Blowing Cellulose Insulation Into Walls


Blowing insulation into attic ceilings is a straight forward and easy DIY job, but blowing insulation into walls is a bit more challenging a task.  Here is some information on doing a good job on insulating walls with blown in insulation.



A few highlights from the Bruce Harley book "Insulate and Weatherize":

Find out what's in the wall now -- existing insulation may make it hard to blow in new insulation.

If using a "filling tube" (see below) you will need to drill a hole large enough to push the filling tube through for each stud bay.  These should be about 1/3 to 1/2 the way up the stud bay.  If the stud bays have horizontal blocking installed, then you will need a hole above and below the blocking.  The holes should be about 2 inches in diameter -- make sure it matches the size of the filler plugs you are going to use to fill the holes after blowing. 

To drill the holes, you will need to remove one row of siding.  Do this carefully so that the board can be replaced.  This is better than drilling holes through the siding and having to patch each hole with patches that will likely show.

The filling tube is used to fill walls more completely and to dense pack the insulation for better R value and so that it does not settle.  The filling tube is and about 8 ft long by 1.25 inch diameter flexible vinyl tube that is taped to the end of the blower hose.   

In use, the filler tube is pushed into the hole you drilled in the stud cavity.  Push the tube to the top of the stud cavity, and turn the blower on.  With draw the hose as it starts to bog down.   You want to pack the cellulose as tightly as possible without clogging the blower.  Repeat the process for the lower half of the stud bay.

When all of the stud cavities are filled, plug the holes with plugs made for this purpose and replace the siding board.

It is also possible to blow the cellulose in from the inside by drilling holes in the plaster dry wall.

You may find that the blower that the home center provides is not powerful enough to blow insulation into wall cavities.  In this case, you may want to rent a blower to do the job (see story below).

You can also blow the cellulose in without using the fill tube.  The Harley book recommends two holes per stud cavity in this case.  The nozzle of the blower is pushed through the hole, and insulation is blown in until back pressure and bogging of the blower is detected, and then blower is shut off and the nozzle is removed. 

I highly recommend getting a copy of the "Insulate and Weatherize" book by Harley.  It has a great deal more detail on this and many other insulating jobs.


Excerpt from: "Cellulose Insulation - A Smart Choice" article:

Here is an excerpt from an article on the Building Materials and Wood Technology section of the University of Massachusetts.  The full article is here:

Blowing fiber into enclosed wall and cathedral framing cavities is different. Here a smaller 1- or 2-inch diameter fill tube is attached to the end of the larger hose. The fill tube is inserted into enclosed cavities through a series of strategically placed holes. The general idea is to drill a series of 2-inch holes horizontally across the structural surface so that the holes are centered in each framing cavity. One or more holes per framing bay are required depending on the length of the framing cavity and the applicator’s fill technique.

Filling walls and cathedral roofs from the outside is the typical practice. Pieces of siding or roofing are removed, holes drilled and insulation fill tubes inserted. Air pressure is cranked up for cavity-fill applications to provide a more densely packed injection called dense-pack cellulose. The narrow fill tube is inserted into the holes and pushed to within a foot of the far end of the enclosed cavity as the blowing begins. When the packed insulation becomes dense enough to stall the blower, the hose is backed out a bit. The blower gears up and filling resumes. The process is repeated until the framing cavity is filled. Then jump over to the hole(s) in the adjacent cavity. The injected fiber compacts tightly around wires, plumbing, and other penetrations providing an airtight insulating blanket with a slightly elevated R-value approaching R-4 per inch. The holes are plugged and the siding and roof covering is patched or reinstalled when the blowing is completed.

Cellulose can be blown into wall or cathedral roof cavities from the inside as well. Remove interior trim, drill – or simply drill holes through the interior drywall surface – and blow. Replace trim and patch the holes after the cavities are filled. In new construction, walls must be enclosed with fiber-reinforced plastic sheeting or drywall before cellulose can be blown into the framing. The plastic sheeting doubles as a vapor barrier. Choose whichever strategy makes the most sense for your situation.

