[Greenbuilding] ***SPAM*** Re: air barrier recommendations

[John Straube]

If you skip the plywood in OSP then you will need to use a standard membrain product such as Tyvek. Is there any reason you would not use the obvious?
Also you were asking about the air barrier but really the most fundamental requirement of a shed or a hospital is rain control. Thus providing lapped housewrap under the siding and over the framing is fundamental and critical. Much more important than an air barrier. We often call the rain control layer in a roof in a underlayment when it is placed below roofing that does not leak too much such as metal roofing and Ashfall shingles. You could use Tyvek for this as well but it is too slippery to work with and hence special grippier products are widely used. The benefit of large sheets of polymer are that they are strong enough and have few enough joints that you can tape them as an air barrier once you have properly gravity lap them for rain control. Providing air barrier continuity between the roof membrane which is above the framing and the wall membrane which is to the exterior of the framing is a bit more complicated but for a shed simply installing blocking and sealing with caulking around the edges should be sufficient.
There is no good answer for under your floor since what you are proposing is just considered "bad practice". That said I have several projects in my own property that to do some thing similar although they do provide at least 6 inches of clear air over the soil. These projects of mine are small 10' x 10' sized buildings that are not intended to be durable and in fact are likely to begin to have floor rot in 10 to 20 years of service. To locate a wood-based floor system directly on grade you would need to use something you probably don't like that is a foam plastic insulation on the underside of the wood protecting it from water vapour flow and keeping the framing at a different temperature than the grade. You will likely say you are not placing it on grade but you are clearly placing it so close to the grade it's almost the same. This one and a half or 2 inch foam layer underneath the wood will allow for a long term durable floor system. In your climate I would not bother putting fibrous insulation between the floor joists.

> On Jul 11, 2022, at 18:53, Reuben Deumling <9watts at gmail.com> wrote:
> 
> Portland Oregon, dry hot summers; high humidity in the winter. 
> 
> I should have mentioned that I am skipping the plywood altogether. I am putting up rough sawn board-and-batten siding over the studs and blocking. My thought was to put that air barrier between those. 
> 
> On Mon, Jul 11, 2022 at 3:35 PM John Straube <jfstraube at gmail.com> wrote:
> Hey there Reuben.  A description of your climate would be helpful before I weigh in..
> In most cases taping up the plywood or osb sheathing would often be a great solution. 
> 
> On Mon., Jul. 11, 2022, 18:15 Reuben Deumling, <9watts at gmail.com> wrote:
> I hope there are still a few of you smart people out there. This list used to be so fun!
> 
> I'm a little rusty on this stuff. So your thoughts are much appreciated. 
> 
> I am building a 250 sf shop in my back yard. I am insulating the floor right away (R19 FG batts) and the walls and 12/12 metal roof plane sometime in the future. My hope is to affix a barrier of some sort (6 mil plastic?) to the underside of the joists (ca. 12" unvented crawl space beneath), and wrap the walls and roof plane with some sort of air barrier, but am not sure what a good product choice would be for 
> - under the floor
> - the walls
> - the roof
> 
> Thanks for all suggestions. 
> 
> Reuben
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