Back in the early 1990s, I worked for a nonprofit agency, overseeing renovation work at several old wood-framed buildings in St. Johnsbury, Vermont. Each of these century-old buildings had between two and five apartments; after renovation, they were rented to low-income families at subsidized rents.
Almost all of these buildings received similar weatherization measures: empty stud bays were filled with dense-packed cellulose; attic floors received a new layer of deep cellulose; and air leaks were reduced with blower-door-directed air sealing. These retrofit jobs convinced me that cellulose insulation was versatile, affordable, and effective.
In 2000, while serving on our local school board, I helped develop the scope of work for energy improvements at our town’s elementary school. The floor of the school’s attic had a thin layer of fiberglass insulation, so I made sure that the retrofit specs called for the installation of a layer of thick cellulose on top of the fiberglass. Once the job was complete, I climbed into the attic to verify that the cellulose met specifications. (Unfortunately, it didn’t — the layer was too thin — so the contractor had to come back and add more.)
Although I’ve been involved with many construction projects that specified cellulose, I haven’t installed any of the stuff myself. (Most of my construction experience dates back to the 1970s and 1980s — the bad old days when fiberglass batts ruled the roost.)
Last year, with the help of friends and family, I built a garage. The garage has a second floor which I am gradually transforming into a guest room. This summer, my brothers and sons finished drywalling the ceiling and walls of the second floor, so the next step was to blow cellulose in the attic. This was an easy one-day job, and there were three willing workers on the crew: my wife Karyn, my brother Clark, and me.
I don’t own a cellulose blower, so I did what most homeowners do: I bought my cellulose from a big box store, which lent me a blower for 24 hours at no charge.
A homeowner’s guide to cellulose insulation
Cellulose contractors usually show up at a job site with a large truck equipped with a cellulose blower that can deliver up to 3,000 pounds of cellulose an hour. Because these blowers often draw 30 amps, the typical truck is equipped with a 10,000-watt gasoline-powered generator.
If you want to install dense-packed cellulose in your walls, and you are aiming to achieve a density of 3.5 to 4.0 pounds per cubic foot, it’s usually best to call up an insulation contractor who owns a big truck-mounted blower. (For more information on this issue, see Eric Habegger’s advice in Comment #3, below. For a better understanding of how insulation contractors do their work, see How to Install Cellulose Insulation.)
However, if all you want to do is blow cellulose on your attic floor, you won’t be dense packing — so all you really need is one of the wimpy blowing machines that are available at Home Depot and Lowe’s. These machines are rated for about 450 pounds of cellulose per hour.
There are at least two advantages to these wimpy machines: they aren’t very heavy, so they’re easy to load into a pickup truck; and their relatively small motors won’t trip your 15-amp breaker.
Air sealing comes first
Before you drive out to the store to pick up your insulation blower, your attic needs to have been properly air-sealed. Don’t leave this work until the last moment. Give yourself plenty of time to inspect the attic carefully. Use as many cans of canned foam and caulk as you need to make sure that your ceiling is as airtight as possible.
Don’t forget to build a plywood insulation dam around the perimeter of your attic hatch. If you plan to install 16 inches of cellulose, make sure that your dam extends at least 18 inches higher than the drywall ceiling.
For more information on this work, see Air Sealing an Attic.
If you are insulating the attic of an old house, remember that installing insulation on top of knob-and-tube wiring is a code violation and a possible fire hazard. If you have knob-and-tube wiring in your attic, you can only insulate on top of the knob-and-tube if you are sure that it has been permanently disconnected. When in doubt, call an electrician to have your wiring evaluated before proceeding.
One person feeds the blower, and one person aims the hose
Two people can lift one of these blowers up and load it into a pickup truck. The machine should come with 100 feet (two 50-foot lengths) of 2½-inch diameter hose.
If it’s not raining, you’ll probably be setting your blower up outdoors. A rental blower can easily blow insulation onto the floor of an attic above a second or third story.
