Facts About Ventilation and Moisture Control

INSULATION FACTS Facts About Ventilation # and Moisture Control 11 Information from NAIMA

In this issue, we examine attic ventilation, crawl space ventilation, and vapor retarders.

oisture accumulation Attic Ventilation within a building struc- NAIMA recommends the following ture can cause prob- widely used ventilation strategies: lems such as water n Provide ventilation for each Mstains, ice damage, peeling paint, separate space to the outside with wood deterioration, mold and ventilating openings protected mildew. Water forms when water against the entry of rain and snow. vapor migrates through the structure and condenses on a cool surface. n The total net free ventilating There are three basic ways to area shall be no less than 1/150 of minimize potential water vapor the attic floor area if no vapor condensation problems in attics, retarders are used, or 1/300 of the floors and walls: attic floor area when a vapor retarder having a permeance of n Provide sufficient ventilation to one perm or less is used. reduce excessive water vapor build-up within the home; n Ventilate building sections so Steps to Determine that excessive water vapor is dissi- Attic Ventilation Needs pated to the outdoor air; n Measure area to be vented. n Use vapor retarders to limit n Divide this number by either water vapor transmission into 150 (no vapor retarder) or by 300 building cavities. (vapor retarder in place). Ventilation and circulation with n Multiply by the appropriate factor outdoor air are the major moisture from Table 1 (on the next page) to control strategies for attics and learn the total gross vent area needed. crawl spaces. In most parts of the country, passive ventilation is n Determine where vents will be important for attics and crawl located. (See examples in Figure 1.) spaces, and mechanical ventilation Cathedral, flat and hip roof/ceil- is desirable for kitchens, laundries ings with vapor retarders need a and baths. All of these strategies vent area of 1 sq. ft./300 sq. ft. of are designed to remove water ceiling. Total ventilation may be vapor from the structure so that achieved by placing half of the condensation will not occur. required vent area at each eave. Vapor retarders limit the migra- Without a vapor retarder, the vent tion of water vapors from warmer area should be doubled. areas to the cold surfaces in walls, The most effective ventilation is roof/ceilings and sometimes areas provided by a combination place- below floors. Vapor retarders ment of the vents with 50 percent reduce the amount of water vapor of the area at the roof peak and 50 available for condensation. percent at the soffits or eaves.

Table 1 tions reflect the current prevalent Obstruction Chart building code requirements. A rectangular crawl space TYPE OF COVERING AREA OF OPENING

1 requires a minimum of two vents, ⁄4” hardware cloth 1 times required net free area

1 located no farther than 3 ft. from ⁄4” hardware cloth and rain louvers 2 times required net free area corners. The vents should be as 1 ⁄8” mesh screen 1.25 times required net free area high on the wall as possible to best 1 ⁄8” mesh screen and rain louvers 2.25 times required net free area capture breezes, and landscaping 1 ⁄16” mesh screen 2 times required net free area should be planned to prevent 1 ⁄16” mesh screen and rain louvers 3 times required net free area obstruction of the vents. The total Caution should always be exercised to assure that ventilators are not obstructed. free (open) area of all vents should Blocking or baffles should be used at the eaves to keep insulation material away from be at least 1/1500 of the floor area. soffit vents and provide a 1” free airspace. In the absence of a ground cover, the vent area should be increased Net Free Vent Area laying the membrane. Overlap to 1/150 of the floor area. The stated vent areas for attics, edges 4 to 6 in., and continue the Ventilation alone should not be roof/ceilings, and crawl spaces membrane up the face of the wall relied upon where soils are known refer to net free areas through to the grade level if the interior to be moist. which air can pass unobstructed. grade is below the outside grade. When screening, louvers, or Traditional building practices rain/snow shields cover vents, the and building codes have included Steps to Determine Crawl area of vent openings should be provisions for crawl space ventila- Space Ventilation Needs increased to offset the area of the tion. In recent years, some n Measure square footage to be obstruction. Use manufacturers’ researchers are finding that crawl ventilated. stated net free areas whenever space vents may not be necessary, available. If no information is avail- especially in southern climates n If ground cover vapor retarder able, use the conversion factors for where vents can introduce hot, is not used, divide the square determining gross area of vent humid air into the crawl space. As footage by 150 to determine openings listed in Table 1. Caution more information becomes avail- needed vent space. Then consult should always be exercised to able, building codes may change the obstructions chart and multiply assure that ventilators are not their ventilation requirements. The venting by the number given for obstructed. Blocking or baffles following vent area recommenda- the type of material used. should be used at the eaves to keep insulation material away from Figure 1 soffit vents. Recommendations for Location of Attic and Roof Vents air space between the roof sheath- ing and the insulation may vary. As a general rule of thumb 1” is often used. (See Figure 2.)

