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Community and Q&A

Final wall stack-up?

Chaubenee | Posted in Energy Efficiency and Durability on

I think I have decided on the final stack up for my walls here in Zone 5. From outside to inside: Hardi siding, firring strips every 12″, 2″ of XPS foam with taped seams, house wrap, 7/16 OSB with taped seams, 5 1/2″ open cell foam (Aloha Foam) to edge of 2×6 (24″ OC) stud interior, sheetrock with latex paint. Of course we are using an ERV in the house, Tripane Marvin Integrity Ultex windows.

I studied in depth ideas to use ZipR, Mooney straps, etc. but found none as appealing as the use of exterior foam to reduce bridging, and open cell foam to add sealing and R value.

As an aside, I am using treated plywood for the bottom two feet all the way around. And am using tape to cover the intersection of the mudsill and wall, followed by ‘made in the field’ aluminum Z flashing to cover the bottom of the foam to protect from critters and varmints. The mudsills/plates will be spray foamed as well.

Any issues with this set up? Appreciate your thoughts.

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  1. GBA Editor
    Martin Holladay | | #1

    There are two unusual aspects to your wall.

    The first unusual detail is the decision to install vertical furring strips on 12 inch centers. Usually, furring strips are screwed through the rigid foam to the studs. I assume that your studs are 24 inches on center, and you want some intermediate support for your siding between the studs. As long as you realize that half of your furring strips will only be attached to OSB rather than to framing, I suppose that there is no harm to your plan. But if I wanted more support than 24-inches-on-center furring strips would provide, I would just switch to 16-inches-on-center framing (for both my studs and my furring strips).

    The second unusual aspect to your plan is the use of spray foam between the studs. Since you have about R-8 of rigid foam on the exterior side of your OSB, and several opportunities to include a good air barrier, I don't see any advantages to the use of spray foam instead of cellulose.

  2. Chaubenee | | #2

    Martin, you are correct, the studs are at 24 OC. I am pretty stuck on 24" OC.The strips that lie in between are simply to provide better backing for the Hardi so it is NOT wavy. They will only get attached into sheathing, THIS I KNOW. But the strips that are attached into the studs do the "real" work. Secondly, as to cellulose. Having spent thirty years living in two houses with cellulose insulation I have found that the dust that comes through the outlets over time, and the settleage that occurs (that you discover when you open a wall for remodeling or some repair) has always bugged me. I know everyone will say that dense pack will prevent that, but I am still not a happy customer of cellulose in walls. Attics are a different story especially if they are sealed off and one never goes in there other than to inspect. There are greener open cell spray foams on the market that provide the benefits of air sealing and R value, as well as sound deadening. I also think that going forward foams will become the norm and in twenty years homes without foam will be seen as the dinosaurs and white elephants. Besides, since I am not doing the work myself, the price difference is not great. I did think about roxul bats too and if I can find one installer who has wrked with the material I would consider a fash and batt application with open cell foam and Roxul batts. But around here there are essentially two kinds of insulation- foam and glass, and the outliers who can do cellulose charge big money because they all are used to doing magical blown in retrofit work. Asking insulators to use roxul is like asking plumbers to be able to use email or to quote your job for less than $250 an hour.
    By the way, my windows will be "innies" since this takes care of the extension jamb difficulties as well as providing a sounder nailing surface and flashing detail for the flanges. I think it is better for the windows too, and at only two inches it doesnt look horrible. Here is a video from Dupont that illustrates the assembly:

    Do you think it all works?

  3. Expert Member
    Dana Dorsett | | #3

    You can't get fire retardent & cellulose dust out of outlets if both the sheathing and outlets are air tight. Using damp-sprayed cellulose also binds most of the fire retardent to the cellulose. Dense packed cellulose is sufficiently air retardent that even with non air-tight outlets & sheathing dust is not an issue. Your prior cellulose experience was likely to be very low density 2-hole method or even "fluffed" retrofit installation.

    On a deep energy retrofit was involved with a couple of years ago when the interior was gutted the ~30-year old 2-hole method cellulose in 3-story balloon framed house had not settled a bit, and looked as fresh as if it if had been installed the prior week (rather than 300+ weeks prior).

    On the other side of the coin, a friend's 2-story balloon framed house showed (via IR imaging) settling of more than 4' in only 10-12 years, which I personally remediated by dense packing.

    Both the equipment and cellulose material has improved over the past few decades, as has the average competence of the installers, but there's nothing wrong with open cell foam. I'll be interested in your review of Aloha's open cell product. (I've read several positive real-world reviews of their 1.8lb closed cell goods.)

