Information Brief - Wet Spray and Dense Pack Insulation

This informational brief covers wet spray and dense pack installation techniques for both fiberglass and cellulose insulation. Some basic characteristics of these insulation types are discussed, including common densities and tested R-values. The specific benefits and drawbacks are also discussed. Consideration should be given to these factors when considering wet spray or dense pack insulation for specific applications.

Common Types of Wet Spray and Dense Pack Insulation

Wet spray and dense pack cellulose insulation is typically installed or applied in a range of densities from approximately 3.0 lbs/ft3 to 4.0 lbs/ft3. Installations below 3.6 lbs/ft3 are typically wet spray, installed with a wet adhesive or water because cellulose at this density is liable to settle unless it is fixed with adhesive or applied when wet. Installations at or above 3.6 lbs/ft3 are termed dense pack and can be blown dry into a closed stud cavity. Because of the high density, settling is not an issue. The R-value of wet spray and dense pack cellulose insulation depends slightly on its installed density and the exact type of cellulose used, but commonly ranges between 3.6 and 3.8 per inch1 (as measured by the standard ASTM C518 test). The R-value per inch decreases slightly as density is increased from 3.0 to 4.0 lbs/ft3.

Wet spray and dense pack fiberglass insulation is typically installed to densities between 1.8 and 2.3 lbs/ft3. In wet spray applications, wet adhesive is added to the fiberglass to help it stay in place. This application is frequently used in new construction with open stud cavities. However, some new fiberglass products are formulated using extra small fibers that resist settling without the use of adhesive. These can also be sprayed into open cavities in new construction. Dense pack fiberglass can be blown dry into a closed stud cavity. This installation is more common for existing construction undergoing an insulation retrofit. Typically, dense pack installations use a higher density application. The R-value of wet spray and dense pack fiberglass insulation increases with increasing density and can reach 4.2-4.3 per inch at the highest densities (as measured by ASTM C518).

Benefits and Drawbacks
In general, wet spray and dense pack insulation provide a number of benefits over traditional batt insulation products. One potential benefit for existing structures is the ability to insulate without removing interior drywall or exterior cladding to gain access to stud cavities. Due to the greater installed densities, R-values per inch are significantly higher than batt products, especially for fiberglass options. However, high-density fiberglass batt products with improved R-values are also increasingly available. Perhaps a greater benefit is the ability to completely fill stud cavities. Compared to batt products, blown-in insulation offers a much improved ability to thoroughly insulate behind wires, pipes, and in difficult-to-reach places and non-uniform cavities. Researchers at Oak Ridge National Laboratories have found that ill-fitting and compressed batts, resulting from improper installation, wires, piping, and non-uniform stud cavities, can reduce the stated R-value of batt insulation by up to 15%.2 A further advantage of dense pack and wet spray insulation is the air sealing benefits they offer. While neither dense pack fiberglass nor cellulose insulation alone qualify as an air barrier, both can help reduce the air infiltration through leaky walls that is commonly found in older homes.

One drawback to wet spray insulation is the added time required for the insulation to dry before wall cavities can be enclosed. Drying times depend on ambient temperature and humidity conditions, but can be 36 hours or more. Wet spray fiberglass typically dries out more quickly. If wall cavities are enclosed before the insulation is allowed to reach a balanced moisture content, excess moisture trapped in the cavity can encourage the growth of mold, creating indoor air quality and durability issues.

Recommended Applications
Wet spray and dense pack insulation are commonly used in areas where space is an issue and the maximum R-value is desired in a relatively thin layer. This makes their use ideal for walls and cathedral ceilings. These installation techniques are rarely used in attics because space for extra insulation thickness is not an issue.

Dense pack insulation is ideal for insulation retrofits in older homes. For instance, it can be blown into closed stud wall cavities with minimal disruption to the exterior cladding or interior plaster. In addition, because the stud cavities are fully closed, it can be blown at high densities, which can improve the R-value of fiberglass insulation and prevent future settling. Finally, since the exterior walls of older homes are commonly constructed with boards rather than sheet goods and don't typically have a continuous air barrier, air infiltration through the wall is often a significant issue. In this situation, the air sealing benefits of dense pack cellulose or fiberglass can significantly reduce air infiltration.

