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Community and Q&A

Tiny House Roof Assembly

NewTinyBuilder | Posted in Plans Review on

First time builder Evan here, putting my sheathing up this week. Yahoo! Would love help solidifying my roof assembly for next week.

Tiny cabin, 80 square feet (8×10)
Vermont, Zone 6
Cathedral shed/skillion roof pitch 3:12

I had updated my roof assembly to be as follows: drywall, 2″ of fiberglass faced polyiso, 3″ of polyiso cut and cobbled in between 2×6 rafters, 2.5″ air for ventilation, OSB with taped seams, 30# tar paper, metal roof.

My limited beginner understanding, based on several articles on GBA and Building Science, was that I needed to do a vented assembly because I could not fit R-40 or more of insulation in the tiny house’s height restricted roof to keep the sheathing warm/dry and prevent ice dams. However, the expertise here on GBA from users Akos, Jason S, and others has recommended unvented tiny house assemblies in zone 6 before, both to me on an earlier thread and in Ari’s tiny house Q&A post back in July.

If I did the unvented approach it would look like this (thanks to Akos): Exposed rafters, 7/16″ OSB with taped seams, 3″ polyiso, 30 pound felt underlayment, 1×4 strapping, metal roof. Edges of the foam would be protected from critters.

My questions about the unvented assembly are: (1) If the cabin needs more insulation, how much air permeable (cellulose, fiberglass, wool, etc) insulation between the rafters can I add later before I develop condensation issues? I was guessing about R-16.5, or half the total roof insulation, but wanted to confirm. (2) Will 3″ of polyiso be enough to prevent ice dams? My limited understanding is that only 3″ would allow a lot of heat to escape and melt snow on the roof, leading to dams. (3) Should I use a peel and stick product on the roof like GAF StormGuard? Lastly, (4) is a roof overhang necessary? I would love to have no/minimal overhang to stay within the 8’6 width for road transport and thought if the roof is all the same temperature without an overhang that perhaps melting snow would not have a chance to freeze at the eaves and dam up.

Thank you so much for your continued support and expertise! I would not have gotten this far without you all.

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Replies

  1. creativedestruction | | #1

    Evan,
    1. Yes, about 50% or less of total R value should be safe for air permeable insulation in the rafters. R15 mineral wool batts would work fine. This assumes your interior relative humidity doesn't exceed say 50% in the winter. Keep that in check--I imagine it can build up quickly in a tiny house!
    2. You shouldn't have ice dams if you get the OSB airtight at the joints, outer edges and any penetrations. If you can vent the gap at the 1x4 strapping without letting in bugs (or water of course) that can act as a safety measure by uniformly cooling the roof surface. No heat buildup, no ice dam. That said, I'm not sure this is possible without overhangs.
    3. Sure, use it on the OSB. Not needed, but a 'nice-to-have'.
    4. No, not needed as long as you have a proper drip edge and the siding & openings are well detailed for higher exposure.

  2. NewTinyBuilder | | #2

    Hi Jason S.,

    Thank you so much for your thoughtful reply. Its good to know I could add more insulation later without issues, as long as I keep the RH down. Assuming, as a new builder, that I will NOT do a great job (thus far Ive made so many mistakes!) with air sealing, detailing, etc, would you go for the 3" poly unvented assembly or the vented assembly with 2" poly below rafters and 3" poly cut and cobbled between rafters with 2.5" vented air space? My impression is that there is more room for error with vented assemblies?

    Thank you so much,
    Evan

  3. NewTinyBuilder | | #3

    Sheathing on the cabin is done! Wooo!!

