What is fiberglass insulation?

What this means for your home project

1. Look for a roll or batt description and any listed facing in the proposed fiberglass product sheet. Source: Vermont Department of Public Service, read 2026-11-06.
2. Ask for a written attic scope listing the adopted-code insulation level, so the proposed thermal resistance is explicit. Source: Building America Solution Center, read 2026-11-06.
3. Ask the installer to explain how attic air sealing precedes insulation in the proposed work sequence. Source: Building America Solution Center, read 2026-11-06.
4. Ask whether removed attic insulation is in good condition before agreeing to reinstall it. Source: Building America Solution Center, read 2026-11-06.

What are the uses of fiberglass insulation?

Fiberglass rolls and batts fit building cavities. Vermont Energy Saver describes installation "between wall or floor joists"; Building America also describes batts on a vented attic floor, with a second layer covering joists. The attic layout addresses heat flowing through the joists themselves (read 2026-11-06). (source)

“Often installed in the cavities between wall or floor joists.” (source) “When attic batt insulation is installed in two layers (Figure 1), the first layer of batts is typically installed between and parallel to the ceiling joists and the second layer is typically run perpendicular to the first layer, to cover the tops of the joists, thus stopping thermal bridging through the joists.” (source)

What is R-Value?

Fiberglass insulation’s R-value describes its resistance to heat transfer. Vermont Energy Saver calls this the ability to "reduce heat transfer" (read 2026-11-06). A higher R-value means better thermal performance, according to ENERGY STAR’s guidance captured on 2026-10-30; compare that rating when reviewing the proposed insulation. (source)

“Insulation’s ability to reduce heat transfer is rated in terms of its “resistance,” or R-value.” (source) ENERGY STAR says (read 2026-10-30): “The higher the R-Value the better the thermal performance of the insulation.” (source)

What is facing?

Fiberglass facing is a vapor-retarder layer supplied with some rolls or batts. Vermont Energy Saver explains that a retarder reduces the rate at which "water vapor can move through a material" and lists Kraft paper, foil-Kraft paper and vinyl. Fiberglass also comes without that facing (read 2026-11-06). (source)

“Sold as big rolls or batts and comes with or without vapor retarder (a material that reduces the rate at which water vapor can move through a material) facings, such as Kraft paper, foil-Kraft paper, or vinyl.” (source)

Do I need a vapor barrier?

Fiberglass vapor-control details depend on the assembly and climate. For existing vented attics, Building America specifies "climate zones 6 or higher" when discussing a Class I barrier or Class II retarder above ceiling drywall, as required by code. Ask which provision applies to the proposed attic assembly (read 2026-11-06). (source)

“In climate zones 6 or higher, a Class I vapor barrier (e.g., polyethylene) or Class II vapor retarder should be installed above the ceiling drywall as required by code.” (source)

Which type of insulation do I need?

Fiberglass and cellulose differ in material and form. Vermont Energy Saver describes fiberglass as woven glass fibers sold in rolls or batts, while cellulose is recycled newsprint, "finely shredded and chemically treated". The source describes cellulose sprayed into cavities and attics; match the material’s characteristics to the intended location (read 2026-11-06). (source)

For fiberglass, Vermont Energy Saver describes: “Glass spun into long fibers and woven.” (source) See the Vermont Department of Public Service passage quoted above. For cellulose, Vermont Energy Saver describes: “Recycled newsprint that is finely shredded and chemically treated to resist fire, corrosion, fungal growth, and vermin.” (source) For cellulose placement, “Often sprayed into wall cavities and attics.” (source) “Types of insulation materials vary, and their characteristics typically determine their best use.” (source)

How do I reach a RESNET Grade I install with batt insulation?

For attic fiberglass batts, Building America specifies "RESNET Grade 1 installation criteria" and air sealing before insulation. The guide’s table cites ICC 2009b and ICC 2012b, historical code editions. Ask the installer to identify the currently adopted code and describe the planned installation in the written scope (read 2026-11-06). (source)

“The batts should be installed in accordance with RESNET Grade 1 installation criteria (See BASC guide: Insulation Installation Achieves RESNET Grade I as well as Insulation Installation [RESNET Grade 1] - Part 1 and Insulation Installation [RESNET Grade 1] - Part 2 ).” (source) “The ceiling plane or attic floor should be thoroughly air sealed prior to installing insulation.” (source) The guide’s historical code table is described as follows: “The table below provides the minimum thermal resistance (R-value) requirements specified in the 2009 IECC (ICC 2009b) and the 2012 IECC (ICC 2012b) based on climate zone for ceiling/attic floor assemblies.” (source) For the installed insulation level, “Install batt insulation to provide adequate thermal resistance and to the levels specified in the current adopted building and energy codes.” (source)

What factors affect how much fiberglass insulation costs?

Fiberglass installation planning depends on access and installation work. Vermont Energy Saver suggests staging areas "harder to access" until another project opens them. Building America notes that attic flooring makes air sealing difficult; ENERGY STAR says insulation types require different installation skill levels. Request a scope that accounts for those tasks (read 2026-11-06). (source)

“For spots that are harder to access, it may be more cost effective to stage upgrades until another improvement project opens those areas of the building.” (source) “The presence of flooring above the ceiling joists in an attic will make it difficult to air seal at the ceiling plane (attic floor).” (source) “Different types of insulation require varying degrees of skill for installation, which can help you make an informed decision on whether to hire a contractor or take on the project yourself.” (source)

Sources

• Insulation — Vermont Department of Public Service; read 2026-11-06.
• Batt Insulation for Existing Vented Attics — Building America Solution Center; read 2026-11-06.
• Batt Insulation for Existing Vented Attics — historical code table context — Building America Solution Center; read 2026-11-06.
• Choosing the Appropriate Insulation Type — ENERGY STAR; read 2026-11-06.
• Recommended Home Insulation R–Values — ENERGY STAR; read 2026-10-30.