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Community and Q&A

Air barrier material and location

Sfair74 | Posted in Green Building Techniques on

I have several questions involving wall systems. I live in Michigan, Sanilac county (zone 6). 7133 HDD, 477 CDD. We are about 1.75 miles inland west of Lake Huron.

I am looking to build a ~1400 Square foot ranch with a 2+ car attached garage.

I have looked at many different construction systems and I keep coming back to stick built. There are many different ways to build a stick built wall. I think i would prefer a 12” to 16” double stud wall with dense pack cellulose. Sheeted in Structural fiberboard covered in 15# building felt and a ¾” rain screen with Fiber Cement siding. I would also like to place the air barrier next to the cellulose and behind a service cavity then cover with â… drywall preferable type X.

I know this wall will not pass the 60/40 exterior / interior insulation recommendation. My understanding is that that is a problem in the winter when moisture laden air and some vapor diffusion condensate on the cold sheeting. With a proper air barrier very little moisture will reach the cold sheeting. And due to the permeability of the sheeting any condensation that does occur will be allowed to dry without causing damage. I want a wall that dries to both sides. If I understand correctly the outer sheeting should be 5 times more permeable than the inner air / vapor barrier in a predominantly heating climate (Zone 6)

Sorry for the wall of text. I have a question regarding the material to use for the air/vapor barrier and how to create a service cavity.

How should the service cavity be made? The simplest way i can think is to place the air barrier on the outer surface of the interior stud wall. This would allow the open vertical cavities to be used. Drilling paths for cable would be necessary and future modifications would be more difficult. Conduit could be added to reduce future modifications costs.

I also like the idea of placing the air barrier on the inner surface of the interior stud wall and then using horizontal strapping. This would make it easier to run utilities and future changes. This would also allow me to use a thinner material for the air barrier because it will be sandwiched between horizontal strapping and the vertical studs behind it. My question would be what method who be preferred?

The second question would be what material would make a good affordable air barrier. Currently i’m thinking plywood since it is more vapor open than OSB and contains less resins so it is likely to off gas less. I am also very interested in Masonite. Masonite is just wood with no binders and can come in assorted thicknesses. I would really like to use it due to it being more natural with no added resins. I haven’t found any information on others using Masonite as an air barrier and am not aware of the perm rating for it. I am also aware of smart vapor barriers but I would prefer to use something more rigid and durable.

I would greatly appreciate any input on the best way to make an effective service cavity with a durable mostly not toxic air barrier. I would really like to know more about using Masonite.

Shane Fairman

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  1. GBA Editor
    Martin Holladay | | #1

    In case you didn't see it, here's an article on service cavities: Service Cavities for Wiring and Plumbing.

    A few notes:

    1. People who have tried to install dense-packed cellulose in a wall sheathed with U.S.-made fiberboard have had problems with the fiberboard bulging out. More information on that problem here: Visiting Passivhaus Job Sites in Washington State.

    In that article, I reported, "According to Mark Dixon, one of the builders on site when we visited, the fiberboard wall sheathing was unable to resist the pressure of the dense-packed blown-in fiberglass insulation. After the insulation contractor finished insulating the walls, the fiberboard had bellied out and was bulging as much as 3/4 inch in some stud bays. The workers eventually managed to force the bellies back, at least partially, but the experience revealed one potential drawback of fiberboard sheathing."

    2. There is no simple answer about where to put the air barrier; both of your suggested locations will work fine.

    3. You don't need to worry about an interior vapor retarder with your wall design. Outward vapor diffusion in winter isn't going to cause any problems to your wall assembly. All you have to do is keep the building inspector happy.

    4. I haven't heard of anyone using Masonite as an air barrier. The only two concerns I would have: make sure that tape will stick to it; and discuss your plan with your cellulose installer. It doesn't make any sense to design a cellulose-insulated wall unless you know how the wall will be insulated. You probably don't want to drill holes in your Masonite to insulate your wall.

  2. user-757117 | | #2

    I also like stick built superinsulated homes.
    And I like service cavities too.
    I think you're on the right track for your climate.
    I don't think you can go wrong with a good, rigid air barrier on the warm-in-winter side of the envelope.

    Have you heard of the Sunrise home?
    You might also be interested in some of the details at my blog.

    The Sunrise home uses a service cavity like the first method you described, while I will be strapping with horizontal 2x3 on edge.