    Time to put in roof rafters - I want to make sure I'm moving forward as recommended and within my skill level as possible. It seems the GBA consensus is to do an unvented assembly with exposed rafters and 3" of polyiso on top of my osb sheathing. I'm a first time builder and don't have confidence in my ability to ensure the assembly is air tight. I purchased 3m all weather flashing for the seams of the osb roof deck - but I'm worried especially about the intersections between the roof and the walls. Turns out the edges of the foam boards are not in the best condition because the rope I tied them down with to get them home cut into the foam a bit. I can use only one layer so can't offset any seams as is recommended, and Im reading that it's hard to tape the felt/fiberglass style of foam I have. I'm also not sure if I can keep humidity down below 50 percent with two people in such a small 8x10 space. Is it still smart to attempt an unvented assembly? I haven't gotten much insight on the vented option so I'm scared to do that one when everyone recommends unvented, but worried I can't do unvented right. Would love to hear what my options are here. Thank you all! Evan

  4. ROBERT OPALUCH | | #4

    If you use an HRV to bring in fresh air, which would be warmed by the exhaust air stream, you would capture a large percent of the heat but exhaust humidity. That might help keep humidity down. Here's a GBA article that lists sources of indoor humidity:
    https://www.greenbuildingadvisor.com/article/moisture-sources-relative-humidity-and-mold

    1. NewTinyBuilder | | #5

      I will look into HRVs thank you Robert! Update: oof those prices are steep! Might be out of my price range for now

      1. ROBERT OPALUCH | | #6

        There are some cheaper models, probably $200-$300 in total for the entire installation.

        Lunos makes innovative models for smaller spaces that you could install yourself, but they are closer to $1,000.
        https://foursevenfive.com/lunos-e/

        You could plan your installation and install later if needed. Pre-wire or frame for future install.

        1. NewTinyBuilder | | #7

          Thank you for the info Robert! I'll look into those budget models. For now, all the advice I get on GBA says to go unvented, yet my gut says to go vented because of the high level of humidity in a tiny house and mold. But I'm new to all this so moving forward with the ideas people share with me has been really helpful!

          1. ROBERT OPALUCH | | #8

            Yes usually its cheaper and less potentially problematic to have a vented attic above the upper floor ceiling, rather than an unvented, insulated roof. However, tiny homes are typically limited in space, so an unvented insulated roof makes sense given that constraint.

            An HRV or ERV would bring fresh air into the house and exhaust stale air. The Lunos models vent directly though walls with no ducting. Makes it simple and easy to install. Just have to run low voltage wiring to the controller/switch, no ducts. Other brands of inexpensive HRV/ERVs have to be mounted somewhere, and run ducts for incoming air from outside, exhaust air to outside, supply air from HRV/ERV to a room, and exhaust air back to the HRV/ERV. But could be fairly simple and not take up much space along your ceiling or through storage areas. Doesn't have to be in an attic or basement area since they are not large.

            If you don't do much cooking (e.g., mostly microwave) and don't have many other humidity-generating items inside your home, and air out the house or use an HRV, and be careful about designing walls be able to dry outwards, you may not experience a problem with humidity condensing and damaging wood in your walls or roof over the years. A smart vapor retarder might be suggested too, to reduce interior humidity from migrating into your building envelope where condensation might result during winter. Even if it does, drying out during the summer can help avoid mold and rot over time.

          2. Expert Member
            MALCOLM TAYLOR | | #9

            NTB,

            The level of humidity in the interior has no bearing on whether the roof performs better vented or unvented. What matters is how good your air-sealing of the ceiling is.

            In a building that size you will have high humility, but also such low heating demand that I wouldn't get too worried about providing mechanical ventilation, and would consider just cracking a window instead. Tiny Houses perform very similarly to their close cousins RVs and Trailers. They are all fine with natural ventilation.

          3. NewTinyBuilder | | #13

            @Malcolm Taylor thank you for educating me. I'm afraid my air sealing of the roof won't be good enough to go unvented - I will tape the osb seams with 3m all weather and spray foam on the inside of the wall to roof transition but my polyiso board edges are a bit torn and I only have one layer so I can't stagger seams. I will definitely plan to crack a window for ventilation. If I don't trust my air sealing ability for the roof, do you have a recommended roof assembly for such a small tiny cabin? I'd be most grateful to hear your thoughts, I plan to install rafters tomorrow eek!

          4. Expert Member
            MALCOLM TAYLOR | | #15

            NTB,

            I think you are fine with a vented assembly if that's what you are most comfortable with. I was just making the p0int that both rely on good air-sealing to work well. That's where you should put your emphasis.

