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Necessity of smart vapor retarder in Tiny House on Wheels in Zone 4A?

Cam Kerr | Posted in Energy Efficiency and Durability on

GBA community,

We’re in the final stages of a 24’ DIY “tiny house on wheels” build (photo attached) and struggling with the ever-complex vapor retarder question. We’re in the mid-Atlantic (Climate Zone 4A, mixed-humid but just coming out of a colder-than-normal winter), so my understanding of code and best practices is that we only require a Class III retarder. However, one ‘counterpoint’ that has me struggling is the fact that even with more than adequate mechanical exhaust ventilation, moisture produced by daily activities in 200 square feet can potentially be an issue for walls with fibrous insulation. Another factor/variable is our use of thin interior paneling, with unknown vapor permeability.

Here’s our wall assembly, from exterior to interior:
-Stained T&G 1×6 cedar siding
-Cor-a-Vent 3/8” rainscreen gap, vented at top and bottom
-Tyvek housewrap, fully taped at all edges and seams as an air barrier
-1/2” CDX plywood sheathing, glued/nailed and taped with 3M 8067 at all seams
-2×4 stud walls, 24”o.c. insulated with Roxul at R-15
Possibly Certainteed MemBrain detailed as an air barrier/smart vapor retarder layer
-5mm (1/4” nominal) birch PureBond formaldehyde-free 3-ply plywood* as interior paneling, primed and painted with two coats of latex paint

*Possibly not thick enough to serve as an air barrier?

Vented cathedral ceiling assembly, from exterior to interior:
-Galvalume standing-seam metal roof
-Grace Ice and Water Shield, High-Temp version covering entire roof deck
-5/8” CDX plywood sheathing
-1” continuous, uninterrupted vented airspace between insulation and sheathing in each rafter bay, from eave to monoslope ridge
-2×6 rafters, 24”o.c. insulated with Roxul at R-23 (furred out 1” on their underside with strips of 1” XPS foam to allow for both the full Roxul batts and vent gap)
-Same PureBond plywood as the walls, noted above

Our mechanical ventilation/HVAC plan:
-Panasonic Whisperwall in the bathroom hooked to an automatic humidity sensor
-Broan range hood in the kitchen above the 2-burner cooktop
-Pioneer 9000BTU mini-split for heating/cooling, with dehumidification capability

As noted, I have been weighing the inclusion (or not) of Certainteed’s MemBrain smart vapor retarder product as a compromise to address my above points/counterpoints (it’s an internal struggle!), to balance what I’ve read/studied about vapor retarding in ‘normal’-sized houses with what I know about tiny house living – all with the additional variables of living in a mixed-humid climate and using a thin, unknown-permeability interior plywood paneling. Needless to say, I would appreciate any/all input, feedback, criticism, etc.!

Thank you in advance!
-Cameron

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Replies

  1. Cam Kerr | | #1

    (Photo of the house didn't attach the first time, so I'm including it here)

  2. Expert Member
    Dana Dorsett | | #2

    If the ventilation keeps the interior relative humidity well under 40% @ 70F during the 10 coldest weeks of winter you don't have too much to worry about from interior moisture drives.

    Run heating & cooling load calculations- 3/4 ton Pioneer is probably way too oversized to hit it's efficiency numbers.

    The minimum modulated output of a 3/4 ton Pioneer is 3785 BTU/hr @ 47F, (see: https://www.highseer.com/pdf/WYS-22-2015.pdf ) which is probably above your design heat load at your 99% outside design temperature of ~15F (give or take a few), which means it effectively never modulates. You may be better off (or at least not much worse off) with half ton PTHP, which is can be easier & cheaper to install & maintain. (A wall sleeve rather than refrigerant lines and wall-brackets.) Since the refrigeration loops are all pre-packaged and built at the factory they are (theoretically, at least) less likely to lose refrigerant, since there are no field-assembled refrigeration lines or refrigerant charging to be done.

    Some models to consider would be:

    LG LP073HDUC

    Amana PTH073G35AXX

    Blueridge BPC07G3

    Gree ETAC2-09HP230VA-A

    GE AZ65H07DAB

    Friedrich PDH07K3SG

    Most have wired or RF wall thermostat options, some have mini-split style hand held remotes, etc.

