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Community and Q&A

2 x 8 stud walls, or staggered?

Nicholas C | Posted in Green Building Techniques on

Preface:
Southwest Nebraska – Zone 5B – I experience all four seasons. Hot summers that can be humid some years. Cold winters.

Designing my home to build. Trying to determine which is the most cost effective method to do the walls. I am young, and intend on living in this house for a very long time.

I have done so much reading on here and other sources that I have honestly confused myself. Sad, isn’t it? I realize what works in Florida will not work in Nebraska for example. However there seems to be a lot of controversy. I have read that using anything more than a 2 x 6 stud gives diminishing returns on the extra material cost to using a staggered stud wall system for ultimate insulation.

I am currently thinking I should go with a 2×8 stud wall with 16″ o.c. – Whatever wall I choose, I want 16″ o.c. for purposes of hanging sheetrock, cabinets, shelves, tvs, etc. 24″ o.c. just leaves too big of space for me to consider, even though 2 x 8 with 16″ o.c. will be overbuild in terms of load, I wouldn’t change it to 24″.

My idea from exterior to interior: Siding or stucco > Firring strips > 2″ Poly Rigid Foam >House wrap> OSB/Plywood > 2 x 8 stud bay with blown in cellulose > no vapor barrier > drywall. My logic is any moisture that gets trapped in the wall would dry into the interior.

How does my method sound? In what ways could I improve this?

I am aware spray foam offers better insulation, but I believe it may be too costly. I would have to hire it done, and contractors are not cheap. I would be willing to use the DIY kits if I knew of something affordable and reputable.

A few options I can think of are to use more rigid foam inside the stud bays, or to use very little spray foam on the inside stud bays to serve as an air seal, then cellulose after that?

Is my method flawed? Have I combined too many methods into one big mess?

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Replies

  1. Dan Kolbert | | #1

    I think you'll find your idea is perhaps the least cost effective method.

    -Materials. Locally, I can get a 2x4x8 for $3.25, a 2x8x8 for $7.45, so the same dimension wall will cost about 14% more for framing

    -Labor. Don't know if you've ever done it, but framing 2x8 walls is a lot of work. Lifting them will require either a lot of people, wall jacks, or a crane

    -R-value. If you do 2 separate walls, you can make them however far apart you want. We have typically built double walls with a total thickness of 11-12". You'd be breaking the thermal bridge with the exterior foam, but you could get rid of the expensive and chemically nasty foam by just making your wall thicker. Plus exterior foam leads to a lot of extra labor for siding and window treatment.

    I wrote a piece on our first double-wall project for JLC several years ago (June 2009 issue). If you can't find it and would like a copy, you can contact me thru our website - kolbertbuilding.com.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Nicholas,
    Installing exterior rigid foam is a good idea. I don't know what you mean by "poly" foam -- probably polyiso (polyisocyanurate). You have to de-rate the performance of polysio in code weather -- let's call it R-5 instead of R-6 per inch. (For more information on this issue, see In Cold Climates, R-5 Foam Beats R-6.)

    So your 2 inches of polyiso will have an R-value of about R-10. If this problem bothers you, you might want to switch to EPS, or a sandwich with polyiso toward the interior, and EPS on the exterior.

    In this type of wall, the rigid foam is doing most of the insulating. (The problem with inserting insulation between the studs is that the studs act as thermal bridges, degrading the performance of the wall.)

    A 2x6 wall with exterior rigid foam makes a lot of sense. If you want a wall that performs better than a 2x6 wall, what you need to do is to add thicker foam (in other words, go to 3 inches of foam). You'll get much better results doing that than upgrading from 2x6 studs to 2x8 studs -- and the thicker foam is a much better use of your money.

    Do you live anywhere near Enders, Nebraska? My mother is a Grosbach, and is related to the Grosbachs of Enders.

  3. Nicholas C | | #3

    @Dan,
    I was under the impression a double / staggered wall would share the bottom and top plate for fire safety reasons, leading me to believe the wall would be just as heavy. What you're saying is to frame the house with standard 2x4 studs, then frame another 2x4 wall inside on a separate bottom and top plate. Is this a risk with fire code?

