2x8 Joists at 16" On-Center

At October, 2010, I have a new solution: Wrap joists with 16" width R11 unfaced batts. Complete filling with R30 kraft batts. I estimate this increases floor system R-value from R21, to R31. Installed cost increases by twenty percent. Payback absent any rebates is slowed from about 2.4 years, to three years, still very acceptable. Despite care in R11 placement, the job is not much slower. There is absolute assurance batts can't fall, and less time thinking to pull batt edges down to cover joists.

Under rebate regulation by Energy Trust of Oregon, R30 is required floor insulation for a crawl space with joists 2x8, or deeper. The rule is here, stating further that batts are not to be compressed, and that less than R30 should be installed, if more would be compressed. This is silly, as all insulation confined with restraint by side loading becomes measurably compressed. The free thickness of an R25 batt is 8.25", yet such batts never came close to filling a 7.25" depth 2x8 joist bay. The free thickness of an ordinary-density R30 batt is 10.25", yet such batts do not over-fill 2x8 bays.

Compression is desirable with the need for retentive force, and is unavoidable. The Johns Manville batts I use are available only in 16" unconfined width, and become messily distorted in a 14 1/2" space. I can not get batt edges to align with joist faces, with any amount of effort. There is low-pressure fill, with good retention and floor intimacy, but bays have hilly surfaces, not full. I use kraft-faced batts, kraft up, to employ added retentive force from stiffness in the bonded face. I will monitor jobs done without wires, for years, and will honestly report any mishap. I WILL use wires, slats, anything needed, to fill oversize, irregular bays. I WILL hack width from batts for fit in narrow bays, but find the cutting sloppy with the low density material, and have concern lesser fill to limit compression, comes with poorer retention.

R30 Fit Without Wires, October, 2008:

Diligence, R30 Fit Without Wires, Inspection, March, 2010:

At seventeen months, the batts, of course, have not shifted at all. I had other purpose for this re-inspection. About 20% of the warm-air ducts had failed bond of the joining tape, Venture Tape 1537WMP-VR, which has chancy bonding with Johns Manville R11 Basement Wall Insulation. I think frequent bond failure is due to mold release on the wrap polypropylene skin. I have a trusted supplier of tapes for conditioned crawl spaceground covering.Vapor-Bond tape by Americover is excellent here. The Americover tape is of desirable 4" width, and just peels from a roll. Separating paper backing from the Venture tape takes a lot of care.

R38c Fit Without Wires:

This is a typical found condition in a crawl space with floor joists 2x8 at 16" on-center. There is an adequate ground cover, and all is dry. The space is well ventilated, contributing to dustiness. It is a bit hostile for spiders. There is no insulation.

As a test case, I will fill 2x8 @16" oc joist bays with Johns Manville Product K1258, kraft-faced, with batt free dimensions 15.5" x 48" x 10.25." Here is the job done, again looking in from the access. Placing an R25 batt over the entrance, I have left a joist face visible to limit claustrophobic feeling. Elsewhere, fill is on-average slightly proud of the 7 1/4" joist depth.

R38c batts cut neatly with a saw. This saw would fall right out of a low-density batt. A sharp knife has a hard time tracking through a tall cloud.

The batts in end view are intimate with flooring, and need little fussing, to align with joist faces. Installation is slick and secure, with very favorable trade of labor to wire up cheaper batts, for the beautiful, consistent, better-value high density batts. All needs are met: intimate contact with floor sheathing and no possibility of pest inhabitation, bays just full with minimal tugging into shape, nothing to snag on in crawling beneath, and free removal of any 48" zone for service.