It is often useful to be able to determine the installed density of dense-pack cellulose in a wall. Because the installer must work without the benefit of viewing the coverage or density, core sampling was devised many years ago to measure these important metrics. If the density is too low, the R-value and air leakage degrade. If the density is too high, insulation and labor are wasted.
Ideally, dense-pack cellulose should have an installed density of 3.5 to 4.5 lb/ft3 (56 to 72 g/L) and the density should be consistent throughout all the wall cavities. Cellulose installed to this dense-pack range will not settle over time, will prevent most air leakage, provides an R-value per inch of 3.2 to 3.5 (0.222 to 0.2428 RSI per cm), and will do a good job of acoustically insulating a wall. Before starting the installation, it is ALWAYS important to thoroughly inspect the inside finish materials for significant holes or cracks. Additionally, check for proper fastening and strength because drywall can crack and paneling can pop off at dense-pack pressures, although this doesn’t happen often.
A number of years ago I spent two months in the six New England states with many weatherization installers discovering the best ways to install dense-pack cellulose in walls. We learned a lot about removing and drilling siding, the right blowing machine settings, tubes and hoses, hose transition fittings, conditioning the cellulose properly, being sensitive to the blowing machine sounds, plugging our fill holes correctly, and reinstalling siding. We had our bad days of cracking drywall and blowing off ¼-inch paneling, but fortunately, no one lost any fingers in a blowing machine. A day-by-day account of this training is available here.
My core sampling often showed that installers had missed sections of a wall and that the density of the installed cellulose varied considerably. Voids in the installed insulation can be “seen” with an infrared (IR) camera, but variations in density, even severe, usually don’t show with IR. Core sampling is the only method to check density with accuracy.
I have found that not only is core sampling important for determining if dense-pack cellulose is installed properly, but it is also a great training tool for installers, inspectors, and weatherization managers.
You can assemble your own core sample kit for around $150.00. Instructions for acquiring the parts and properly using the kit are available on this website. You can also purchase a core sample kit made by GreenFiber for $2000.00 (this kit also includes a moisture meter).
Where should the density be tested? Multiple samples should be taken for any insulation job. Drill the core sample hole at least two feet away from any insulation fill hole. See this document for detailed instructions for drilling the core sample hole and extracting the cellulose. It’s a good idea to photograph the location of each core sample hole and encode this to the small plastic bag of cellulose you extracted from that core. Always fill the empty core with cellulose and cover your tracks by plugging/replacing the sheathing and replacing the siding.
In order to determine the density of your sample, you must know the relationship between the core sample weight, the inside diameter of your core tool (mine is 2-inch diameter DWV copper tubing, see my density chart), and the core depth.
Other useful dense-pack cellulose resources include:
- Wall Insulation Density Calculation Forms, 2003
- RED Calc Free tool: Dense-Pack Insulation
- Wall Insulation Experiments with Cellulosein Maine, October 2003. This is in slide presentation format
- Sidewall Blowing Tips and Tricks, by Bob Pfeiffer, Tom Wilson, and Jim Fitzgerald, 2003
- Siding Removal and Wall Insulation Techniques, handout for a presentation by Chris Allwein and Dennis Biddle, April 2002