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Roof and wall detailing and barriers

user-7674151 | Posted in Plans Review on


Roof and wall detailing

I’m in the process of designing my roof and wall assemblies in climate zone 6 for a small 400sq ft building. I had an original plan of using 2” of most likely XPS or polyiso on the exterior of my wall and fill the cavities with fibreglass batts. My roof would have open rafters with 5” of exterior foam board. 

My question comes to my use of my air/vapour/water barriers. I have been doing some research in regards to the PERSIST method and having a continuous barriers lapping over my roof and wall connection. I was just going to use Tyvek housewrap on my wall sheathing after taping my seams and then have a peel and stick water/ice shield membrane lap over from my roof onto my walls. (I would cut the rafters so that my roof and wall sheathing would be flush according to the PERSIST method and install the eaves and rakes later). 

My first question is if the combination of the taped seams and lapped peel and stick would cover my air and water barriers. 

Second question is in regards to a need for class 2 or 3 vapour retarder (such as membrain) at all in either my roof or walls due to the high r values established by my exterior rigid foam. I thought it couldn’t hurt but then am not sure how I would detail its connection from my wall to roof as it would sit in front of my framing in the walls but then i’m not sure where it would be in my roof if I have open rafters. 

Third question is in regards to risk of the building being too airtight in such a small space that humidity might become an issue. Since drying would occur to the inside vs the outside because of the foamboard thickness, I’m a bit worried that i’m setting myself up for loads of unnecessary interior humidity problems. 

Fourth question is if I could get away with doing outie windows if my WRB is under my exterior foamboard. I know Martin recommends having the WRB on the outside of the foamboard if doing outie windows but then I would struggle to have a continuous water barrier with my roof. 

Any input is hugely appreciated!



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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Kelly,

    In your situation, you can detail the wall and roof sheathing as the air barrier using sealants where needed at the perimeter of panels and tape over all seams. Lapping the peel-and-stick ice-and-water membrane from the roof sheathing to the walls would take care of air sealing that transition (under the roof underlayment and over the WRB on the walls for shingle-style lapping). Then your roofing underlayment and housewrap would be your water-resistive barriers.

    If you put your insulation (assuming rigid foam, as you said) outside of the roof and walls, you do not need an interior vapor retarder, though your painted drywall will be a class III interior vapor retarder, which is fine. These walls will need to be able to dry inward and they will be able to.

    May I suggest not using XPS. It is the least environmentally-friendly option for rigid foam and both polyiso and EPS can work. Many GBA members recommend sourcing reclaimed roofing polyiso as a much better option.

    I'll let others weigh in on the humidity issue. You will want balanced ventilation though, if you build a very tight house.

    I've seem builders do outie windows with the WRB behind the foam. I've only done innie windows myself, a decision based on the fact that I feel more comfortable getting the flashing details right and building some simple extension jambs than doing the opposite. Keep in mind that some rigid foam qualifies, according to the ICC-ES, as a WRB. So if you use the right foam and detail it as a WRB, you don't need housewrap over the foam.

  2. user-7674151 | | #2

    Thanks Brian for the response!

    Another thought we were toying with was having a 3.5" x 3/4" t and g ceiling that could potentially double as our sheathing. In this case we would still be covering it with the ice and water shield -however I'm concerned that the dimples of the T and g will make it quite difficult to get a good seal. Should we be layering some plywood sheathing overtop of this for ease of installing the peel and stick ice and water layer and then proceeding with the foamboard from there?

    Another thought is still filling the cavities both in walls and roof with fibreglass. In this case the same would still stand about not needing an interior vapour retarder or some kinda of smart retarder? We wouldn't be putting up drywall, most likely T and G that would be finished without latex paint for our interior walls. As I understand with the adequate rvalues for walls and roof in ratio to the cavity insulation with no latex paint, we STILL would not need a vapor retarder, correct?

    I think regardless if we keep the housewrap on the wall sheathing underneath the foamboard for the walls that we would still attempt to seal our foamboard as best we could and then I suppose would have some redundancy with our WRB... which we figure if we choose to do outie windows might not be a bad idea!
    Thanks for the input, it's much appreciated

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