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Insulation Alternatives: Sprayed Fiber Insulation

Cellulose, fiberglass, or mineral wool sprayed in a moist state into an open stud cavity

View the ToolBase TechSpecs- Alternative Insulation Materials PDF file for an overview of this technology.

A contractor sprays fiber insulation between wall studs using a high-power hose.

Four entries in the PATH Technology Inventory describe alternatives to conventional fiberglass batts or roll for use in open vertical cavities:

  • Non-fiberglass batts
  • Sprayed foam insulation
  • Sprayed fiber insulation
  • Blown or Foamed through a membrane.

This document deals with sprayed fiber insulation.

Cellulose, fiberglass, or mineral wool (rock wool and slag wool) is sprayed in a moist state into an open stud cavity. Moist cellulose creates its own glue, but some manufacturers add adhesive. Both mineral wool and fiberglass need added adhesive. After application, stud edges are scraped clean with a special milling tool made for that purpose. As long as the "salvaged" insulation is free of debris it can be sent back to a mixing machine for reuse. Properly installed, it completely fills the cavity, suppressing air leakage.

Sprayed-on cellulose is a mature technology. The chief concern with its use is to carefully monitor the water content of the material, and to refrain from closing in the cavity until its moisture content has dropped to a stable level, typically around 10 percent. Cellulose is treated with borate fire-retardant for fire and vermin resistance.

Fiberglass can also be sprayed into wall cavities with some water and adhesive. Although is has a slightly lower R-value than cellulose, it fills the cavity and suppresses air movement. It is sprayed at 24% moisture content, but because the fiberglass does not absorb moisture, it dries quickly.

Mineral wool (rock wool and slag wool) is sprayed into stud cavities with water and adhesive at between 10% and 20% moisture content. In place, the material is a non-combustible fire-retardant, and can be used to create 1-hour and 2-hour fire-rated walls.

Many companies make more expensive products (with substantial amounts of adhesive binder) from cellulose, wood fibers, fiberglass, or mineral wool that can be spray-applied directly to almost any substrate. These products typically are used for acoustical or thermal insulation in commercial applications, where the material is left exposed (as in a hanger, warehouse or recording studio). At least one manufacturer of sprayed-on mineral wool markets a direct spray product, using Portland cement as a binder that can be applied to crawlspace walls and left exposed.

Energy Efficiency

By completely filling wall cavities, sprayed fiber insulation can reduce air circulation within a wall cavity and create a uniform insulating layer within a wall.

Environmental Performance

Sprayed cellulose insulation is a recycled product.


With the proper equipment, sprayed fiber insulation is widely available and installation is relatively straightforward. Some materials, such as mineral wool, may be difficult to source.

Cellulose wall-spray insulation costs about $1.20 per s.f. for an R-19 wall.
Depending on the supplier and geographic regions, installed cost for in-cavity sprayed-on cellulose, fiberglass and mineral wool can be about 50% higher than a typical fiberglass batt installation.

Not Applicable

Section R316 of the 2003 International Residential Code covers insulation. All insulation products must meet ASTM standards for fire and thermal resistance.

Not Applicable

To prepare the jobsite, the area should first be swept clean (to prevent foreign debris from being drawn into the insulation hopper) and areas that should not be covered with insulation, such as windows and electrical boxes, should be taped off.

Fiber insulation material, used for open wall cavities, is sprayed through an applicator hose with water added at the nozzle. The material is oversprayed; then a wide scraper tool is drawn along the studs or joists to clean off the excess. Clean material is vacuumed back into a hopper and re-used. A typical job includes three workers: one to spray the material, one to scrape, and another to vacuum excess.

Applicators should follow manufacturer guidelines on the number of bags, blowing density, and moisture content to achieve the proper adhesion, R-value, and to reduce the chance of settling. After application, the material must be allowed to dry before being covered. Many manufacturers do not recommend vapor barriers. However, if a vapor barrier is required, it is imperative that the insulation material is adequately dry before covering.

Neither fiberglass nor mineral wool absorb water, so the water used for application (23% by weight for fiberglass, 17% by weight for mineral wool) can dry out in less than a day. Cellulose, which is sprayed at between 25% and 35% moisture content, absorbs water and dries more slowly—requiring about 36 hours drying time under normal circumstances. Regardless of the type of fiber, drying time will vary according to ambient conditions, thickness and width of that wall cavity, and the amount of moisture added during the spray.

It is important to follow manufacturer’s guidelines on the amount of moisture added while spraying: if the mix is applied too dry, it tends to fall out of the cavity; if applied too wet, it will increase drying time. However, in humid conditions, especially in cool maritime climates, the material may never dry enough to cover it safely. Spraying into an overhead cavity is problematic because of the tendency for the material to fall out because of vibration, before a finish can be applied.

Surface sprayed material can be sprayed directly against almost any clean substrate. The most common products are not abrasion resistant and, therefore, cannot be left exposed. For exposed applications, there are alternative products that contain additional adhesive or a cementitious admixture for enhanced durability. One brand of mineral wool fiber mixed with portland cement is marketed for use on crawl space walls and on basement walls when covered by a wall finish. The material is commonly used exposed in overhead applications as acoustic and thermal insulation. Most materials will adhere overhead up to 3" in thickness. Surface sprays can also be aerated on discharge to reduce their density.

Warranties vary by manufacturer; a typical warranty will offer lifetime coverage against material defects.

Properly applied, cellulose insulation completely fills the wall cavity, minimizing air circulation within the wall and providing some acoustical attenuation. No membrane or netting is necessary to hold the product in place. Mineral wool provides excellent fire resistance and the highest acoustical rating of the three materials, and it also is totally inert. Borate treatment of cellulose provides it with fire protection and vermin resistance.

Depending on the supplier and geographic regions, installed cost for in-cavity sprayed-on cellulose, fiberglass and mineral wool can be about 50% higher than a typical fiberglass batt installation. The cost of mineral wool varies with the distance from a source of minerals. Mineral wool is not typically stocked by the dealer network normally used by homebuilders.

Installed cost for surface sprayed commercial products that can be left exposed is typically higher than in-cavity sprays because they contain substantial amounts of adhesive binder.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.