Cellulose and Foam Comparison
By Paul Johnson, LEED A.P.- Evergreen Contractors
The insulation world today is full of competing claims about the comparative effectiveness of foam
and cellulose, the popular methods of sealing structures against air infiltration and noise at higher
effective R value, which are rapidly supplanting fiberglass. We are currently installing cellulose and
foam insulation. Each type of insulation has its advantages. Our goal is to put the right type of
insulation in the right location for maximum performance on a range of characteristics at minimum
cost.
The debate over the comparative benefits of foam and cellulose center on the issues listed below.
Although cellulose is considerably less expensive than foam, there are situations in which foam is
nevertheless a better choice. Genuinely comparative data are hard to find, as there are few unbiased
sources. After extensive research of our own, we have reached the following conclusions:
Thermal Performance
Cellulose has an R-value of 3.8 per inch, open-cell foam has an R-value of 3.6 per inch. You will be
able to get an additional R-2 in a ten inch cavity with cellulose.
Sound Attenuation
Open-cell foam and cellulose are both sound attenuators but, based on comparable figures, cellulose is
significantly more effective. This is because it is denser, at 3.5 lbs per cubic foot, than foam, at .5 lb
per cubit foot. The optimum density for a sound-absorbing material is about 3 lbs per cubit foot.
Cellulose has an STC (sound transmission coefficient) rating of 45 in a 2x4 wall, compared to only an
STC rating of 37 for foam.
Fire Safety
Both cellulose and foam meet fire retardancy specifications, but cellulose will give you a higher
rating and can provide a two-hour firewall. The tests here are conclusive and not debatable.
Cellulose is clearly better. Although foam will not burn when flame is removed, it will support the
flame while it is present. Some sources say that foam gives off somewhat toxic gasses in a fire.
Cellulose, on the other hand, makes a structure virtually fireproof. A clump of it in your hand will
only glow in a small area when a torch is applied, leaving a char. In some fires the cellulose insulation
remained standing while the framing burned away. Tests have shown that 14.5 inches of cellulose
exceed the performance of wood fire blocking. The bottom line here is that cellulose would provide
more time for people to escape a burning building and for fire fighters to extinguish a blaze.
Mold/Rot Resistance
Neither foam nor cellulose support mold growth. Foam has no food value to support mold, while
cellulose includes an anti-mold natural substance in the fire retardant used - borate. Foam has no
chemicals at all that would prevent mold if it got and stayed wet for awhile.
Insects and Vermin
Foam does not have food value for insects, mice or rats; however, they may nest in it. Cellulose,
critics contend, is what these buggers eat. This would be true except that cellulose is loaded with the
natural fire retardant – Borate - which is also a natural insect and rodent repellant. You can buy it at
the hardware to sprinkle around your kitchen to keep carpenter ants away. Borate is very safe to
humans. It is less toxic than table salt. Insects and pests do not eat, or nest in cellulose with Borate.
They run away from it.
Site Specific Benefits
Neither foam nor cellulose is perfect for all site situations. Foam may be better for tight spaces like
rim joists, crawl spaces, etc. Closed-cell foam with an R 6 per inch, although much more expensive
than open cell foam, is the only option when the stud depth is not enough for adequate thermal
insulation at R 3.6-8 in cellulose or open-cell foam. Closed cell foam being a true moisture barrier is
the only option in damp areas like basements or under radiant floor slabs. Closed cell foam is great in
hurricane zones because it makes the structure more rigid. Some say it makes a structure strong
enough that with it you can use 2x4 rather that 2x6 walls.
Air Barrier
This is a principal area of contention. Foam and cellulose have not been tested at the same time and
under the same conditions, but it is generally accepted that foam has a slight edge on cellulose as an
air barrier. The difference in air penetration is significant only in sustained high-wind conditions such
as affect the average structure only a few hours per year unless it is located on the beach or top of a
hill. (A true air barrier is only achieved by closed-cell foam, at 2 pounds per cubic foot), it meets this
standard. Unfortunately closed cell foam is almost twice as expensive per R as the popular open cell,
½ pound per cubic, foot foam.) Realistically, if you hold a hair dryer to one side of a wall and your
hand on the other, you will notice no air blowing through either cellulose or foam. Both succeed in
making your home so air tight – at only 2 air exchanges per hour – that mechanical ventilation is
recommended to assure a healthy environment. Thus any discussion about possible differences
between the two products is moot.
Moisture Retardance
In this area there are fewer contentions. Cellulose and foam are both regarded as moisture retarders
rather than moisture barriers. Both are sufficiently effective that neither requires a vapor barrier
on the heated side to meet code requirements. Manufacturers of foam and cellulose tend to agree
that they are pretty close here. The big difference is the way in which each material functions, the way
it manages moisture when it gets wet or faces high humidity. The difference between the two is in
their capacity to wick water.
Foam is hydrophobic. It does not wick. The moisture stays more or less where it is until it evaporates.
The benefit of this is that if you have a leaky roof or pipe, you can readily locate the problem from the
wetness of the wall.
Cellulose is hydroscopic. It wicks water, as is evident when you spill any liquid on a sheet of
newspaper. Cellulose manufacturers regard this as a benefit because if humidity is extremely high, or
if rain blows under an area of siding or shingles, the liquid will quickly disperse throughout the
framing and never be wet enough to create mold.
Improper installation problems
Critics of cellulose contend that it settles. It does, when open blown-loose fill, into attics where it is
calculated and expected to do so over a period of 2 weeks. However in sidewalls with wet spray, or in
cathedral ceilings and floors where it is netted and dense packed, the cellulose is blasted in under high
pressure and cannot settle. (U.S. Dept. of Housing standard) This is because it is packed in at 3.5 lbs.
per cubic ft. – 2.5 times its normal settled density as in the attic. If however, god forbid, it is done
improperly and a void occurs where moisture can develop, the moisture will be wicked away by the
cellulose. Foam on the other hand has its own potential installation problems. If the temperature,
pressure or mix of the foam A&B components is not correct for the temperature of the air or plywood
sheathing, voids will develop between the sheathing and the foam. Unfortunately you cannot see these.
Moisture will develop here and unlike cellulose, which disperses moisture, it will sit their for
sometime, a potential breeding ground for mold.
Environmental Impact
Cellulose contains over 80% recycled material and requires one-twentieth the amount of energy
required to produce foam. Foams are manufactured from mostly petrochemicals having under 5%
recycled content. Also, the overspray for spray-applied cellulose is recovered and reused, while the
overspray from foam is trimmed off and taken to the landfill.
Copyright Paul Johnson 2006. No reproduction without written permission.