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Moisture concerns with double stud in 4a?

Clay Whitenack | Posted in Energy Efficiency and Durability on

Hi all,

First off, this is a great website! I have been reading all that I can on super-insulated homes and this site has been a huge help.

We are in the planning stages to build a new home (which we hope to be “THE” home that last the rest of our lifetime). We would like to be as efficient as possible with energy use, as well as from a construction cost standpoint. I am willing to pay extra for a system that will pay off down the road, but also don’t have an unlimited budget (who does?) so I have to be smart where we spend our money. I live in central KY, which is zone 4a, so I would like to build a “Pretty Good House” with R-30 walls and R-60 attic. I also want a brick exterior if that matters.

I have read about the pros and cons of double stud walls vs. traditional framing with rigid foam added to the exterior and have waffled back and forth as to what seems to be the best in my area. I have read that double stud seems to be a bit cheaper to build than rigid foam on the exterior, and is easier to install the exterior features like windows, doors and fixtures. However, I worry about moisture build-up. I know that zone 4a doesn’t have that much of a concern, but it also seems like opinions vary from person to person and year to year about whether some rigid foam is needed. It seems to be a unanimous opinion that rigid foam won’t hurt, and for Zone 4a, any thickness of foam is enough to control the moisture. However, those articles that I have found that say any thickness of foam board is acceptable for 4a are only talking about traditional framing techniques, and not double stud. I don’t think I have found an article the explicitly says that exterior foam insulation is not necessary for double stud in 4a.

So, to get right to it… For zone 4a, if I go with double stud wall construction, do I also need to install rigid foam board on the exterior? And, if I do, does that lessen the cost savings of going with double stud construction enough to the point where I might as well go traditional 2×4 or 2×6 with a couple of inches of exterior foam? It seems like going double stud PLUS rigid foam board on the exterior will be way more than the R values I am needing for my area, but I can’t figure out if that is still cheaper than going 2×4 or 2×6 with 4″ of exterior foam.

Any help would be greatly appreciated. (I also have a question about HVAC, but I’ll save that for another time).

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Clay,
    The use of exterior rigid foam is incompatible with double-stud walls.

    Double-stud walls need to dry to the exterior. Foam-sheathed walls can't dry to the exterior; they are designed to dry to the interior.

    You need to choose which way to go -- don't mix-and-match these two approaches.

    If you plan to install brick veneer, I strongly suggest that you frame your walls with 2x6 studs and install exterior rigid foam. Rigid foam is a great match for brick veneer, since it protects the studs from water intrusion and stops inward solar vapor drive in its tracks.

  2. Clay Whitenack | | #2

    Thanks for the reply Martin! I was hoping you would respond. I have been reading your articles and was hoping to get your opinion. Sounds like the clear and obvious choice is rigid foam over 2x6 studs.

  3. Expert Member
    Dana Dorsett | | #3

    The brick veneer also has a severe thermal bridging factor to contend with at the top of the foundation- it'll undercut the high thermal performance of the walls if you don't take measures to deal with it:

    https://www.greenbuildingadvisor.com/sites/default/files/Exterior%20basement%20insulation%20is%20incompatible%20with%20brick%20veneer.jpg

    Pouring the foundation with a ~1' stepdown to the exterior side of the framing to support the brick, and running the rigid foam in the cavity all the way down to the step helps. Then insulate the foundation on the interior side with rigid foam, then over the top of the foundation sill as well. You still have a thermal bridge at the band joist, but it's a fraction of the size that it would be otherwise.

    This is how it would look in a slab-on grade with a step in the grade beam:

    http://www.buildingscience.com/documents/insights/content/bsi-059-slab-happy/images/bsi059_figure_04_web2.jpg

    (Couldn't find a really great pic of a full-basement variant.)

    You can also deal with this by stacking autoclaved aerated concrete blocks (which are insulating) under the foundation sill and insulating on the interior with rigid foam, but it's a somewhat messier build.

    A 2x6 wall w/blown cellulose and 3" of foil-faced polyiso will end up with the ~R30 whole-wall performance you're looking for. You'll have to keep the bottom edge of the polyiso off the concrete or it'll wick moisture (even a 1/4" gap is good enough, but you could also just put down a 1" strip of EPS or something.)

    A common issue for this type of wall is finding the appropriate masonry ties that can accommodate that much foam. There are a few companies out there starting to address this market (but I haven't used any of them). With 3" foam you'll probably want to use a type that get's attached to the framed wall before you put up the foam, so you can ensure that it's really tied to a stud, since it's easier to miss the stud for the drill-through types as the foam gets ever thicker. eg:

    http://www.h-b.com/index.php?main_page=product_info&products_id=106

    (There are others...)

  4. Clay Whitenack | | #4

    Thanks for the extra advice Dana. This surely helps. If I am looking at the diagram correctly, it look like the rigid foam extends down the exterior wall and onto the foundation and stops just above the top of the step. Correct? What about the white block that sits on the step under the brick?

    I like the look of a stone veneer foundation (aiming for the house to look like it was built in the 1800's). I spoke to the builder about this and he mentioned needing to have a stepdown on the foundation for this, so it sound like this might not interfere? I assume the insulation would need to go between the stone veneer and the foundation it is sitting on? Can this sit below grade?

  5. Jerry Liebler | | #5

    Consider thin brick, ! It is 1/2" thick bricks that are "glued" to the walls. I plan on using the http://ambrico.com/ system with NO exterior sheathing. The outer stud wall, of the double stud pair,, after installing insulation, will be covered with "Tyvec" then a layer of "Greenguard DC-14 (which serves as a ventilated drainage plane as well as blocking solar driven vapor) , then the Ambrico metal substrate panels, brick & Mortar. Using thin brick won't save much money but it allows thinner walls.

  6. GBA Editor
    Martin Holladay | | #6

    Jerry,
    I'm not sure that I agree with you that your house has no wall sheathing. It sounds like the metal substrate panels are used as sheathing.

    This is a proprietary system that I am not familiar with. I would advise Clay and other GBA readers to investigate the system thoroughly before considering its use.

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