Download PDF version (324k) Log In or Register to view the full article as a PDF document.

Insulating With Spray Cellulose

- Continued Drying time. Unless the cellulose is installed too wet, there's no need to worry about nail pops or other drywall problems. In our climate, we find that insulated walls can be closed in within 24 hours of spraying. This seems to hold true for us even during the occasional spell of damp rainy weather. The residual moisture will migrate out through the wall through vapor diffusion, and the cavity will dry completely over the next month or two. The borate content of the cellulose prevents any mold growth during that time.

Filling in With Fiberglass

Cellulose is difficult to use in some areas. Kneewall framing, for example, is often left open on the back side, leaving nothing to spray against. In such cases, we resort to carefully fitted kraft-faced fiberglass. We also make limited use of batts around rim joists, blocking, and some difficult-to-spray corners. This approach might upset cellulose purists, but it's a necessary compromise in our competitive marketplace. In our market, many customers are unwilling to bear the cost of installing the mesh or rigid-foam backing needed to make spraying possible in that situation. Even though sprayed kneewalls are thermally superior, we'd rather see a customer invest in airtight ductwork and a duct blaster test instead if there's no room in the budget for both. Air and vapor barriers. Except in kneewall areas, where the batt facing provides a localized vapor barrier, we don't use a vapor retarder or additional air barrier. We feel that spray cellulose contains so few voids that there's little convective movement to allow moisture-laden air into the wall. The combination of an outer layer of housewrap and sheathing and an inner layer of drywall is enough to control air penetration.

Interior Partitions

Where interior soundproofing is called for, we often spray interior partitions as well. Many of the general contractors we work with tell us that cellulose performs much better than fiberglass batts in this application. In addition to being three times as dense as the batts, spray cellulose leaves fewer voids, which helps resist sound transmission at electric boxes and other small openings (Figure 8).


Figure 8. Spray cellulose is ideal for sound insulation in partition walls containing plumbing and wiring, because it contains few voids and seals small openings that allow sound to pass from room to room.

Mesh and drywall. Many of our customers install drywall on one side of interior walls to provide backing for us to spray against, and we've never had any problems with excess moisture soaking the drywall. That approach does require the drywall sub to make an extra trip to the job site, though. If that's too much trouble, we can staple spray mesh to the studs instead. The key to using spray mesh — which is actually not a mesh but a porous, nonwoven material something like the filter fabric used in footing drains — is to get the material taut, so it won't belly out beyond the studs and complicate life for the drywall crew.


The excess material that the scrubber shaves off the wall is referred to as "recycle." The traditional method of dealing with it is to shovel it into clean garbage cans and dump it back into the hopper on the truck (Figure 9). This works well, although it can be a lot of work if the truck is some distance from the house; if that's the case, we'll often assign a fourth member to the crew.




Figure 9. The low-tech method of dealing with excess cellulose is to collect it in clean garbage cans (top) and hand carry it to the hopper (center), where it's mixed with virgin material for reuse (bottom). Newer equipment includes a vacuum system that eliminates most hand work. Either way, it's important to start with a clean, well-swept floor to keep sawdust and other debris from contaminating the insulation.

The easy way — which we're able to use with the newer of our two truck-mounted spray rigs — is to suck up the recycle with a powerful vacuum hose that sends it back to a dedicated recycle hopper. This predampened cellulose is automatically blended with the virgin material in a separate dry hopper. In addition to saving a lot of labor, this makes it easier to maintain a consistent moisture content, which improves quality control and keeps dust down. The only disadvantage is higher cost: Not counting the trucks, our older rig, without the vacuum system, cost us about $18,000, while the newer one set us back $40,000. That's a big investment, but we — and our customers — are convinced that the results are worth it.