Crawl Space Ventilation Recommendations The Building Foundation Design Handbook (Labs et al. 1988) notes that ground cover membranes such as 6-mil (0.006 in.) polyethyl- ene that restrict evaporation of soil Figure 2 moisture are the single most Clearance of Eave Vents important way to prevent conden- sation and wood decay problems in crawl spaces. The ground cover material should have a perm rating of no more than 1.0 and must be rugged enough to withstand foot and knee traffic. All debris must be removed and the soil should be leveled before

If ground cover vapor retarder What Does A Vapor retarders in ceilings. When it is is used, vented area may be Retarder Do? convenient to do so, vapor reduced to 1/1500 of the floor area. Occupants of buildings, appli- retarders should be installed in ances, and plumbing equipment existing buildings. In some cases n Determine location of needed generate moisture that is carried in local building codes may conflict vents. (See Figure 3.) the air as a vapor. As moisture with common building practices vapor moves from a warmer inte- regarding the use of vapor Figure 3 rior through construction materials retarders. Always confer with local Location of to a cooler surface, the moisture building departments before begin- Crawl Space Vents may condense as water, which ning construction. may damage the building. It is for this reason that vapor retarders, which retard the flow of moisture Cautions! through construction materials, are Always follow these rules when installed in buildings. By locating working with vapor retarder-faced vapor retarders as close as possible insulation: to the warm-in-winter (heated inte- n Many facings are flammable. rior) surface of the building, the Because of this, when vapor moisture vapor flow is restricted retarders face building spaces before it has a chance to condense intended for human use, they must to water. (See Figure 4.) Vapor Retarders be covered. These spaces include garages, storage rooms, utility A vapor retarder (barrier) is rooms and laundries. NAIMA rec- defined by ASTM C 755 as a mater- Vapor Retarder-Faced ommends that this statement be ial that has a water vapor perme- Insulations printed on all combustible facings: ance (perm) rating of one (1) or The vapor retarder facing less. The terms vapor retarder and should be installed toward the “This vapor retarder is flam- vapor barrier are used interchange- warm-in-winter side of the con- mable and should not be left ably and describe the same materi- struction except where noted. This exposed. Special care should be als. For consistency, the term means that in ceilings the vapor taken when working close to the retarder will be used here. retarder faces down, in walls it facing with an open flame.” faces the inside, and in floors over unheated spaces it faces up. With (For exposed applications, insu- Figure 4 the exception of reverse-flange lations with special, strong, rein- Water Vapor Movement and insulation, the stapling flanges are forced flame resistant facings are Vapor Retarder Location on the same side as the vapor available.) retarder. In warm, humid climates n Batt and blanket insulations local practice should be followed may be installed one on top of the regarding the use of and/or place- other in ceilings where there is ment of vapor retarders. adequate space. Only the bottom Note: The ASHRAE Handbook layer should have a vapor retarder, of Fundamentals, Chapter 23, which should face down toward Thermal and Moisture Control in the space that is warm-in-winter. Insulated Assembly Application, Additional layers should be contains information on the unfaced. If unfaced insulation is placement and use of vapor not available, use the faced type retarders in various climates but remove the facing completely including humid climates such as before installation. Florida, the Gulf Coast, or Hawaii. n Repair damaged vapor retarders. Rips or tears in the vapor When Is A Vapor retarder facing may be repaired by Retarder Required? covering the damaged area with Good construction practice calls scrap vapor retarder material and for installing vapor retarders in taping it in place or, in the case of walls of all new buildings. Local small rips, by using duct tape or building codes should be followed polyvinyl tape. Gaps around win- with regard to the need for vapor dows, doors and other openings

