Why cellulose and fiberglass behave so differently
Cellulose is shredded recycled paper treated with fire retardant. It’s heavy, it soaks up humidity, and it packs together easily. Fiberglass is spun glass fiber. It’s light and springy, and it comes out of the bag compressed hard, so it has to be opened back up before it can do its job.
That difference drives everything about how you set up the machine. With cellulose, the challenge is moving heavy material down the hose without plugging it or having too little throw at the end. With fiberglass, the challenge is opening the material up and placing it at the right density without blowing it into a cloud, creating a static mess as if you are in a snow globe.
| Cellulose | Fiberglass | |
|---|---|---|
| Installed weight | Heavier, roughly two to three times denser in an attic | Light and lofty |
| Main machine job | Break apart material with no clumps | Open up compressed fibers |
| Feed (gate) | Typically partly closed | Typically wide open |
| Common failure | Plugged hose, overly dense install | Clumps, or blown too light for the R-value |
| Humidity | Absorbs moisture and clumps | Not absorbent, but static can be a nuisance |
The four settings that actually matter
Every blowing machine is a little different, but nearly all of them come down to the same four controls. Think of the first two together: what you’re really adjusting is the ratio of air to material.
1. Feed rate (the gate or slide). This controls how much material drops into the airstream. Fiberglass usually runs with the gate wide open, since it’s light and needs volume to keep up. Cellulose usually runs with the gate partly closed, since it’s heavy and plugs the hose if too much drops in at once. Too much feed causes plugs with cellulose and chunky, uneven coverage with fiberglass. For fiberglass, open the airlock all the way. For cellulose, close the airlock about 1/4 of the max. If you have a hyper speed shredder, then you can open the airlock all the way
2. Air. Whether your machine adjusts air with a pressure control, a throttle, or the number of blower motors running, this is your other half of the ratio. Cellulose needs enough air to push heavy material the full length of the hose. Fiberglass needs air to loft it, but too much makes it hard to aim, throws it past where you want it, and can leave it under-dense.
3. Agitation and conditioning. Fiberglass comes out of the bag tightly compressed. If the machine’s agitators or shredder don’t break it up well, it comes out of the hose in clumps that settle unevenly. Cellulose breaks up easily, but in humid weather it can clump in the bag, so break bags apart as you load the hopper.
4. The hose. Hose length, diameter, and layout matter as much as anything on the control panel. Every foot of hose and every bend adds friction. Keep runs as short and straight as you can, avoid kinks and sagging loops where material can settle, and remember that cellulose plugs more easily on long runs than fiberglass does.
The bag chart is your real target
Machine settings are only a means to an end. The goal is the R-value printed on the bag, and you only get it by installing the right amount of material at the right thickness. Every bag label includes a coverage chart that lists, for each R-value, the number of bags per 1,000 square feet and the minimum thickness. Cellulose labels also list settled thickness, because cellulose settles after it’s installed.
Use that chart on every job. Measure the attic, figure out how many bags it should take, and install depth markers so you can see thickness across the whole space. When you finish, compare the bags you used with the chart.
This is where fiberglass catches people. With too much air, fiberglass can reach the target depth while using fewer bags than the chart calls for. The attic looks right, but it’s under-dense and the R-value falls short. Hitting depth isn’t enough. You have to hit depth and bag count together.
Do a coverage check at the beginning of the job
Chose a 10×10 square area in the attic. Blow your material into that area to the proper depth. Then compare the coverage chart on the bag. It is FAR better to test the coverage with a small area than finding out at the end of a 3,000 sq ft job that you wasted material.
Compare that to what the bag label calls for. If it’s too light, reduce the air or open the gate a bit. If it’s too heavy, do the opposite. Retest until it’s right. It takes about ten minutes, and it can save you from callbacks, wasted bags, and attics that don’t perform.
Dense-pack walls: Cellulose vs Fiberglass
Dense-packing closed walls is a different job from blowing an open attic. It calls for high air pressure, a low feed rate, and a fill tube pushed into the cavity, so material packs tightly enough that it won’t settle and leave gaps at the top of the wall.
Cellulose: A density around 3.5 pounds per cubic foot is a common target, but follow the manufacturer’s specification for the product you’re using.
Fiberglass: Fiberglass compacts far more than cellulose. Measure the PSI to around 2.5psi. If you do not measure the PSI, you can easily pack a wall way too tight and waste a lot of material.
Switching materials on the same machine
Most blowing machines handle both materials, but don’t switch without a reset. Run the hopper empty, clear any packed cellulose from the airlock and hose, and check the airlock seals. Then set the gate and air for the new material and run a fresh density check.
Storage matters too, especially on the Gulf Coast. Keep cellulose bags dry and off the ground. Bags that have soaked up humidity in a hot trailer clump in the hopper and plug the hose.
Troubleshooting quick reference
- Hose keeps plugging (usually cellulose): too much feed for the amount of air, a hose that’s too long or kinked, damp material, or worn airlock seals.
- Fiberglass coming out in clumps: feed set too high, not enough agitation, or bags not broken up as they’re loaded.
- Material clouds up and won’t land where you aim: too much air.
- Target depth reached with fewer bags than the chart: the install is under-dense. Reduce air or increase feed, then recheck density.
- Using far more bags than the chart: material is going in too dense. Back off the feed or add air, then recheck.
- Output drops partway through the job: check airlock seals, filters, and the hose for a partial plug.
Choosing a machine that runs both well
If your crew blows both materials, look for a machine with enough air to carry cellulose over long runs and enough agitation to condition fiberglass properly. When you compare output ratings, check which material each rating refers to, since a rating for one doesn’t translate directly to the other.
Not sure your current settings are right, or ready to step up to a bigger machine? Call Barry at (810) 614-0072 or email barry@insulationmachines.net.
Frequently asked questions about blowing cellulose and fiberglass
Can you use the same insulation blower for cellulose and fiberglass?
Yes. Most blowing machines handle both, but they need different settings. Empty and clean the machine when switching, reset the gate and air for the new material, and run a fresh density check before starting the job.
Should the gate be open wider for cellulose or fiberglass?
Generally wider for fiberglass and narrower for cellulose. Start from the machine manufacturer’s recommended setting for each material, then fine-tune with a density check, since hose length, humidity, and the specific product all affect the right setting.
Why does my fiberglass reach the right depth but use fewer bags than the chart?
The insulation is under-dense, usually from too much air or too little material for the amount of air. It looks right but won’t deliver the R-value on the bag. Reduce the air or open the gate slightly, recheck density, and match both the depth and the bag count on the chart.
What causes cellulose to plug the hose?
The usual causes are too much feed for the amount of air, a hose run that’s too long or kinked, damp or clumped material, and worn airlock seals that let air escape before it reaches the hose.
How do I check blown insulation density?
Blow material into a box with an inside volume of one cubic foot, strike it level, and weigh it. Subtract the empty box weight to get pounds per cubic foot, then compare that to the density the bag label calls for and adjust your settings as needed.
Does blown cellulose settle after it’s installed?
Yes. Cellulose bag labels list both an installed thickness and a settled thickness. Install to the full installed thickness on the chart so the attic still meets its R-value after the material settles. An Internal Wetting System (IWS) will help prevent settling as it creates a crust throughout the material. Loose-fill fiberglass settles much less.
