Unvented Roof Construction & Ventilation in Tiny House
I have a few questions as I’m in the midst of redoing my tiny house roof (for the 2nd time) due to mold. I’m in western WI – Climate Zone 6A.
History of Build:
Original Roof Construction:
We were given faulty advice from green building materials consultant to put a non permeable Vapor barrier on the exterior of our sheathing roof and walls and with no interior vapor. I am an inexperienced builder and didnt understand a lot of this in the beginning. So roof and walls were covered in non-permeable vapor barrier. Roof pitch 1.5/12 (also didnt understand implications of this). Interior to outside: Pine tongue and groove, 2×6 rafters filled with r-19 wool insulation, exterior grade plywood, denny foil vapor barrier, metal barn siding for roof. Roof was unvented (unintentionally). We heat with wood. No attic. Some mold happened on interior side of roof sheathing.
2nd Roof Construction:
We were given advice that we needed to vent our roof and take the vapor barrier off of the outside of the house, especially without having one on the inside. Roof from interior to exterior: pine tongue and groove (no mold or moisture damage to original), 2×6 rafters with wool batting, cardboard baffling to give a 1-2 inch air space on top of insulation, new exterior grade sheathing, metal barn siding roof. I drilled 2 inch holes on lower side of rafters of which there is a 1.5 ft overhang. I vented the high side. Significantly more mold happened than the first time and was most heavy around the wood stove area.
3rd and current roof construction:
I have now learned through reading some building science type articles that with no attic and such a low pitch single slope we want an unvented warm roof to prevent condensation from happening in rafter space. I have also learned the metal roof material (the barn siding, not the seamless kind) is not good for our pitch. So here is our current plan:
Current build plan: from inside to outside: pine t&g, some kind of vapor barrier/retarder, 2×6 rafters with wool batt insulation r-19, OSB, 7″ of polyiso = R20-24 (3″x4x8s aluminum backing, 4″x4x4s paper backing), 1/2″ fiberboard, 45 ml epdm rubber.
Additionally I’ve purchased a through wall erv (TwinFresh Expert” from US-Vents) to help circulate and control air.
**Our hope was to have a draftier house to allow for good air flow as opposed to building it air tight. We are learning that may not have been the route to go but are now trying to creatively deal with it on a budget. Biggest concern is mold and moisture not heating our space. Despite the drafts from windows and door our house is warm (sometimes hot) in the winter as long as we keep the wood stove going when sun isnt shining and our lower sleeping space is cool which we like.
Questions I have are:
1) what vapor barrier/retarder would be recommended to put on the rafter side of the pine tongue and groove?
b) will I be okay installing this barrier in the ceiling only, for this winter or do I need to make sure I get in the wall as well? We mostly have had very little mold in the walls at this point but I am concerned with a vapor barrier in the ceiling that this will push moisture out into walls. I am hoping ERV system can help alleviate this. I’m balking at putting vapor barrier in walls as it means tearing down the shelves, t&g there, stairs, counters, etc -while trying to live in it and colder weather coming and with a 4 year old.
2) I’m installing the polyiso on roof right now. I’ve been layering it all up, putting a bit of liquid nails to hold the different layers (CDX ,aluminum faced foam, paper faced foam, fiberboard) in place, Good Stuff foam in the cracks, and planning on using a 9-10 inch screw with a plate on top through the layers of foam and fiber down into sheathing/rafters to really hold it all in place before sticking the rubber down. Am I going to create a bunch of little metal thermal tranfers with the screws running from just under the epdm down through my foam and into sheathing (they will stick out of sheathing into interior of space if I miss the stud). Is this a problem; is there a smarter way to do it?
3) any best practices for where to install the through the wall ERV? When we called US-Vents and described our place (8.5×24 ft trailer, 11 ft tall tiny house, shed style roof with 1.5/12 pitch, and lofts at both ends, 1 higher and 1 lower) they answered really quickly that anywhere would be fine and didn’t educate us in any way beyond that. Left us feeling uncertain, but the item still seems like best one for our situation and budget. We have no bathroom in the tiny house, so no shower vapor. The natural airflow goes from the kitchen side, which is close to the wood stove where we also cook from time to time, to the opposite side where we did have significant mold in the corner of the wall sheathing. We currently have a low force fan installed into the wall on that corner.
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