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Rigid foam insulation advice

[email protected] | Posted in Green Building Techniques on


I need to replace some siding on my house so I was thinking this would be a good opportunity to add some insulation. I have done quite a bit of research and it seems adding some rigid foam would be a good way. I thought about just having some dense pack cellulose blown in to my walls but that doesn’t appear to be worth it since there is already fiberglass in the 3.5” bays.

I am thinking of adding 1” of rigid foam under LP Smartside. I would love to add more insulation but the roof overhang doesn’t extend far enough to allow this. I thought about extending the overhang but I think that is more than I can bite off right now. I am planning on doing most of the labor myself and I have limited time. I am in climate zone 5 so based on my research on this site 1″ meets the minimum thickness.

Here are the steps I’m planning to take:
Add 1″ foam over the existing OSB sheathing.
Add a WRB over the foam. I have read on here that the foam can be used as the WRB but being a novice I don’t want to screw something that important up.
Add 7/16” thick plywood or OSB vertical furring strips (for air movement).
Attaching new windows over foam to studs with long screws (with proper flashing into WRB).
Nailing the siding through the furring strips and foam then into the studs.

Now for the questions:
1. Does this sound like a good plan?
2. Is all this worth it for 1″ of foam?
3. If this plan is good what method would everyone recommend for fastening the siding to the studs? I’d love to use a nail gun but not sure if that’s a good idea or not. I would need at least 3″ nails (any recommendations?).
4. How important are the furring strips? I was thinking it would be a little easier not to use them but the air gap seems to be worth the added trouble.
5. Would it also be good to add dense pack cellulose to the stud bays while I have easy access to them? Or, maybe just add the dense pack only and skip the foam?

Thanks in advance for the help!

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Does this sound like a good plan?"

    A. Yes. However, if you can find a way to increase the foam thickness to 2 inches, it would be even better. Sometimes you can increase your roof overhangs at the rakes and eaves by just installing wide drip-edge flashing.

    Q. "Is all this worth it for 1 inch of foam?"

    A. Probably. But I would still aim for 2 inches if I were you.

    Q. "If this plan is good what method would everyone recommend for fastening the siding to the studs? I'd love to use a nail gun but not sure if that's a good idea or not. I would need at least 3 inch nails (any recommendations?)."

    A. I think you should use 1x4 furring strips and you should fasten the siding to the furring strips rather than to the studs. For more information, see How to Install Rigid Foam Sheathing and Fastening Furring Strips to a Foam-Sheathed Wall. [Later edit: It turns out that Louisiana Pacific requires furring strips under Smartside to be at least 1 1/2 inch thick.]

    Q. "How important are the furring strips? I was thinking it would be a little easier not to use them but the air gap seems to be worth the added trouble."

    A. You're right: the air gap provides many benefits, and will probably make your siding last much longer. If you decide to omit the gap, you will first have to check with Louisiana Pacific to see if the manufacturer allows Smartside to be nailed through foam.

    Q. "Would it also be good to add dense pack cellulose to the stud bays while I have easy access to them? Or, maybe just add the dense pack only and skip the foam?"

    A. If your stud bays are already filled with fiberglass batts, I wouldn't bother with the cellulose. Just concentrate on doing a good job with the exterior rigid foam.

  2. GBA Editor
    Martin Holladay | | #2

    More information from Louisiana Pacific: here is some relevant information from the Smartside application instructions:

    "LP SmartSide Sidings may be installed over low-compression rigid foam or fiberglass sheathings. The following precautions must be followed:
    a) Adequate bracing of the wall in accordance with the International Codes or other ruling building code is required.
    b) For rigid foam sheathing up to 1" (25.4 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 in-creased to ensure a minimum 1-1/2” (38.1 mm) fastener penetration into the structural framing.
    c) For rigid foam sheathing greater than 1" (25.4 mm), a minimum 1-1/2” (38.1 mm) thick by 4” (101.6 mm) wide vertical strapping 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” OC (406 mm) with a minimum nail penetration of 1-1/2 inches (38.1 mm) and a maximum nail spacing no greater than the width of the siding."

    Note that 2x4 furring strips wouldn't be wide enough; you would have to use 2x6s for furring strips if you go that route.

