b'THE OFFICIAL NFBA MAGAZINEcontinued from page: 23is found in Moisture Control Guidance for Buildingtemperature even though inside relative humidity level Design, Construction and Maintenance from USEPA. is acceptable.The inside dew point temperature being The appropriate design and construction details dependhigher than the outside air temperature is the foundation upon where the building is located and the followingfor indoor air infiltration-based moisture problems.three strategies to protect the building components: When warm, moist inside air infiltrates into an insulated 1.Vapor migrationinstalling a vapor retarder in properexterior wall, the infiltrated air will usually come in contact location. with a surface or location inside the cavity that is cooler than the dew point temperature of the inside air.When 2.Indoor air infiltrationpreventing inside air leakingwarm, moist air comes in contact with building materials into exterior walls and/or insulated ceilings. that are at or below the inside air dew point temperature, RESEARCH & TECHNOLOGYmoisture will condense out of the infiltrated air and causes 3.Outside air penetrationpreventing outside aira variety of moisture problems.Exterior insulated walls can leaking into exterior building components. be protected from indoor air infiltration-based moisture Vapor Migration problems by addressing the following construction details:Moisture that is permitted to migrate at a sufficiently highSeal all electrical boxes located on exterior walls.Air rate through an insulated building component will oftencan easily infiltrate through unsealed electrical boxes condense inside the insulated building assembly.Thisinto many types of insulation resulting in moisture condensed moisture inside the insulated assembly willproblems inside exterior wall cavities.result in a variety of moisture problems.A vapor retarderSeal all holes in the top plate of exterior walls.Sealing minimizes the moisture migration rate through an insulatedthe top plate of exterior walls reduces any inside air wall or ceiling.Moisture will migrate through an insulatedthat can leak into an exterior wall cavity because air building component from a high vapor pressure to a lowwill not enter into a wall cavity if it cannot escape vapor pressure.However the climate where the buildingfrom the cavity.is located will dictate whether the inside air has a higher vapor pressure or a lower vapor pressure.Climates whereSeal the inside exterior wall surface to the top and the inside is usually warmer than outside, the inside vaporbottom plates or use spray foam insulation to create an pressure is normally higher than outside.Conversely,air barrier at the inside insulation surface.Convection climates where the inside is usually cooler than outside,currents can develop when inside air can enter into the outside vapor pressure is normally higher than inside.an exterior wall cavity, cools and then can exit from Buildings located in Climate Zones 5, 6 and 7 should havethe bottom of the same exterior wall cavity.When a vapor barrier, with a perm rating less than two, locatedwarm inside air cools inside an exterior wall cavity, near the inside of an insulated assembly and no more thanmoisture problems will typically result.one third of the total insulation value of the assemblyInsulated ceiling assemblies can be a source of moisture should be located on the warm side of the vapor retarder.problems in attic spaces.Warm air will naturally infiltrate Buildings located in warmer climates should use a vaporup through any hole or crack in the ceiling surface system retarder, with a perm rating of greater than two, and caninto the attic space.When inside air enters the cold air in be located on either the inside or outside of the insulation.the attic, moisture carried by the inside air will typically A more detailed discussion on vapor retarders is locatedcondense out the infiltrated inside air and cause moisture throughout Moisture Control Guidance for Buildingproblems in the attic space.Adequate attic ventilation is Design, Construction and Maintenance from USEPA.generally not capable of removing the amount of moisture Indoor air infiltration moved into the attic by leaking warm air from the inside The exterior building envelope needs to be protectedof the building.Air infiltration into an attic can be from indoor air infiltrating into exterior insulated wallsminimized by addressing the following construction details:and insulated ceiling assemblies.Buildings located inSeal all attic accesses to minimize any air leakage from Climate Zones 4, 5, 6 and 7 are ones which definitelythe inside into the attic.need to minimize inside air infiltration.The air insideSeal all holes in the top plate of all interior walls.If air is typically relatively warm compared to outside air,cannot escape from an interior wall cavity, air will not especially during cold weather periods.The dew pointleak from the inside into the attic via interior walls.temperature is often defined as the temperature at which moisture will condense from air onto a surface exposedSeal all electrical fixtures and any recessed light fixtures to the air.During cold weather, the inside dew pointlocated in the ceiling.Unsealed recessed ceiling light temperature is normally higher than the outside air 24 / FRAME BUILDER - NOV2023'