FAIRBANKS - Building envelopes have a hard job in Interior Alaska—keeping us warm, dry and healthy at 40-below. CCHRC studies a variety of building designs and products to see how they perform in this environment. We recently looked at cellulose insulation to see how it compared to other types, like fiberglass and rigid foam insulation.

First, let’s look at a conventional wood-framed wall with 2x6 or 2x4 studs and an interior vapor barrier. This system has historically worked in the Interior because the vapor barrier limits the moisture allowed into the walls and moisture that does sneak in remains frozen through most of the winter. During the spring, the walls thaw and dry to the outside. 

But with rising energy costs over the past decade, many homeowners added exterior foam insulation to their walls to save money. That also changes the moisture behavior of the wall. The foam acts as a second vapor barrier, preventing the walls from drying to the outside.

Is this good or bad for your house? Depends on how much you add. If you add enough exterior insulation (in Fairbanks, two-thirds of the total wall R-value, or six inches of EPS foam for a 2x4 wall), the wall framing will stay warm enough to avoid condensation, improving your overall moisture control. If you don’t add enough, however, you move your wall sheathing into the danger zone—above freezing and very humid.

We’ve learned from earlier studies how to use fiberglass and EPS and XPS foam in various wall systems to improve energy efficiency while avoiding moisture problems (See cchrc.org/safe-effective-exterior-insulation-retrofits).

This latest study looked at how cellulose performed in different wall scenarios over an 18-month period. These were not standard walls—they intentionally lacked a vapor barrier because we wanted to force moisture into the walls.

Cellulose insulation is made primarily of recycled paper. As a local, affordable product, it has recently become more popular in building in Interior Alaska. “Dense-pack” cellulose is blown into a wall to a density of 3.2 pounds force per cubic foot, which is designed to prevent the insulation from settling over time. Dense-pack cellulose has an R-value (or insulation value) of 3.7 per inch—slightly higher than fiberglass batts and slightly lower than EPS foam.

Our study showed that cellulose handles moisture better than fiberglass or EPS insulation. The test wall that used cellulose as both interior and exterior insulation maintained lower humidity levels (and was less likely to see condensation or mold) than the test wall that used interior fiberglass and exterior foam.

That can be partly attributed to material properties of cellulose. Dense-pack cellulose is actually less permeable to air flow than fiberglass batts. So when used as interior insulation, it reduces the amount of moisture that migrates into the stud cavity.

Cellulose also has the ability to absorb and release water vapor, allowing it to moderate moisture levels within a wall and prevent the large spikes in relative humidity that cause moisture damage.

It’s also more permeable to water vapor than EPS or XPS. The test wall with exterior cellulose had lower humidity levels than the wall with exterior foam, because it allows faster drying to the outside.

Based on this study, dense-pack cellulose can provide a good alternative for exterior insulation besides rigid foam board. In future studies we plan to look at the minimum amount of exterior cellulose needed to keep the sheathing warm and dry.

Read the report at http://www.cchrc.org/sites/default/files/docs/CelluloseSnapshotFinal.pdf