  3. [email protected] | | #3


    Thank you for your quick responses. I was guessing you would say the thicker foam the better. I'm going to look to see if I can get another 1" to work without too much difficulty.

    I have also read the instructions from LP which is why I was thinking 1" thick foam would be easier since I could avoid the added depth of "2 by" furring strips. In my reading here it seems most people are using 1x4 furring strips. I would much prefer that to 2x6s. Do you know which siding allows for this? I would like to use lap siding. Also, good catch on the 2x4s not being wide enough! I missed that little detail in my reading.

    One last question. If I can find a way to use a total of 2" of foam is it ok to use 2" thick pieces and make sure the seams are air tight or is it significantly better to use two layers of 1" foam so the seams don't overlap?

    Thanks again for your help!

  4. GBA Editor
    Martin Holladay | | #4

    Many builders have successfully installed a wide variety of siding types on 1x4 furring strips. One possible option is to ignore the manufacturer's recommendations. Although this would be a code violation and might result in a loss of warranty protection, you wouldn't be the first builder to go that route.

    Another alternative is to install fiber-cement siding.

    Here is more information on the issue (from my blog, Fastening Furring Strips to a Foam-Sheathed Wall):

    "Until recently, many siding manufacturers recommended that siding nails penetrate 1 in. or 1¼ in. into wood. That’s beginning to change, however. The Vinyl Siding Institute requires only ¾ in. of fastener penetration for vinyl siding; James Hardie Corp. accepts only 7/16 in. of penetration for fiber-cement lap siding.

    "According to building scientist Joseph Lstiburek, if you have any doubts arising from the fact that your siding nails penetrate into only ¾ in. of wood, just switch from smooth-shank nails to ring-shank nails. Unless you’re building near the coast in south Florida, ring-shank nails will be more than adequate, even when penetration into wood is only ¾ in."

  5. Expert Member
    Dana Dorsett | | #5

    +1 on the use of ring-shank nails when nailng into furring.

    Not only does it have better retention, going with the shorter nails yields much lower thermal bridging into or through the foam. Furring through-screwed to the studs 24" o.c. or even 18" o.c. studs has but tiny fraction of the performance robbing thermal bridging of a few thousand long-nails.

    +1 on bumping the thickness to 2" too.

    If the pre-existing fiberglass has foil facers or there is sheet poly between the insulation & interior gypsum you'll have to pay close attention to what foam & facers you use on the exterior, lest you create a moisture trap. This is much less of an issue (but still something to consider) if the fiberglass has kraft facers. (Kraft facers on batts run about 0.4 perms when dry, but higher when wet)

    The best R/inch you get with rigid foam would be polyiso at ~R6/inch, but it's most commonly sold with foil facers, which are a true vapor barrier. But some foil-faced iso is R10 @ 1.5" which may be easier to accomodate than 2", so long as you're sure what's on the interior side of the assembly. If you have poly or foil on the inside you could still use polyiso by going with roofing foam with a fiberglass or paper facer, most of which run between 0.5-0.8 perms, a modest but real drying capacity. The fully black asphalted paper versions have much lower vapor permeance and not such a great choice. XPS runs about 0.6-0.8 perms @ 2"/R10, but unfaced Type-II EPS runs a healthy ~1.5 perms @ 2"/ R8.4, and would offer the best possible drying rate. (Rigid rock wool would have even higher drying capacity, but it's a pricier proposition.)

    If the pre-existing fiberglass is pre-1970 or even newer R11s rather than recent vintage R13s it may be worth drilling 1-1/4" holes spaced every 3-4' along the stud bays on the exterior and blowing cellulose until the blower stalls (assuming it's a single stage blower that won't pop the gypsum off) moving along to the next, etc. The old stuff or low density stuff is less air-retardent than R13s, and even though the R-value of the assembly won't be changed much by adding the cellulose, the air tightness will, even though it won't hit dense-pack density. Many homes with low-density fiberglass (including sections of mine) will have developed mouse-nest voids over decades, and filling with denser cellulose will restore the fill, and substantially block mouse-superhighways (like the ones in my mid-1980s 2x6 addition that I'll eventually get around to dense-packing.) With a box-store rental blower you'd probably have to cobble up your own blowing nozzle/reducer from plumbing fixtures, and use copious duct tapes to get it over the ~2.5" flex hose, since they usually only have them set up for open blows in attics. With the multi-hole approach you won't hit 3.5lbs dense pack densities, but you'll be bumping on 3, a density with substantial air-retardency. (Of course this is easy for me to recommend, since I have a beat up but functioning insulation blower in the corner of the garage to haul out for projects like that.)

