Tiny house with metal roof: vapor shield?
I’m building a 20’ tiny house on a trailer with a 3:12 shed vented roof vented. My current plan for the roofing and ceiling structure consisting of 2×6 rafters is as follows: metal roof->water and ice shield->5/8” plywood->XPS foam between rafters->tongue & groove beetle kill planks. I am located in zone 5b with cold snowy winters. Is this layering plan sufficient to prevent condensation issues in the winter? The question is, do I need a vapor retarder between the plywood and xps foam, or between the tongue and groove and the xps, or skip a separately distinct vapor retarder? Thanks for any insights you can provide – I am a complete rookie. For what is worth, the heating will be small wood stove.
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Sorry, the layers in my post above missed the 1" layer of xps between the T&G and framing.
To be safe, this type of roof assembly needs a vent channel between the uppermost layer of insulation and the roof sheathing. The vent channel needs to be connected to the soffit vents near the eaves and the ridge vent near the peak.
The method you have chosen is called the cut-and-cobble method -- generally not recommended for roofs, buy possible if you have a clear vent channel. For more information, see Cut-and-Cobble Insulation.
By far the most important layer in this assembly is the interior air barrier (needed to prevent interior air from entering the assembly). Note that tongue-and-groove boards are extremely leaky. In theory, a continuous layer of XPS between the boards and the underside of the rafters could be used as your air barrier, as long as the seams of the XPS are carefully taped with high-quality tape -- but some experts note that XPS may shrink, pulling away from the tape.
For more information on XPS shrinkage, see Using Rigid Foam As a Water-Resistive Barrier. (Scroll down to the section under the subhead that reads "Do rigid foam panels shrink?")
Q. "Do I need a vapor retarder between the plywood and XPS foam, or between the tongue and groove and the XPS, or skip a separately distinct vapor retarder?"
A. No, you don't need an additional vapor retarder, because XPS is already a vapor retarder. What you need is a durable air barrier.
Thanks a ton for your quick response. Noted on the need for an air barrier on the inside. Is there a better option than foam board? Also, I'm trying to stay away from dry wall due to vibration concerns when moving the structure. I'm not 100% sold on the need for tongue and groove and would also be open to the idea of plywood sheathed interior instead of T&G.
I just read your suggested links and I'm now wondering if I should consider roxul instead of foam board in the roof due to the shrinkage issue and PITA install factor. I unfortunately can't get a spray foam installer up to my location and I'm leery of trying DIY spray foam options.
Can you tell us your name?
Your 2x6 rafters on have enough room for about 4 inches of insulation once you include a vent channel and a baffle. That's not much -- about R-16 if you install mineral wool insulation, or about R-20 if you use the cut-and-cobble approach. A continuous layer of interior XPS adds another R-5, so you end up with about R-21 to R-25. That's much less than minimum code requirements.
The inferior R-value may not matter in a tiny house, because a tiny house is usually easy to heat, but it's still worth mentioning in case you were unaware of the issue.
For an interior air barrier, the first choice is always taped drywall. Taped XPS would work, as long as you don't worry about future foam shrinkage. Another option is one of the European air barrier membranes (for example, Intello Plus).
Thanks again Martin! My name is Jon, but I unfortunately haven't figured out how to change my name on this board.
Intello Plus does seem like a good option for my needs.
Given that a tiny house will have more concentrated levels of water vapor creation from the inside occupants, shower, cooking, etc should I also be looking for a roofing underlayment that provides a higher level of permeability than grace water & ice shield? I realize a metal roof has essentially zero permeability, so maybe the permeability of the underlayment is of less of a concern?
Q. "Given that a tiny house will have more concentrated levels of water vapor creation from the inside occupants, shower, cooking, etc., should I also be looking for a roofing underlayment that provides a higher level of permeability than grace water & ice shield? I realize a metal roof has essentially zero permeability, so maybe the permeability of the underlayment is of less of a concern?"
A. You won't get any outward drying through your metal roofing, so the permeance of your roofing underlayment is irrelevant.
Roofs don't dry toward the exterior, metal roof or no, so don't bother with permeable underlayments.
By the time you've paid for the cut'n'cobble foam a Intello you will have been better off buying a few sheets of 3" or 4" reclaimed roofing polyiso and installing R23 rock wool or R21 fiberglass in the 2x4 bays and go un-vented. A 4x8 sheet of used 3" polyiso in near perfect condition usually goes for between $15-25 in my neighborhood, 4" goes for $20-30/sheet, sometimes less. Double-layering it with 2 layers of 2", seams overlapped works too. With 4" foam the you'll need 6" pancake head timber screws to hold it all down with an OSB nailer deck onto which you would mount the roofing, at about 75 cents per screw in small quantities, eg: http://www.diyhomecenter.com/fastenmaster-headlok-heavy-duty-wood-screws-50-box?screwlength=34 . (Consult a nailbase manufacturer's installation guide for the optimal screw patterns and spacing, eg: https://www.hunterpanels.com/docman-categories/product-documents/hpanels/speciality-products/1388-h-shield-nb-application-guide/file )
With 4" of polyiso (R24-ish at fully rated R) and R23 rock wool in the rafter bays you'd still be shy of R49, but it would still meet code performance on a U-factor basis. At a derated R5/inch for dew point control evaluation purposes you'd still have R20 out of R43, about 47% of the total R outside the structural sheathing, which is plenty for a zone 5 climate, even at higher humidity. (For regular houses 40% is usually enough.) At only 3" of exterior polyiso it could be a bit marginal for R23 rock wool in the cavities, but still OK with R20 or R21 fiberglass.