    Installing the furring horizontally 16" o.c. would likely deliver a flatter wall than vertically 12" o.c. with every other board attached only to the sheathing.

  4. Chaubenee | | #4

    Dana, if i ran the furring horizontally, I would be concerned about the ventilation of the rain screen behind the Hardi siding. Do you think the air space alone would be fine? Or would you recommend some breaks in the furring every few feet to allow drainage? Also at 16 inches apart how would I attach Hardi lap siding that needs to be nailed every 8" or whatever? You have me confused. I can see how doing it with the panels (batten boards style) would work but for lap siding I am not sure what you mean.

  5. dankolbert | | #5

    If you can find someone to do Roxul I wouldn't bother with flash and batt. The advantage of that approach is the air seal of the flash, but if you have exterior foam it shouldn't be hard to get a decent air seal either at the foam or sheathing.

    And it sounds like you're settled on it, but I'll echo Martin & Dana's recommendation to use cellulose. We dense pack old and new houses pretty much constantly and have never had clients complain about dust nor have we seen any signs of settling.

    Make sure Hardie accepts whatever furring detail you come up with. They have very good technical resources, both printed and human.

  6. Expert Member

    Attaching the intermediate furring to the OSB will be fine. In fact if you switched to 1/2" plywood you could attach all the furring to the sheathing without worrying about hitting the studs. Use ring-shanked trim air-nails.
    You won't get much if any drying through the Hardi siding and fastening to horizontal furring would add a layer of real difficulty to the installation, so I'd keep it vertical. If you form the bottom flashing to cover both the foam and the cavity, and get it made from perforated U stock, the wall can dry, or drain at the bottom.

  7. dankolbert | | #7

    And I will also add that we gave up on fiber cement siding a long time ago. I assume you're using their clapboard product? I would recommend looking in to LP Smartside - most of the FC installers I most respect switched over to that product years ago.

  8. Chaubenee | | #8

    Malcolm, in regards to plywood, I think you are correct. I was intending to use 1/2" treated plywood for the bottom two feet anyway. But going to the plywood makes for a better product in this regard and it may be the answer to the furring out dilemma. I appreciate that advice... Bear in mind I have 157 sheets of sheathing. Also: I was intending to either make a Z flashing on sight that would drain to the rear or have the metal shop bend me some perforated U or simple L channel to place at the bottom for critter control. I like to be able to protect that bottom edge of foam and aluminum screen isnt that tough. I really like the idea of gravity to drain the plane under the siding and horizontal furring kind of defeats that. The corrugated furring made of plastic might be an exception. As to LP Smartside vs. Cement Board (Dan) I think they are both excellent when used with a fully ventilated assembly which can drain. I am doing both clap on the bottom and boards/battens up top with a band of trim in the half way up middle. I really like Miratech trim. The trim around the windows will be made indoors as boxes and fastened on once the foam gets attached. It is nice that there are only three sizes of window on the house. The nice thing about this assembly especially this time of year, is that it allows us to get dried in fast with wrap, and windows in place. Then the foam, furring and trim work, which is slow and pains taking is not slowing the drying in process. Plenty of the trim work can be done indoors and then fastened up on clear days. Sometimes, formulating a battle plan is the hardest part.

  9. Chaubenee | | #9

    Dan, upon further thinking, I agree with your comment on the Roxul not needing flash and batt. I am working in pricing both, and my competent carpenter is able to do the work with the batts, I believe. However, with this assembly, framing and drying in and sheet foaming will be the same no matter which of the three methods of insulation I so choose. What I intend to do is to get her dried in, and sealed and have various contractors come on premises to measure accurately, bid and discuss the insulation options in that regards. All of these vapor permeable insulations would handle drying out properly with this stack-up if I am not mistaken, and none would be risky or require fancy gimmicks or vapor membranes, etc. The only thing ruled out is fiberglass which if I am to believe my sources in the environmental testing and legal community, will be considered the asbestos/lawsuit whipping boy of the future. I will certainly report back my findings and ask for more wisdom.

  10. AlanB4 | | #10

    Dana, how did you dense pack the balloon framed house, did you have to remove the existing loose fill insulation first?

  11. Dana1 | | #11

    The house with the severe settling issue was dense-packed from the interior with a single hole midway up the wall. The dense packing tube was inserted initially downward air-only as far as it would go. When the 1-stage blower stalled the tube was pulled back a foot or so until it stalled again until there was only a couple feet of tube in the wall. From then the tube was inserted upward in the cavity air-only until it jammed, then pulled back, repeat. It took a LOT more material than I had initially anticipated, and at one point I was wondering if it was filling up between floor joists(!).