    Also, how would I manage an abnormal wall thickness when it comes to door and window extensions?

    Would it still be cost effective if I wanted to frame with 16 o.c. when building basically two walls? The gears are turning with the idea of having an extra thick wall.

    @Martin,
    I have some doubts with using the foam on the exterior. It is definitely not common practice around here it seems. I think anything over 2" with EPS or PolyIso would be troublesome working with and attaching to the exterior walls.

    Would it be more cost effective to use Dan's suggestion and skip the rigid foam? I would love to have siding or stucco right up against the plywood sheeting. But I fear with electric and gas prices, that's shooting myself in the foot for a long term house.

    If you were given a limit of 2" Rigid Foam (EPS or PolyIso), and a fair share of cellulose, what would be the most practical way to insulate while maintaining the consciousness of moisture needing to escape wall cavities? I am leaning towards 2x6 walls as you suggested with cellulose inside and foam outside, or perhaps 2 sets of 2x4 walls with a custom spacing...depending on if it's a fire safety risk.

    Yes, about 30 miles East of Enders. I am probably too young to know the family name, though.

    Edit: The brand of rigid foam I was talking about is the FOAMULAR series. I can purchase the 150, 250, or 500 models. I was assuming the FOAMULAR 250 4' x 8' x 2" would be okay on the exterior.

  4. Charlie Sullivan | | #4

    I'm not familiar with the fire safety issues you allude to with a double stud wall, but the way I have seen it done as two separate walls with separate top and bottom plates. Generally one or the other is chosen to serve as the load bearing wall.

    If you like the idea of skipping the foam and going with a double stud wall, you certainly don't need to worry about undermining performance--you can make it thick enough to perform better then the 8" stud wall plus 2" foam.

    Foamular is XPS (extruded polystyrene). It's made with high-global-warming potential blowing agent (>1000X that of CO2) in the US. That's a good reason to opt for EPS instead. EPS has 200X lower global warming impact, and is lower cost. Lower cost per inch thickness, and also lower cost per unit R-value.

  5. Nicholas C | | #5

    Charlie,
    The fire safety issue I am regarding is the fact there has to be something above so there is not just a void between the two walls that opens to the attic. Perhaps a piece of plywood, fire sheetrock, or a top plate? I do not know about the exact code here, but I also believe I have to have 'fire blocking' inside any exterior wall. I will have to check into it more. Is there a normal way to address this, or is my assumption just misinformation about the subject?

    I like the idea of skipping foam for cost and conventional siding methods. However, I think the foam is crucial to have energy efficient walls in my area. I'm guessing my walls would need to be spaced 8" apart for accumulate for the loss in R-Value from not using any foam.

    Thanks for the information about Foamular. I am not familiar with what EPS options I have in the area. There is a Menards within decent distance. They offer R8 2" x 4' x 8' Expanded Polystyrene Insulation. I am guessing it is cheaper because it is less R-Value per inch? Even if this is the case, I'd probably not want to use over 2" exterior. 3" of foam seems like it would be very complicated when attaching firing strips to, along with door & window flashing?

  6. Charlie Sullivan | | #6

    Ah, got it. It makes sense that you'd want plywood or sheetrock there for that reason, but I don't know the code.

    EPS starts with a process that makes tiny beads with compressed gas embedded in them. It's shipped in that high-density form to regional operations that expand it into the final shapes. That two-step process saves some shipping cost because it's lower volume in the first shipping step. But it means that there are lots of smaller operations that do the expanding, and it seems that they sometimes sell more directly rather than going through home centers which do their purchasing from larger national manufacturers.

    The stuff at Menards is a fine option, but you might be able to find a better deal direct from a company in your area. There's a directory of the here:
    http://www.epsindustry.org/eps-manufacturers-map

  7. Dan Kolbert | | #7

    Think of the framing as a house built with 2x4 walls. Then you just build another wall around the perimeter of the house before your interior partitions. No fire issues - the decks on each floor block things.

    And framing with 2x4 walls is fast, cheap, easy. The double wall flies up. You could easily go to 24" OC on the double wall, since it's not doing anything structural. Want to stick to 16"OC on exterior wall.