should be covered with vapor References About NAIMA retarder facing or taped. For more information consult NAIMA is a trade association of n Avoid electrical wiring when the following: North American manufacturers of stapling vapor retarders or facings. fiber glass, rock wool, and slag wool 1. ASHRAE Handbook of Care should be taken not to staple insulation products. NAIMA’s role is Chapters 20 into electrical wiring when Fundamentals, to promote energy efficiency and and 23, American Society of environmental preservation through installing faced batts and blankets the use of fiber glass, rock wool, and Heating, Refrigerating and Air or separate vapor retarders. slag wool insulation products and to Conditioning Engineers, Inc. encourage safe production and use Atlanta, GA. of these insulation products.

2. C755-Selection of Vapor For additional information on Other Vapor building insulation contact: Retarder Materials Retarders for Thermal The use of separate vapor retarders Insulation, ASTM, NAIMA 44 Canal Center Plaza, Suite 310 with unfaced insulation is becom- Philadelphia, PA. Alexandria, VA 22314 ing more common. They should Tel: 703/684-0084 3. Covering Vapor Barriers, face the warm-in-winter side of the Fax: 703/684-0427 framing. Four-mil, or thicker, poly- NAIMA, 1988, Alexandria, VA. E-mail: [email protected] Website: http://www.naima.org ethylene sheeting, available in 4. Mineral Fiber Batts and rolls, is rolled out horizontally and Blankets, NAIMA, 1989, stapled to the face of the framing, NAIMA BUILDING INSULATION Alexandria, VA. at top, bottom and the side. If one COMMITTEE MEMBERS: sheet of poly is not wide enough 5. Loose Fill Mineral Fiber, CertainTeed Corporation to cover the area and a second NAIMA, 1989, Alexandria, VA. PO Box 860 sheet of polyethylene is required, Valley Forge, PA 19482 800/233-8990 they should be overlapped on the 6. Moisture Control Handbook, stud space where they meet and ORNL/SUB/89-SD350/1, Oct. Johns Manville stapled to adjoining studs. ‘91, Oak Ridge National PO Box 5108 Denver, CO 80217-5108 Foil-backed gypsum board is Laboratory, Oak ridge, TN. 800/654-3103 also an effective vapor retarder. Other options for providing a 7. National Center for Knauf Fiber Glass vapor retarder include gypsum Appropriate Technology, One Knauf Drive Moisture and Home Energy Shelbyville, IN 46176 board manufactured with a 800/825-4434 vapor retarder or vapor retarder Conservation, 1983, Butte, MT. type paint, which may be used Owens Corning 8. Building Foundation Design One Owens Corning Parkway in renovations. Handbook, Oak Ridge Toledo, OH 43659 800/GET-PINK National Laboratory, 1988. Additional, more current, references

Figure 5 suggested include the following: Vapor Retarder Installation 9. Builders’ Manual, Canadian Home Builders Association, Ottawa, Ontario, 1994.

10. EEBA Field Guides for Different Climates, Energy Efficient Building Association, Minneapolis, MN, 1997.

11. CABO One & Two Family Dwelling Code, 1995 Edition. The Council of Building Officials, Falls Church, VA 1995.

12. “Moisture Control Handbook,” J. Lstiburek, J. Carmody 1991.

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