What is the minimum acceptable thickness of closed cell spray foam?  The spray foam will be on the inner side of OSB applied on the outside of 2x4 24" OC framing forming "service cavities" the foam is intended as primary air boundary and vapor retarder.

Depends on the climate and the total stackup & venting of the assembly.

Can you do 2"x4" at 24" on center?

You can do any stud spacing you like for a service cavity, since it's non-structural. Those sorts of service cavities are done as a "false ceiling" all the time, as well as on the interior side of foam-insulated basements, etc.

But the minimum thickness of the foam still depends on your climate and the stackup, including the roofing or siding type & underlayment type, vented air-gaps or "rainscreen" etc.

Dana1,
I think you may be miss interpreting my question. My intention is to use the foam mostly as the air seal or vapor retarder as a substitute for taped house wrap or a liquid applied vapor retarder. I'm in zone 6 and the ceiling will have 20" of loose cellulose with a vented 'attic' above roof will be plywood covered by standing seam metal.

A nominal-inch flash is (more than) fine for an air-seal & vapor retarder on the underside of cellulose insulated, vented attic in zone 6. The combined vapor retardency of 1" of foam + OSB is less than 1 perm, making it a class-II vapor retarder, and the amount of moisture that would get into the cellulose or attic via vapor-diffusion alone would be miniscule, WELL within the drying capacity of a vented attic.

Resurrecting this thread. So with this flash and bat approach, one would not need to wrap the outside of the OSB wall sheathing with a Tyvek or 1/2" foam sheet?

Some sort of Water-Resistive Barrier (WRB) is needed outside of wall sheathing, especially if it is OSB. Code requires it, and as is the case most of the time (TIC), code does so for good reason. Wind-driven rain can and will get behind almost any kind of siding, and a WRB is needed to protect the sheathing from moisture damage. While Tyvek and other wraps can be detailed to provide part of an air barrier system (unless it's perforated by zillions of siding nails), the primary purpose of wraps is as the WRB. In addition to having a WRB in place, many or most building scientists now advocate having an air gap of at least 1/4" between the siding and the wrap, and vented at least at the bottom. The gap prevents liquid water from being held against the wrap by capillary action; it allows free drainage and promotes rapid drying. Here's a good read on this: http://www.buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh.

with this flash and bat approach, one would not need to wrap the outside of the OSB wall sheathing with a Tyvek or 1/2" foam sheet?

Also note that a wall is quite different than the ceiling discussed above that had 20" of cellulose to stop thermal bridging by the joists. I'd use rigid foam, usually more than 1/2".

What Dick said. A flash'n'batt or flash'n'fill only addresses the potential for interior moisture drives from acculating in the sheathing, but does nothing for protecting the sheathing from exterior bulk-water wetting.

Even a small air gap be is a sufficient capillary break between the siding & weather resistant barrier (WRB) to limit saturated siding from grossy wetting the OSB sheathing, but without the WRB wind driven rain draining off window & door flashing would still soak the OSB. Using a crinkled texture type WRB provides a better capillary break from drain-plane liquid moisture than the flat-sheet or #15 felt, and is highly recommended in rainy coastal climates.

The amount of flash-foam necessary to limit moisture accumulations from interior moisture drives is also climate & wall stackup dependent.

Well it looks like I need WRB (Tyvek wrap or similar) as well as some sort of capillary rainscreen between my hardi plank cement fiber siding and OSB wall shathing. I;ve asked this on another forum, but I'll present the quesiton here too.

Climate is NoCal coastal, Trinidad, Zone 4; 4400HDD; annual rainfall between 45-60"

2x6 walls with either flash and batt or no interior foam and just blown in cellulose fiber.

I am unsure if I need to also use Tyvek WRB wrap on the OSB in addition to Tyvek's "DrainWrap" product or is the DrainWrap a dual product (WRB and capillary rainscreen).

Also, I suspect the Tyvek DrainWrap product is not nearly as effective as some of the more properly engineered rainscreens (Ben Obdyke, Mortairvent)? The Tyvek Drainwrap is just a bunch of random crinkles... but the other products actually seem to have defined vertical capillary channels?

Go to buildingscience.com and look up "mind the gap" it goes into how little of a gap is needed to drain bulk water. When using a flash and batt you have to keep the inner surface of the foam above the condensing temp. This dependant on the R value of the wall and the climate zone. Look it up at green building advisor.

And do those articles say that the drainage gap is sufficient to let the wood or cement siding dry thoroughly between storms? Bulk water is NOT the only issue here.

Mind the gap says that crinkle wrap helps with redistribution of moisture. I think it depends on where you live if it will be enough. If you live in a very wet climate you should probably go with something more robust maybe even a rain screen.

The thickness of foam has to do with keeping the inner surface warm enough that there will not be condensation. Personally I am not a fan of flash and bat. It does nothing to address thermal bridging and there are other ways to air seal.

The thickness of foam has to do with keeping the inner surface warm enough that there will not be condensation. Personally I am not a fan of flash and bat.

I agree. Even if you avoid condensation, you might not avoid mold. Consider EPS rigid foam instead.

Flash'n'batt also reduces the drying rate toward the interior- it's a compromise.

It takes only a miniscule of exterior insulation for dew point control in a Trinidad CA climate, and with rainscreened siding it doesn't take ANY exterior insulation, so the benefits of a flash'n'batt are far less here than they would be in say, Winnipeg or White Horse.

In areas with a moderate to high brush-fire potential using EPS (even under fiber cement) is less advisable than using polyiso, or better yet, rigid rock wool for sheathing insulation. Unlike polystyrene, polyiso does not melt (even when burning), and has a relatively high ignition point. But rigid rock wool doesn't burn at all- it's fire-protection. High denstity rigid rock wool has about the same R/inch as EPS, doesn't change dimensions with age, won't be picked apart by ants, and has HUGE drying capacity into the rainscreen gap.

So why doesn't everyone use rock wool for everything. In fact, I just saw in on the exterior of a new foundation wall here in Minneapolis...

Rigid rock wool still has availability issues in most markets, but it's competitive with XPS in $/R-foot terms in some US markets. (Roxul ComfortBoard-IS was only released in the US about a year ago, and has a limited number of thickness options.) The Roxul DrainBoard product is suitable for ground contact, which is probably what you saw on the outside of that foundation.

In cooling dominated climates (and behind brick/stucco cladding even in cold climates) the extreme vapor permeability of rock wool is a problem leading to high moisture accumulation issues on the cooler air-conditioned interior side of studwall cavities.

Rock wool is THE standard for fiber insulation products in Europe, with cellulose a distant second. While available there, spray foam insulation has never caught on to the same degree as it has in some parts of the US.

See also:

http://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/mineral-wool-boardstock-insulation-gains-ground

Thank you kindly.

So why doesn't everyone use rock wool for everything.

Twice the price of EPS and quite heavy?