If you have a home that was insulated years ago with inadequate levels of insulation, you are not out of luck. Skilled cellulose professionals can snake fill tubes into a wall already filled with fiberglass batts. The installer fills the cavities with dense-pack cellulose in a way that crushes the existing insulation without balling up the batts, achieving a full uniform application of the new cellulose fiber. The goal on any application is to assure complete coverage that is installed at a density that will not settle over time.


Video on Dense Packing vs Non-Dense Packing on a See Through Demonstartion Wall

The video shows the techniques for dense packing a wall with cellulose for various hole locations. Also shows the diffrerence between a stud cavity that is not dense packed to one that is dense packed.

A Post from Greenbuilding Digest on blowing cellulose into walls:

The item below was recently posted on the Greenbuilding Digest list service.  It gives a good description of blowing cellulose insulation into an existing wall cavity.

Two years ago we insulated our 1929 Craftsman 
bungalow with Cocoon insulation.  We did it 
ourselves, and none of us had ever blown 
insulation before.  It was very easy to learn. 
The main problem we had was the machine we 
originally got for free from Home Despot was not 
strong enough to fill the wall cavities.  We went 
to a rental store and got another machine and it 
did fine.  For the wall cavities, we would keep 
the hose in the hole and the blower going until 
the machine started to choke from being unable to 
push more material in, then we would stop and go 
to the next hole.  There would always be a little 
puff of insulation when we took the hose out.

After we filled our original walls, we put sheet 
rock over the new frame that we had constructed 
5.5 inches inside of the original exterior walls, 
and filled those holes.  So now we have 9 inches 
of cellulose insulation in our walls, and we like 
it a lot.

A year later, consumed with curiosity, I drilled a 
half dozen holes in the walls to see if the 
insulation had settled or turned into green goo or 
oozing mold or any of the other numerous horror 
stories that people tell about cellulose 
insulation.  Each wall we checked remained tight 
packed.  There had been no settling.  I think some 
of those stories originate in the marketing 
departments of fiberglass insulation corporations.

We also did our attic.  That was easy, but it is 
very dirty for the person in the attic.  An 
excellent respirator and goggles are mandatory.  A 
coverall also helps as the cellulose gets all over 
the clothing.

It is a 2 person job -- one to wield the hose, one 
to run the machine outside.  We rigged a rope that 
the person inside would pull when he wanted the 
machine turned off, as the one we rented didn't 
have a control that could be used inside by the 
worker with the hose.


I suggest simply following the manufacturer's 
instructions.  I am not a professional builder, 
but as an amateur I found cellulose insulation to 
be easy to install according to the manufacturer's 
instructions. Just keep that hose blowing into the 
wall until  it won't take any more.

If your hose clogs a lot (like every 20 minutes or 
so), go get another blower.  We wasted a whole day 
futzing with the Home Despot machine, trying to 
make it work, when the plain fact was that it was 
under-powered for the job.  About every 20 minutes 
it would clog.  The machine from the rental store, 
however, worked like a dream and only clogged a 
couple of times during the rest of the work.  The 
Home Despot machine would have been fine probably 
for the attic, which is probably what most people 
used it for, but it was underpowered for walls.

The winter after we did that insulating, we hardly 
needed any backup heat (we also tore off the south 
wall of our utility room and added windows to 
collect solar heat).  Last winter we needed more 
backup heat, but we also had an unusual (for us) 
amount of cloud cover.  We don't regret the work 
at all and are glad we did not yield to the 
numerous "nay-sayers" who constantly went on about 
how we were installing too much insulation, and it 
would never work and the cellulose would settle 
and turn into green ooze and kill us with nasty 
diseases. Yadda, yadda, yadda.  Me, I look at my 
$50/month electric bill and consider the work and 
money to be an investment that is paying me 
tax-free interest (in the form of energy I no 
longer have to buy to stay comfortable in my 
house) for the rest of my life.

Some More Links on Blowing Cellulose in Walls:

This slide presentation is geared toward people who want to do 
this for a living, but it does have a lot of good installation pictures, and some 
helpful hints on "challenges" you may encounter:
Dense Packing Cellulose Insulation Slide Show...
This is from


Gary 6/2/07 Updated 9/13/2009