Insulating between the rafters in my attic truss of my non attached garage located in NE Oregon zone 5. Garage has a 10/12 roof so the attic truss is a nice storage area, has a finished floor in the attic and asphalt shingles on roof. Garage is wrapped with tyvek on the outside sheeting, sided with hardie board. Primary purpose of the insulation is to keep the attic area cooler during summer. I will be heating the garage in winter also but not that often. So I'm insulating from the inside. Rafter cavities are vented from eave to peak and I built vent chutes with osb running the length of the rafter cavities with a 1.5" space to maintain air-flow (caulked and foamed for tight seal). Plan is to install batts between rafters against the osb vent chute and sheet over the rafters with osb for the finished ceiling. The only insulation I can get locally is unfaced batts. With all the truss connections to the rafters running poly to face the insulation will be a pain. Is it necessary to have this vapor barrier with this type of insulation design. I guess same question for the walls downstairs and upstairs, do they need a vapor barrier as well?

Class III (like OSB) is fine in zone 5 with vented exterior OSB. But tape the seams for air sealing.

As this isn't an occupied dwelling, there isn't moisture being generated that would lead to issues. Humidity inside and out would essentially be the same, except at times when it was being heated. Then the relative humidity would be even less inside. So I can't imagine you would need a moisture barrier

OSB isn't a class-III vapor retarder, it's a "smart" vapor retarder. It doesn't reach Class-III vapor retardency until/unless it's moisture content is high enough to support mold. That's the problem the interior side vapor retarder is intended to solve. When the OSB chute is colder than the dew point of the interior air it takes on moisture as adsorb. It was a poor choice to use for the rafter chutes- fiberboard would have been better since it's more vapor permeable at any moisture content.

But as an interior side vapor retarder it's fine. If you give it a shot of latex paint and make it air tight at the seams it'll be more vapor retardent than the chute OSB, which will protect the chutes from rotting, without absolutely blocking the drying path to the inteiror.

Roofs run colder than walls in winter (due to night time radiant cooling) which makes the OSB chute somewhat more susceptible to interior moisture drives than OSB sheathing an exterior vented "rainscreened" wall assembly with a vented air space between the sheathing and siding, but in an unoccupied building Nashvegas has it right, it doesn't much matter.

On the walls, if you have a vented air space between the Hardie and the wall sheathing you don't need anything more vapor retardent than standard latex paint on wallboard to be moisture safe, in a zone 5 climate, but without the rainscreen it becomes an issue if the building occupancy goes way up. Here 2-mil nylon (eg Certainteed MemBrain) under the wallboard would be a much better choice than a true vapor barrier- JUST SAY NO TO POLYETHYLENE SHEETING! :-) Somewhat better than OSB, 2 mil nylon becomes more vapor open than latex paint when the cavity air becomes humid enough to support mold, but is more vapor tight than OSB when the cavity air is dry.

In my area you can buy it off the shelf as some of the big box store outlets, but you can probably order it online and get it drop-shipped to your local store, if that's the way you decide to go. Its about 10 cents per square foot from distributors, but something like 1.5-2x that much at box stores.

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1-2 perms (1 is the lower limit of class III) is a pretty good number for interior OSB at normal interior humidity levels. One can argue more, which is also class III and still fine as an interior side vapor barrier (in this case, see the link) or one can argue slightly less, which is even better as an interior side vapor barrier.

Dry OSB testing a 1-2 perms would be an ASTM E96 wet-cup test, which is only appropriate under high humidity conditions. ASTM E96 dry cup tests typically yield sub- 1 perm, sometimes sub 0.5 perm at 7-10% moisture content (typical of wood kept indoors in conditioned space- not sure about an unheated garage.)

Actual permeance varies by the density of the samples used, but all become more vapor open at higher moisture content, and really does behave as a smart vapor retarder. So does half-inch CDX, which behaves a bit more like MemBrain, becoming much more vapor open than OSB at high moisture content:

http://buildingscience.com/sites/default/files/migrate/jpg/BSI087_Figure_03_sheathing_perms_web.jpg

http://media-s3-us-east-1.ceros.com/orange-tap/images/2015/04/27/5d1d0896036a3f1317f1896554f1efc3/membrain-brochure-permeancegraphonly.png

Even not-so-relevant dry cup OSB is typically > 1 perm. Numerous other sources (and my ruler on the linked graph at the point marked dry cup) agree.

Better data:
http://www.norbord.com/na/cms/wp-content/uploads/Moisture%20Vapor%20and%20Perms%20J450.pdf

Note to anyone looking at the graphs: wood % moisture and %RH are not equivalent.

Thank you for your response and for all the information. It appears to me you have a wealth of knowledge in this area. When you say it would have been a better choice to use fiberboard instead of osb for my vent chutes are your referring to something like hardboard tempered panels? I just did a quick search @ homedepot to try to find an affordable type of fiberboard and was wondering if this is what you meant. I can easily remove the osb and I have another use for it if I decide to replace it with some type of fiberboard.

I don't believe HD carries asphalted fiberboard. The midwestern box store Menards does.  For asphalted one-side fiberboard the asphalt side goes to the exterior.

If the OSB is already cut and installed there's not a whole lot of rationale for swapping it out at this point.  It's fine just as it is, but from a good / better/ best point of view asphalted fiberboard would have the edge.

Just out of curiosity would eps have been a better choice than osb for creating the vent chutes? R-value of course is higher but unsure of the air and moisture perm values. Only reason I'm asking is I'll be doing the same type of insulation project on my dad's garage later this fall.

Depends on the R-value of the EPS, the total R value and your climate zone.

In zone 5 for residential building it takes 40% of the total R to be air-impermeable to have dew point control at the foam/fiber boundary.

Ideally a vent chute it should be over 5 perms (like OSB at a high moisture content), over 10 perms is better. Unfaced Type-II EPS runs about 2.5-3 perms @ 1", or about 5-6 perms @ 1/2". But half-inch EPS isn't very sturdy stuff. Asphalted fiberboard is typically 15 perms or more at a high moisture content, 5 perms when dry.

EPS with facers is typically under 1 perm, sometimes under 0.1 perms. It has to be unfaced to work.