Whether or not you need a vapor retarder hinges on three main factors your climate, your home and the location of the wall you're insulating. Map: Insulation Institute. What is my climate? If you live in a mixed climate — hot and humid with several heating months in the winter, you probably need a vapor retarder. Specifically, if you live in climate zones 4C marine , 5, 6, 7 and 8.
Not sure your climate zone? You can check here. What is my cladding type? According to the U. Census Bureau statistics, more than half of all new homes are clad with absorptive materials such as brick, stucco, wood, fiber cement or stone. These moisture-retaining claddings can worsen moisture challenges in the wall cavity. They have the potential to release moisture into the structure, creating an inward vapor that doesn't exist with vinyl siding.
Where is the wall located? If you are adding an exterior wall, anywhere in the house, and you answered yes, to either of the above questions you should consider using a vapor retarder. In certain climates vapor retarders can be a vital part of the wall construction.
Most authorities now agree that vapor barriers are important under certain conditions, but not necessarily as a whole-house solution for every home. In circumstances where conditions inside a home or office are much different than outdoor conditions, water vapor is likely to move through wall cavities and can get trapped inside, and a well-installed vapor barrier is recommended.
Vapor barriers can also be important for certain rooms where moisture levels are especially high. Water vapor can pass through building materials in several ways, including direct transmission, and by heat transfer, but studies suggest that fully 98 percent of the moisture transfer through walls occurs through air gaps, including cracks around electrical fixtures and outlets, and gaps along baseboards.
Thus, installing vapor barriers on wall surfaces must be done in conjunction with sealing these air flow-gaps in walls and ceilings, and along floor surfaces. Note that a poor effort at establishing a vapor barrier may be worse than no effort at all. The goal of vapor barrier strategies is to prevent moisture from collecting and corrupting building materials.
Improperly installed, a vapor barrier may actually trap moisture inside a wall, while a wall that is more porous can breathe effectively and be less susceptible to long-term moisture problems. This condition is especially problematic where vapor barriers are installed on inside as well as outside wall surfaces, as such a wall cannot breathe at all.
Once thought to be essential throughout a home or office, vapor barriers are now strongly recommended only for certain conditions, and methods for creating a vapor barrier must be tailored to fit the climate, region, and type of wall construction.
For example, the recommended vapor barrier in a home or office in a humid southern climate built with brick varies greatly from creating a vapor barrier in a cold climate in a home built with wood siding.
Always refer to current local code recommendations when deciding if and how to install vapor barriers. Avoid adding interior vapor barriers where the outer wall construction already includes a material with vapor barrier properties.
Most authorities recommend vapor barriers in certain situations:. If vapor barriers are warranted by local building practices and code recommendations, keep the following practices in mind:. To assist builders in controlling moisture, various building materials are rated according to permeability and are assigned a perms rating.
Water vapour constantly diffuses through building materials from the warm, humid interior of a house toward the cold, dry exterior. As water vapour passes through a wall, ceiling or other barrier and meets a surface that has a temperature below the dew point when water vapour condenses , then it becomes condensation — and a threat to the integrity of your building materials.
Sources: Ecohome. According to sustainability expert and architect Daniel Overbey , water vapour transmission is an important but rather confusing issue. The difference in vapour pressure between two sides of a building envelope assembly is the driving force behind vapour transmission. As the Canadian Mortgage and Housing Corporation CMHC points out, many everyday human activities such as laundry, cooking and bathing release water vapour into a building and increase its humidity.
This air then naturally seeks to find a way out of walls, ceilings, etc. The same is true for commercial buildings, even though the activities taking place inside may be different. Some may ask, is a vapour barrier necessary? In many colder North American climates, vapour barriers are a required part of building construction. You may find that vapour barriers are often not required in warmer climates. And, if installed in the wrong climate or on the wrong side of building materials, a vapour barrier can cause more harm than good.
This circumstance may prevent water vapour from drying, which in turn can cause rot and mold. Source: Dupont. For example, the National Roofing Contractors Association NRCA recommends vapour barriers on the interior side of a roof in any climate where the outside average January temperature is below 40 F 4 C degrees and the expected interior winter relative humidity is 45 percent or greater. Vapour barriers are installed along, in or around walls, ceilings and floors to prevent moisture from spreading and potentially causing water damage.
Vapour retarders also are commonly referred to simply as vapour barriers. Class II vapour retarders greater than 0. Class III vapour retarders greater than 1. The IRC divides North America into eight climate areas for the purposes of determining when a vapour barrier might be needed in a building. However, if you air-condition your house in the summer, your might trap condensation in your roof or walls for part of the year. If this is the case, be sure to use a Class-II vapour retarder on the interior of the wall.
You also can use a Class-III vapour retarder on the interior paired with spray foam insulation on the interior of the wall or roof. Source: Fine Home Building. Experts say that most condensation problems occur because of air leakage , not vapour diffusion, so be sure you properly seal penetrations such as flashings for air leakage using an air barrier.
Some compare a vapour barrier to a raincoat, whereas an air barrier is more like a windbreaker. In a lot of cases you may not need a vapour barrier, but instead use an air barrier to prevent water vapour from migrating through air currents. This is the number one way for water vapour to travel into homes and assemblies such as walls or roofs.
In fact, air flowing through holes and cracks is 30 times more likely to transport water vapour through building assemblies than through simple diffusion of the water vapour. On the other hand, a vapour barrier helps prevent the second most common form of water vapour movement: vapour diffusion.
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