  6. [email protected] | | #6

    Ok, I think I’m starting to get convinced I need to use thicker foam with furring strips. I will do a little more research on local codes to see if there are any other types of siding that can be used with the ¾”. I’m a little nervous to use the Hardie lap because it seems easier to screw up and I would need to get some more tools. I don’t mind learning new things but I also realize I’m not a professional and experience goes a long way with this type of thing. Am I overthinking it and should I just do it?
    Martin, thanks for sharing the link to your furring strips article. I had read it before but I forgot about the last part. It does give me confidence that ¾” would hold my siding even if all codes don’t acknowledge it yet. I find it strange that hardie siding (which I would think would be heavier and need more support) allows for half the penetration as manufactured wood. I’m definitely no engineer though…
    Dana, Thank you for the detailed response! The fiberglass batts have Kraft paper on them. I was planning on using the foil faced polyiso. Based on my understanding of your post that is still ok because I have the Kraft batts. Hopefully I’m understanding correctly.
    I looked into building my own reducer to use with the rental cellulose machine and it ended up being more money than to just pay someone. Maybe I’ll call to get a quote. I know a very cool guy that will let me do all the prep/finishing work to save money.
    I thought of something else now that I’m planning to use the furring strips. Does it matter if the nails penetrate the WRB I was planning on putting over the foam? It seems like there will be lots of little holes unless I get the perfect size nail. I would tend to lean towards a longer nail to make sure it gets the full ¾” penetration in to the furring strip but I don’t want to mess anything up. Am I being to anal?

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Does it matter if the nails penetrate the WRB I was planning on putting over the foam?"

    A. No. As long as you have an air space between the WRB and the back of the siding, liquid water will follow the path of least resistance. The fact that the nails penetrate the WRB won't cause any problems.

  8. Expert Member
    Dana Dorsett | | #8

    As long as there's nothing more vapor retardent than the kraft batt facers on the interior and the flashing details are attended to properly, foil faced goods on the exterior are fine.

    Note, the thicker you make the exterior foam, and the more air-tight the assembly, the lower the overall risk. A square inch of air leak can more wintertime moisture into the cavites than a whole house-worth of vapor diffusion through kraft facers. But if half of the total center-cavity R value is outside the sheathing, the wintertime moisture from minor air leaks or diffusion accumulates in the sheathing, because it remains above the dew point of that air for more than 75% of the time, even during the coldest weeks. So with R12 on the exterior the drying rate toward the interior doesn't have to be nearly as high as when you only have R5 (or less) on the exterior, where there would be at least some seasonal moisture accumulation to purge.

  9. [email protected] | | #9

    Martin and Dana, once again thank you for your replies!

    I heard back from the building inspectors office after I sent my plans and they said I would be up to code. I now have to figure out a way to 1.5" to 2" of foam attached to the walls but thanks to your suggestions I will find a way.

  10. [email protected] | | #10


    Sorry to revive an old thread but I thought it might help others who are doing the same project as me to keep it all together. I am just about to start on this project and I have been thinking about how I am going to flash the windows. Just to recap, I am planning on putting a WRB over 2” of foam. I will then put 1x4 furring strips over that.

    My question is about flashing the windows. I am planning on putting a “picture” frame of furring strips around the window so I have something to attach the nailing flange to. There will be foam under the furring strips. I have attached a rough drawing of how I was planning to flash the windows. I’m not very good with SketchUp but hopefully you get the idea. Is this plan a good plan?



  11. kevin_in_denver | | #11

    Larger gutters are another easy way to extend the overhang.

  12. GBA Editor
    Martin Holladay | | #12

    Pete B.,
    If I understand your sketch correctly, you're on the right track. Just make sure that upper layers lap over lower layers.

  13. [email protected] | | #13

    Thank you. I just wanted to make sure I wasn't doing something inherently wrong. I will definitely make sure the upper layers overlap the lower layers. Thanks again for all your help!

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