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Following up tiny house & vapor retarder question

Kai Mikkel Forlie | Posted in Green Building Techniques on

Hello all!

My partner and I are also building a “tiny house on a trailer” nearby in Northern Vermont and also want to use v-joint tongue & groove hemlock to panel the interior walls and cathedral ceiling (we may instead use white cedar T& G in the bathroom, nailed on strapping). I’ll leave out the floor (which in this case we’re treating like a wall) for the purposes of this discussion.

1) We plan to follow “tight building” practice/technique. Therefore, we need a layer of something on the warm side of the wall/ceiling underneath the T&G to act as both an airbarrier and vapor retarder.

2) We’ll be using a rainscreen wall (heart redwood clapboards nailed on strapping) over 1″ of taped T&G un-faced XPS, over plywood sheathing, over reclaimed native 2X4 framing in the walls, and over store-bought 2X6’s in the roof framing.

3) We are unwilling for several reasons to install the paneling over taped and primed sheetrock as Mr. Riversong has suggested and we wonder if this “layer” really requires anything as fancy (and perhaps unproven/unknown) as Mr. Riversong’s suggestion of Certainteed’s “Membrain” polymide paper. I know Goretex begins to fail as soon as it gets dirty so I fear (maybe wrongly) the same lack of longterm durability from this recent newcomer high performance “fabric.”

I’d love to hear If any one has had success with this product.

I wonder too if it is more resitant to tearing during installation (when compared to Kraft-faced fiberglass batts) – similar to Tyvek maybe?

But what about the alternatives (if there are any). I’ve read (and reread) everything I can get my hands on from BSC but I regularly end up somewhat more confused than before I delved in. However, from my research, it seems that Kraft paper also has the characteristic of having a variable perm rating depending upon the relative humidity at any given time. So maybe carefully installed Kraft-faced fiberglass batts (caulked and errant holes taped) would solve both problems too.

Then, of course, there’s the issue of what’s “greenest”. IOW, which qualifying product features the lowest embodied energy, which off-gases the least and which is safest in a fire (non-toxic when burned). Since whatever we use is basically going inside the living space, this makes a difference. And before anyone pipes in, we can all agreee now that XPS is nasty stuff during its manufacture and when it burns. However, I do like the fact that it can someday be reused (we ourselves will be using reclaimed XPS on the cold side of the walls and roof, for what its worth).

I look forward to any and all replies. Thanks! 🙂


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  1. J Chesnut | | #1

    - Is the "tiny house on a trailer" going to be used as a residence replete with plumbing and electricity?

    - What about insulation in the wall cavities? You're in Northern Vermont and you only mentioned using 1" XPS on the exterior.
    Have you read this blog?

    Note that according to an article by Alex Wilson EPS is preferable to XPS because its manufacture has a far less Global Warming Potential because XPS uses hydroflourocarbon (HFC) as its blowing agent.

    The plywood wall sheathing is a candidate for part of the air tight enclosure (which needs to be contiguous with an air tight layer on the floor and ceiling/roof).

    With XPS or EPS on the exterior of your sheathing, it is advisable NOT to have an additional vapor barrier on the warm side interior.

  2. GBA Editor
    Martin Holladay | | #2

    Will you have insulation between your studs? If so, what type of insulation? What R-values are you aiming for?

    J Chesnut raises a valid concern: since your exterior rigid foam has such a low R-value, you are risking wintertime moisture accumulation in your sheathing -- especially if you cut corners on your interior air barrier. T&G boards are notoriously air-leaky; you'd be better off forgetting the interior boards and just installing airtight drywall.

    If you insist on the interior boards, you should still install the drywall under the boards, following the airtight drywall approach. (Of course, the boards give you thick walls -- a drawback for a house on a trailer.)

    And thicken up that exterior foam!

  3. Riversong | | #3

    And do NOT use fiberglass batts - there is no worse insulation on the market. Vulnerable to temperature extremes, convection, moisture diffusion, insects, rodents and fire. It's a known carcinogen, typically contains formaldehyde, and nearly impossible to install tightly.

    If you must use batts, then use recycled blue jean batts treated with boric acid, which is a fire retardant, mold retardant, insecticide (though non-toxic to humans), and rodent inhibitor.

    You should have a continuous air barrier and the easiest way to achieve that is with taped drywall. What are the reasons you've ruled that out?

