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Community and Q&A

Can brick/stone veneer coexist with double wall construction?

Alex House | Posted in Green Building Techniques on

Before I started reading articles here on double walls and dew point issues in the OSB/Plywood sheathing I had intended the following wall construction on a 10″ foundation (4″ ledge + 6″ concrete) wall – 3″ stone veneer, 1″ drainage plane, Tyvek, plywood sheathing, 2×6 load bearing wall, 2″ inch XPS layer, 2×4 interior non-load bearing wall, drywall. Forget all that now.

Clearly that plan was a disaster waiting to happen in that the sheathing would run afoul of the guidelines regarding proportion of exterior insulation with respect to insulation inboard of the sheathing.

The problem appears to be the brick/stone layer limits the ability of a builder to layer on exterior insulation to address the dew point issue without having to expand the brick ledge, inch for inch. I’m not really keen on having to pay for a 13″+ inch thick foundation wall in order to free up 3″ (or whatever is needed to address the dew point issue).

So, are these two features compatible? If so, how to design it?

Secondly, instead of pouring concrete to thicken to foundation wall to create space for the exterior insulation, can the 2×6 load bearing wall be shifted inwards to some degree? Does the entire 2×6 wall have to rest squarely on the foundation?

Thirdly, I came across a reference which noted that shear panels can be located on the interior plane of the exterior, load bearing, wall. Any thoughts on this? That would put the sheathing 5 1/2″ interior to the 1″ drainage plane giving lots of opportunity to rebalance the insulation distribution between outboard to sheathing and inboard. How would this be framed though?

http://www.abag.ca.gov/bayarea/eqmaps/fixit/ch3/sld053.htm

Lastly, where even a fraction of a inch could save money on a foundation pour, how much of the 3″ stone has to rest on the brick ledge. I seem to recall that corbels work on a 2/3 on plane vs 1/3 extension rule, so can the same be applied to the entire stone wall, can a 3″ stone wall rest on 2″ of brick ledge, thus freeing up an additional inch for exterior insulation?

I’d appreciate any insights on how to make this work without increasing the thickness of the foundation walls too much, or at all.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Alex,
    First of all, I'd advise you to hire an architect or an engineer to draw up these details. You shouldn't depend on any advice you read here to build your wall.

    A few general points:

    1. Double-stud walls have to dry to the exterior, so in general they should never have any exterior rigid foam.

    2. It's possible to design a double-stud wall so that the exterior stud wall is the bearing wall. If the bearing wall is over the foundation wall, the inner stud wall (which isn't load-bearing) can be located wherever you want.

  2. Alex House | | #2

    If a double stud wall needs to dry to the exterior AND shouldn't have exterior rigid foam AND simultaneously needs insulation exterior to the sheathing in order to have the sheathing avoid dew point condensation, then how is that achieved?

    Does it boil down to double studs and brick/stone are mutually incompatible?

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Alex,
    If you want to build a wall with exterior sheathing that stays above the dew point, you need to frame your wall with 2x6s. This type of wall is designed to dry to the interior.

    Double-stud walls have cold sheathing. They are designed to dry to the exterior, not the interior. To speed the drying of sheathing when it gets damp, a double-stud wall should have a ventilated rainscreen gap between the siding and the sheathing.

    If you built a 12-inch-thick double stud wall, and then tried to add enough exterior rigid foam to keep the exterior sheathing above the dew point, you would end up with a ridiculous amount of rigid foam -- maybe 7 or 9 inches in some climates -- and the wall would be unbuildable. That's why double-stud walls are designed to dry to the exterior.

  4. Kent Jeffery | | #4

    Alex - I'm not certain your design is the non-starter you believe it to be. In fact, with the exception of the foam layer location, that is how I built my exterior walls. You correctly identify the critical issue of where to locate the "condensation" plane - or foam layer serving as your vapor retarder.

    As Martin points out, double-stud walls are typically designed to dry to the exterior. In my case, I have a wall that dries to the interior. With effort, I was able to persuade the building code folk to allow me to build my double stud wall without an interior "warm side of the wall" vapor retarder.

    Thus, my wall is contructed - moving from outside in - as follows: stone veneer, lathe & scratch coat, 3/4" rain screen, WRB house wrap, 1/2" OSB sheathing, 3" ccSPF foam filling most of exterior load-bearing 2x4 wall, interior 2x4 wall placed 1.5" inboard of exterior wall, 5-1/2" dense packed blown fiberglass filling balance of double stud wall space, 1/2" gypsum drywall, vapor retardant primer/paint.