          5. NewTinyBuilder | | #16

            @Malcolm Taylor thank you for checking back in. That's really helpful to know, I had thought wrongly that air sealing was less important for a vented assembly. My gut says keep it simple and cut and cobble 3" or 4" polyiso between the rafters, spray foam the edges, and leave 2.5 or 1.5 inches air space below the osb to allow it to dry in both directions, but I have learned that cut and cobble isn't a good solution and air sealing would still be an issue. Is there an assembly that you would recommend given my inexperience with air sealing? I don't know what is good enough, it sounds like I will have potential moisture related issues either way with less than perfect air sealing. Does one assembly over another give more room for error?
            *Update: I just read an article that makes it seem like only the roof sheathing needs to be air tight, not necessarily both the sheathing and the foam above it. If I put grace ice and water on the osb sheathing would that get me air tight enough that I don't need to worry so much about the foam insulation's seams etc?

          6. Expert Member
            MALCOLM TAYLOR | | #17

            NTB,

            I don't know about best, but this is what I did in a similar situation.

            Several years ago I built an insulated she-shed for my wife. It was a bit different in that the wall structure was made of plywood bookshelves which were structural, but the roof was fairly simple:

            - I sheathed the rafters with G1S 1/2" plywood. Taped the top and caulked the seams below.
            - I fastened a 2" high perimeter around the plywood.
            - Filled the area with EPS foam.
            - Sheathed over the foam with exterior grade plywood fastened through to the rafters.
            - Added an underlayment and snap-lock metal roof.

            This gave me a finished ceiling, easy air-sealing, and a simple non-vented assembly.

          7. Expert Member
            MALCOLM TAYLOR | | #18

            The walls got 2" of exterior EPS too, leaving the interior wall cavities as storage.

          8. NewTinyBuilder | | #19

            @Malcolm Taylor,

            Thank you for sharing this info and photos with me. The shed you built is incredible! I hope to one day be able to build like that. Your post gave me a lot of ideas. I have the rafters up now but I am still not sure where to put the fascia/sub-fascia and drip edge if I use battens - I was also told to use battens for ventilation instead of solid decking like you used, but perhaps I should switch? Im not sure, I just drew up a sketch for solid decking like you did but worried without cooling the metal roof could lead to ice dams. Here is what Im thinking for next steps if I do battens:

            1. Sheath roof and tape seams (I like your idea of caulking underneath - worried it might interfere with the 1/8" OSB spacing).

            2. Extend wall sheathing up to meet roof sheathing.

            3. Install Grace Ice and Water over all roof sheathing to ensure air tightness, and over the wall-roof transition

            4. Build 3" high perimeter around sheathing (doubled 2x4)

            5. Install sub fascia and fascia to outside of 3" perimeter

            6. Install foam

            7. #30 felt over foam, perimeter, and 3" down fascia board

            8. Install battens parallel (and on top of) rafters with 8" timberlok screws through foam and into rafters

            9. Install aluminum drip edge on battens along rake

            10. Install battens perpendicular to rafters

            11. Install aluminum drip edge on battens along eaves

            12. Screw down metal roof

            Thank you for all of your help! Evan

          9. Expert Member
            MALCOLM TAYLOR | | #20

            Evan,

            That sounds good. I'd do a couple of things differently for simplicity.

            - Make the perimeter out of a cut down 2"x4" (3" high) and install vertically.

            - Don't worry about using caulking between sheets. If the sheathing needs to expand, it will.

            - I'd go straight to purlins (battens) perpendicular to the rafters, and skip the two layers (or use plywood as I did). Between the foam, the air-sealed sheathing, and the I&W, you have a great un-vented assembly.

            - With most metal roofing you want your gable and peak trim to be installed after the panels. Bend up the edges of the panels at both ends and top, then install the flashing. i would use something similar to the gable flashing in the link for both situations: https://www.westform.com/application/files/4215/8585/3144/Prolok_Flashings_NP_2020.pdf

          10. NewTinyBuilder | | #21

            @Malcolm Taylor Great tips! I will do all of these as you said. Follow up:

            1. What caulk would you recommend for the underside of OSB sheathing seams? I was planning to use DAP 230 at base/top plate seams, might work here too?

