Considerations When Insulating With Dense-Pack Cellulose – GWC Mag

In this Q&A post, GBA member Peter G Engle PE writes to say that he’s working on a Pretty Good House build in Vermont (Climate Zone 6A). The double 2×4 walls are framed 24 in. on center (o.c.) and insulated with dense-pack cellulose; the walls are 10-1/2-in. thick. His insulation contractor wants to staple netting to the sides of the studs (a technique known as “lip stitching”) and fasten 1×3 horizontal strapping to the studs to contain the cellulose; and he recommends using 5/8-in. drywall instead of the more conventional 1/2 in. The problem with adding the 1×3 strapping and changing to 5/8-in. drywall is that the additional thickness of the interior walls will cause conflicts with planned finishes and fixed equipment. As he doesn’t have a lot of experience installing dense-pack cellulose, Engle would like to hear suggestions from the forum on how best to contain the cellulose.

Job of the net, battle of the bulge

Most of the respondents have experience with installing blown-in cellulose. Jollygreenshortguy leans on the drywall side of the equation, reassuring Engle that 1/2-in. drywall is adequate to span 24-in.-o.c. framing, as long as it’s a reputable name-brand product, rather than less-expensive imported drywall. He also suggests an alternative: using 1/2-in. Type C drywall that’s stiffer and more fire-resistant than the normal. He references the Fine Homebuilding article “Dense-Pack Cellulose Done Right.” On the topic of drywall, Mike Maines thinks 1/2 in. or 5/8 in. is a matter of preference, and reminds Engle to add extra blocking for cabinets and trim.

Both Maines and norm_farwell write that lip stitching is a typical technique for double-stud walls and that Engle shouldn’t be concerned about any performance loss. Maines admits that he prefers walls 16-in.-o.c., but suggests separating the exterior and interior walls…