    Key points:
    1. My exterior sheathing is going to freeze, get wet, and be at risk. No two ways about it.
    2. But I went to great lengths to see that my rain screen allowed for unobstructed air flow from bottom to top to allow drying, and I only installed the stone veneer of the sunny south & west sides of the home.
    3. The thickness of the foam layer was designed to account for at least 40% of total R-value of wall, thereby confidently keeping the interior surface of the ccSPF above the dew point. (foam R-value = 6*3" or R18, dense FG R-value = 4.2*5.5" or R23)

    With respect to your situation, Alex, the foam is the issue too. You could forego the foam, insulate the entirety of the double-stud wall with whatever fill you are choosing (which is what, by the way?), and have a wall that dries to the exterior. The veneer would not require a ledge to rest of... it can be "hung" from the rain screen strapping as I have done. If you want to add foam to the exterior, the challenge becomes how to support the weight of the veneer with a 3" moment arm. (Incidently, I did this too, on my shop which I built first to experiment with exterior foam. I used FastenMaster structural screws - not cheap!)

  5. Alex House | | #5

    Roxul's Comfortboard should address the foam issue, correct? The remainder of the insulation would be Roxul Comfortbatts, in both the 2x6 exterior wall and the 2X4 interior wall.

    The stone is real stone, 3" thick, so it most definitely needs a brick ledge to bear down upon and this feature limits ability to put any insulation outside of the exterior sheathing. A 10" thick foundation wall, 4" of which is reserved for the brick ledge.

    The message I'm reading from Martin is that every double wall will experience dew point condensation in the exterior sheathing, and per other articles here that sheathing will dry out over the course of the year. Am I interpreting you correctly? There is nothing to be done to eliminate the sheathing going through this annual wetting cycle?

  6. Andy CD Zone 5 - NW Ohio | | #6

    Kent's first key point (#4 above) is "my exterior sheathing is going to freeze, get wet, and be at risk. No two ways about it." In consulting with this site while designing my wall system, I was NOT comfortable with conceding this point without a fight, and I went to great lengths to mitigate risk to my sheathing.

    I also went with full-thickness stone veneer and ventilation gap, outboard of.. wait for it... SIP walls. Here, the OSB is not just sheathing, but structure as well. (Some people here strongly discouraged me from doing this, and they were probably right, but that's a whole nother story...) Point is, I've got cold OSB that I want to make DAMN sure doesn't get wet. Leaving aside the question of interior moisture migrating through poor SIP joints, exterior rain and humidity will threaten the outer OSB.

    The ventilation gap is critical, and since I used natural stone, mine is wildly variant. Generous vents top and bottom. The main enemy, as far as I understand, apart from keeping rain out of the gap, is inward vapor drive caused by the sun. (Here is where I'm not sure that the south and west sides are less risky, as Kent suggested.) After a rainstorm, rapid drying of the exterior of the masonry will make it literally rain on the backside of the wall, as absorbed moisture is driven through the stones and mortar. This phenomenon happens less in stone than in brick, since it's far less of a "reservoir", but it happens nonetheless.

    I decided that standard housewrap was not good enough. I used the StoGuard liquid-applied system, which is more money and more labor, but the advantages seemed worth it. It totally integrates flashing around rough openings, a high-perm vapor retarder, AND an effective air barrier. (Of course, the critical air barrier with SIPs is always on the inside, but I figured another wouldn't hurt.)

    The really crazy thing is that I then added another WRB for extra insurance, I think it might have been Dana Dorsett's suggestion: I used a standard masonry shingle-lapped drainage screen/mesh product, turned backwards, creating a secondary air gap that could never be blocked by mortar droppings. Thus my effective WRB was, from outside in: Typar housewrap, 1/4" plastic mesh, StoGuard LAWB. This gave me two permeable WRBs with a 1/4" gap between, to absolutely minimize the chances of my OSB getting wet, and to give it the best chance to dry out if it did. Overkill?

  7. Alex House | | #7

    Andy, thanks for those details. I appreciate your "extra insurance" strategy - it makes sense, but it is becoming clearer to me that the two features don't work well together. Even with your extra insurance, the design of your wall acknowledges your sheathing experiencing dew point condensation, in addition to risk of wetting from moisture which penetrates the masonry.

    With other wall designs the way to remedy this is to balance outward insulation against inward insulation in order to sandwich the sheathing at a point safe from dew point condensation. So, if we can't rebalance the insulation distribution, then that logically leads to moving the location of the sheathing, but to my knowledge I've never seen that done on a framed house- sheathing the interior side of the exterior wall in a double wall system, then filling the 2x6 will roxul or insulation of choice and then constructing the masonry wall.

    See this article.

    http://www.ecohome.net/guide/interior-sheathing-better-air-vapour-barrier

  8. Andy CD Zone 5 - NW Ohio | | #8

    Alex, the home shown in the article you mention looks like a giant pile of trees. 2x4 strapping, then 3/4" OSB, then 2x8 structural walls, then a 4"-thick wood-fiber product? Europeans look upon our lumber intensive conventional methods with alarm--I can't imagine what they'd think of this.