    The density isn't as even nor is it as high as if it were done in one go with a 2-stage blower. I'd estimate 2.8-3lbs density, but it's impossible to know without having the initial volumes or taking cores. It's dense enough that it's not going to settle any time soon, but it's less than ideal to be sure.

    After 10+ years of settling it's unlikely that the portion below that which was dense packed is going to settle further forming a gap in the middle. If he still owns the house in 10 years it would be interesting to do more IR imaging to see if any voids formed.

  12. AlanB4 | | #12

    Interesting, thanks for the reply

    I also have loose filled and was considering dense packing it someday, though once the vinyl siding wears out (a few decades i assume, its 10 years old now) i considered removing it, removing the wood siding under it (there is no sheathing) filling the true 2x4 voids with roxul then sheathing and maybe roxul boards or foam board (not sure if i can foam exterior, its a century old house with 2x4, 1 1/2 inch barnbaord then plaster, lath, plaster again and who knows how many coats of oil based paint.

  13. Chaubenee | | #13

    I have a question related to the furring strips. I think I know what to use, but if there is a choice between 1x3 furring strips (3/4 inch stock) or 1/2" plywood ripped into strips or 3/4" plywood ripped into strips- what would you use over the foam to nail the cement board siding into?

  14. Expert Member
    MALCOLM TAYLOR | | #14

    We use nothing but 1/2" plywood strips, but that's without exterior foam. I don't think 1/2" could be trusted to lay flat when fastened through the foam. I wonder if 3/4" could? I prefer plywood to solid stock because you don't have to worry about the fasteners splitting the strips, but again maybe the greater stiffness of a 1"x3" on the foam outweighs that.

  15. Chaubenee | | #15

    I am leaning to 3/4 plywood strips ripped to 3 inches. I think they are less likely to split, and the multiple layers of laminate and the glue, like a good set of chaps, give more stick in the saddle, (for the nail) to borrow an expression from the days when I rode like a cowboy. Stiff as my back these days? Not so much, but less likely to split in my opinion. Can anyone with serious background in nailing to furring over two inches or more of exterior foam share their preference and why so?

  16. Chaubenee | | #16

    I found an article here, plus this and both say that 24oc is fine, but 1x4 is preferred. Also the six inch screws must be no more than every 33 inches apart. So predrilling the strips in stacks with premeasured holes marked out is the way to go, apparently.

  17. Adam Emter - Zone 7a | | #17

    Joe, I used 1x4 on my house, spaced 16"o.c. Screws through the foam were spaced 24-30" then used ring-shank nails to attach wood lap siding. Only had one or two small splits in the wood. The 4" gives you plenty of room to stagger nail holes as you attach siding going up the board. Once I gripped the top of a piece of siding and hung my body on it. Very solid. I did gang drill about 6-8 pieces of furring at a time. Then came back and drilled a small countersink on each hole to make sure the screw was countersunk (but I had 6" of foam and 10" timber screws, so you may not need to do that step with 2" of foam and smaller screws.)

  18. Chaubenee | | #18

    My thought on the strips at 12oc was that the strips that sit in bteween my 24oc studs would simpky be backers to keep the wavyness in check and to provide support in btween if soneone pushed or something. Maybe they are unneeded? If they are just backers to lend support snd nothing is attached then i imagine I can get away with some of the 1x4 ripped in half and screwed more like every six feet. I was not sure if I needed this 'tweeners everywhere but maybe more just where it made sense like on areas like the covered porch or near entry doors, in case there is pressure (kids leans on the building or porch rocking chairs bumping up against the wall!) and also perhaps on the west elevation where we might get lots of wind, and the very few places also where we might see longer and uninterrupted windowless spans of siding. There is also the matter of the batten boards, and posting these directly under the 12oc battens as well in the gables.. It also occurs to me to perhaps do reverse battens then the battens are part of the rainscreen furring. Any thoughts? These are in the gables primarily.

  19. Chaubenee | | #19

    Back to cellulose, which is not out of the question seeing as you all so heartily endorse dense pack. I cant find anyone in the Albany, NY region who does it other than one larger statewide outfit. Does anyone habe a handle on installers in Albany or Saratoga areas? It is unreal how far behind the times this area is compared to other regions in building techniques and science. With this wall design, I can use any products that dry inward.

  20. Expert Member
    MALCOLM TAYLOR | | #20

    Hardi is still king here. What are the main advantages you've found with LP Smartside?

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