Exterior Paint: Effective Exterior Vapor Barrier Solution?

The use of exterior paint as an exterior vapour barrier is a highly debated topic. While some people argue that exterior paint can act as a vapour retarder, others believe that it is not sufficient and a poly vapour barrier is necessary. Ultimately, the decision depends on various factors, including the climate and construction of the home. In this paragraph, we will explore the different perspectives on this topic and provide insights into the effectiveness of exterior paint as a vapour barrier.

Latex paint can be used as a vapour barrier

The classification of a material as a vapour barrier or a vapour retarder is determined by how much water vapour passes through the material under specific conditions. Latex paint can be used as a vapour barrier in some climates, but it is not a fixed rule. Building practices should always be determined by the climate zone in which one is building.

In hot and humid climates, a vapour barrier should be installed on the outside of the insulation. In cold climates, the vapour barrier should be on the inside of the insulation. In both cases, the vapour barrier is tasked with preventing warm, humid air from shedding its moisture as it meets a cool surface.

There are several latex-based paints on the market that can be used as a vapour barrier, including Sherwin-Williams' Moisture Vapor Barrier Interior Latex Primer/Sealer, Benjamin Moore's Super Spec Latex Vapor Barrier 260, and Start Right Interior Latex Vapor Barrier Primer.

Oil paint can be used as a vapour barrier

However, it is worth noting that oil paint is not commonly used anymore due to its negative health and environmental impacts. There are now more environmentally friendly alternatives available, such as latex vapour barrier primers, which can be just as effective as oil paint in preventing moisture from absorbing into wall cavities.

When considering a vapour barrier, it is important to also think about the air barrier. The air barrier controls air leakage, while the vapour barrier controls vapour diffusion. Well-sealed drywall can act as an effective air barrier.

In addition, the decision to install a vapour barrier and the type of vapour barrier used should be informed by the climate of the region. For example, in hot and humid climates that rely on air conditioning, a vapour barrier on the interior of the wall may not be ideal as it can trap moisture.

Polyethylene is a common vapour barrier

Polyethylene is classified as a vapour barrier or a vapour retarder depending on its vapour permeance. The National Building Code of Canada stipulates that for residential buildings, a vapour barrier must have a water vapour permeance of less than 60 ng/Pa*s*m2 or 1.0 Perm. Polyethylene typically falls under the classification of a vapour barrier, with a vapour permeance rating of 3.4 ng.

Polyethylene is commonly used as a traditional approach to preventing water vapour from penetrating walls in homes. However, it is more suitable for extreme northern communities in Canada and may not be ideal for warmer climates. In the US, any material with a perm rating of 1 or less is considered adequate for residential construction.

While polyethylene is a widely used vapour barrier, there are alternative options available, such as latex vapour retarder paint, which can be more effective and provide cost savings.

The effectiveness of a vapour barrier depends on the climate

The number of heating degree days (HDD) in an area is typically used to determine the need for a vapour barrier. In cold climates, a vapour barrier is placed towards the interior, heated side of insulation in the assembly. In humid regions where warm-weather cooling predominates within buildings, the vapour barrier should be located towards the exterior side of the insulation. In relatively mild or balanced climates, or where assemblies are designed to minimise condensation, a vapour barrier may not be necessary.

In cold climates like Canada, the vapour barrier should be on the inside of the insulation for most of the year. In hot climates, such as the southern United States, it should be installed on the outside of the insulation. In both cases, the vapour barrier is designed to prevent warm, humid air from shedding its moisture as it meets a cool surface.

The International Residential Code (IRC) divides North America into eight climate areas to determine when a vapour barrier is needed in a building. The IRC recommends builders install a Class-I or -II vapour retarder on the interior side of homes in climate zones 5 (Cold) and north, and in the Marine 4 zone. However, if the house is air-conditioned in the summer, a Class-II vapour retarder should be used on the interior of the wall. In hot, humid climates (zones 1 to 3), a vapour retarder should not be used on the interior side of the wall.

The effectiveness of a vapour barrier also depends on the type of material used. A true vapour barrier completely prevents moisture from passing through its material, while a vapour diffusion retarder allows a prescribed amount of water vapour to pass through. Materials such as elastomeric coatings, paper-backed aluminium, polyethylene plastic sheets, asphalt-coated Kraft paper, and vapour retarder paints can be used as effective vapour barriers or retarders.

It is important to note that a vapour barrier is not the same as an air barrier. A vapour barrier is designed to retard the migration of water vapour, while an air barrier is intended to retard the migration of air. Air movement accounts for more than 98% of all water vapour movement in building cavities, so ensuring proper air sealing is crucial.

Airtightness is more important than vapour barriers

The traditional approach to preventing water vapour from penetrating walls in homes is a 6 mil polyethylene vapour barrier. This is an ideal building practice in the extreme northern communities of Canada, less so as you move southwards. Despite being used extensively in residential construction, it can be overkill in most Canadian homes and can cause problems of its own.

The vast majority of Americans and Canadians live in a temperate climate, so for most, a vapour barrier (or more accurately, a semi-permeable vapour retarder) that allows a certain amount of water vapour to pass through a wall could actually serve better over the course of the year.

In order to prevent condensation from forming, a vapour barrier should be placed on the warm side of the insulation to stop warm, moist air from condensing on a cold surface inside the wall. In cold climates, for most of the year, the vapour barrier should be on the inside of the insulation. In hot climates, it should be installed on the outside of the insulation.

Water vapour diffusion through building materials accounts for only about 2% of moisture penetration through walls, and a vapour retarder primer can be twice as effective as it needs to be. Polyethylene is some 15 times more resistant to water vapour diffusion than it needs to be; it's expensive to buy and install; is environmentally questionable; and it can actually cause problems in the summer months.

In much of the country, you could take the time and money you would have spent on installing polyethylene on the entire exterior wall of your home, and instead put those resources into a latex vapour retarder paint on primer and a properly sealed air barrier. There are hard cost savings to be had doing this, and an improvement in both performance and durability.

Airtightness, which focuses on the level of air leakage, should be considered as part of a fabric-first approach to building, designed to limit reliance on space heating, creating houses that are thermally efficient at their core. Air leakage is the uncontrolled movement of air into and out of a building that is not for the planned purpose of exhausting stale air or bringing in fresh air, like a mechanical ventilation system would.

Airtightness is aimed at stopping this uncontrolled movement of air by closing the gaps and cracks in the construction of the building (the fabric, the windows and doors, and around cables and pipes, for example) through which warm air can escape and cold air can get in.

Airtightness is the most important and most difficult challenge in achieving California's 2020 energy-efficiency and indoor-air-quality (IAQ) goals. Currently, many new-home blueprints and specifications don't even identify the air barrier as a building component. To meet California's zero-net-energy (ZNE) requirements, builders must minimise air leakage. ZNE standards require air-leakage reductions of at least half – from roughly 4 ACH50 in current production homes to 2 ACH50 or less. This air-leakage reduction represents about a 15% energy saving compared to conventional new homes.

Air leakage often carries pollution from the ground, the outdoors, or from intermediate zones like crawl spaces, attics, and attached garages. However, in some cases, the building envelope can filter outdoor air entering the home, which benefits by reducing particle pollution. Airtight homes provide better indoor air quality because an effective air barrier blocks dust, pollen, smoke and other outdoor pollutants.

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