    And why the west coast virgin redwood clapboards when we have radial-sawn spruce clapboards produced at the Ward Clapboard Mill in Moretown VT, which are among the best in the world but also local and "green".

  4. Kai Mikkel Forlie | | #4

    Thanks everyone!!

    I apologize, in advance, for the length of this reply.

    So, to answer everyone's questions:

    Yes, the 8.5' wide X 22' long tiny house with loft and two small shed dormers and eleven (11) small double-glazed fiberglass casement and awning windows will be used as a full-time residence (while we're not travelling) complete with:

    - Point-of-use plumbing (a marine foot-powered kitchen faucet and single bowl stainless sink and stainless counter with water storage tank under sink & modified 4 gallon stainless steel garden sprayer shower and/or Ortlieb water bag with shower head attachment);

    - A stainless steel shower surround and solid surface shower pan;

    - A marine urine-separating single-batch 'composting' toilet (Nature's Head) directly vented through the roof via a low cfm marine solar/battery fan (that runs day and night) but also plumbed to allow the selection of outdoor air in place of indoor air, thus avoiding overpowering the small solar fan when usin the gable-end mounted high cfm fan and preventing backdrafting of the toilet exhaust);

    - A small passive adjustable through-the-wall make-up air vent located in the lower part of the wall near the woodstove;

    - A brand-new 10,000 to 28,000 BTU Little Cod woodstove (Enterprise Fawcett Foundry) with steel flue & cast iron flue damper all od which exhausts through the roof;

    - A high cfm Panasonic through-the-wall fan mounted in the gable-end (for summertime cooling and for use in winter while cooking, with a window or two also open, so as to not backdraft the woodstove);

    - An Origo 6000 marine alcohol range (two burner with oven); and

    - A small 'dorm-sized' 110V refrigerator.

    As far as electricity goes, the house will be lightly wired for both 110V AC and 12 volt DC. AC power will enter via an RV service (an extension cord from a residence) or via an inverter. DC will come via a few solar panels, possibly a small wind turbine and a small battery bank.

    Cavity insulation will be either unfaced or faced fiberglass batts, rock wool batts or dense packed cellulose - the final selection of which is somewhat dependent upon the outcome of this thread.

    I have read
    and I note the negative environmental footprint of XPS - but since we are limited (by DOT max-width regulations) we need the most R-value per inch we can get that is also semi-permeable (thus no polyiso for us). We will use reclaimed XPS.

    The coment was made that "[t]he plywood wall sheathing is a candidate for part of the air tight enclosure (which needs to be contiguous with an air tight layer on the floor and ceiling/roof). With XPS or EPS on the exterior of your sheathing, it is advisable NOT to have an additional vapor barrier on the warm side interior."

    We hear this but are mindful that this is NOT your typical large home with the accompanying greater water storage potential. Plus, its my understanding that 1" of XPS is not a vapor "barrier" but rather a vapor "retarder". Keep in mind that this is a 140 square foot residence with two adult residents who just by their (our) breathing alone will produce relatively large amounts of interior moisture. Add stove top water heating, cooking and showering and there is a huge interior water vapor potential (hence our multi-point ventilation strategy and attempts to insulate the cold side of the walls/roof/floor as best we can given our dimensional constraints).

    Given this, we are exploring the use of a vapor semi-permable membrane behind the T&G interior paneling as an additional strategy to mitigate this interior moisture making its way into the wall assembly but one which would also allow for drying. We realize that the lion's share results from poor air sealing (i.e. air transported moisture) thus our need to at the very least put an air barrier (NOT primed sheetrock - too heavy and why not just a thin film of something?) behind the interior paneling. I'll repeat that we do not want to use sheetrock anywhere in this building.

    Vapor diffusion is also a concern, although minor in comparison. But its still a concern.

    Trust me, if we had the space to slap the recommended 2" of rigid on the exterior we would (we might be able to squeeze 1.5"), but we're hamstrung by the max-width regulations and the starting width (8') of our all ready purchased trailer. For this reason alone we'll go with 1/2" strapping (rain-screen wall) instead of 3/4" to maximize the room left for rigid insulation. I fully understand the concept that if we can keep the entire wall assembly above freezing, condensation cannot occur and thus management of excess moisture can be dealt with via the drying nature of wood heat and via our mechanical and passive ventilation strategy.

    I hope this better (and more fully) describes our plans. I look forward to follow-up comments/questions. And I look foward to reading of other's experience with "Membrain".

