Cellulose vs. Fiberglass vs. Spray Foam Insulation
By Henry Parker
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Insulation is what stands between your house and excessive heat loss or heat gain. It’s a basic fact of physics that heat energy is always on the move from a warmer zone into a cooler zone. When it comes to your household comfort and monthly utility bills, heat usually moves in the wrong direction.
In winter, heat energy produced by your furnace naturally escapes to the colder outdoors, boosting heating costs; during summer, outdoor heat infiltrates your air conditioned living spaces and cooling expenses rise as your A/C runs longer cycles to compensate.
Insulation inhibits heat transfer that occurs by radiation and conduction through building materials such as wood. It’s rated by R-value , a numeral that represents relative heat resistance. The higher the R-value, the better the material inhibits heat transfer.
The target area for most home insulation upgrades is the attic. Because heat naturally rises, winter heat loss through the ceiling of living spaces into the colder attic is very high. In summer, conversely, solar energy overheats the attic, which in turns radiates heat downward through the ceiling into rooms below. Fully 25% of heat loss or gain in most houses occurs through the attic.
How Much Is Enough?
As energy costs climb, Department of Energy recommendations for the proper amount of insulation are continuously updated. If your home is 10 years old or more and you haven’t upgraded insulation, it’s probably under-insulated by current DOE standards.
A well-insulated, tightly-sealed house will typically have heating and cooling costs that are up to 40% lower than a home with very poor insulation. Upgrading to the DOE recommended level of insulation can pay for itself in lower energy costs in as little as four to five years.
Home insulation comes in three common varieties: fiberglass batts, cellulose loose-fill and spray foam.
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The most common residential insulation is supplied as fiberglass batts. It’s that stuff that looks like fluffy pink cotton candy rolled up at the local home center. Fiberglass consists of a layer of spun glass fibers contained between paper or foil backing and pre-cut to standard sizes that fit between ceiling joists in the attic and framing studs inside walls. Because it rolls out easily into place, fiberglass batts can be installed quickly.
Standard fiberglass insulation in the attic provides an insulating R-factor of about 3.2 per inch. Multiply the total depth in inches of installed fiberglass batts by 3.2 and compare to the Department Of Energy insulation zone map to determine if you need to add more to reach current minimum standards.
Benefits of Fiberglass Insulation
Low cost. The most affordable insulating material, fiberglass is readily available on the consumer market.
Straightforward installation. Installation of pre-cut fiberglass batts in the attic requires no special tools and required skills are not very demanding. Any insulation or HVAC contractor can handle the job quickly and efficiently. Many homeowners choose to upgrade insulation themselves by adding additional fiberglass batts atop existing layers to reach the recommended depth—a relatively easy DIY project if you do it on a cool day in the attic.
Drawbacks of Fiberglass Insulation
Of the three types, fiberglass provides the lowest R-value.
Because fiberglass batts are one-piece blankets, achieving total coverage of the entire attic floor is often problematic because the batts don’t fit well into assorted odd-sized nooks and crannies present in most attics. This can be overcome by cutting the material into smaller shapes and sizes to fit each situation, but total coverage is still difficult to achieve.
While the attic floor is a prime destination for fiberglass batts, the inside of existing walls is not. Walls would need to be opened up completely to install batts. Unless you are doing a complete home renovation and already taking apart the walls, upgrading wall insulation with fiberglass batts isn’t practical.
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Cellulose is a loose-fill product composed of small particles of pulverized paper and cloth, treated with a fire retardant. It’s blown into the attic and wall spaces under air pressure through hoses. Once installed in the attic, cellulose loose fill insulation looks like mounds of freshly fallen snow covering the attic floor. Cellulose has an R-factor of at least 3.8 per inch, moderately higher than fiberglass batts.
Benefits of Cellulose Insulation
Because the loose particles blown in fill any and all odd-shaped areas of the attic floor, cellulose offers superior coverage without the need to custom fit pieces.
Cellulose is often the easiest choice to upgrade insulation in an attic. Blowing a layer of loose-fill insulation in to cover an existing layer of fiberglass batts is easily done and adds excellent R-value.
In addition to inhibiting heat transfer, a thick layer of cellulose also reduces air leakage somewhat (caulking all identifiable cracks and gaps in the ceiling and between walls is still recommended to seal leaks.)
Cellulose can be blown into existing walls through small access hoses without dismantling the entire wall as with fiberglass batts.
Cellulose in walls and the attic also has some soundproofing properties that reduce noise transmission through the house.
Downsides of Cellulose Insulation
Costs slightly more than fiberglass.
Due to the requirement for a motorized hopper, air pressure and large diameter hoses, installing cellulose is usually a job for professionals only, not a do-it-yourselfer.
If cellulose becomes soggy and saturated from some source of water–a leaky roof in the attic, for example—it does not readily dry and may form an environment for mold growth. If the wet area is extensive and toxic mold is an issue, the material may require removal. Roof condition should be checked and repaired if necessary before installing cellulose.
Spray Foam Insulation
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Spray polyurethane foam insulation (SPF) is formed by mixing two liquid chemical components to produce a chemical reaction as it is sprayed onto a surface such as the underside of a roof or into the inside of wall voids. The applied mixture rapidly expands to fill cracks and gaps, then dries to a stiff cellular constituency with good to very high insulating properties.
SPF is available in two types: closed-cell and open-cell. Closed-cell is the more affordable option while open-cell provides superior insulating performance, better water resistance and higher strength and rigidity.
Benefits of Spray Foam Insulation
Very favorable R-factor. While open-cell typically provides about R-3.5 per inch, closed cell R-factor often exceeds 6.0 per inch, making it the superior insulating option.
Adaptable to almost any surface. Wherever SPF is applied it forms a continuous insulating barrier that coats uneven surfaces, curves and corners.
Fills gaps and provides excellent air sealing.
Good soundproofing properties.
Can also be sprayed into wall cavities through access holes without opening up the wall.
Spray Foam Insulation Drawbacks
It is the most expensive option, with costs running as high as two or three times as much as fiberglass batts or cellulose loose-fill.
Properly mixing and applying spray foam insulation requires substantial expertise and can only be done by trained professionals.
Potential for excess application and additional cost due to overspray. If spray is not carefully and consistently applied, too much volume may be applied, resulting in excess depth and expense.
When spray is applied to the wooden underside of roof sheathing, it may seal moisture into the wood that has migrated through the shingles. This can accelerate deterioration and rot of these materials.
Some susceptible persons may be sensitive to chemicals included in the spray foam formula and exhibit allergic reactions.