            2. If I go with only the perpendicular-to-the-rafter purlins, should I cut kerfs in the bottom to allow bulk water to drain with gravity between the purlins and the foam? I dont want to trap moisture that might get in.

            3. Wow I had no idea that the drip edge would get install afterwards. I bought aluminum 6" roll to make my own drip edges I will try to mimic that image you sent for the gable sides and top end.

            Here is a photo of me installing rafters yesterday - only a few weeks until snow here in Vermont! Down to the wire, appreciate all of your support.

          11. Expert Member
            MALCOLM TAYLOR | | #22

            Evan,

            - The primary reason to caulk the underside is aesthetic. The tape above (and in your case the I&W) do the heavy lifting. Anything will do.

            - If enough moisture accumulates under the roofing that it requires draining and won't dissipate, something has gone badly wrong. That said, that's why I prefer roof sheathing with the panels fastened directly to it. If there is no air space, there can't be any moist air.

            - Good luck with your build!

        2. NewTinyBuilder | | #24

          @Malcolm Taylor - roof is sheathed! Ready to put ice and water down, then foam, felt, battens, and metal roof. Just realized a quick question - can I stick the ice and water a few inches over the wall transition now or do I need to wait until I have the house wrap felt on, after the roof is done, to make sure the ice and water is lapped over the house wrap? I thought it didn't matter because I'll have felt going on above the ice and water but wanted to check. Thanks!

          1. Expert Member
            MALCOLM TAYLOR | | #25

            Evan,

            It's always best to lap your layers so they will shed water. If you don't want to wait, run a 12" piece of house wrap along the top of the wall. You can tuck the rest of the house wrap underneath when you install it later - much as you do with the house-wrap at window sills.

          2. NewTinyBuilder | | #26

            @Malcolm Taylor Great suggestion - I was able to do this by keeping a few inches of backing paper on the ice and water where it runs off the roof deck until I get the wall felt WRB up and tucked underneath. Woohoo! Learning every day how to do this.

            For the roof, I priced out 3 sheets of ply/OSB sheathing for the above-foam layer like you did instead of 12 foot purlins and its only a $50 difference total, seems worth it. Would also let me staple/nail roof felt underlayment directly on the top layer of sheathing instead of through the foam, saving money on longer cap nails and put fewer holes in the foam. Win-win. I am using a non-standing seam metal roof with ribs that I got for free - looks like photo attached - so it has more space between the metal roof and the decking.

            Question: Is it worth adding 1x4 or 2x4 on top and parallel to rafters before the top layer of solid decking? This could make the decking/metal roof cooler, and give the metal roof more wood to screw into than just the sheathing, but the air space would expose the top of the foam insulation to critters etc. Here it is written out interior-exterior:

            Option 1 (ISO is more protected by being sandwiched between sheathing)

            Rafters
            7/16" OSB
            Grace Ice & Water
            3" ISO
            1/2" ply or 5/8" OSB screwed with headlok to rafters
            #30 felt
            Metal roof screwed only to the sheathing

            Option 2 (Cooler metal roof with air space below sheathing, Metal roof is fastened to more material, foam is more exposed)

            Rafters
            7/16 OSB
            Grace Ice and Water
            3" ISO
            1x4 or 2x4 straps parallel to rafters screwed through foam to rafters
            1/2" ply or 5/8 OSB screwed to strapping
            #30 felt
            Metal roof screwed through sheathing into straps

            Thank you for your guidance!

          3. Expert Member
            MALCOLM TAYLOR | | #27

            Evan,

            As I said (post #22) I'm not big on air cavities under roofing, unless they can be detailed to provide good ventilation - which yours can't. However, you are using an exposed fastener roofing, which makes a good substrate more important, as the screws backing off mean a leak. If it were me, rather than spend the extra time and money on another layer of strapping, I'd beef the roof sheathing up and use 5/8" plywood.