    Thanks again! :)


  5. Kai Mikkel Forlie | | #5

    We'd planed to use local white cedar but then, thanks to the miracle of the internet, came across a deal we could not pass up. We went with reclaimed heart redwood boards, 5"W X 3/8" thick in varying lengths that was milling "waste" from a huge downcountry project . We bought 2000 linear feet of it for .25 (that's twenty five cents) a linear foot. It will be left to weather on its own - no finish of any kind. We'll still use local white cedar for the trim. Who amongst us could pass that up?

  6. Kai Mikkel Forlie | | #6

    I note the hatred of fiberglass. I too won't use it (I included it to draw everyone's comments) and look forward to finding a membrane of somekind ("Membrain"?) to hermetically lay on top of the studs and then blow dense pack cellulose behind (DIY friendly over wet spray which we'd have pay someone to do, plus tough to work with in the winter in Vermont), sealing and taping up all of our holes afterwards before installing the T&G.

    We're set on interior T&G boards (which much of Sandinavia swears by) and so have to find something to use in lieu of sheetrock as an airbarrier, if not also a vapor retarder.

    Thanks again all!

  7. Riversong | | #7


    It sounds as if you've done exceptional planning and design work for this project, and I appreciate your concern about the potential for anthropogenic moisture problems in such a small space.

    A couple of your statements require correction:

    I fully understand the concept that if we can keep the entire wall assembly above freezing, condensation cannot occur and thus management of excess moisture can be dealt with via the drying nature of wood heat and via our mechanical and passive ventilation strategy.

    The basis for the "warm sheathing" approach to moisture management is to keep the sheathing above the dewpoint (not above freezing), which is generally around 45° in a moisture-controlled house but probably significantly higher in your little space when the indoor RH rises. And it requires the ability to dry to the interior, since drying to the exterior is limited by the foam board.

    Wood heat is no dryer than any other heat. This is a common misperception. What makes houses dry in winter is infiltration, and wood stoves that use interior combustion air encourage infiltration. You should not use any woodstove that uses interior air for combustion. That's illegal even in much larger mobile homes and very unsafe in such a small and tight space as yours with exhaust fans and the stack effect of a high ceiling.

    If you don't want to use drywall as an air/vapor membrane, then there is probably little choice but a material like Membrane (with which I have no personal experience). It is not like Goretex, but rather just the opposite - it is designed to not be vapor permeable until the RH rises. Housewrap is like Goretex - keeps out liquid water and wind but lets vapor through.

    Whatever air/vapor membrane you decide to use, it will be important to monitor and control indoor RH, since the wood paneling will be vulnerable to mold.

  8. Kai Mikkel Forlie | | #8

    I check the remarks on the dangers of using a non-closed combustion wood stove in such a small space. Unfortunately, no one here (or abroad - trust me, I've looked)) makes ultra-tiny closed combustion wood stoves, so we are stuck with the approved-for-use and time-honored marine version. To combat disaster, we plan to install redundant CO detectors (likely at least one combo CO/smoke AC/DC unit and one standalone CO only battery-powered model) and follow the manufacturer's suggestions for a marine installation, including never leaving the stove unattended, cracking a window when its in use (or in our case making good use of at least one through the wall make-up air vent) and always being very mindful (as anyone aboard ship would be) of the dangers of combustion by-products in a small enclosed space. As far as the solar exhaust fan on the toilet goes, its CFM rating is miniscule and hopefully not enough to overpower or "confuse" the woodstove draught. If we find there is a conflict, we can always open the vent more, crack a window and/or install a second makeup air vent somewhere else. We chose to make war on complacency for the ability to use free waste biomass for all of our winter heating. IOW's, we'll just have to be very careful and keep a watchful yet critical eye on our detectors.

    Thanks also for the corrections to my mispeak. Yes, dewpoint and not freezing point, is the concern. I'm glad you pointed that out to prevent inaccuracy.

    As far as the wood heat and drying goes - I'll have to take your word for it (but it sure feel dryer!) :)

    Finally, I guess the threat of mold on the back of the T&G will have to be met with reasonably agressive venting and, as it was said, quality monitoring of the indoor RH. I suppose we could consider eating up a litle more interior space by slightly furring out all of the the interior T&G and even adding some high and low vents to encourage convective drying behind the paneling. Could be kind of a neat solution.

    Thanks again for all the good questions, answers and concerns.

    And, of course, I'd still love to hear from anyone who has any experience (good or bad) with Certainteed's "Membrain" product or has any other suggestion(s) for what we might consider using underneath our interior T&G paneling in place of sheetrock.