    The air barrier in both cases is taped plywood sheathing but executed differently.

    In my case I located the sheathing on the inside face of the inner wall - the inner wall is structural and so is the plywood sheathing.
    On the exterior side, I omitted sheathing altogether (similar to the Sunrise home) so the assembly is relatively vapour open to the exterior - although if you're set on using felt I'd probably include some kind of sheathing...
    Cladding is 5/4x6 tamarack "dolly varden" and no rainscreen.

  3. user-757117 | | #3

    I was thinking about your queations re: service cavities.
    Both approaches have their merits...
    And either can be insulated or not...

    I guess that the framing system you choose will determine which style of service cavity makes the most sense.

    I know I am looking forward to not having to bore holes for cable and pipe.

  4. user-1137156 | | #4

    In considering a similar wall system, I encountered the following: Structural fiberboard is only specified for installation on 16" OC framing, I couldn't find a contractor willing to dense-pack cellulose in a double stud wall.
    Wanting to stick with 24" OC framing I've arrived at the following plan: Outer stud wall erected without sheathing (will be added later). Inner stud wall is "structural" and sheathed with plywood on exterior face & detailed as primary air barrier. Insulation is 'Roxul" bats installed after walls are erected and window/door 'bucks' installed. 25/32 fiberboard exterior sheathing installed after insulation, then WRB, rainscreen & siding. I'll also insulate the wall service cavity with 'Roxul' bats. This puts the "smart" vapor retarder (plywood) at about the 1/3, 2/3 spot good for heating dominated climates and it serves as the shear bracing as well.
    Surprisingly the material cost for Roxul equal to or lower than cellulose. I'll install the Roxul as a DIY project with hired help.

  5. Sfair74 | | #5

    Thanks for taking the time to help me out..

    I have read the article on service cavities along with many other great articles on GBA. I have a lot more to learn. This is just the planning stage I will not start building until the spring of 2014. I have plenty of time and I figure I have at least 9 months before I need to pick a general contractor and get initial plans to an engineer.

    I think I really want to run horizontal strapping for the service cavity. But i’m not sure if the additional lumber and cost of installing horizontal strapping will outweigh the simplicity of just leaving the inner stud wall cavities open and drilling holes. In reality I really don’t plan to change any electrical. My concern is with data cables and the inevitable change that will occur. I could just run conduit runs to 5 or 6 locations where I plan to run data cables and use the simpler vertical cavities.

    I also want to run a service cavity in the ceiling for the heat recovery ventilator. How much space is needed for a typical HRV runs? I really haven't done much research on the topic. With my current plans I could have up to 12” service cavity in the ceiling. i’m not sure if it is better to run strapping over the air barrier perpendicular to the trusses. If I included several gaps of up to 24” (Equal to truss spacing) this option could take the minimum space.

    The other option I see is to run a bulkhead that attaches to the walls inline with the trusses with additional space between the air barrier and the false ceiling. This option would take more space and lower the height of my ceiling. I have an interior wall that run the full length of the plans so a bulkhead would only need to span 10’ on one side and 15’ on the other.

    I am not stuck on using any particular material. I am willing to change my mind as i learn about a better product or technique. When it comes to WRB i strongly prefer 15# felt to house wraps such as Tyvek. Building felt has been around for over 100 years and it holds up well and does the job. It is also more affordable.

    I think I am being a little too concerned about avoiding off gassing with OSB and plywood. What is the preferred material for an interior air barrier?

    As far as dense pack cellulose goes I really want the inner air barrier to be near 100% perfect. This would make dense pack cellulose difficult without compromising the inner air barrier. Would it be reasonable for the external stud wall to be placed without sheeting and then insulweb to be installed on the exterior facing? This would allow the insulation crew to properly dense pack the wall and inspect the density. This would likely increase the labor costs of the wall assemble because it is much easier to secure sheeting before standing the wall up. If i can find a contractor that is willing to hang the sheeting while the wall is vertical would it be worth it?

    I intend to use 2x4 walls 16” OC. Will 25/32” structural fiberboard hold up to dense pack with tighter stud spacing and thicker sheeting? Does bulging matter if the cellulose is installed behind insulweb and rolled flat before sheeting is applied?

    Thanks again for your time and the links to additional material.

