Q: Standard-width batts don’t fit in I-joist floors or steel-stud walls. How should these be insulated?

A: Nathan Shirai, principal of Insulation Unlimited, based in Chattanooga, Tenn., responds: I-joists? Steel studs? 19.2‑inch framing layout? Fear not! There’s a batt for that.

Judging by what’s available off the shelf in the big box stores, and even most lumberyards, you can easily get the impression that batt insulation is available only in 15- and 23-inch widths. And before the proliferation of framing products like I-joists, metal studs, and open web trusses, that’s all anyone would have needed. Nowadays, we might encounter cavity widths of 10 1/2, 12, 14 1/2, 16, 19, and 22 1/2 inches, sometimes all in the same house. But the insulation manufacturers have adapted to evolutions in framing over the years and now offer batts in many widths for various applications.

We’re all familiar with standard 2-by framing on 16- and 24-inch layouts, and the associated 15 1/4- and 23-inch-wide batts that friction fit in cavities of those sizes. Every now and then, you might run into 12-inch layout, and there are 11-inch-wide batts available that prevent the need to rip a 23-inch batt in half.

Use full-width batts for insulating between I joists.
Photo by Steve Easley & Associates Use full-width batts for insulating between I joists.

Floor, ceiling, and roof assemblies using I-joists might pose concern for the conscientious builder who wants a “Grade 1” job (no voids, gaps, or compression) but may not have the budget for anything more than the humble fiberglass batt. There’s no need for concern, though. Batts are available in full 16- and 24-inch widths that fill the additional space created by the narrower OSB web of an I-joist on standard layout—no need to rip down a 23-inch batt for a 16-inch-wide cavity and waste the offcut, or crosscut a regular piece into 16-inch lengths that you then painstakingly cobble into the run sideways to fill the width. Putting a 15 1/4-inch-wide batt between those I-joists is simply not an option because the gap that results will allow air movement beside the batt that can reduce its effectiveness by a staggering degree.

Sometimes, you might see I-joists on 19.2-inch layout (an even division of 96 inches—the length of OSB and plywood panels). There’s a batt for that too. Most floor and ceiling batts (R19, R30, R38, etc.) are available in a 19-inch width that fills this cavity perfectly.

A loose-fill approach, such as dense-packed cellulose or fiberglass BIBS (blow-in blanket system), or spray polyurethane foam, will, of course, result in a more perfect fill around the irregular profile where the I-joist chord meets the web that translates into better performance. But when budget constraints eliminate those options, the full 16-, 19-, and 24-inch-wide batts can make for a correct and convenient installation with minimal compromise in performance.

The 16- and 24-inch-wide batts are technically manufactured for metal studs so that the extra inch of width can fill into the C-shaped profile without leaving a gap. It’s critical to mention that if you are insulating for thermal control, cavity insulation by itself is almost pointless without an exterior thermal break for steel metal framing. Most metal stud applications are found in commercial projects where sound attenuation, not thermal protection, is the goal.

All in all, fiberglass insulation is available in sizes including 11-, 12-, 15 1/4-, 16-, 19-, 23-, and 24-inch-wide batts. The variety of offerings can vary slightly from manufacturer to manufacturer. For example, not all R-values are available in all widths, and vice-versa. The higher-density batts (R15 as opposed to R13, for example) tend not to have as many options compared with standard-density batts. As an insulation contractor, I am able to order the most commonly used R-values in just about any width required from my distributor, and often help supply small quantities for local builders and remodelers who may have a project too small to justify subcontracting the insulation work.

If you’re having trouble finding the size batt you need for an unusual framing cavity in a small project, try reaching out to a local insulation contractor. If you’re subcontracting the insulation work, ask your contractor how they plan to handle areas where unusual-size material may be needed, and always insist on a proper “Grade 1” cavity fill.