  9. James Morgan | | #9

    If my house was on a trailer, I think I might just spend the winters in Florida and not bother with the woodstove at all ;-).

    Seriously, I find myself not much interested in the how of this project but rather interested in the WHY. A home detached from the earth and with no sense of place is not entirely a mystery to me but I find it hard to reconcile this with the trappings of permanence expressed in these specs: siding and stovepipes, XPS and paneling, composting toilet.... I find myself wondering if it's about living lightly on the earth why not a yurt? If it's about mobility why not a tent? If it's about comfort, why not a cabin? I'd love to learn from Kai more about the context of this concept, what kind of life will be wrapped in and around it.

    But perhaps I'm reading too much into this. Is it just a 'because we can' project?

  10. Kai Mikkel Forlie | | #10

    Good question! Why?

    Partly out of neccessity. Our tiny house skirts zoning and building code because its on wheels. Where we live, a yurt, like a tent, is a regulated structure, and prohibitions exisit that prohibit their long-term use.

    Partly out of a need for mobility. Our tiny house will be self-contained, on-the-grid and off-the-grid capable and easy to move to a nearby piece of land or a backyard. A cabin can't offer this.

    Partly out of a need to simplify and live lightly. A building this size regularly sleeps six or more in most of the rest of the world. We've decided we don't need most of what we own and a tiny house forces us (willingly) to minimize our belongings while increasing our opportunities at the same time. We liken the result to "living large in a tiny space" since we'll have more time and money to pursue the better things in life, not strapped with a mortgage, huge utility bills or constant maintenance expenses, etc. On the carbon footprint side of things, while a yurt or tent would be great, neither is a suitable structure for a northern climate in this age of resource depletion and climate change. We prefer to burn as little wood and homeade cooking alcohol as possible; a tight shell and modest insulation affords us the ability to meet this goal.

    All in all, living tiny will give us more time, energy and money!

    Thanks for asking! :)

  11. James Morgan | | #11

    Good answer!

  12. J Chesnut | | #12

    Very interesting. First please tell me where I can find a partner that will tolerate such interesting conditions; I want one!

    Since you are only talking about 140 sf here and a lifestyle where I can imagine you are not spending all your time indoors I think too much emphasis is being placed on a high performance envelope. For such a small space with tighter hygrothermal and air quality tolerances I would suggest erring towards an diffuse open assembly that is less air tight. While you compromise a little on R-value we are only talking 140 sf of space.
    When you are occupying a structure that small the strategy could be to heat your bodies rather than condition the space (except of course to maintain a minimum ambient temperature to keep things from freezing and to properly manage the RH to care for your wood finishes and such.)

    I'd like others to verify this by why not stick with a 2x6 stud wall with a dense pac cellulose or mineral wool infill. Sheathing with 1x boards covered with #30 felt paper. No rain screen and don't paint your redwood. When the house is parked extend your roof overhangs generously to protect the small home from direct rain and splash back.
    Forgo the rigid foam insulation, or if you already have it can you put in on the interior of the wall assembly?

  13. J Chesnut | | #13

    Let me retract my advise not to paint your redwood clapboard.
    I've seen successful unfinished wood exteriors that have maintained well over 15 years but they were 1" nominal in thickness and did not have a high degree of sun UV exposure.
    Because of the house's changing conditions you will probably see a situation where your clapboard is highly exposed to the sun and with a thinner siding it could degrade and cup sooner than you want.

  14. Jesse Thompson | | #14

    I'm with J.

    Ditch the foam, 2x4 wall with horizontal cross strapped 2x for a 5" thick thermally broken wall, dense packed with cellulose. Interior plywood for a structural air barrier if you like more toughness and racking resistance than air tight drywall (especially if moving your house around much). Exterior: either diagonal pine boards covered in felt paper and redwood for a vapor open affordable and rigid exterior skin, or metal diagonal strapping, typar and your redwood siding for a Riversong style outside skin. Either way your walls can dry to the outside, and most likely have a warm, cozy and affordable house in the winter.

    Your house is small enough to not need to be super duper insulated as long as you have a good thermal break on each surface.

    All one person's opinion, of course...