You’ll need at least two people to operate an insulation blower. One worker opens the bales of insulation and feeds the insulation into the hopper of the blower. The other worker is in the attic, aiming the hose.
How many bags should you buy?
Cellulose bags usually include a table to help you calculate how many bags you need to buy (see Image #8, below). For example, the brand of cellulose I bought comes with a table that explains that an R-60 installation requires an initial installed thickness of 18.4 inches. Each 18.1-pound bag is adequate to insulate 9.3 square feet of attic floor to R-60. Such an installation would require 2.06 pounds of cellulose per square foot.
For R-49, you want an initial installed thickness of 15.2 inches. An 18.1-pound bag will cover 12 square feet to R-49; this results in an installation with 1.6 pounds of cellulose per square foot.
If you live in a warm climate, you might be satisfied with only R-38 insulation; that would require an initial installed thickness of 12 inches. An 18.1-pound bag will cover 16.4 square feet to R-38; this results in an installation with 1.16 pounds of cellulose per square foot.
You should probably buy more cellulose than you think you need. When you return the blower to the store, you can return any unused bags for credit. (This approach is easier than running out of insulation halfway through the job.)
To insulate our 25 foot by 20 foot attic, I bought 70 bales; that’s 1,267 pounds of insulation.
If the blowing machine were truly capable of delivering 450 pounds per hour, it should have taken about 3 hours to deliver that much insulation. It actually took us more like 5 hours.
Feeding the hopper
The blower grinds up the cellulose insulation and blows a mixture of air and cellulose out the cylindrical steel fitting that extends horizontally from the bottom of the hopper. The rate of insulation delivery is controlled a simple sliding sheet-metal gate. The instructions advise adjusting the gate so that it is about 3/4 open, and that advice makes sense. If you try to open the gate all the way, the hose will probably clog, and delivery will stall.
If you can’t find the coupling used to connect the two lengths of hose, they can be easily connected with ordinary duct tape.
Blowing cellulose is dusty work, so wear a dust mask. If you are filling the hopper, don’t try to cram in too much at once. If you overfill the hopper, the cellulose can jam at the top, creating a bridge that hides an invisible void below; if this happens, the hose will deliver nothing but air. You want to be able to see the safety grate about halfway down the hopper. Break up the cellulose near the top of the hopper with your fingers or a short 2×4, but make sure you don’t stick your fingers where they don’t belong. If an object other than cellulose falls into the hopper, hit the “off” switch immediately.
Watching the drifting snow
If you’re holding the hose in the attic, you’ll probably get bored. The insulation comes out like snow, and it can take a while for drifts to form. If you have a big spring clamp, you may want to fasten the end of the hose to a rafter or truss chord, and let the insulation pile up for later raking.
You don’t want the hose to get away from you and disappear through the attic hatch to the floor below. One way to prevent this is to pull a few extra feet of hose into the attic, and wrap the hose around a framing member to provide friction on the hose. (Don’t wrap it in a full circle, of course — tight corners impede airflow.)
Do you need a set of walkie-talkies to communicate with the operator below? Not really. The insulation comes out slowly. If you have a way to secure the hose safely in the attic, you can even climb down the ladder and chat with your co-worker feeding the hopper.
When do we quit?
The difference between “almost enough cellulose” and “a bit more than you need” is subtle. You can expect cellulose to settle at least 13%. If you want a settled thickness of 16 inches of insulation, you should aim to install at least 18 inches. To me, that means at least 19 inches. Blow as much as you think you need, and then blow some more.
Don’t skimp. Aim the hose at the corners and edges of your attic; look for crannies that may need special attention; and be generous. A little too much does no harm at all. Remember: if you have the blower on site and the hose in your attic, it’s never going to be easier to add a little extra cellulose than it is right now.
Once you are all done, rake the high spots smooth and admire your work.
Martin Holladay’s previous blog: “Martin’s Pretty Good House Manifesto.”