I have installed insulation for my own projects and for customers and the question comes up about which type of fiberglass insulation is the best for the attic.
Attic insulation: blown-in fiberglass vs fiberglass batts, which is better. Blown-in fiberglass insulation will perform better against fiberglass rolled/batts. To get the full R-value of insulation it must be in contact with the conditioned space. Any gaps or spaces next to the ceiling or joist will produce a convection air current that reduces the effective R-value of fiberglass rolled/batts.
When installing attic insulation there are things that should be done to get the full value of the insulation you paid for. In the rest of the article, I will try to explain what I have learned with the training that I have received along with what I have experienced.
What is blown-in insulation
Blown-in insulation can be made out of different materials. The most common materials found in today’s blown-in insulations are:
- Fiberglass
- Cellulose (which is ground up newspaper)
- Rock-wool
A machine is used to install this insulation. The insulation machine has a hopper, a blower fan and a hose that delivers the insulation to the area where it is needed. 2 people are required to operate the machine, 1 to fill the hopper and 1 to operate the hose.
What is insulation rolled/batts insulation
Insulation rolled/batts is insulation that comes in strips that fit between wall studs, ceiling joist or rafters. The insulation may come with a paper or foil layer on the outside that is known as a vapor barrier. The barrier comes with a little flap on each side that is used to attach the insulation to the stud, joist or rafter with staples.
The materials that rolled/batts can be made out of are:
- Fiberglass
- Rockwool
- Denim (recycled blue jeans)
- Hemp
Rolled insulation comes in a roll and must be cut to length.
Batt insulation comes pre-cut to length.
There is only one way to install this type of insulation and that is by hand.
R-value per inch of blown-in and batts insulation
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Fiberglass loose (blown-in) R 2.2 – 2.9
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Fiberglass rolled/batts R 2.9 – 3.8
The r-value is “the resistance of the flow of heat thru the material” and the density of the material will affect the R-value, that is why there is a range for each material.
Does the insulation itch
Fiberglass insulation has come a long way and itching is really not as much as a problem as it was in the past. You will get some itching when installing the insulation products, but not like the old stuff.
Please make sure you read and follow the recommended safe installation instructions provided with the product and wear your personal protection.
What will reduce the effective R-value of attic insulation (blown-in or rolled/batts)
This is how fiberglass insulation works. The fibers create air pockets in the material. This trapped air is what makes the material resistant to heat and the trapped air is what makes the material insulation.
This is important to understand because now you know that the movement of air thru the insulation will reduce the R-value of fiberglass insulation.
This is a list of things that will reduce the effectiveness of fiberglass insulation.
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Wind-wash
Wind-wash is caused by the lateral movement of air inside the insulation. The air is injected into the side of the insulation by the wind blowing against the wall of the building and continuing up into the eaves. The way you can tell if this is happening is the interior ceiling of the room that is being affected will condense moisture on it and in time mold will start to grow.
The way to stop the wind-wash is to block the edge of the insulation with an air barrier. Foam sheet insulation is commonly cut to size and installed on the top of the wall at the edge of the insulation.
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Gaps
For insulation to work it must be in contact with the area that you want to reduce the transfer of heat. Any gaps in the insulation will create a path for air to circulate. This movement of air reduces the R-value of the affected insulation.
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Compression
When the insulation is compressed it losses the air pockets that give the fiberglass insulation its R-value. Insulation is compressed when it is improperly installed under electrical wiring or bracing. Rodent activity in the attic area will also compress the insulation.
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Air infiltration from ceiling penetrations
The ceiling penetrations come from the holes that the electricians, HVAC, and plumbers create to pass the wires, ductwork, and pipes from the conditioned space to the unconditioned space.
To solve this issue air-sealing must be done with caulk or foam in each of the penetrations that are creating air movement.
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Thermal-bridging
Thermal-bridging is caused by a material that has a lower R-value inserted into the insulation layer. Each of the ceiling joists is a path for thermal-bridging to occur because wood has a lower R-value per inch than fiberglass. Wood has a value of R1 per inch.
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Holes in the insulating layer
Holes or areas that have no insulation and is caused by improper installation of the insulation, remodel work or rodent activity. The holes not only let air flow through the insulation and also insulation can not work if it is not there.
“fiberglass insulation has a nickname with those who work with it, filterglass”
Mice and bugs, do they like blown-in or batts better
Fiberglass insulation does not contain any additive that deters rodents or insects. Blown-in, rolled or batts they will tunnel into it or crawl over it.
There is not much difference between the two when it comes to a mouse, both make a great home.
Which one is easiest and fastest to install
Blown-in insulation is the easiest and the fastest to install. I installed 1475 square foot of ceiling insulation to a depth of 20 inches in 4 hours last week. This gave the customer an R-value of 60. Thermal-bridging is at a minimum because of the insulation material covers the ceiling joist and any bracing.
Cost for blown-in insulation per square foot for materials
Using R-30 for reference a bale will cover 65 of costing $42 at a major retailer in October 2018 making the cost of materials close to $0.65 per square foot
Cost for batt insulation per square foot for materials
Using R-30 for reference a 58 square foot roll cost $61 at a major retailer in October 2018 making the cost of materials close to $1.00 per square foot
Do you need to do anything special for blown-in insulation in the attic
- Yes, you need to have a ceiling to hold the insulation up in the attic.
- Maybe, you might need to build a box over the top of any recessed lights that are not “IC” insulation contact rated
- Good practice, build a box over bathroom fans for future replacement of the fan unit.
Simple answer to the question
Blown-in is better than rolled/batts insulation
Blown-in fiberglass insulation is much easier, faster, and gives a better R-value because of the way it is installed over any potential thermal-bridging areas and no gaps.
And it cost less to buy and install.
Helpful tidbit of information
What about cellulose insulation
Let’s compare the two and you can decide
- Fiberglass weighs less per same R-value than cellulose…R-30… fiberglass .4lbs. per sq.ft. cellulose .9lbs. per sq.ft.
This makes fiberglass a good choice for ceilings that may sag under the weight of the added insulation
- Cellulose cost about $0.40 per square foot fiberglass is $0.65 per square foot
- Cellulose has an additive to repel insects and rodents, fiberglass does not
- Cellulose has an additive to make it fire resistant, fiberglass does not burn but will melt
- Cellulose does settle over time, fiberglass does not settle as much
- Cellulose does provide some air-sealing after the material settles, wind-wash will not be a problem
- Fiberglass and cellulose are installed with the same machine
I hope this article has answered some of your questions. Good luck with your project.
Gary
10/25/2018