  15. J Chesnut | | #15

    I'm with Jesse,
    His suggestion is superior as it reintroduces the thermal break and the point about using the plywood to stiffen a mobile structure is smart.
    I think the 2x cross strapping would then be better on the outside so the plywood could tie to the vertical studs. A nice enough grade of plywood could act as your interior finish saving you another layer. A gasket behind the seams on the studs and top/bottom plates would establish the air barrier on the wall.

    What was your plan for insulating the cathedral ceiling and floor?

  16. Kai | | #16

    Hmmm. Working off the last post by J.T., how about everything J.T. said but with a couple of alterations?

    [From the outside of the studs to the outside of the trim, we only have 3" to play with.]

    So, (from the inside - out) T&G hemlock boards over the full dimension 2X4 native wall studs (or the storebought 2X6 rafters), over 1" thick horizontal strapping, over 1/2" plywood with holes in it (the 4.75" cavity created then blown full of cellulose), over Tyvek, over 1/2" thick vertical strapping (AKA rainscreen wall), finished with redwood? [Can somone say Halleluiah?!?! :)]

    We need to keep the redwood off of the wrap or the tannins will eat right through it and we need plywood on one side of the wall for the racking strength it provides (being a house on a trailer). We want boards on the interior so that leaves the exterior.

    I would love for Mr. Riversong to weigh in on this idea! :)

  17. Riversong | | #17


    There's no Mr. Riversong here, just Robert.

    You would still need an air/vapor barrier, continuous from walls to ceiling, and it would be more effective on the inside so I would still advise the Membrain.

    And I would delete the horizontal strapping so the the plywood is attached directly to the studs with 8d ring-shank nails for reliable bracing (would also use construction adhesive). With full dimension 2x4 studs, you'll have enough R-value, or if you're worried about thermal bridging you could put the horizontal strapping on the inside and install your T&G vertically.

    Another option is to install the XPS on the inside with taped seams for your air/vapor barrier, strap over the studs vertically and apply the T&G. Then you could install the redwood over #15 felt over plywood without bothering with the rainscreen.

  18. Kai | | #18

    Thanks! How about if we glue and screw the plywood sheathing to the horizontal strapping (and glue and screw the strapping too)? That's got to be beefy anough for structure on wheels that will likely only move to one or two different places in its lifetime (at least its lifetime with us). We'd move the strapping to the inside but are loathe to shrink the interior dimensions at all given that we would like to benefit from all the interior square footage we can.

    Also, I note that since a single layer of latex paint has a perm rating of 5, then the use of a single layer of "Membrain" would be the way to go behind the interior T&G boards.

    Also, to answer an earlier question concerning the roof and floor, the roof will match whatever we do in the walls or possibly utilize an exterior layer of foam underneath the standing seam metal roof to deal with condensation under the metal. As far as the floor is concerned, it will be [from the outside - in] Masonite (held in place with galvanized roofing screws and caulking), 1/2" thick unfaced XPS, then wooden 2X6 joist and wooden 2X6 joist hangers (the joist hangers are sistered to the steel frame but we held all the wood ~ 1'2" high for a thermal break and because we had to level the floor with a dumpy level since the steel frame was so out of whack), 2" thick unfaced XPS cut to fit between the joist and foamed in place, dense pack cellulose (filling the rest of the cavity), plywood subfloor, then reclaimed 1" thick maple flooring.

  19. Riversong | | #19


    That exterior screw/glue strapped sheathing detail would probably be OK, but if the framing and the strapping is 16" oc, then the fasteners are in a 16" grid, which is very wide spacing. A good plywood shear panel must be oriented vertically, attached to the bottom and top plates and with 4" oc nails at all edges, 8" oc in the field.

    Changing the strapping from outside to inside should make no difference in the usable interior space if you build the wall to meet exterior width limits and simply shift its location on the floor.

    As for your floor section, I've never seen XPS (which is always unfaced, by the way) in 1/2" thickness, and adding that little foam to the exterior of the joists will not give much of a thermal break so I wouldn't bother. And there is no real advantage to inserting one layer XPS between the joists since the R-value (5/inch) is not that much more than dense-pack (3.8/inch) and cellulose performs best in an assembly that breathes. With caulked masonite on the bottom and plywood (air/vapor retarder) on the top, I would just dense-pack the entire cavity. Use construction adhesive on the subfloor for strength and rigidity.

    And, for either subfloor or sheathing, with the use of construction adhesive (I would recommend PL Premium urethane) there is no advantage to screws (and a disadvantage if you use drywall screws which have little shear strength). Ring-shank, cement coated or hot-dipped galvanized nails are sufficient.

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