Q&A / 

Cellulose vs. Fiberglass

Be sure to read the special Author's Note at the end of this column. It provides updated information on this topic.

DEAR TIM: I would like to upgrade my attic insulation. However, I understand that fiberglass insulation is now considered a suspected carcinogen. Cellulose insulation, I've been told, possibly is a fire hazard. Can you help with this dilemma? Also, are there any factors that affect the R-values of each material? C.T.

DEAR C. T.: The information you heard about concerning fiberglass being a suspected carcinogen is correct. In July of 1994, the federal government labeled fiberglass as a substance that quite possibly could be a carcinogen. However, the U. S. Department of Health and Human Services, in a letter to Congress, exclaimed that they felt that fiberglass does not pose a threat to persons in their daily lives.

The risk appears to be centered around the fact that when installing either blown or batt type fiberglass, small glass fibers become airborne. These fibers can then be inhaled into your lungs. This can in fact happen. Years ago, as an uninformed rookie, I installed fiberglass without using a facial mask. After working with the material for several hours, I could feel the fiberglass in my throat.

However, once installed, fiberglass is almost always covered with some material. Or, it is in an attic space where it can not be easily disturbed. This is why it does not pose a serious health threat to the average person.

Fiberglass manufacturers have responded to this concern. You can now purchase fiberglass batt insulation that has a combination plastic and kraft paper covering. The back side usually has small perforations in the plastic to prevent the buildup of water vapor within the insulation.

Cellulose insulation is made primarily from recycled newspaper. Yes, newspaper is very flammable. However, cellulose insulation is treated with either sodium borate, boric acid, or ammonium sulfate. These chemicals, which have been deemed safe for humans, make cellulose insulation fire retardant. These chemicals also repel rodents, insects, and mold.

However, I have a concern about quality control when the chemicals are applied to the ground up paper. What happens if the chemicals are not mixed correctly? What happens if not enough chemical is applied? Is it possible for there to be a malfunction at the factory and you receive a bad batch? How can you test for yourself that enough chemicals have been applied?

These chemicals apparently retain their ability to retard fires in the hot temperatures found in many attics. However, there have been instances when cellulose has indirectly caused a fire. This usually happens when either type insulation is installed in direct contact with an older style recessed lighting fixture. The insulation traps the heat created by the light bulb. This, in turn, causes the lighting fixture to overheat. Some modern recessed fixtures have thermostats within the fixture that turn the light off if it begins to overheat.

Settling of blown insulation can affect its long term R-value. R-value is the term used to measure the flow of heat through an object or material. A higher number indicates that the object or material has a higher insulating capability.

Most blown-in fiberglass and cellulose insulations settle, or compact, after installation. Cellulose seems to settle more than fiberglass. Some fiberglass manufacturers have developed newer loose fill fiberglass that evidently does not settle. Once it has settled, fiberglass has an R-value of 2.1 - 2.7 per inch, while cellulose has an R-value of approximately 3.0 per inch.

Loose fill blown fiberglass insulation has another slight problem. As the temperature difference between the living space and attic increases, the R-value of blown fiberglass diminishes. In extreme situations, such as the upper Midwest, this reduction in R-value approaches 50 percent.

This phenomena seems to occur as a result of thermal convection. The trapped air molecules in the insulation are pulled up through the insulation into the colder attic air. This problem has been successfully solved by installing fiberglass batt insulation over the top of loose fill or blown insulation.

The bottom line is that cellulose can burn, but fiberglass will not support combustion. Fiberglass may melt in an extreme fire, but will not add to the fire load. Choose your insulation wisely.

Author's Note:

Routinely I am blessed to hear from professionals and industry experts who share in-depth and updated information about a topic. You'll absolutely want to read the letter I received from the President of NAIMA (North American Insulation Manufacturers Association). It really helps clarify some of the points I just touched on in this column.


I received this very interesting email from E. G. who is a remodeling contractor. He related to me a frightening incident that happened on one of his jobs in 1997, in Champaign, Illinois:

"I just read your article comparing the virtues and vices of Fiberglass vs. Cellulose Insulation. Let me share with you an anecdote from my own personal experience.

A few years ago, I was remodeling a room in a client's home -- moving a wall, adding a skylight, stuff like that. I finished mudding the drywall about noon and headed home for the day. Two hours later, I got a call from the homeowner, telling me that the job would be delayed for awhile.

After I left, the electrician doing some work in the attic had set his trouble light down in the cellulose insulation. He then went down to his truck for a bite of lunch.

I'm sure you can tell where this is going: the "fire-retardant" cellulose insulation caught fire, and by the time the fire trucks arrived, a big hole was burned in the roof, and my lovely new work was ruined.

You say in your article that the fire hazard of cellulose insulation has been magnified. Perhaps it has. But having seen it with my own eyes, I can't trust the ground up newspaper insulation any more. For my money, it's fiberglass all the way. I've never heard of it catching fire."

Eric Gentry


2 Responses to Cellulose vs. Fiberglass

  1. Is it ok to put blown in cellulose installation on top of blown in fiberglass insulation? We have squirrels in the attic that have made mounds from the fiberglass insulation. Some areas are 3" and some are 12+". We were advised that putting in cellulose insulation would help with the problem. Any advice would be appreciated. Thanks. Dania

  2. Yes it is ok to blow cellulose over fiberglass. I recommend packing down the fiberglass prior to blowing in the cellulose. This packing makes for a better foundation for the cellulose. I have done this on my personal home and on remodels. On my personal home, we noticed a significant increase in home comfort and thermal imaging improved significantly. If there is any duct work in the attic, make sure it is completely covered. Make sure any exposed metal duct is covered or you risk condensation and mold issues. I spray 2" of closed cell foam on any exposed duct in attics then overspray with loose fill. We have also dense packed cellulose in garage ceilings where fiberglass batts did not completely fill the space between the I-joists. The dense packing crushed the R19 bats down to less than 2" thickness. The homeowners reported a noticeable improvement in the comfort of the bedrooms above the garage. I am a huge fan of cellulose in certain circumstances. In attics, I find it is a better product in older homes that have not been air sealed. When dense packing garage ceilings and when dense packing walls. The only real drawbacks to cellulose is the dust and if there is a roof leak or water leak of any kind, cellulose holds a tremendous amount of water and must be removed and replaced if saturated.

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