Condensation Fact Sheet METAL BUILDING MANUFACTURERS ASSOCIATION The Condensation Process In metal building systems, we are concerned with two different Condensation occurs when warmer moist air comes in contact areas or locations: visible condensation which occurs on with cold surfaces such as framing members, windows and surfaces below dew point temperatures; and concealed other accessories, or the colder region within the insulation condensation which occurs when moisture has passed into envelope (if moisture has penetrated the vapor retarder). Warm interior regions and then condenses on a surface that is below air, having the ability to contain more moisture than cold air, the dew point temperature. loses that ability when it comes in contact with cool or cold surfaces or regions. When that happens, excessive moisture Signs of visible surface condensation are water, frost or ice on in the air is released in the form of condensation. In metal windows, doors, frames, ceilings, walls, floor, insulation vapor buildings, there are two possible consequences of trapped retarders, skylights, cold water pipes and/or cooling ducts. To moisture in wall and roof systems: (1) corrosion of metal effectively control visible condensation, it is necessary to reduce components and (2) degradation of the thermal performance of the cold surface areas where condensation may occur. It is also insulation. important to minimize the air moisture content within a building by the use of properly designed ventilating systems. Dew Point and Relative Humidity Dew point is the temperature at which water vapor in any Signs of concealed condensation include damp spots, stains, static or moving air column will condense into water. In other mold and/or mildew on walls or ceilings, delamination of words, the air is saturated and can no longer hold the moisture laminated surfaces, bubbles or blisters in asphaltic surfaces, at this temperature. When the air temperature drops below peeling paint, and damp insulation. Concealed condensation is its dew point, excess moisture will be released in the form of the most difficult to deal with and can be the most damaging. condensation. Condensation problems are most likely to occur This type of condensation may be controlled in metal buildings in climates where temperatures frequently dip to 35°F or colder by the proper use of vapor retarders and by minimizing moisture over an extended period of time. content within the building by proper ventilation. Additional condensation control can be accomplished by venting the Relative humidity is a percentage measurement of the amount cavities of the walls and roof. of water vapor present in the air in relation to the amount it is capable of holding at that temperature. For example, 50% Vapor Retarders relative humidity indicates the air is carrying one-half of the A vapor retarder is used to inhibit the passage of warmer maximum amount of moisture that it is capable of containing at moist air into the inner regions of the roof or wall system. The the given temperature. There is a relationship between the dew proper selection and installation of the vapor retarder can help point and relative humidity. A high relative humidity means that control condensation problems in a building. Vapor retarders the dew point is near the current air temperature. Therefore, a are rated by the amount of moisture that can pass through relative humidity of 100% indicates that the dew point is equal them. The lower this rating, called a perm rating, the less vapor to the current temperature. This relationship between dew point transmission will occur and the more effective the vapor retarder and relative humidity is given in Table C-1. will be. Water vapor transmission rates (perms) are determined using ASTM E 96, Standard Test Methods for Water Vapor Visible and Concealed Condensation Transmission of Materials (ASTM, 2005). Two things must be present for condensation to occur: warm moist air, and cool surface temperatures below the dew Types of Vapor Retarders point. The proper control of these two factors can minimize There are various types of vapor retarders available, such as: condensation. 