Wet spray insulation is recommended for new construction. Because of the lack of closed stud cavities, dense pack insulation is rarely used. Instead, insulation contractors must use either a wet spray application and shave off excess insulation, or staple a strong membrane to the wall first, which can hold dry-packed insulation in place. This membrane system is commonly called a "blown-in-blanket", or BIBs system. However, both wet spray and BIBs installations lose some of the benefits dense pack insulation confers in existing construction. For instance, with both wet spray and BIBs installations, the insulation density cannot match the density used in retrofit situations with enclosed stud cavities. This can reduce the R-value of the insulation when using fiberglass. More importantly, since new exterior walls are commonly built with sheet goods and polyethylene air barriers, air infiltration through walls represents a small component of overall house air leakage.3 In this situation, the installation of high-density cellulose or fiberglass insulation has virtually no effect on air infiltration.4 To significantly reduce air infiltration in a new house, a more robust air barrier like closed-cell spray foam or a completely different construction system like SIP panels or ICFs, that completely air seals joints in the exterior wall, is recommended.

Comparing Fiberglass and Cellulose
There are some important differences between cellulose and fiberglass insulation.

Compared to traditional fiberglass batts with an R-value of 3.2-3.3/inch, dense pack and wet spray cellulose have higher R-values, at 3.6-3.8/inch. But dense pack fiberglass, such as Johns Manville Spider insulation, can have R-values up to 4.2-4.3/inch. However, the R-value of fiberglass insulation varies more significantly with changes in density and, to achieve these high R-values, the insulation must be blown to densities of at least 1.8 lbs/ft3.

Mold and Moisture
To avoid moisture and mold-related problems, it is important to understand the wall being insulated. Is it new or old? Does it have a vapor barrier? What is its overall condition in terms of holes and leaks (on the inside and outside)? Homes with potential moisture problems should not be retrofitted with additional insulation of any type until the moisture issues are identified and thoroughly addressed.

In new construction, a properly designed exterior wall with carefully constructed moisture management details on both interior and exterior surfaces will prevent the vast majority of water and water vapor from entering the wall assembly. It will also allow drying and safe storage of the small amount of water or moisture that does enter the wall. In this case, neither fiberglass nor cellulose will encourage mold growth or create problems for long-term durability of the wall.

However, many walls, especially those found in older homes, allow significant amounts of water and water vapor to migrate in and out of the wall. For older walls, this is not usually a problem because there is little insulation and therefore plenty of heat available to help the wall dry out. However, when new insulation is added to an older wall, its drying potential is decreased and mold and rot can become a problem. For several reasons, fiberglass insulation may exacerbate an existing moisture problem. Although the fiberglass itself does not support mold growth, it has a very limited ability to store water. This essentially concentrates moisture on the most important and susceptible components of the wall, the wood sheathing and studs. On the other hand, cellulose provides some protection because it has a greater capacity to store moisture. Cellulose absorbs and transports moisture away from a specific leak, spreading the moisture out and storing it safely at lower levels until it can be dried out. For this reason, cellulose is often a safer choice for insulation retrofits in older homes that may have minor moisture problems. Nonetheless, it is important to seal holes in the envelope, but recognize that it may be financially prohibitive and physically impossible to find and seal every hole or create a true air and vapor barrier. In renovation applications, it is important to work with an experienced contractor who understands the wall as a system.

1 Yarbrough, David. Oak Ridge National Laboratories (ORNL). Personal correspondence, November 2009.

2 Christian, Jeffrey and Jan Kosny. "Calculating Whole Wall R-Values on the Net." Home Energy Magazine Online. November/December 1999.

3 Colliver, D.G. "Evaluation of the techniques for the measurement of air leakage of building components." Final report of ASHRAE Research Project RP 438.

4 Johns Manville, Technical Bulletin FGBI 43. April 13, 2005.