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Tiny house on wheels — building envelope (vapor-open) suggestions

GreyWolf92 | Posted in PassivHaus on

Hello all,

I am in the process of building my tiny house on wheels (My home will be in Zone 3b (In between Los Angeles and San Diego) or Zone 3c), with a local builder. He is covering most of the build, but I am helping out when i can.

I have been doing loads of research on passive/breathable homes and want to make sure we do our building envelope correctly! I am extremely allergic to mold, so this is even MORE important to me than the average person.

There is a lot of information online for how to build tiny houses but rarely do you see good information on the building envelope…Which I think is one of the important things to consider (especially in such a small space).

I consulted with a very knowledgeable passive house designer, and he gave me a solid foundation of suggestions to start with.We are going for a completely, vapor open wall assembly to allow to dry exterior and interior. The home will be water tight, air tight, but vapor permeable. Of course, there are plenty of windows and proper Hvac for good ventilation.

Here is what we are thinking for our building envelope:

Wall assembly: Gypsum board OR Wood –> Intello plus (vapor retarder) –> Insulation (Roxul)->Sheathing (Plywood)->rain proof barrier-> air barrier->WRB (mento plus)-> Western cedar siding

Roof assembly: Gypsum board OR Wood –> Intello plus –> Insulation (Roxul)->Sheathing (Plywood)->rain proof barrier-> air barrier->WRB (mento plus) –> ventilation gap (1×3 lumber) –> metal roof

Now, the floor envelope is what I am really struggling with, because my metal trailer (Iron Eagle PAD series) has a galvanized steel bottom pan. In other-words, completely vapor closed.

After speaking with some tiny home companies and brainstorming myself, these are my two options:

Dry towards exterior: Poke small holes in bottom of bottom pan with air gap, to allow moisture to dry and drain out bottom. Order would be: Metal pan (with holes) –> Air vent gap –> rigid foam –> vapor barrier –> Advantech or plywood

Dry towards interior: Completely vapor/water/air tight (like a boat) and a vapor open assembly. Order would be: Metal pan (with holes) –> Air vent gap –> roxul–> Advantech or plywood.

What do you guys think about my current building envelope and what do you suggest for the floor? Is it ok to have a vapor open wall assembly with a different floor/roof envelope? Also, is it common to use a different insulation for flooring, than the rest of the house?

Thank you so much for taking the time to read.


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  1. user-2310254 | | #1

    Why did you not include strapping on your wall assembly? The gap would help to promote drying.

    What are the R-values of your various insulation layers?

  2. GreyWolf92 | | #2

    Hi Steve, thanks for the response.

    Are you just referring to an air ventilation gap? I didn't think it was needed because the walls are a vapor open assembly but now that you mention it, it definitely won't negatively affect anything.

    Walls: r15
    Roof: r23

    Not too concerned over R-value because of the climate I will be in.

    What's your thoughts on my floor assembly?

  3. GBA Editor
    Martin Holladay | | #3

    You really don't have to worry much about whether a floor assembly is vapor-open or vapor-closed, for a variety of reasons (one of which concerns the stack effect: due to the stack effect, moisture problems are greater in walls and roof assemblies than floors).

    My advice: Don't poke holes in the steel bed of your trailer.

    The best type of floor will be a site-built SIP: OSB or plywood, rigid foam, and OSB or plywood.

    -- Martin Holladay

  4. GBA Editor
    Martin Holladay | | #4

    I have no idea what these three layers are (on your wall), all between the sheathing and the siding:
    1. Rain proof barrier
    2. Air barrier
    3. WRB (Mento Plus)

    That's a lot of layers! What are they?

    Typically, the plywood sheathing would be the exterior air barrier, and the drywall (or in your case, perhaps the Intello) would be the interior air barrier.

    The "rainproof barrier" is really two layers of defense: (a) siding and (b) the WRB.

    So what did you have in mind for all these layers?

    -- Martin Holladay

  5. GreyWolf92 | | #5

    Hi Martin,

    Yes, I wanted to edit my original post, but I realized this is not an option.

    The wall assembly should be: Gypsum board OR Wood--> Intello plus (vapor retarder)--> Roxul insulation->Sheathing (Plywood) -> Mento plus (wrb)--> furring strips (rain screen)-->Western Red Cedar (siding).

    Pretty much the exact assembly as assembly 10 on Joseph W. Lstiburek article:

    The mento plus will be the water/air barrier. The rainscreen will allow ventilation for exterior wall assembly to dry. Does that clarify?

    Regarding the floor assembly, do you still not foresee any issues even with the floor being two feet off the ground? Will this not draw increased condensation on the bottom metal pan (which would then get in the floor assembly)?

    What is the benefit of doing a site build SIP vs: Metal pan -> Rigid foam -> Plywood (same thing without plywood beneath insulation)? And you recommend not to use a vapor barrier above or are you classifying the metal pan as the vapor barrier?

    Note, that I will be in hot-dry climate zone 3b

  6. GBA Editor
    Martin Holladay | | #6

    It's easier to detail the plywood sheathing as your air barrier than to try to detail your housewrap as an air barrier. (The plywood is more durable and less likely to get ripped.)

