Why vapour barrier ceiling

This barrier must be applied onto the inner side of the heated zone, in order to prevent damp air from penetrating the insulation. When you are insulating the house on the inside, a vapour barrier is always required. The only exception to this is insulation which is already provided with a vapour resistant layer. In this case, it is important to seal the gaps and seams with a special vapour barrier tape to make sure that vapour still cannot get through.

The weakest points of a house are the ceilings, together with damp rooms like the kitchen and the bathroom. It is advised to ventilate these rooms sufficiently.

A vapour barrier can easily be realised with a plastic foil like Polythene at least 0. This can be attached with staples beneath the rafters or trusses on the inside. The seams must be sealed with tape and the edges of the foil should overlap a little a few inches. If the roof is insulated with foil-faced insulation blankets, the aluminium side can serve as a vapour barrier at the same time.

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.

Condensation is caused when warm air cools as it moves through building materials like insulation and drywall. Source: Ecohome. So, although a vapour barrier must be continuous, unlike an air barrier, a vapour barrier does not need to be as tightly sealed. These are often used in humid, southern climates where moist exterior air is frequently found.

Vapour retarders frequently are used in flat roof construction to prevent moist air from the inside of the building condensing onto the roof assembly and potentially causing damage to materials. Source: NRCA. Mark the plastic sheeting with a marker to create a cut guide. Always cut it slightly larger than necessary to avoid mistakes.

Using a straightedge to guide you, cut the plastic to size with a utility knife or scissors. The process is similar to cutting wrapping paper from a roll of gift wrap. Lift the moisture barrier and hold it against the ceiling in one corner. Position it so that several inches extend past the ceiling, onto either wall formed in the corner. Attach with staples every 4 to 6 inches in the corner. Work across and back from the original corner, pulling the vapor barrier taut and stapling to the joist face every 8 to 10 inches.

Don't pull the vapor barrier enough to strain the plastic, which will enlarge the staple punctures, but don't allow wrinkles and sagging either. Overlap any seams created by additional pieces of barrier about 12 inches. Some debate still occurs over how necessary vapor barriers are, but a consensus is growing closer.

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.