2 x 8 Wall Construction questions
I’m building a house next year – single-story 2300 sqft ranch in Zone 6 (Maine), and want to use 2 x 8 wall construction, 24″ O.C., R-30 Roxul, sheathed with 7/16″ OSB, a WRB, and D4 vinyl siding for cladding. The last new house I built was in the mid 1990’s, and I’m doing my best to come up to speed with the current building science and methods.
I’m looking for advisement on moisture control for my walls, I’m assuming that I will have some type vapor barrier or retarder facing my inside surface of my wall studs and ceiling truss bottom chords (R-60 blown in celulose in the attic). Do I need a smart vapor retarder? Any preferred brands? Is 6 mil poly out of the quesiton?
Also, any advice on a good WRB?
Thanks-
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Replies
Without the correct ratio of continuous exterior insulation to cavity insulation, building codes will require a class I (poly) or class II (kraft facing or smart) vapor retarder for your climate zone. Though the poly will work in you climate, the industry is suggesting the class II option, I would look into Certainteed's Membrain, though there are a lot of others on the market.
https://www.greenbuildingadvisor.com/article/building-codes-update-vapor-retarder-requirements
As far as WRB's go, my advice would be based on where you plan on detaining your air barrier. Some WRB's make great air barriers, both the self-adhered and fluid applied products work well. They are more expensive and harder to work with than a mechanically fastened product.
https://www.greenbuildingadvisor.com/article/a-fresh-look-at-water-resistive-barriers
Another question you may want to be asking is do you need a rainscreen.
https://www.greenbuildingadvisor.com/article/all-about-rainscreens
Randy, thanks for the excellent advice and also for the reference to additional articles--which led to references to more excellent GBA articles. I've got more reading to do.....
The house will be heated with 1/2 radiant on grade slab and 1/2, and HWBB over a full basement. A centrally located heat exchanger will provide some AC during the ~20 days in summer that we need it. I'll have an HRV with exhaust in the baths and kitchen, with 20/40/60 minute timer switches at those locations. As for air infiltration, my goal is to have a tight, well sealed and continuous VR on the inside and a well-flashed and taped WRB on the outside.
So my thinking (today) is:
--Switch the OSB to 15/32 plywood sheathing. This adds ~ $1000 to cost, but worth it, and this morning's reading substantiated my issues with OSB.
--Research a good WRB, consider one that provides drainage. (At first search, availability of this product seems to be an issue in my area ) I read that vinyl siding is said to be 'self venting', and the manufacturer's instructions, if my memory is correct, advises that it be applied over a continuous surface, so no furred-out rain screen.
--Inside, a Smart Vapor retarder on my 2x8 walls, 'Tremco' sealed at the bottom plate, airtight electrical boxes, taped laps.
-- I'm still contemplating my ceiling detail, talking myself out of 6-millpoly, and leaning to a Smart VR stapled to the bottom chords, 1x3 strapping and gypsum. All attic space will be flat-bottom chord trusses, with an R-60 blown-in Cellulose blanket, and vented at the 1' soffit overhangs and Aura brand static ventilators sized to provide a 50/50 ratio of intake and exhaust.
Does a Smart VR stop all moisture from permeating into the insulation during the winter months?
Any and all input is appreciated.
Thanks!