    The steel components of your trailer won't attract moisture. In general, if the steel is at the same temperature as the outdoor air, it won't be a condensing surface -- and even when it is, which will happen in some weather conditions, that fact won't cause any problems. (Roofing is far more likely to get dew than the underside of your trailer, because roofing experiences nighttime radiation cooling -- the heat is radiated to outer space, allowing roofing to drop to a lower temperature than the outdoor air temperature.)

    If you have a trailer with a smooth bed, you can create a floor assembly without a lower layer of plywood. You can adhere rigid foam directly to the steel bed if it is smooth. Just think through the water management and drainage details -- you want to be sure that rain isn't channeled into the trailer at any point.

    -- Martin Holladay

  7. GreyWolf92 | | #7

    That's a good point, Martin. So when you say "detailing housewrap" are you referring to sealing all the bottoms and edges to make it air tight?

    If I use plywood as the primary air barrier, do you suggest I tape the core seams BUT leave the bottoms and edges open to allow water do drain out/down?

    The trailer will have a smooth galvanized bottom pan. All four corners are slightly open currently (to allow rain through when it was being shipped).

    I want to clarify, you suggested not to poke any holes, but are suggesting a drainage system? So should I make sure the bottom pan is completely airtight or keep those corner drainage gaps? I would think I need a completely vapor/air closed bottom pan to allow any moisture in (especially if condensation won't be an issue).

    Also, do you think there would be any benefit of having a air ventilation gap between the bottom metal pan and the rigid foam (so if any moisture did get in, the foam won't be sitting in it)?

    Last question here, would be if advantech would have any advantage over plywood because it's more water repellent? Or would it be worse because it's not as vapor permeable (to dry on the inside)?

    Thanks so much for your help!

  8. GBA Editor
    Martin Holladay | | #8

    "Detailing an air barrier" means making sure it is airtight -- at the seams, at the penetrations, and at transitions to other parts of your air barrier system. For more information, see Questions and Answers About Air Barriers.

    Your house needs a foundation or a secure base. If you are using a trailer bed, your base needs to be able to support the floor loads and the wall loads.

    You need to know how your house will manage water. The roof keeps some of the rain off your walls, and a lot of water will drip off your eaves. The walls will occasionally have to handle lots of wind-driven rain. Where will all this water go? I don't know -- but you need to know the answer to this question.

    Ideally, your siding overhangs the trailer, so water dripping off your bottom course of siding doesn't end up in your trailer bed.

    -- Martin Holladay

  9. GBA Editor
    Martin Holladay | | #9

    Q. "If I use plywood as the primary air barrier, do you suggest I tape the core seams BUT leave the bottoms and edges open to allow water do drain out/down?"

    A. No. The point of an air barrier is to make everything as airtight as possible. You don't want holes or unsealed seams. And you certainly shouldn't have enough water in your stud spaces that it needs to "drain out."

    -- Martin Holladay

  10. GBA Editor
    Martin Holladay | | #10

    Q. "Would there be any negative to having the WRB AND the plywood as an air barrier?"

    A. No. It would also be good to have an interior air barrier as well.

    Q. "Assuming I go with plywood as the air barrier, Should I seal the top of the house wrap, but not the bottom (to allow drainage)?"

    A. You can tape both the top and the bottom of the housewrap. If you have enough liquid water behind your housewrap that it needs to drain out of your wall, something is seriously wrong with your house -- something that drainage won't cure.

    Q. "My current plans don't call for the siding to overhang the trailer because of width requirements."

    A. So where is the bottom course of siding designed to drip?

    Q. "Would that mean everything needs to be overhanging that is put on after the plywood sheathing (including WRB)?"

    A. My point is that you don't want to direct any rainwater to the steel bed of your trailer.

    Q. "Can you spell out the floor assembly for me? I want to make sure I am understanding what you suggested. I'm not sure if you are suggesting to have a completely air/water/vapor tight floor pan and seal the corners or leave those open."

    A. My original suggestion was for a site-built SIP. A SIP is airtight and close to vapor-tight. You later amended my suggestion by eliminating the bottom layer of plywood or OSB and substituting the steel bed of your trailer.

    In either case, you need to detail your floor assembly so it doesn't get wet. If it doesn't get wet, it won't need to dry out.

    Your choice of flooring will dictate whether your subfloor can dry to the interior. The biggest problem will be spills that you fail to mop up before the spills damage the subfloor. That's what happens to subflooring near a bathtub or shower. Good housekeeping practices will make your house last longer.

    -- Martin Holladay

  11. GreyWolf92 | | #11

    1) Would there be any negative to having the wpb AND the plywood as an air barrier?

    2) Assuming I go with plywood as the air barrier, Should I seal the top of the house wrap, but not the bottom (to allow drainage)??

    3) My current plans don't call for the siding to overhang the trailer because of width requirements. Would that mean everything needs to be overhanging that is put on after the plywood sheathing (including wpb)?

