exterior of the sheathing, then 3/4 inch x 4 inch plywood furring strips, then LP Smartside lap siding. We knew we had to use the 76 series Smartside product for the 24 inch centers. Then I discovered that LP’s installation instructions specify 1.5 inch x 3.5 inch furring strips for foam greater than 1 inch in thickness, so I guess that would basically mean a 2 x 4 stud for every 2×6 stud in the wall. This is more material/expense than we had bargained for, and perhaps also more increase in wall thickness than we want for window installation, etc. Due to some ambiguity in their instructions, I called LP support just to verify this, and yes, that’s what we need to do to be warranty-compliant. (However, their instructions also seem to indicate that you need 16 inch centers if fastening to furring strips over foam. The rep told me that 24 inch centers are fine in this scenario as long as you use the 76 series.) I asked him if we could fasten the siding through the foam and furring strip with long nails or screws biting into the underlying 2×6 studs. (Wanted to keep the furring strips so we’d still have a rainscreen air gap behind the siding.) He said that sounded like a possibility but I would need to talk to some of their higher up tech people and get a warranty variance in writing to protect myself. And then you’d have the expense of all those long fasteners and not sure how that would add up vs the thicker furring strips? So I’m kind of confused about which route to take. Here’s what I see my options to be:
1) Install according to LPs specs and use a lot more lumber. Would the improved thermal performance of the wall be worth all this?
2) Install according to original plans and hope I never need to use warranty. Anybody have experience fastening Smartside lap paneling to 3/4 inch furring strips?
3) Switch to 1 inch of polyisocyanurate for a similar r value boost, and forget about the rainscreen gap. (For 1 inch of foam or less, LP says you can nail directly through the foam to the studs.)
4) Try to fasten siding through furring strips and foam with long nails or screws. Anybody tried this? Any ballpark ideas on expense of longer fasteners vs just using 2×4 furring strips?
We’re kind of stuck and don’t have a lot of experience to guide us. Any thoughts would be greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    I don't know what stage of construction you are at, or whether you have purchased your siding yet. But your case is a good example of the need to plan ahead: It's important not to select a siding until you're sure that the siding installation details will work for your planned wall assembly.

    Q. "Should we install the siding according to LPs specs and use a lot more lumber? Would the improved thermal performance of the wall be worth all this?"

    A. Switching from 1x4 furring strips to 2x4 furring strips will give the siding fasteners more "meat" to attach to, but the thicker furring strips won't improve the wall's thermal performance.

    Q. "Should we install the siding according to original plans and hope I never need to use warranty? Anybody have experience fastening Smartside lap paneling to 3/4 inch furring strips?"

    A. I don't recommend this approach, but I'm interested in hearing what other GBA readers think. It should be noted that installing building materials in a way that doesn't comply with manufacturer's installation instructions is a building code violation.

    Q. "Should we switch to 1 inch of polyisocyanurate for a similar R-value boost, and forget about the rainscreen gap? (For 1 inch of foam or less, LP says you can nail directly through the foam to the studs.)"

    A. It's a very bad idea to butt the back of your siding tight to a layer of rigid foam. You need the rainscreen gap for siding longevity.

    Q. "Should we try to fasten siding through furring strips and foam with long nails or screws? Anybody tried this?"

    A. I wouldn't attempt this unless you get an OK to proceed from the siding manufacturer. This approach sounds expensive because of the cost of the long screws.

    -- Martin Holladay

  2. John Clark | | #2

    Slicker® can be applied to the insulation and then you can attach the siding directly to the studs through the foam.

  3. William Costello | | #3

    Thanks for your quick reply. OK, so we’ll definitely rule out Smartside on top of foam with no rainscreen gap. And your note about violating building code was something I hadn’t thought about and eliminates that option. To clarify, I didn’t assume we’d get any thermal benefit from the 2x4 furring, just asking, with all that cost and material, is it worth the increase in thermal performance gained from the foam (as opposed to eliminating the foam altogether.) And I’m thinking what you’re thinking about the cost of the long fasteners.
    So, back to the drawing board I guess … We haven’t purchased siding yet, haven’t even finalized design – so still in planning stages – but I do think we’ll stick with 2x6s on 24 inch centers and foam over the OSB. Maybe we’ll go with 2x4 furring strips, but I feel like we’d need more foam to make this all worthwhile. All more expense of course. Another option might be Hardie board – I think I’ve read that Hardie only requires 1/2 inch of furring strip “bite” for the fastener, so our 3/4 inch furring would be more than sufficient. Not sure about 24 inch centers though. Worst case we’ll have to go to vinyl, which we really don’t like aesthetically, but I gather we’d be able to fasten it through even 2 inches of foam right to the stud, and still have air space behind of course.
    Thanks so much. We couldn’t have made it this far without this site, which I’ve devoured over the past few months.

