Do I need another air barrier?
I installed 3 inch and 7 inches polyiso foamboard tightly between my studs and ceiling in my tiny house. I plan to live mostly in California. No AC and wood burning stove in winter I plan to do wood shiplap on the inside walls and ceiling. I calked the cracks around the foamboard bigger than 1/16 on the outside and installed the plywood sheathing. I purchased a permeable air and moisture barrier (Vaproshield Wrapshield SA) for the outer walls with a 1/4 rainscreen beween the outter cladding. I plan to do a pvc or tpo roof on top of my ice and water barrier membrane since my roof is flat. Should I be concerned with trapping moisture in the sheathing of walls and roof since air will not really dry to the inside of the house very well and definately to the outside on the roof because of the foamboard and non permeable ice water shield? If so what can I do? Should I use different products as far as barriers on the outside or inside?
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Replies
Darryl,
Your method is a bit eccentric. I'm not sure whether you tiny house needs to comply with any building codes, or whether it is too late to change the details.
For your walls, the best way to establish an air barrier would be to tape the seams of the plywood sheathing with a high-quality tape. You would also need to consider potential leaks at the bottom of the wall (where the sheathing attaches to the bottom plate) and at the top of the wall (where the wall air barrier transitions to the ceiling air barrier).
On the interior of your wall, most building codes require that any rigid foam be covered with a layer of 1/2-inch drywall for fire safety. The 1/2-inch drywall, properly installed, can be an air barrier. Once that air barrier is established, you can install boards on your walls (on the interior side of the drywall) if you want. Note that boards are not an air barrier.
You have decided to use the cut-and-cobble method to insulate your flat roof assembly. That method isn't recommended for unvented roofs -- it is associated with moisture problems in cold climates. Whether or not you end up with moisture problems in this roof depends on the climate -- in other words, it depends on where you decide to park your tiny house.
If you haven't installed your roofing yet, you can still salvage the situation by installing an adequately thick layer of rigid foam insulation above your roof sheathing. The thickness of this exterior rigid foam depends on your climate, so you need to figure out where the tiny house will end up before you install this rigid foam. For more information, see Insulating Low-Slope Residential Roofs.
Thank you for your response Martin! Yes no code requirements.
1. So should I not use the vaproshield barrier on the outside? Just tape the plywood seams/top/bottom and maybe just a typar material as a housewrap along with the rainscreen?
2. I would normally install drywall, but being that this project has a weight variable for the trailer, is there something else? The house is 32 feet long and the drywall would add a pretty significant amount of weight on top of everything else. Im pushing the width and height allowed for highway travel also. This is the reason for doing your cut and cobble method as to gian the maximum space on the inside.
3. I read your article...so being that I will mostly park the home in climate zone 3 I can install a minimum R5 on top of the sheathing. Im assuming I can use either polyiso foam or eps to achieve this R5? The order to the outside would be cut and cobble, sheathing, impermeable membrane, R5 rigid foam, then PVC roofing?
Darryl,
If you have already purchased your VaproShield (and I notice on re-reading your question that you have), go ahead and use that product as your exterior air barrier and water-resistive barrier (WRB). That will work.
If you care about fire safety, you should install the drywall. It's good enough to act as a finished surface, so if you want to keep down the weight of your house, you can skip the shiplap boards on the interior.
If it's not too late to redesign your roof, put all of your rigid foam on the exterior side of the roof sheathing. That's much better than cutting the rigid foam into narrow strips and installing it between your rafters (since the cut-and-cobble method doesn't address thermal bridging through the rafters). If you have enough rigid foam above your roof sheathing, you won't need any rigid foam between your rafters.
Ok thank you for the help Martin