Blown Cellulose Insulation: Passive House Insulaton

Blown Cellulose Insulation: Passive House Insulation

use of spray wet cellulose in passive house

Beyond using SIPs and Superior Walls as our initial exterior insulating material, and selected amounts of spray foam insulation (primarily for air-tightness), we selected blown-in "wet" high-density cellulose as our additional insulating materials within our secondary interior studded wall system.

Blown cellulose offers many advantages over other insulation media:
  • Cellulose is extremely cost-effective (even though its R-value is somewhat low)
  • Cellulose is environmentally friendly consisting of approximately 80 percent post-consumer recycled material, such as newspapers and blue-jeans
  • While not strictly airtight, cellulose provides an level of air sealing other conventional insulation (like fiberglass batts) does not
  • Borate treated cellulose, itself, has the highest (Class I) fire safety rating
  • Cellulose enhances fire-resistance to the building, as it nearly eliminates the primary food of fire, air--behind the wall and in the cavities between the studs
  • Borate treated cellulose offers a level of insect control
  • Cellulose adds additional levels thermal mass
  • Cellulose improves sound-deadening
  • Spray-applied cellulose (wet-spray cellulose) improves airtightness, density, moisture (ie; mold) resistance, and potential settling issue
  • Cellulose has extremely levels of embodied energy (energy used for its manufacture)
  • Cellulose uses no VOCs, CFCs or HCFCs and does not off-gas, for improved interior air-quality, especially important for air-tight passive houses
In the attic spaces, we used a slightly "less-wet" stablized form of blown cellulose and also applied the material in two-steps allowing a settling to take place with the first application.  The reason we chose this approach was to improve the overall insulating performance within our depth of ~24 inches.

We also chose to spray cellulose within all of our interior walls as well, to further increase thermal mass and to offer additional levels of sound-isolation between the rooms.  To further help in sound-deadening of the drain pipes over head in the ceiling of the first floor, we netted the floor joists and filled them as well.

proper installation of sprayed wet blown cellulose insulation in passive house
Keep in mind that you should allow for a certain amount of time for "wet" sprayed-cellulose to dry.  We allowed 24-48 hours between our 2x4 interior studded walls and nearly a week for the 11 inches of material used in our basement double-wall system, given the additional thickness. In our case, we had our drywall crew install the drywall over a two step process.  We installed one side of the drywall on our interior walls, sprayed the cellulose, let it dry, and then installed the other side of the wall.

One additional point worth nothing:  Using this material added a significant amount of moisture (humidity) inside the conditioned building envelope.  Since the home was already air-tight and given the time of year, we relied on using de-humidifiers and fans to speed up the drying process (more on humidity levels in a later article).  For a time, RH percentage levels increased to the mid-to-upper 60s and even >70% for a short period and required close monitoring and management for a short-time.

finished appearance of wet spray cellulose insulation

However, between the 5/8" drywall and the use of sprayed (wet) cellulose, we built a fortress.  Of all the energy efficiency initiatives during our construction, I believe its use provided the biggest bang for the buck.


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