  4. William Costello | | #4

    Thanks John,
    Slicker seems like a simple solution. So you're thinking that slicker would give me all the drainage and drying I'd need next to the fairly impermeable polyiso? Would it be as good as a 3/4 inch air gap, or would you see it more as a compromise solution?

  5. Brendan Albano | | #5

    Hardie on 1x4 @ 24" O.C. seems pretty common. You'll of course want to confirm that the manufacturer approves it and all that, but if there is a Hardie product that fits your aesthetic, that might be a good way to go without having to alter much else.

  6. John Clark | | #6

    "Would it be as good as a 3/4 inch air gap, or would you see it more as a compromise solution?"

    IIRC the gurus at Building Science Center recommend a min gap of ~1/4 inch (approx. 6mm) when dealing with siding other than vinyl siding*. Perhaps more is better but it's not necessary. 1x4 furring strips are just readily available.

    *Vinyl siding doesn't need a rain screen gap.

  7. Jonathan Lawrence CZ 4A New Jersey | | #7


    My wall design has 3" of Roxul with 2x4 furring and Hardie Artisan siding. I recall that the furring and fastener requirements were the same for Roxul and foam. The Artisan siding is about 2x the weight of regular Hardie which required me to go with 2x4 furring because I needed 1.75" of fastening depth on both the furring and the stud. Had I gone with regular Hardie, I could have used 1x4's. We also spec'd 7.5" HeadLoK fasteners spaced 16" OC vertically and horizontally.

    This a blurb that I sent to my structural engineer when trying to determine the furring and fastener size.

    "Hardie has a technical document (#19, attached) regarding exterior foam assemblies (they include mineral wool/Roxul in this category). Using this document along with 2 other documents that it references (Ter No. 1009-01 and ESR-2290) we can determine the required furring thickness, cladding fastener size and length and furring fastener."

    You may want to check these documents or call Hardie tech support to confirm what they recommend.

  8. Douglas Epperly | | #8


    It appears that LP's Technical Note No. 18 addresses this issue. It allows use of a 3/4 inch furring, but requires an additional 1/8 inch overlap on the siding and two nails per furring strip. The nails must penetrate the furring a minimum of 1/2 inch.

    I plan a similar wall structure as you. If I am missing something regarding this Technical Note, please let me know.

    Here's a link to the Technical Note:


  9. Douglas Epperly | | #9

    So, I just realized Technical Note No. 18 specifies using Southern Pine with a specific gravity of .55 or higher. Since we're in Montana, that may be a challenge for us.


  10. William Costello | | #10


    Thanks, I don't know that I ever would have found that technical note. Unfortunately, it specifies 16 inch centers. We plan on 24 inch. But think I'll give LP another call just to verify. Their basic installation instructions also seem to specify 16 inch centers (with 2x4 furring strips), but the rep I spoke with said that 24 inch centers are ok as long as you use their 76 series product.

  11. Matthew Vrazel | | #11

    Mr. Costello,

    I hope I am not too late with this answer. I work in the siding technical service group for LP Building Products, and I think you have two options for installing the siding according to LP's requirements.

    Another person correctly answered with a link to Technical Note 18; This document specifies using 1x4 southern pine with a specific gravity of at least 0.55 as furring strips. The average specific gravity across the 4 major species of southern pine (loblolly, longleaf, shortleaf, and slash) is about 0.55. The specific gravity is important because it is directly correlated with fastener withdrawal resistance. Therefore, using a furring strip with a lower specific gravity will have lower wind load resistance. Technical Note 18 states that furring strips must be installed every 16-in. on center for 36 Series Strand SmartSide products. However, 76 Series Strand SmartSide lap is rated for 24-inch stud spans, as you noted, and can be installed in accordance with Tehcnical Note 18 with the furring strips spaced 24-inches on center.

    You also mentioned using a 1-in. thick poly-iso insulation board without furring strips and thus eliminating the rain screen. You can use this option in combination with a drainable housewrap between the insulation board and siding which will create a drainage plane behind the siding. With this design, you'll be able to fasten the 76 Series Strand SmartSide through the foam directly to the 24-inch OC framing according to the installation instructions:

    I hope this information is helpful to you.

    Best Regards,

    Matt Vrazel
    North American Field Technical Manager
    LP Building Products

  12. Charlie Sullivan | | #12


    Thanks for joining the discussion and providing an detailed answer!

