In Oregon at 2008, weatherization sponsor Energy Trust requires R30 for crawl space insulation. For floor framing of 2x8 at 16" on center, the available Johns Manville batts are 16" by 48." This photo shows placement of a kraft-faced batt within 2x10 joists. Although free-standing batts are 10.25" thickness, batts stably compress to less than the 7.25" joist depth. Side compression and friction give sustantial retentive force. The batt is forced against floor sheathing, and is fluffed down to just fill the cavity. R30 Cathedral (high density) batts are inferior in this application. Contractors who place twine to hold up batts are misguided and do a disservice. I blame irresponsible advice of Bonneville Power Administration for much wasteful placement of floor insulation. BPA calls for R19 batts resting on twine or wire, making tempting habitat for critters, until the insulation falls down, and having no insulating effect. Twine will do nothing to force floor contact of a batt R30 or less in 2x8 framing, even at 16" oc. An R38 batt might be retained in floor contact, but not with joist spacing more than 16" oc.
Requirements do not specify whether batts should be kraft-faced or unfaced. I will choose kraft-faced, with belief that flooring often contributes to house infiltration. Foundation vents, even with typically-crummy plugs, leave the crawl space as the entry point for the chimney effect of air circulation. The kraft facing will be an imperfect barrier, but may help. Wouldn't we do more good where the crawl space is substantially open to outside air, to run house wrap under the insulation?
I have tried both R30 and R38 Cathedral, high density batts. For appearance and security of fit, I prefer the R38c, to ordinary-density R30, but find the added cost not justified. The R38c better fit is in part from manufacture at 15.5" width. I wish there were more choices of width for R30 batts.
This illustration for floor framing of 4x8 on 48" centers expresses a dilemma. Whether kraft faced or unfaced, R30 batts are available to me at only 16" and 24" width. I want batts to be retained without billowing down below the joists, because space is often limited, and slack batts are at risk of destruction by a challenged, snagging worker. Insulation must somehow be kept in intimate contact with floor sheathing, everywhere; billowing batts with slack wire or twine, must not be allowed. But, what of the batt interference illustrated, where batts are tried without any custom sawing?
At 9/13/2012, note that better information can be maintained on a web page. Here are pages for several joist configurations:
4x6 At 32" On Center
4x8 At 48" On Center
2x8 At 16" On Center
At 5/2/2016, consider this proposal submitted for 2018 revision of the International Energy Conservation Code:
Revise Table R402.1.2 Footnote g, and apply it for all climate zones.
g. Floor insulation shall be in full contact with overhead sheathing and shall be retained by an underside air barrier. If the air barrier is set against joist bottom faces, bays must be over-filled, generally compressed to much less than batt free thickness. An excess of insulation, more than label R30, shall be required for common 7.25" joist depths if batts and air barrier material are not raised to make contact with the subfloor, above joist bottom faces. Where a crawl space or unconditioned basement does not follow outdoor temperature extremes, with outdoor vents safely, easily and assuredly closed during colder weather, air barrier-faced batts shall achieve floor R-value 19 in any climate zone .
My code suggestion derives in part from success with hard-covering of insulation in a crawl space overhead for immunity to mice and rats:
The proposal needs substantial revision to look like "code."
The outcome of passage might be be manufacturer offering of fibrous, compliant insulation material bonded to OSB in useful sizes including 43" width for the difficult 4x8@48 situation. Perhaps a standard R19 or so, for any climate zone with CS not vented in extreme Winter.
In the matters of climate zone and practices of winter closure of vents of an unconditioned crawl space, note that venting for radon is a seasonal concern. My home in Climate Zone 4 with vents closed experiences radon levels above 4 picocuries per liter only in wet spring weather with ground water-saturated and is well below action levels in other seasons. I am getting to know how my crawl space behaves while devising the means to condition it as unvented year-round.