Does untreated OSB with peel-and-stick taped joints qualify as a Water-Resistant Barrier?
We have untreated OSB on the walls, with the intention to tape all joints, corners, window bucks, joint to roof sheathing, etc.
This house is in the Northeast, and our wall section is siding on vertical strapping (rainscreen) on 3″ shiplap Poly-Iso (R21) insulation on OSB on 2x6s at 24″ OC with cellulose fill (say R17, for a total of ~R38-40). The roof section is metal on cross purlins on 2x4s on 2 sheets of 2″ XPS (R20) on ZIP sheathing on 12″ TJIs at 24″ OC with cellulose fill (say R37, for a total of ~R57-60). The basement with walkout has 4″ XPS on the exterior walls for now, and when finished we’ll add another R10-15 cellulose or XPS. Under the slab is 2″ XPS.
Is there a dew point potential inside the wall and roof cavities? We’ve got more than the 1/3 R-value rule of thumb for interior vs exterior insulation. The mechanicals are high-efficiency propane furnace, HRV, A/C (future if needed), and a masonry heater. The house is a simple gable, with a footprint of 1300 sf on 2 levels plus a full basement and unfinished attic (for future use), for a family of 3-5. There is no swimming pool or green house indoors, though there will be some plants and the masonry heater with wood storage. Appliances & bathrooms have typical mechanical vents to the exterior. Does any know of resources for on-line dew point analysis?
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Standard OSB is an absorbent material and would not qualify as a WRB.
With your wall section, 68°/40%RH indoor conditions, the outside of the OSB would be at local dew point at outside conditions of 20°/85%RH (common winter humidity in the NE), and the inside of the OSB would be at local dew point at 17°/85%.
But that shouldn't be a concern, as long as you have a tight air barrier (ideally on the inside of the thermal envelope), use no interior vapor barrier or retarder less than about 1 perm, and control indoor humidity.
Neither your wall nor roof assembly can dry to the exterior, so it's imperative that they be able to dry to the interior. The cellulose insulation (which, by the way, should offer 3.8 R/in dense-packed) is highly hygroscopic and can safely buffer considerable quantities of moisture, as long as it can dry when conditions change.
Thanks for your comment Robert. For the 'tight air barrier' on the interior, would the drywall need to be caulked at plates and sealed at outlets? For the exterior, with the relective exterior plane of Poly-Iso taped, would that qualify as a WRB? Have exterior Air Barriers been replaced by WRBs? Would adding Tyvek prior to the insulation do any good?
Chris,
1. If you expect your drywall to act as an air barrier, you will indeed need to use caulk or gaskets at the plates and choose a method for air sealing the electrical boxes. To learn more about these details, Google "airtight drywall approach."
2. Taped foil-faced polyisocyanurate can be used as a WRB. There is one disadvantage, however; if you use polyiso as a WRB, there is no way to lap the WRB to flash penetrations (for example, windows and doors). If rigid foam is your WRB, you depend on the adhesive in tape for watertightness of horizontal joints; if you use asphalt felt or plastic housewrap, you depend on gravity and lapped joints, which are more forgiving and long-lived.
3. A WRB is not necessarily an air barrier. The main purpose of a WRB is to shed wind-driven rain that gets past the siding. Many perfectly good WRBs are not air barriers.
4. If you want to use Tyvek as your WRB, I would put it on the exterior of the polyiso in order to allow it to be easily lapped over window and door flashing.