    I have a question about the specific gravity spec. The specific gravity will vary with the moisture content of the wood. The same strip of southern yellow pine might be above 0.55 specific gravity at 14% moisture content and below 0.55 at 6% moisture content. Is your spec for a specific moisture content? And more generally, is your expectation that everyone should be measuring and checking that spec, or is it enough to be sure it's really southern yellow pine, and not some other generic unspecified SPF species that would have a lower specific gravity?

  13. Steve Knapp CZ 3A Georgia | | #13

    Yellow pine might be hard to locate in some parts of the country. Would strips of CDX douglas fir plywood work as a furring material? It supposedly has a specific gravity of .57?

  14. Inger Peters | | #14

    I don't want to hi-jack this thread but I would like to talk with Matthew Vrazel with LP--I have similar questions but in regards to LP Smartside Panels over osb, rigid foam and 1x4 furring. Hope he looks back at this...if not maybe Martin can help make the connection. Agreed with William note 18 doesn't appear on LP's website....

  15. GBA Editor
    Martin Holladay | | #15

    I don't have contact information for Matthew Vrazel, but my Googling reveals that he works in the St. Louis, MO area. You could probably call the national headquarters and try to reach him by phone. His job title is North American Field Technical Manager at Louisiana Pacific Corporation.

    Louisiana-Pacific Corp. national headquarters
    Nashville, TN

    -- Martin Holladay

  16. William Costello | | #16

    Hope you can post back here what you find out. I'm wondering whether you can use furring strips from any of these pine species, or if you have to know the specific gravity of that particular lot of lumber. I'll give a call if you don't ...

  17. Jonathan Lawrence CZ 4A New Jersey | | #17


    I know you are looking for LP and not Hardie, but it my case with Hardie the specific gravity of the furring and wood framing was dictated by the technical bulletin for the fastener required by Hardie based on the type of framing I am using. The only option for 2x framing per Hardie's technical bulletin is FastenMaster. Hardie provided the link to the appropriate 3rd party technical evaluation report of the FastenMaster HeadLok screw to attach cladding and furring to wood framing through foam sheathing. The following is a footnote in the chart that shows fastener spacing for various combinations of cladding weight, stud spacing and foam thickness.

    2) Wood framing and furring shall be minimum Spruce-Pine-Fir or any species with specific gravity, G, of 0.42 or greater.

    6) Furring type and thickness shall be selected based on the cladding manufacturer’s installation requirements (e.g. required fastener penetration into furring).

    So back to Hardie and another technical report to determine the required fastener penetration and thereby the furring thickness

    It was almost a circular exercise, but now I know how they determine all of this. However, I discovered the easiest thing to do is call Tech Support, they already know the answer, so unless you really want to figure this out on your own, I would call LP tech support first.

    Btw, the FastenMaster report also has a chart that shows allowable design wind pressures by furring size and fastener spacing.

  18. GBA Editor
    Martin Holladay | | #18

    GBA has an article on this topic; here is the link:
    Fastening Furring Strips to a Foam-Sheathed Wall.

    -- Martin Holladay

  19. Jonathan Lawrence CZ 4A New Jersey | | #19

    That's it Martin. I wish I had seen that article before I spent the 3 hours trying to figure all of this out.

  20. Inger Peters | | #20 into Matt. Left a msg. I'll ask him to revisit this Q.A. and perhaps respond to the additional questions. Thanks Martin, Thanks Jonathan.

  21. Inger Peters | | #21

    Never got in touch with Matt. I plan on attaching 3/4" furring through 1" rigid foam, 7/16" OSB and then 1-1/4" into studs using 3-1/2" screws. Then LP 8" lap siding attached to the furring per Tech Note No. 18. CZ 3.---oh...and I'm not getting in the weeds about specific gravity of wood species. Sorry.

  22. Jay Johnson | | #22

    Hi, I want to bump this question up to ask about the siding nails specified in the LP technical note.

    The technical note says that ring shank nails are required with a diameter of .120". However, the only ring shank siding nails that I can find that meet this requirement are 3 inches long. So that means they'll be piercing through a significant amount of the foam if I use 4'' of foam, or pretty much all of the foam if I end up going with 2'' of foam.

    Is that OK? Or has someone found a source of siding nails that meet the diameter requirement but are closer to 1.5''?

  23. Walter Ahlgrim | | #23

    Might as well do anything you want LP warranty is a joke. LP has made bad batches of siding it swells and chips just like there hardboard siding did. It seems it will take 20 years to settle a warranty claim. If you take the failing siding off you get a reduced payment if you live with the ugliness for 20 years you get a check for that is for what you paid for the cost of the material at the time of purchase not a dime for labor.

    I say buy Hardieplank siding


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