    You and I are really into incorporating stone into our houses, but it does get us into trouble. Kind of like building one house, then building another on top. And trying like the dickens to make them not fight each other. If I had to do it again, I would seriously consider going with structural stone... or ICF?

    Ever seen what this guy is up to? http://hopeforarchitecture.com/

  9. Alex House | | #9

    Andy, the take-away point from that article shouldn't be the framing design of his wall instead of the fact that his sheathing is applied on the inside of the wall.

    Why isn't interior sheathing the automatic solution for the dew point issues associated with double walls?

    Simple question - Doesn't interior sheathing solve the problem entirely, a 2x6 insulation filled cavity EXTERIOR to the sheathing?

    The fact that I'm having trouble finding many articles on this practice leads me to suspect that I'm not seeing the entire picture and the negatives, whatever they may be, likely outweigh the positives. So, anyone up to pointing out the negatives?

  10. Malcolm Taylor | | #10

    Alex, Moving the sheathing into the middle of a double wall has benefits and downsides.
    The benefits are as you say: It is safe from moisture issues,and if it is the primary air barrier it is less susceptible to damage than poly or drywall are.
    The downsides are that depending on where it is placed in the assembly it can be a lot harder to seal than a continuous layer on the outside. It may determine which wall you can make load-bearing. It may disrupt the sequence of construction, in that you now have insulation which can only be applied from the exterior as well as the regular interior work. And probably the most significant downside is that as well as offering shear strength, sheathing is also used as a backer for the WRB, flashing, siding and trim. All of which become a lot harder to install without some substrate on the exterior.
    I think it is a good idea, but one that has consequences which need thinking though.

  11. Kevin Shenefiel | | #11

    Alex, your original idea wasn’t bad the 2” of XPS in the middle would be the most vapor restrictive layer. There’s nothing preventing the interior 2 by 4 wall from drying to the interior, or the exterior 2 by 6 wall from drying to the exterior. In winter the XPS will be an adequate interior vapor retarder to prevent condensation on the plywood. With about 70 % of the insulation to the outside it would take some rather cold weather or high indoor humidity too get condensation on the inner face of the XPS. With about 55 % of the insulation to the interior humid summer weather shouldn’t cause condensation on the outer face of the XPS.

    There isn’t any dew point problem with having no insulation to the exterior of the sheathing. As long as you don’t neglect the interior vapor retarder, the wood sheathing won’t exceed an acceptable moisture content. The exterior of a well insulated building is routinely exposed to dew point conditions; every foggy day and any morning with dew. There will likely be a few instances when the sheathing drops to the dew point as well. Wood is resilient, brief exposure to water followed by good drying conditions doesn’t cause decay. In a cool unusually foggy climate there may be some benefit to exterior insulation to keep the sheathing and framing warmer thus drier, But adding exterior foam insulation to your wall design will slow the drying process. I don’t see any problem with the vapor permeable rock wool board.

    Secondly, per ICC code a wood frame wall with 3’ stone veneer and no exterior insulation only needs an 8” foundation wall. The sill plate resting on the foundation wall doesn’t need to be more than a 2 by 4 regardless of the wall thickness above.

  12. Malcolm Taylor | | #12

    Kevin, If a good vapour retarder is enough to prevent moisture build up in the exterior sheathing of double walls, why do the Building Science tests on cold sheathing show such high levels?

  13. Alex House | | #13

    Kevin, thanks for the detailed response. I never did see the problem with having the XPS in the middle of the wall assembly. I do see the problem with a sandwich of XPS, mid-wall plus on exterior wall in order to cover the sheathing, but rock wool board, as you note, is vapor permeable.

    The longer I'm sitting with the idea of moving the sheathing to the interior plane of the exterior wall the more comfortable I'm becoming with that solution combined with the rock wool boards on the exterior as a final thermal bridge and one last barrier between the tyvek + drainage plane + masonry wall and any moisture which penetrates.

    If there is a wall assembly template which actually minimizes or altogether avoids the seasonal wetting and drying cycle of the wood components in the wall, then I want to strive to find that template. ISTM, sheathing on the interior surface works to that end.

  14. Malcolm Taylor | | #14

    Alex, As long as you are planning a one story house having the sheathing on the inside of the outer wall sounds build-able. If it is two story, I'm having a problem imagining how you detail the wall/floor intersection.

  15. Alex House | | #15

    Does anyone know of data which measures the performance of the sheathing materials used on the exterior of double walls?

    I understand that OSB is the most problematic and 1x8 dimensional lumber is best in terms of resilience but other than this rule of thumb are there hard numbers available.

