Here's a secure means of attaching insulation where a crawlspace has 4x8 joists on 48" centers. Note rigid full thickness of Johns Manville R25 kraft batts. There is minimum projection below floor joists. Wires wound onto shingle nails about 2" above joist faces are loaded with lengths of wood lath to retain the insulation There is nothing to snag on in passage under joists and insulation.
Here is a better plan I will try at my next opportunity.
Even Johns Manville batts sag, and effort is needed to draw batts into contact with the floor, and away from interference with a person wanting to pass under.
New R30 Requirement:
At Spring, 2008, Energy Trust of Oregon increased the crawlspace insulation requirement to R30. I am critical of the unstated reasoning, since R25 batts are available in especially convenient shapes. I think the purpose is to increase the possibility that wimpy or poorly-fit batts might be in contact with floor sheathing, where insulation is held up only by wires or twine nailed up to joist faces. Whatever the reasoning, we must comply, and batts will be R30 hereafter. I have responded with trials for joists on 16" centers, using Johns Manville kraft batts in convenient 48" lengths.
The photo at left shows an R30 batt, 16" wide, set in 2x10 joists. The batt would securely fill 2x8 joists. R30 seems to be an odd item however, often thin by 20%, whereas R25 is very consistent. Expecting better consistency with high-density batts, I tried R30c (c- for cathedral) and was very disappointed. Note the further 2" under-filling of 2x10 joists.
I then tried R38 high-density batts, R38c, 15.5" wide, set in 2x8 joists. I was very pleased with the ease and security of installation. When batts are pressed up into firm contact with the floor, edge forces keep them compressed. Yet, it is easy to fluff edges into alignment with joists. In the photo at the right, note very little crowning below joist faces. The visible joist side is beyond an R25 batt set at the crawlspace entrance, where underfill makes entry more comfortable. From my supplier, the cost per unit area of R38c is 90% higher than for R30. I will let the homeowner make the choice, and will hope for improving consistency in R30 batts, where chosen.