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Is Condensation in Larsen Truss Wall System a Problem?

Hannah Haselton | Posted in Green Building Techniques on

We are intending to build a wall system composed of a 2×4 and a 2×3 Larsen Truss with Zip system sheathing on the outside. We are concerned about the potential for condensation in the cellulose with which we fill the cavity if we don’t have insulated sheathing on the exterior.

Is this a legitimate concern and is there something we can do about it without applying insulated sheathing?

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Replies

  1. Lucas Durand - 7A | | #1

    Hannah,
    What climate zone are you in?

    Is the zip sheathing the only air barrier in the assembly?
    If it is, then it is possible that your proposed assembly could end up being a "turd".

    It's hard to say though without some more detail...
    Have you got any drawings or sketches you could share?

    I'm puzzled by the few details you've provided...
    Normally a Larsen truss does not require any exterior insulated sheathing at all...

  2. GBA Editor
    Martin Holladay | | #2

    Hannah,
    As you probably know, some building scientists have raised concerns about high moisture levels in exterior sheathing on very thick insulated walls. (If you haven't seen it yet, you may want to read this article: How Risky Is Cold OSB Wall Sheathing?)

    My own advice: including a ventilated air gap between your siding and your sheathing will go a long way toward reducing the risk of this type of assembly.

    For more information on Larsen truss walls, see All About Larsen Trusses.

  3. Albert Rooks | | #3

    Hannah,

    Depending on what climate your in, I also think that this could be a potential problem. Take a look at the thread that Martin has suggested. I'm a big fan of "diffusion open to the exterior walls" so that they will stay healthy without needing a layer of foam. I don't like foam and freely admit that it's "simply my personal bias".

    It would be a good idea if you had your proposed assembly checked by WUFI for your location. If you don't know someone who can do that for you, we can. http://www.smallplanetworkshop.com/why-a-wufi-check-up/

    One option that the thread points out is a highly permeable sheathing. There are a few options for that. We import a 5/8" thick very solid fibre board, that is 18 perms, and made for high perm exterior sheathing applications. We use it, and sell it to builders in the Western US. Another possibility is a product like dense deck gold or other exterior drywall.

    A larson truss is a good wall. One of the best things about it is that the truss is designed to break thermal bridging internally and not require a layer of exterior foam.

    I'm sure we are all curious where this project is so we can understand how the climate will play into this wall.

  4. Aj Builder, Upstate NY Zone 6a | | #4

    Good posts gents.

  5. John Klingel | | #5

    Curious. Is it not possible or advisable to sheath the inside of a L Truss (under sheet rock, or whatever the interior finish will be) and then wrap the house in Tyvek or MagCoat (?)? You could then possibly install a few inset diagonal braces in the corners of the exterior wall. This would be similar to Thorsten's wall.

  6. Albert Rooks | | #6

    Thanks AJ :)

    John Klingle, You read my day today. I got a call from an intrepid gal in KY that has done that detail on one project and is planing on doing it again on the next project. I like people like her that aren't afraid to challenge the norms.

    Thorsten Chulpp up in Fairbanks Alaska has done that quite a few times. The amazing part of it is... When he does it, it's a 24" deep wall truss. Really for him, it's more of a necessity since his sheathing gets soooo cold.

    Btw... It's Majcoat. Thanks for trying... :)

  7. Floris Keverling Buisman | | #7

    In most heating climates vapor open exterior walls are recommended as mentioned above to prevent condensation - an option is to use a gypsum based sheathing on the exterior (USG) or similar and cover it with SOLITEX MENTO - a propject that is using this method is BLDGTYP's house in Wisconsin.

    Another option is to use no exterior sheathing (use cross braces or interior sheathing) and use the reinforced vapor wrb SOLITEX MENTO Plus on the exterior as blow in membrane. See this blogpost regarding a double stud wall setup like this.

    Chris Corson's current project is using a similar method (airtight taped OSB sheating with TJIs bolted to the exterior, covered with MENTO Plus as blow in membrane for cellulse). Picture of trusses setup before membrane installation is attached. It doesn't get much more vapor on the exterior than this.

    I happen to import these products with http://www.foursevenfive.com

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