What Is Blown-In Insulation?

Blown-in insulation is most often made of cellulose, fiberglass, or mineral wool. It looks and feels like small bits of cotton or feathers. The name blown-in refers to the application method – whether installed wet or dry, all of the products are fed into a machine that blows them into attics and wall cavities.

What is Blow Insulation

Uses of Blown-In Insulation

Blown-in insulation is associated with attics. Contractors often blanket the attic floor with blown-in insulation to prevent heat from escaping into the attic. It also prevents attic temperatures–warm or cold–from seeping into the home’s living area.

Loose-fill insulation will settle over time, but even in a more compact form, it does not lose any R-value (thermal resistance). Manufacturers provide information on the number of bags required to achieve a specific R-value.

Installers can also add blown-in insulation between wall studs in two ways. During new construction, they can use the insulation to fill the cavities between studs. If they use a dry loose fill, a net or other type of retainer is attached to the insides of the studs to hold the material in place while it is being blown.

Cellulose offers the option of wet application. Installers add water to the loose fill, enabling it to stick to the backside of the wall sheathing and the studs. It fills any gap and hole and adheres to electrical boxes, wires, and plumbing pipes to provide complete coverage. There is no need for a retaining net, and the product is dry in 24 hours.

Types and R-Value of Blown-In Insulation

There are three types of blown-in insulation–cellulose, fiberglass, and rock wool. Each has benefits and drawbacks to consider before purchasing. Blown-in insulation has an R-value of 2.5 per inch to 3.7 per inch, depending on the type.

Cellulose Blow-In Insulation

Cellulose insulation is the most eco-friendly product. It contains up to 75% recycled newspapers and cardboard. Manufacturers run it through a hammermill, where it is turned into a light fluffy product resembling cotton. It’s then chemically treated with boric acid to make it fire, mold, and insect resistant.

  • Approximate R-value of R-3.7 per inch.
  • Requires just over 5” to equal an R-20 fiberglass batt.
  • Most versatile. Use in attics, new construction walls, finished walls, and on ceilings.
  • Dry application. Use as loose-fill insulation in attics and finished walls.
  • Wet application. Use to spray onto new walls during construction and on unfinished ceilings. Will stick and not fall off.
  • If cellulose gets wet after insulation, it will become less fluffy and loses some of its insulation value.

Fiberglass Blow-In Insulation

Fiberglass insulation traps tiny air pockets in the spun glass material. The air provides the insulation value–not the glass. Compressed fiberglass insulation–loose fill or batts–is not as effective, and wet fiberglass has no insulation value.

  • R-2.5 per inch.
  • Manufactured from heated spun glass. Up to 60% recycled product.
  • Requires approximately 7 ½” to get the same R-value as an R-20 batt of fiberglass that fits into 2 x 6 framing.
  • Excellent loose-fill blown-in insulation.
  • Does not work as well as wall cavity blow-in.
  • Not suitable for wet applications.

Rockwool Blow-In Insulation

Rock wool, or mineral wool, is made by spinning blast furnace slag and molten rock into a light air-holding product. Rock wool resembles raw sheep’s wool or cotton candy.

  • Approximately R-3.3 per inch.
  • Requires approximately 6” to equal the R-value of an R-20 fiberglass batt.
  • Very fire resistant. It’s quite often specified in parts of a building covered by building fire codes, such as furnace room walls or walls between the house and garage.
  • Best soundproofing qualities of the three products.
  • Uses up to 75% post-industrial recycled material.
  • More expensive than fiberglass but less expensive than cellulose.

Is Blown-In Insulation DIY-Friendly?

Blown-in insulation is DIY friendly for attics. Blowing insulation into existing wall cavities–either loose-fill or dense-pack–may require a professional. Loose-fill insulation can go over existing batts and most other loose-fill products, but you should remove polystyrene beads, vermiculite, and perlite first.

How to DIY Loose-Fill Attic Insulation

Blowing loose-fill insulation into attics is easy for anyone who can remain bent over for periods of time, kneel for periods of time, and is not claustrophobic. Most home improvement stores carry the products, and some have blowing machines for rent. Be prepared to spend a day on this project. 

Prepare for loose-fill insulation by considering the following:

  • Helper. You need someone to feed insulation into the blower from the main floor or outside. It blows into the attic through a long hose. 
  • Light. Attics are rarely equipped with enough light to illuminate the furthest recesses. Use an LED headlamp, trouble light, or work lamp to see your work.
  • Protective Equipment. Blowing loose-fill insulation is dusty and can make sensitive skin itch–even with the oil that is added to reduce the problem. Wear gloves, a mask–preferably N-95–or a respirator, eye protection, and either old clothing or a disposable hazmat-type coverall.
  • Recessed Lights. Recessed ceiling lights can be very hot. Build a drywall box to put over them–then blow over the drywall. Never cover recessed lights with insulation; they are potential fire hazards.
  • Plywood Platforms. Standing and kneeling on ceiling joists or trusses gets uncomfortable. Also, they are easier to slip off of. Take two pieces of plywood–approximately 2’ x 3’–into the attic to stand or kneel on.

Blown-In Wall Cavity Insulation

Blowing wet or dry insulation in new construction projects is a job for professionals. Most builders will not have anyone but licensed and insured installers because of liability concerns.

Adding insulation after the drywall is also best for professional installers. Adding insulation to existing drywall requires someone to drill holes at the top of each cavity–from the outside if possible– and then patch them after the job is done. The loose-fill product–usually cellulose–must fit tightly above, below, and around any obstructions. The same is true for dense pack installations.