1. Structural membranes, including rigid steel sheets or other Dew Point Temperature (oF)1 Relative Design Dry Bulb (Interior) Temperature Humidity 32oF 35oF 40oF 45oF 50oF 55oF 60oF 65oF 70oF 75oF 80oF 85oF 90oF 95oF 100oF 100% 32 35 40 45 50 55 60 65 70 75 80 85 90 95 100 90% 30 33 37 42 47 52 57 62 67 72 77 82 87 92 97 80% 27 30 34 39 44 49 54 58 64 68 73 78 83 88 93 70% 24 27 31 36 40 45 50 55 60 64 69 74 79 84 88 60% 20 24 28 32 36 41 46 51 55 60 65 69 74 79 83 50% 16 20 24 28 33 36 41 46 50 55 60 64 69 73 78 40% 12 15 18 23 27 31 35 40 45 49 53 58 62 67 71 30% 8 10 14 16 21 25 29 33 37 42 46 50 54 59 62 20% 6 7 8 9 13 16 20 24 28 31 35 40 43 48 52 10% 4 4 5 5 6 8 9 10 13 17 20 24 27 30 34 1 Chart adapted from ASHRAE Psychometric Chart, 2005 ASHRAE Handbook of Fundamentals Table C-1: Dew Point Temperature (oF)1 verses Relative Humidity impermeable materials. The list below is not exhaustive, nor is it Besides the most popular facings, the industry provides special an effort to limit the designer, but these membranes may include facings that have high impact resistance for gymnasiums, facings the following in roof and/or wall construction: for high UV applications and black colored facings for structures a. Steel formed panels, properly sealed on edges and where ceilings are not used. ends. b. Steel, zinc alloy, copper or aluminum sheets with caulked Sealing Vapor Retarders and formed standing seam edges and ends. Careful attention must be paid to the insulation seams at side c. Vinyl siding applied to suitable structural substrate or and end-laps to maintain the integrity of the vapor retarder. sheathing. Some common methods used to seal the seams include rolling d. Tilt-up concrete panels, suitably sealed at end and side and stapling the side-laps, peel and stick tabs at side-laps, and laps, with a painted exterior. sometimes insulation tape. The use of insulation tape alone to e. Vinyl, metalized plastic and similar overlays on rigid seal the seams may not be advisable. Job site conditions such as board insulation. humidity, dirt, and access to the underside of the insulation can f. Foam insulated metal panels, caulked and sealed at ends make this difficult. Where used, tape should be at least the same and side laps. quality as the vapor retarder, and the tape should be approved by g. Bituminous spray or trowel-on coating on concrete or the insulation supplier for the particular product. In the case of a masonry. membrane type retarder, any punctures or tears in the material should be repaired using the self-adhesive repair tape supplied 2. Flexible membranes, such as foils, coated papers, or plastic with the insulation. More information and downloadable literature films. Usually, these membranes are rated by “perm” of 1.0 or can be found on the NAIMA website via http://www.naima.org, less, per ASTM E 96, Standard Test Methods for Water Vapor or visit National Insulation Association (NIA) at Transmission of Materials (ASTM, 2005). The most common http://www.insulation.org. applications for metal buildings are membrane retarders laminated to fiber glass blanket insulation. Plain white vinyl with a Because there is a growing trend to add insulation to existing perm rating of 1.0 is not an effective vapor retarder, especially in metal buildings, a proper vapor retarder is of critical concern. buildings with a high relative humidity. One of the most common methods is to add an additional layer of insulation to the bottom flange of the purlin system. This can 3. Coating membranes, which includes paints, trowel-on create an air space where moisture laden air can accumulate bituminous coatings, epoxy and urethane foams. if the integrity of the new vapor retarder closest to the warm insulating surface of the building has not been maintained. It Laminated facings for fiber glass batts serve several purposes is important that an intact vapor retarder is not left within the other than appearance. They prevent the blanket from sagging. insulation mass. They prevent most water vapor from penetrating the fiber glass batt. They provide resistance to impact and provide reflectivity Ventilation and emissivity benefits. Most facings have three parts: All metal buildings require some level of ventilation, and more often this ventilation is becoming the responsibility of the 1. A base made up of natural or white kraft paper or metal building contractor. A lack of ventilation can create an aluminum foil. uncomfortable working condition through elevated heat levels, 2. A fiber glass scrim netting is provided for reinforcing and excessive humidity, and stale air. It can also contribute to sag prevention. condensation problems. Ventilation can best be represented 3. An exterior film made up of polypropylene, vinyl, metalized by the number of times per hour the building air is replaced polyester, or aluminum foil is applied on top. with outside air. This is referred to as air changes per hour. The number of air changes required per hour widely varies per Table C-2 lists some of the most popular types of facings and application. shows their water vapor transmission rates (perm ratings). The North American Insulation Manufacturers Association (NAIMA) The following example illustrates a calculation for ventilation recommends that metal building insulation be faced with a vapor requirements: retarder having a permeance of not greater than 0.10 perms.