    Even with 2x8 walls, you're thicker than standard. We set our windows to the exterior; our clients always love the deep window wells (and especially the deep windowsills).

  8. Malcolm Taylor | | #8

    Nicholas, you may want to look at the double wall details here on GBA and elsewhere. There are a number of variants - some have the load bearing wall as the exterior, some the interior. This can have implications for your foundation width and the extent of thermal bridging. There can also be concerns with cold sheathing with such thick walls which may affect which materials you use and where they are in the wall assembly.
    Our code allows the insulation to act as a firestop so no continuous top plates are required, but it's probably a good idea to see what your code says.

  9. Nicholas C | | #9

    @Charlie - That makes sense on the price difference. The nearest manufacturer is 4.5hours away from me, so I probably won't see much of a savings unless they do deliveries. The ones at Menards do seem reasonably priced. They also don't seem to have a brand name, which may or may not be good. I am curious about the method of installing them compared to the Foamular panels. These EPS panels don't seem to be tongue & groove. Is there a specific tape used for EPS panels? I was wondering if housewrap over the EPS is still okay to do as if it were with the other type of rigid foam?

    @Dan - I follow the process after reading more about them. One thing is that I would want to frame my interior walls 16" o.c. also, because it makes sheetrock and hanging things much more enjoyable. I realize it will cost a little extra, but I would not be building much more than a 1200 sq ft area. One thing the articles don't seem to address is working with doors and windows in detail. Do you install all doors and windows before the second wall? I can see it being troublesome having to reach through the interior wall to do those things. When it comes time to upgrade a door or window, is it much more complicated when you have in essence...two walls? Those are probably my leading questions when comparing to a 2x8 wall.

    @Malcolm - I read a lot about the double wall method. I found a handful of variation between using a single top and bottom plate between two walls, framing two walls, and also framing each wall so the studs were staggered equally. I would definitely build the exterior wall to serve as my load bearing wall. I would fasten my single top plate on the interior wall to the bottom cord of the truss, and the bottom plate to the subfloor. Perhaps a double top plate so standard studs can be used w/o many cuts involved. The only part I scratch my head about is the void between the two walls' top plates. I think a piece of 3/4" plywood would take care of that (other suggestions welcome). I read an article here about cold sheathing, and the suggestion was to use plywood on the outside of the interior wall. That would be interesting, but my exterior wall is load bearing and would require the sheeting. In my Zone 5B, what would alternatives be? Would it be a problem with 2" EPS on the exterior sheeting? I'd hate to have the double wall plan backfire with mold problems.

  10. User avatar GBA Editor
    Martin Holladay | | #10

    Nicholas,
    Here's a link to a good article on fire blocking: Fire-Blocking Basics.

  11. Joseph Malovich | | #11

    Alternative to a double stud wall is a larsen truss: http://www.greenhomebuilding.com/articles/larsentruss.htm which is basically a conventional 2x4 house with an offset balloon framed shell that can be a 2x3 or less sometimes. Similar to this is a wall built with a conventionally framed wall composed of TJI Joists for the studs. These will be more dimensionally stable and lighter than 2x8, can go thicker for not substantially more cost, and reduce thermal bridging by using thin OSB instead of a full thickness 2x. There is no need to use anything more than a 2x4 for any wall now-a-days. Plywood window bucks and non-flush-mount windows do wonders, mount the windows near the half way point of the insulation thickness.

  12. User avatar
    Stephen Sheehy | | #12

    Nicholas: reclaimed XPS is widely available and is an economical and environmentally good choice, if you decide to use foam. I used it under the slab.

    My new house, currently under construction, has two 2x4 walls, studs 24" o/c. It's a single floor on a slab, total wall thickness is 12 1/2" with the outer wall carrying the structural load. One feature I like is that the primary air barrier is a membrane attached to the outside of the inner wall. Most of the plumbing and wiring is inside the inner wall, and thus don't penetrate the air barrier.

  13. Dan Kolbert | | #13

    If you have a good rainscreen detail, I don't think you have to worry that much about "cold sheathing."

  14. Nicholas C | | #14

    @Martin - Thanks for that article. It seems I am correct about worrying about any gap between the double framed walls leading to the attic. Since the house is a single story build over a basement, that won't be much of issue.