    If sheathed with 1x8 what kind of spacing between boards is needed to accommodate the seasonal expansion of the lumber and what effect would this have on the air tightness of the house?

    Does dimensional sheathing dry so much quicker than plywood and OSB that the seasonal drying actually resolves the dew point problem?

  16. User avatar GBA Editor
    Martin Holladay | | #16

    Alex,
    Q. "Does anyone know of data which measures the performance of the sheathing materials used on the exterior of double walls?"

    A. I'm not sure what you mean by "the performance of sheathing materials," but here is a roundup of the data on the moisture content of exterior sheathing on double stud walls: Monitoring Moisture Levels in Double-Stud Walls.

    Q. "I understand that OSB is the most problematic and 1x8 dimensional lumber is best in terms of resilience but other than this rule of thumb are there hard numbers available?"

    A. Monitoring studies provide hard numbers on sheathing moisture content, but longevity predictions are estimates rather than hard numbers.

    Q. "If sheathed with 1x8s, what kind of spacing between boards is needed to accommodate the seasonal expansion of the lumber and what effect would this have on the airtightness of the house?"

    A. When you install board sheathing, you should butt the boards tight. If anything, the boards will shrink, not swell. Board sheathing is not an air barrier, so you will need to achieve airtightness with other layers -- for example, your WRB or your drywall.

    Q. "Does dimensional sheathing dry so much quicker than plywood and OSB that the seasonal drying actually resolves the dew point problem?"

    A. Board sheathing can rot, just as OSB and plywood can rot. If the wetting rate exceeds the drying rate on an annual basis, your sheathing is headed for problems. To reduce the chance that your sheathing gets wet:
    (1) You need good flashing details, a well-installed WRB, and a ventilated rainscreen gap between the siding and the WRB.
    (2) You need to install your gypsum wallboard and electrical boxes with attention to airtightness.
    (3) You should consider installing a smart vapor retarder (like MemBrain) on the interior side of your wall assembly.

  17. Alex House | | #17

    I'm hoping the following is a solution: An ICF foundation with insulation sandwiched in the middle. This neatly creates a plane for the loading from the stone work, creates a plane for insulation in the foundation and, importantly, for insulation on the exterior of the sheathing, and then creates a load plane for the exterior wall +roof loads.

    Now, my question. is this too good to be true? Can a 3" brick ledge carry the load of a 4" stone wall (1 inch of the stone extends outward from the plane) and are the wall clips able to cross through the 4" of foam insulation to the 5" foundation wall on the other side and make the foundation wall one structural unit?

    For the moment I'll assume that this can work, so the wall + foundation would have this section: 4" of stone loading onto 3" of foundation, 1" drainage plane above 1" of foundation foam insulation, 2" of Roxul Comfortboard directly over 3" of XPS foam in the insulation, the Roxul affixed to 1x8 diagonal sheathing (covered with a (WRB) which in turn is affixed to 2x6 framing filled with r-22 Roxul batts, this part of the wall bearing down on a 5" foundation.

    Inboard will be a 3.5" space filled with Roxul R-14 batts, then an interior 2x4 wall also filled with R-14 Roxul batts.

    Exterior to the foundation wall will waterprooofing which in turn is covered with 2" of Roxul Drainboard. Interior of the foundation wall will have a 1/4" air gap, followed by a 2x4 empty wall cavity into which wring and plumbing can be run, the wall attached to floor and ceiling rather than against the foundation wall.

    Question: For the main wall assembly, where to put the vapor barrier, should I look to an intelligent vapor barrier product?

    Question: On the main wall, I will have R-8 (2" Comfortboard) exterior to the sheathing with R-50 of Roxul interior, in addition to a 1" drainage plane between the Comfortboard and the 4" stone cladding. Will that R-8 make enough difference to keep the dimensional sheathing warm enough to avoid dew point condensation?

  18. Malcolm Taylor | | #18

    That's a 20" wall!

  19. Alex House | | #19

    Indeed, though 5" of the wall is stone and air and has no insulative value.

    What really burns my shorts is that the permit fee and then the subsequent property taxes count the wall thickness as "living space." A home built with a 2x4 wall and wood siding has more liveable space than a double-wall home built on the same perimeter so tax policies are misaligned with energy saving policies. I'm being penalized with higher tax bills for building a house with thicker walls.

    The middle layer between the exterior loading bearing 2x6 and the interior 2x4 doesn't have to be 3 1/2" thick but I can't find any Roxul batts with are thinner but I do want to incorporate a thermal break between the studs of the interior wall and the studs of the exterior wall, so I'm ready to slim down that gap thickness if I can find a DIY means of insulation (unlikely to be able to rent a cellulose blower to achieve dense pack results) I might put 1" Roxul Comfortboard in there but I need to check on pricing compared to their batts.

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