  6. user-1137156 | | #6

    Consider a 'hybrid' insulation system with mineral wool bats in the 'service cavity" and cellulose in the space between walls & the outside wall's stud spaces. The insulweb on the outside before sheathing sounds quite viable as it gives the insulator good visual feedback and should work well if rolled flat before installing the fiberboard. However, a heavy driving rain between insulation and sheathing could really mess things up. Have you contacted cellulose installers?

  7. user-757117 | | #7


    ...i’m not sure if the additional lumber and cost of installing horizontal strapping...

    I've always thought that the "extra cost" argument for additional lumber to furr-out a service cavity was a bit overblown.
    I think I spent about $400 to buy enough 2x3 for my place - If I weren't doing the job myself, I guess there'd probably be something close to that cost again for labour.

    How much space is needed for a typical HRV runs?

    A 2x8 ceiling joist ought to give you enough space for a 6" trunk.

    What is the preferred material for an interior air barrier?

    Like Jerry I used pywood in my house.
    I like plywood because it's generally a better product than OSB and airstop tapes will stick better to either side (I don't know of any tapes that will stick to the "rough" side of OSB).
    I used a 1/2" t&g plywood, but I would recommend using square edge (except as a roof sheathing) because it goes up faster and the t&g really isn't that neccessary.

    The detail below is from the ARCTIC profile as practiced by Thorsten Chlupp.
    This approach might work well for you since you seem to be interested in a deep service cavity at the ceiling.
    Instead of the 2x4 ceiling joists in the detail, substitute larger lumber - 2x8 or maybe 2x6 if you are able to limit the duct size in the ceiling to 4".

    Notice how the top of the ceiling joists are sheathed with plywood...
    I really like "rolling trusses made easy".

    Do you have a builder in mind?
    Since you seem to have a lot of interest in some fairly subtle details, you may want to start early on finding the right team of people to help you execute your project.

  8. Sfair74 | | #8

    After some more thought I think I would be best to not use Structural Fiberboard on the exterior.
    I think Plywood will make a better nailbase for exterior cladding and will hold up to dense pack cellulose.

    Here is my revised wall and air barrier locations from Exterior to Interior:

    Fiber Cement Siding or MgO siding
    ¾” rainscreen gap strips on 12” OC (maybe 16” or 24”)
    15# or 30# Roofing Felt as the WRB
    ½” CDX plywood taped outside and gaskets inside for exterior air barrier
    2”x4” 24” OC OVE exterior stud wall
    5.5” Cavity between walls (dense pack cellulose prefered, rock wool as an alternative)
    2”x4” 24” OC OVE Interior stud wall
    Insulweb for dense pack cellulose
    ½” CDX plywood taped outside and gaskets inside for interior air barrier
    2”x2” or 2”x3” horizontal strapping on 16”OC for service cavity
    Drywall ½” or ⅝”

    As far as the deep ceiling service cavity goes I think I can scrap that idea and save substantial money by sticking with 8’ ceilings. I have the house designed as rectangular and I can easily create a interior double stud wall to act as a duct wall for the HRV. This would also provide decoupling on a wall that separated public area and private area to increase STC value for soundproofing. This would allow me to stick with 8’ walls instead of upsizing to 9’ or 10’.

    I would still like to keep a small service cavity in the ceiling. I was thinking keep it similar to the walls and run 2”x2”s or 2”x3”s run on the bottom on the trusses. In the two bathrooms the ceiling strapping could be 5.5” or 7.5” to provide room for proper HRV exhaust location instead of using the wall.

    Should the CDX air barriers be taped on the face or gasket between the stud and the sheet? Is it worth it to do both?

    I would like to leave the service cavities open for future changes if necessary. I have not contacted cellulose installers or even a GC yet. I will likely start trying to find a GC in a few weeks when they slow down for the winter. I am concerned about a driving rain so I will likely avoid the unconventional exterior fill before exterior sheeting.

    Thanks for the info. I think the general consensus is to just use CDX and not worry about the off gassing. I guess the extra cost is minuscule in comparison to the total cost to build. I guess horizontal would make me sleep better at night knowing I did it right the first time. I guess I will need to hurry up and start contacting reputable builders in my area.


  9. user-1061844 | | #9

    See attached detail of a double studs walls with service cavity that you might find useful. More options can be found here on the website.

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