    @Joseph, that was the article that got me started on this path. I dislike how unconventional that framing is. I realize it's less burden with one story. However I don't see a lot of differences (besides the 1x4 that holds the two walls together in place) compared to just a secondary interior wall. It seems like more hassle. Interestingly, the article states "The three air barriers (drywall, dense pack, housewrap) make the walls virtually impermeable to infiltration." which would mean a double wall like I am describing should be the same...making the cold sheathing a non-issue, right? I wonder what the inspector would think of floor joists as walls. I don't see them being a cheaper method than double framed or a single 2x8 wall here, though. Prices are raised in the area. I have read about installing the windows at the halfway point with thick walls. I wonder what the most effective way is to weatherize the exterior part of the window buck in an attractive way, though.

    @Stephen - I will check around. What kind of slab did you have? Was there a reason you used the XPS over EPS...moisture reasons? I like the sounds of your wall method, it sounds like the route I was leaning towards after this post. What membrane did you use for the outside of the inner wall? Do you plan on using any house wrap or rigid foam on the exterior wall after the sheathing? I'd be interested to know the details to your choice :)

    @Dan - I guess I thought the cold sheathing issue was relevant to winter time when the chilled air condenses on the interior wall (why, I can't understand yet?). If I had 2" of EPS Foam over the exterior anyway, plus the house wrap, I think that would be a good rainscreen right there, don't you think?

    Appreciate all the input here. Not many if any local builders use anything like what is being discussed here for their builds, so it is hard to see it in first person and figure out what does and doesn't work.

  15. User avatar
    Stephen Sheehy | | #15

    Nicholas:
    Reclaimed XPS was readily available from a few sources. It is commonly used as insulation on commercial roofs. I don't know why it isn't reused when replacing such roofs. It is priced at about half the cost of new and, other than being dusty, is in great shape. Just Google "reclaimed foam."

    From outside in, our wall consists of
    White cedar shingles
    Cedar breather
    Typar, seams taped
    Advantech, seams taped
    2x4 studs
    5 inch space, to be filled with cellulose
    Siga Majpell membrane, applied to
    2x4 studs. Membrane stapled to studs while the walls are on the floor, seams taped and staples also taped. (We used a lot of tape)
    Cellulose
    Drywall.

    My blog has more info and some photos:

    Stephen1147.wordpress.com

  16. Nicholas C | | #16

    Stephen, unfortunately I am unable to obtain affordable reclaimed XPS in my area. I am in a low density population area so many products are limited. The only options for rigid foam so far from a builder's store are basically limited to the Menard's chain stores.

    I did read each post of your blog. Interesting system. I am currently trying to find a safe way to build the double wall system.

    In my research, it seems the double wall systems talked about never bring up cold sheathing. Each builder has their own theory. One theory that seems to hold up is having 2/3 of the R value on the outside of the wall, so dew point is never reached. I am slowly starting to doubt having the extra thick walls filled with cellulose because of controversy of moisture problems due to having cold exterior sheathing.

  17. Malcolm Taylor | | #17

    Nicholas, John Straub seems to think that the sheathing will be alright as long as you allow for good drying ability by including a rain screen cavity. If you are uncomfortable with that you can always eliminate the sheathing altogether, or move it to the centre of the wall.
    http://ourhouseuponmoosehill.blogspot.ca/p/details.html

  18. User avatar
    Stephen Sheehy | | #18

    Nicholas: whether 2x4, 2x8, single wall or double, sheathing in a cold climate will get cold. Put a lot of insulation outside the sheathing if it concerns you. I think for the 99% of houses being built without much or any insulation outside the sheathing, water and air management are most important. Keep moisture out of the wall and if it still gets in, let it dry either to the interior, exterior or both.

  19. Patrick Walshe | | #19

    we did a staggered 2x4 stud wall on 10" plates prefabbed at a panelized construction place, blown with dense pack cellulose, airtight drywall, rigid roxul board on outside of plywood sheathing, 1x4 rainscreen = warmer sheathing and drying potential in or out (also generous overhangs, careful housewrap laps, flashing etc of course)

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