    4) Can you spell out the floor assembly for me? I want to make sure I am understanding what you suggested. I'm not sure if you are suggesting to have a completely air/water/vapor tight floor pan and seal the corners or leave those open..

    From my understanding, you are suggesting: Metal pan -> Rigid foam (Inside/between 2x6 joists) -> plywood. Which
    would be an interior drying assembly, am I correct?

    Won't take too much more of your time after we clarify this :)

  12. GreyWolf92 | | #12

    I'm going to go ahead and use the plywood sheathing as my air barrier on the walls.. All seams will be taped. For my wpb, I Am going to tape all seams. but nail the bottom of the barrier, to allow any potential moisture out. This seems to be a common practice, but do you see any issues with this because it will be travelling on the road occasionally?

    Regarding the floor assembly I actually called the trailer manufacturer and they recommended closed cell spray foam OR to tape/caulk all the metal pan seams and use rigid foam.

    So that would be:

    Option 1: Metal pan --> Spray foam (which seals any seams in metal pan) --> Sill sealer over metal joists --> Plywood

    Option 2: Metal pan (tape/caulk all seams to create air/vapor/water barrier--> Rigid foam --> Sill sealer over metal joists --> Plywood

    The last questions I have are:

    1) Do you think one of these options is more preferable over the other?

    2) If I chose option 2, my only option is to dry to the interior right? Because I wouldn't want a vapor barrier at the bottom (metal pan) and at the top (below subfloor) because that would allow to way for any moisture to escape.. Or would I want to use a visqueen vapor barrier above the metal joists (below the plywood subfloor) as well? From my understanding, two vapor barriers on each side is bad, but maybe this is different for flooring.

    3) I am very concerned with the off-gassing / voc of spray foam and rigid foam. Are there any proven, non-voc, green spray foams or rigid insulation's? I've heard of soy spray foam..but looks to have the same issues (petroleum).

    4) Is there any way I could pull off a roxul insulated floor assembly without moisture/mold concerns? I would love to use roxul but I believe it needs to be in a vapor open envelope.

    Perhaps it is important mentioning that I will be using marmoleum flooring, which is not water resistant at all.

    Your advice is greatly appreciated!

  13. GBA Editor
    Martin Holladay | | #13

    Your floor assembly doesn't have to dry to the interior. There will be no inward vapor drive through your steel trailer bed. Nor will there be any rain that enters from that direction.

    The idea that every side of your house has the "breathe" is a myth. For more information on this issue, see Worries About Trapping Moisture.

    -- Martin Holladay

  14. GBA Editor
    Martin Holladay | | #14

    If you are worried about off-gassing, you should probably choose rigid foam over spray polyurethane foam. I have never heard of any off-gassing problems from rigid foam. Using recycled (reclaimed) rigid foam would be best, of course.

    A very small number of spray foam jobs are associated with odor complaints. While these incidents are rare, they are serious.

    -- Martin Holladay

  15. GreyWolf92 | | #15

    You are right- no moisture should enter from the bottom from rain. Just want to be extra cautious, should there be a sudden temperature difference to allow that metal to condensate. Unfortunately, I am not able to view that article.

    My other concern would be if there was an accidental liquid spill in the floor, to allow it to dry to the interior.

    To sum this up- are you saying there shouldn't be a problem if I have a vapor barrier on each side (bottom metal pan) and on top of the insulation (below plywood)?

    I agree, I think it's best for me to use a rigid foam. Aren't the recycled products just made of xps/eps though?

  16. GBA Editor
    Martin Holladay | | #16

    Lots of building assemblies, including SIPs, are vapor-impermeable. As long as the assemblies are put together in a relatively airtight manner, such assemblies can perform very well.

    Subflooring can rot, as I mentioned before. We prevent subflooring from rotting with good housekeeping practices (mopping up spills promptly).

    If you are designing a house, and if you have multiple building science questions that keep bringing you back to the GBA site, I suggest that you invest $15 in a one-month membership so that you can read the articles I link to. Otherwise, my answers won't help you very much.

    As I have repeatedly noted, I think that your insistence that all of your building assemblies must be vapor-permeable is misguided and based on generalizations.

    It is certainly possible to buy recycled (reclaimed) EPS or XPS. Try Craigs List.

    -- Martin Holladay

  17. GreyWolf92 | | #17

    Thanks for all of the help/suggestions, Martin. I will look into signing up for a membership.


  18. AlexTinyBuilds | | #18

    Hi Martin,

    Question on this topic, I just installed my rigid foam above the metal bottom pan on my tiny house trailer.

    The rigid foam is beneath my 2x6 floor joists, And resting on the bottom pan. I did not glue them down however to the bottom pan, they are just sandwiched in beneath the joists.

    I noticed there may be air gaps between the pan and rigid foam where i didn't glue it down.

    Could this air gap cause condensation issues??

    I read above , and you say the metal floor pan should have minimal condensation because it will be the same as the outdoor temp.

    This is true right! I should be ok?!

    Thanks, really enjoy this website with all the articles, and support.


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