What is the Purpose of a Vapor Barrier – A Homeowners Guide

insulation vapour barrier

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Vapor Barrier – Unseen Yet Essential

Depending upon where you live and what materials you are using for your construction project, a vapor barrier can be a very important part of your structure. And since most vapor barriers are not obvious to the homeowner, they are an often neglected part of many do-it-yourself construction projects. Even if they are put in place during construction, if they are not installed correctly, a vapor barrier can easily do more harm than good. But before you start wrapping your structure with plastic; let’s get familiar with vapor barriers and what they are used for.

What is a Vapor Barrier?

By basic definition, a vapor barrier is any material that is designed to prevent the flow of air into a structure. The main idea behind preventing air flow through floors, walls and ceilings is so that the water vapor that is contained in the structures interior air doesn’t condensate as it flows out of the structure. That’s why a vapor barrier is so important—they do exactly as their name implies—they prevent water vapor from entering into the structural envelope where it can cause significant problems and permanent damages.

remodeling vapor barrier

Basic Diagram of Vapor Barrier Installation

There are three main reasons for installing a vapor barrier and are as follows:

  • To protect the envelope of the structure from water damages resulting from condensation
  • To prevent air flow through the building envelope
  • To maintain interior humidity levels

What is Condensation and Why Should I Be worried about it?

Condensation may not seem like a big deal—especially since you can’t see it—but without a good vapor barrier in your structure, condensation can rot wood, crack concrete and rust metal parts in the interior cavities of the structural envelope. But the effects of condensation can easily be seen under the right conditions.

If you’ve ever filled a glass with a few ice cubes on a hot day, you’ve seen the effects of condensation on the outside of the glass as water droplets form from the vapor in the ambient air supply. The same effect can take place inside your structures walls. For instance, as warm air from the inside of your home leaks through the permeable materials of your wall (i.e. drywall, plywood, concrete, etc.), as the warm air gets closer to the cooler air on the outside of the home, it drops in temperature. As the warm air starts to cool and it reaches the ambient temperature of the exterior dew point, the water vapor begins to condense; typically inside of the structure where it is absorbed into the permeable materials and begins to cause damages.

Vapour barrier system

Vapor Barrier in Cold and Warm Environments

Where do I Install a Vapor Barrier?

Condensation only works in one direction—warm air turns cold and condensates. Typically, the vapor barrier will be installed on the warm side of the insulation—in the interior of the structure over the insulation. If a vapor barrier is installed on the cold side of the insulation, it will trap moisture inside of the insulation. And when insulation gets wet, it lowers its R-value, increasing the effect of condensation even further.

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To compound the matter even more, depending upon the structures location, you may or may not need to place the vapor barrier on the warm side of the structures envelope. In cold locations, it’s always best to place the vapor barrier on the warm side of the home. But in areas where high humidity and fluctuating high temperatures are a problem, it’s best to simply leave the vapor barrier out altogether. If a vapor barrier is installed in a humid and hot local area, it’s best to use structural materials in the envelope like glass, plastic, vinyl or aluminum that can withstand periodic moisture without decaying or rotting away.

With that reasoning, it seems to make perfect sense then that you should install a vapor barrier on both the inside of the home and outside. However, that assumption can seriously damage the structures envelope by trapping water vapor inside of the two barriers permanently and should be avoided at all costs.

renovation vapour barrier

Renovation showing Vapor Barrier

What Materials do I Use for a Vapor Barrier?

A vapor barrier is by definition an impermeable membrane that prevents water vapor from condensation inside of the envelope. That really means that any materials that are impermeable to water intrusion can be used as a vapor barrier. A good example of a commonly used vapor barrier material is polyethylene plastic film between .002 and .008 mil thick. Because it’s so cheap, easy to use and transparent, it is commonly employed as a vapor barrier in many homes.

insulation vapour barrier

Vapor Barrier Installation

Polyethylene plastics are easily attached to the interior wall and ceiling studs after the insulation is in place using staples and sometimes mastic. Once in place, the interior cladding can be applied in the usual manner.

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Other materials can be used as a vapor barrier as well. Extruded foam and rigid foam boards double as a vapor barrier as well as insulation, essentially completing two tasks with one product. However, this only works if the foam boards seams are covered using an impermeable material such as foil tape or butyl tape.

House wrap materials are NOT vapor barriers and should never be used as such. These materials are designed to be placed on the outside of the structure and are permeable. They are strictly used for air flow prevention and can cause serious moisture problems if they are used on the interior of a structure.

vapor barrier vancouver home

Ventilation and Vapor Barrier

Ventilation and Your Vapor Barrier

So what happens to all of that water vapor that’s in the air that gets blocked out by the vapor barrier? It still turns into condensation if the air is not properly ventilated out of the structure. When installing conventional insulation, a path for the water vapor to escape is required and is usually done in the form of attic and soffit ventilation. Soffit and ridge ventilation is essential to alleviating vapor from the envelope of any structure whether a vapor barrier is installed or not. In cases where a permeable material is on the exterior of the home (i.e. brick, stucco or concrete) moisture is pulled into the home versus being pulled out of the home. Ventilation and drainage in crawlspaces are essential to preventing damages from water infiltration from ground sources.

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