          4. NewTinyBuilder | | #28

            @Malcolm Taylor Thank you for the reminder about air spaces. I was worried that my metal, with the extra ribs, would by default create air cavities that trap moist air more than your standing seam if not vented, since it wont lie flat on the decking, but perhaps I am over thinking it.

            Edit: When I nailed down the OSB decking I used nails 6" OC on the edge and 12" in the field. I was planning to use headlock fasteners to attach the top layer of decking through the foam to the rafters - do I need to use the same nailing schedule as the lower decking (tons of long nails) or can I just put a headlok every 16" like I would for strapping?

            I will order the plywood sheathing and not the strapping - should arrive tomorrow!

          5. Expert Member
            MALCOLM TAYLOR | | #29

            Evan,

            The small amount of air under the r0ofing striations (which are open at each end) will dissipate. That's another animal altogether than an intentional air-gap.

            Go to 12" oc on the screws. In general, when nailing off sheathing on roofs or walls, more fasteners are better. Here in the PNW I nail off sheets at 3" oc, on the perimeter, and in the field.

  5. dlfdk | | #10

    As someone currently living in a 160 sq. ft. tiny house, I would caution against skipping on mechanical ventilation. In such a small space - and even more so when it's just 80 sq. ft. - the CO2-levels quickly jumps to problematic levels as soon as doors and windows are shut. At least if it is reasonably air tight. And to avoid moisture issues it probably should be.

    Having a permanently open window is of course the cheapest option up front. I don't know how comfortable it is in the long run however.

    Just as you did, I found the available options on the market too expensive when I built my tiny house, so I ended up making my own ventilation unit. While I can't exactly recommend that (it was way more complicated than I had anticipated), having mechanical ventilation is really fantastic. It keeps a check on both CO2-levels and humidity without me having to worry about it.

    Regarding the roof overhang: I had the same concerns as you do now. Every inch spent on overhangs is one less for insulation and floor space. I ended up installing gutter and flashing in a way where I can rather easily detach both, should the house need it for transport. It works well, although 3 foot overhangs would of course be better. At least the maintenance job is proportional to the house size.

    1. NewTinyBuilder | | #14

      Hi @dlfdk! Thank you for this recommendation, your experience living tiny is very important to me. I'll look into a co2 monitor for now to gauge my window venting. I'm guessing my cabin won't be very tight, I've never built anything before and my door came off an old cabin a neighbor was tearing down hahaha. So maybe I'll get some good ventilation there. I'll also look into making my own ventilation unit and take your not-recommendation of that route seriously. I'll also look into gutters. I'm learning so much!

  6. NewTinyBuilder | | #11

    @Robert I think my gut is telling me to go the vented route for the same reason you stated about it being potentially less problematic, even just to cut and cobble polyiso between the rafters and leave 2" of air space between the back side of the foam and the osb roof sheathing. But I'm inexperienced and trying to take all the advice I get about going unvented seriously since I know it will get better heat retention without the thermal bridging. I'll look into mechanical venting again after getting some income :) I'm not planning to cook or shower or anything inside the cabin so I might be ok moisture wise, although if I follow the recommendation to put foam boards on the wall exterior (perhaps a bad idea?) My walls wouldn't be able to dry to the exterior. I never realized humidity would be such an issue! New builder lessons

  7. NewTinyBuilder | | #12

    @Malcolm Taylor thank you for educating me. I'm afraid my air sealing of the roof won't be good enough to go unvented - I will tape the osb seams with 3m all weather and spray foam on the inside of the wall to roof transition but my polyiso board edges are a bit torn and I only have one layer so I can't stagger seams. I will definitely plan to crack a window for ventilation. If I don't trust my air sealing ability for the roof, do you have a recommended roof assembly for such a small tiny cabin? I'd be most grateful to hear your thoughts, I plan to install rafters tomorrow eek!

  8. NewTinyBuilder | | #23

    Question for anyone monitoring the thread - how far below the roof deck and down onto the wall sheathing would you run the grace ice and water? I want to run it down a little bit at least to ensure the transition is air tight, but not sure if it should be just a few inches or more? Thank you! Evan

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