Installation Tips for Dense-Pack Cellulose Insulation

Originally published by: Journal of Light ConstructionMarch 29, 2018

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Dense-blown cellulose is an important problem-solver for high-performance builders and remodelers. In a retrofit situation, when you blow the material into a leaky wall cavity, you can reduce infiltration losses at the same time you boost insulation values. But in high-performance new construction, you start out with an airtight cavity, using dedicated air barrier materials such as OSB, drywall, membranes, and tapes: when there's a choice, nobody recommends relying on the properties of insulation as the primary means for stopping air leaks. So in an ideal world, your cellulose installer is pumping the cellulose into a cavity that is already airtight.

This photo from German equipment maker X-Floc shows the company's dry injection system for cellulose insulation in action. The back-pressure relief valve allows air to escape so that insulation can be uniformly delivered and compacted, while the red filter bag contains dust.This photo from German equipment maker X-Floc shows the company's dry injection system for cellulose insulation in action. The back-pressure relief valve allows air to escape so that insulation can be uniformly delivered and compacted, while the red filter bag contains dust.

But an airtight cavity can actually make it hard to get your cellulose in place, because back-pressure of air in the cavity keeps the cellulose from getting into the space being insulated. Passive House builder Chris Corson of EcoCor encountered that problem in his panelizing shop in Maine, where he makes pre-insulated wall and roof assemblies for high-performance houses. Back-pressure in his 12-inch wood I-joist wall cavities was slowing the work and hampering the job quality. Corson adjusted by drilling holes in the sides of I-joists and covering them with mesh filter cloth, allowing air to escape from the far end of the cavity, away from the insulation hose (see: "A Visit to a Passive House Panelizing Shop," Coastal Contractor 6/14).

Vermont remodeler and home-performance contractor Jim Bradley took a similar approach in a retrofit situation: In order to effectively fill large double-stud wall bays, Bradley attached filter cloth across the thickness of the walls between the inner and outer studs, making small individual bays that would contain the cellulose while allowing the air to escape (see: "Major Surgery for a Failing Fat Wall," JLC 6/15).

Modern air barrier manufacturers are well aware of the issue. Keith Bidwell, a regional sales manager for Swiss membrane and tape manufacturer Siga, doesn't recommend using Siga's interior vapor barrier membranes to contain blown cellulose: not only does the material's airtightness make it hard to deliver cellulose at the proper density, he says, but the air pressure in the cavity can stretch or tear the fabric at staple locations into the wall studs. Instead, Bidwell says, installers should use open netting to contain the cellulose during the blowing operation, and install the air-tight vapor retarder membrane as a second step after the insulation is in place.

But what if those approaches are too complicated, or not feasible? German manufacturer X-Floc, a supplier of cellulose blowing equipment, has a solution: a cellulose injection head with its own air-pressure relief outlet, built into the nozzle at the end of the hose, complete with filter bag to keep the living space dust-free during the operation. With this adapter, you can get good, uniform insulation density even in a tightly sealed cavity.

Energy consultant Peter Yost got a chance to put the equipment through its paces in a demonstration setup at the shop facilities of American Installations, a full-service insulation firm based in western Massachusetts. “The built-in passive ventilation for the cavity gives you the ability to handle the air that is being expelled from the cavity, and it provides a more uniform product," American Installations partner Wes Couture told Yost.

Yost posted his observations and some demonstration videos on the BuildingGreen website (see: "Installing Insulation With the X-Floc Ventilated Dry Injection System," by Peter Yost). "The demonstrations show how the X-Floc dry injection system achieves a uniform cavity fill with cellulose," wrote Yost. "Does the way the insulation fines fill cracks mean that the wall insulated with this installation system achieves significantly greater airtightness? I think the answer to that question depends on the results of future air leakage testing."