Choosing A Fiberglass Insulation Type

The type of insulation you use to install will be determined by the nature of the spaces in the house that you plan to insulate.

For example, since you cannot conveniently “pour” insulation into an overhead space, blankets, spray or board products, or reflective systems are used between the joists of an unfinished basement ceiling. The most economical way to fill closed cavities in finished walls is with blown-in insulation applied with pneumatic equipment or with foamed-in-place polyurethane foam.

Here is a concise summary of the appropriate applications for the various types of thermal insulation:


FormMethod of InstallationWhere ApplicableAdvantages
Blankets: Batts or Rolls

  • Fiber glass
  • Rock wool
Fitted between studs, joists and beamsAll unfinished walls, floors and ceilingsDo-it-yourself Suited for standard stud and joist spacing, which is relatively free
from obstructions
Loose-Fill (blown-in) or Spray-applied

  • Rock wool
  • Fiber glass
  • Cellulose
  • Polyurethane foam
Blown into place or spray applied by special equipmentEnclosed existing wall cavities or open new wall cavitiesUnfinished attic floors and hard to reach placesCommonly used insulation for retrofits (adding insulation to
existing finished areas) Good for irregularly shaped areas and around obstructions
Rigid Insulation

  • Extruded polystyrene foam (XPS)
  • Expanded polystyrene foam (EPS or beadboard)
  • Polyurethane foam
  • Polyisocyanurate foam
Interior applications: Must be covered with 1/2-inch gypsum
board or other building-code approved material for fire safety Exterior
applications: Must
be covered with
Basement walls Exterior walls under
finishing (Some foam
boards include a foil
facing which will act as
a vapor retarder.) Unvented low slope
High insulating value for
relatively little thickness Can block thermal short
circuits when installed
continuously over frames or
Reflective Systems

  • Foil-faced paper
  • Foil-faced polyethylene bubbles
  • Foil-faced plastic film
  • Foil-faced cardboard
Foils, films, or papers: Fitted between wood-frame studs joists,
and beams
Unfinished ceilings, walls, and floorsDo-it-yourself All suitable for framing at standard spacing. Bubble-form suitable if framing
is irregular or if obstructions are presentEffectiveness depends on spacing and heat flow direction
Loose-Fill (poured in)
Vermiculite or Perlite
not currently used for home insulation, but may be found in older homes


When installing fiberglass insulation, it is important to know that the different forms of insulation can be used together. For example, you can add batt or roll insulation over loose-fill insulation, or vice-versa.

Usually, material of higher density (weight per unit volume) should not be placed on top of lower density insulation that is easily compressed. Doing so will reduce the thickness of the material underneath and thereby lower its R-value.

In cold climates, some low-density loose-fill insulation allows air to circulate between the top of your ceiling and the attic. This air circulation can decrease the effective thermal resistance of the insulation and may be significant for regions with more than 5000 heating degree days, or north of a line running from New York to Pittsburgh to St. Louis to Topeka to Santa Fe to Reno and up to Portland, Oregon. You can eliminate this air circulation by covering the loose-fill insulation with a blanket insulation product or with a higher density loose-fill insulation.

See Also: How is Insulation Made