How Does Fiberglass Blanket Insulation Work?

Published March 26, 2018 by Whirlwind Team

fiberglass blanket insulation

Fiberglass insulation, which can come in blankets or loose-fill, is the most common and least expensive of the insulation options available for most structures, regardless of frame material. The frames and finishes of homes and buildings cannot reduce or block the transfer of heat adequately.

The addition of insulation reduces the heat transfer and decreases the load on the heating and air conditioning systems.

How Is It Made?

Fiberglass is a fiber made primarily of thin glass filaments. Molten glass flows past a series of high-pressure jets, which "blow" the glass into filaments that are then bound together with a liquid binder. The filaments form a blanket that is moved down a conveyor belt into hot ovens for curing.

After curing, most fiberglass blankets are cut into specific sizes and one side is bonded to a facing. The composition of the facing determines its permeability, flexibility and durability.

Fiberglass blanket insulation is delivered in rolls to be cut to size or as batts in standard-size rectangular pads. The insulation comes in a variety of thicknesses; thicker blankets and batts reduce heat transfer better than thin blankets. The batts and blankets can be attached to a barrier made of paper, plastic or metal sheeting to create a vapor, air or radiant barrier along the insulation exposed to the outer environment.

How Does It Work?

Fiberglass blanket insulation traps an unmoving layer of air next to a heat source, preventing the transfer of warm air towards the cooler air.

Insulation with a reflective barrier attached reflects the heat back to the source and is often used under roofing to prevent solar gains in warm climates or below floor joists to retain heat in cold climates.

Defining R-Value

R-value is a measure of the relative efficiency of slowing heat transfer. In other words, the R-value indicates the insulation’s resistance to heat flow. The R-value must be disclosed by the manufacturer and is typically found on the packaging or material data sheet.

The R-value is proportional to the insulation’s thickness.

  • One inch of fiberglass blanket or batt has an average R-value of 3.2.
  • One inch of loose-fill cellulose has an average R-value of 3.5.
  • One inch of sprayed on polyurethane foam has an average R-value of 5.9.

As you can see, higher R-values indicate greater energy efficiency and have a greater impact on the energy consumption of the structure.

Put another way:

  • 5-inch thick insulation has an R-value of 11
  • 6 inch to 6.5-inch insulation has an R-value of 19
  • 12-inch thick insulation has an R-value of 38.

The thickness of the insulation (the R-value) and the material it is made from dictate the cost of the product. Insulation without a vapor barrier cannot block moisture.

Wet insulation has no R-value until it dries and if the moisture remains for long, mold may begin to grow within the insulation, creating a health hazard and necessitating the replacement of the insulation. If the building has suffered a water leak, all insulation should be inspected for moisture and mold.

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Insulation can be placed in several areas of a building.

  • Interior cavities or on rough surfaces
  • Covered with finished such as drywall
  • Applied to unfinished walls including foundation walls, floors and ceilings
  • Fitted between studs, joists and beams where they are relatively free of obstructions

Batt and rolls are available in widths to accommodate the standard spacing of wall studs, attic trusses or rafters and floor joists.

  • 2-inch X 4-inch walls can fit R-13 or R-15 batt
  • 2-inch X 6-inch walls can fit R-19 or R-21 batt

Rolled blanket insulation is hand cut onsite and trimmed to fit the space. Insulation is available with and without facing. Facing provides the vapor barrier and helps with fastening the insulation during installation.

Facings can be made flame resistant to reduce the ignition and spread of fire. Fire retardant insulation facing is often found in basements and other areas where the insulation may be left exposed.

Cover any unfaced fiberglass with a 3-mil polyethylene vapor barrier. Fasten the vapor barrier to the wall or roof framing.

Evaluate the installation by measuring the batt thickness and checking for gaps between batts and between batts and the frame. Make sure the fit is tight around any building components that penetrate the insulation, such as electrical boxes and air conditioning lines.

Fiberglass blanket insulation is an effective yet inexpensive method of reducing the flow of heat. In cold environments, the insulation keeps the structure warmer without overworking the furnace. In hot environments, insulation blocks heat transfer into the structure. With a radiant barrier attached, the heat is reflected outward, away from the structure, reducing electricity needs.

To achieve the maximum energy ratings quoted for metal buildings, insulation must be installed in the interior walls and the ceiling or roof. Insulating the floor is optional but can help reduce heating bills in the winter. Fiberglass blanket insulation, the type most commonly used on construction projects, reduces heat transfer and, with the appropriate facing, reflects heat away from the blanket.

Fiberglass insulation is inexpensive and easy to work with. Performance is measured by the R-value, which increases with the thickness of the insulation. The thicker the insulation, the higher the R-value and the more heat transfer is reduced.

Appropriate insulation is critical to energy efficiency and savings on utility bills. It is an easily installed product that can save you money for decades to come.

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