    For a lot more money you can get higher efficiency and modulation out of a half-ton Mitsubishi FH06NA, which can throttle back to less than half the minimum modulation of a 3/4 ton Pioneer. For some that might be "worth it" from a comfort point of view, but it wouldn't necessarily "pay back" on efficiency even over it's lifespan.

  3. Cam Kerr | | #3

    Mr. Dorsett,

    I appreciate the quick response! Our ventilation should help keep indoor moisture levels down, we think (that and plenty of operable windows for cross-breezes). As for the mini split, I agree that it's likely oversized but unfortunately it was one of the only ones with a reasonable SEER rating (and one that could run on 120volts with minimal amp draw, since we're trying to minimize concurrent electrical loads) and at a reasonable price. It also seems to be popular with other tiny house dwellers in similar-sized homes, who report that it's perfect for their homes (anywhere from 20 to 28' long). Because we're building what is essentially a studio apartment with cathedral ceilings, there's more cubic footage to our space than a typical 200 sq. ft. room with 8' walls, so we figured that made the 9000BTU Pioneer our best option.

    Unfortunately, we don't have much floor or wall-space available for a PTHP, though I appreciate the detailed recommendations!

    Back to the smart retarder question to make sure I'm understanding: are you saying that with the aforementioned wall/ceiling/ventilation setup we'll be ok without MemBrain? I have flipped and flopped on both sides of the argument for/against, especially considering how thin our interior paneling will be, but then I'm reminded of the fact that we have a properly vented cathedral ceiling AND a vented wall rainscreen and will be painting the interior with latex paint. Also, do you think the 5mm birch plywood for interior paneling will be thick enough to serve as an interior air barrier in lieu of the MemBrain (provided we detail it properly as such)?

    Thanks again!
    Cam

  4. Expert Member
    Dana Dorsett | | #4

    A PTHP doesn't take much more wall space than a 3/4 ton mini-split head, but you have the dimensions, I don't. Most PTHPs are also 208-265V too, not 120V.

    Plywood is pretty air tight at any thickness, so your 5mm birch should work just fine.

  5. Cam Kerr | | #5

    Upon further research into whether plywood would be able to serve as an air barrier (not that I didn’t believe you, Mr. Dorsett! I just wanted to learn more about the subject in general), I found the following:

    -Information indicating that plywood under 3/8” thickness is not pre-approved as an air barrier (at least for commercial applications): https://www.energycodes.gov/sites/default/files/documents/cn_meeting_the_commercial_continuous_air_barrier_requirements_for_ashrae_90.1-2010_and_the_2012_iec_iecc.pdf

    However, though the article indicates that while plywood over 3/8” is pre-approved as an air barrier, it does not state that 1/4” plywood is un-qualified as an air barrier - just not pre-approved automatically. At least that’s how I read it.

    Later, I read comments (from Martin Holladay and Sam Glass) in the following GBA article which indicate thinner plywood can serve as an air barrier, based on ASHRAE data: https://www.greenbuildingadvisor.com/blogs/dept/musings/osb-airtight?page=1 (comment #63) and https://www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/98109.htm

    I also noted Martin Holladay’s comments indicating that thin plywood will suffice as an air barrier in enclosing fibrous batt insulation in a vented cathedral ceiling: https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/40109/building-wrap-air-barrier-above-cathedral-cei

    So, in conclusion: just to be safe and to compromise (and perhaps add a tiny bit of structural rigidity and improved racking resistance over our already-installed 1/2” ply sheathing), we’re going to use 1/4” AC sanded plywood (installed using an adapted ‘airtight drywall approach’) as our interior wall and ceiling paneling over the Roxul. This will, according to some of the above sources and others, easily meet air barrier requirements when fully spackled and will also serve as a Class III vapor retarder when primed and painted with two coats of latex paint. Hope this’ll work!

    I had originally been concerned about formaldehyde off-gassing from non-Purebond plywood (especially in a small space like 200 sq. ft.) but the more I read* the more I learned that APA PS1-09 phenolic-resin adhered plywood (anything exterior grade, ironically) will be fine for use as interior paneling, as long as there is no added urea formaldehyde and it’s not sourced from China or anything.

    *including several articles and Q&A discussions on this website, which has been immensely useful for us during this absolutely over-thought and over-analyzed build!

  6. Expert Member
    Malcolm Taylor | | #6

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