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  • Writer's pictureJulia Kaufmann

Healthy Homes - Part 2: Mold Prevention through Vapor Retardants

Welcome to the exciting world of vapor retardants.


Last week we started to get a first little glimpse into the measures we can take to reduce the risk of mold and mildew growth in our homes. The first option, and a fairly straight forward one, is ventilation, either natural or mechanical.


This week, we’ll look at where we want which body of air to move to, and how to make sure it doesn’t cause more trouble than good.


Enter: vapor retardants.


Vapor retardant is a broad term used to describe building materials that hinder or decrease the movement of moisture within a structure. Various materials can serve this function; plywood, paints, coatings, and membranes are all commonly used.


In a climate such as ours, with very long, cold winters, vapor retardants play a crucial role in mold prevention. It once again comes down to relative humidity. Relative humidity describes the amount of water suspended in the air relative to the potential of how much water that same body of air could absorb before it hit the dew point, i.e. 100% relative humidity, at which point we would see condensation. A relative humidity of 40-60% is considered comfortable and healthy. However, the relative humidity depends on the air temperature. If the air gets colder, it can hold less water. When a body of air at 65 degrees with 50% relative humidity cools down, the same amount of moisture in the air constitutes 100% humidity at 45 degrees. Meaning at 44 degrees we’re going to start to see condensation, i.e. tiny water droplets that are no longer gaseous but instead liquid water. Anyone who’s ever slept in a tent has seen and felt the wet rain guard where our warm moist air from the night cooled down on the cold tent structure and condensated.


If air is allowed to freely travel through a structure, it will bring with it the water it holds. If warm, relatively moist (50%) interior air permeates into our walls, it cools down and the condensation water will cause moisture in our insulation and on our framing members. Walls are not typically the best vented elements in any house, resulting in moist organic matter (wood studs, and potentially organic insulation such as cellulose) in a poorly ventilated space, which is the ideal breeding ground for mold and mildew.


Consequently, it is important to prevent warm interior air to travel into zones of high thermal differences such as our ceilings, and walls. Zones of high thermal differences are areas in our structure where the temperature changes drastically over a small distance. Walls during winter month are a prime example, with the interior of the wall being close to room temperature (depending on insulation levels), and the exterior of the wall being close to the exterior air temperature. One way of achieving that goal is to clad the walls and ceilings with continuous sheets of a building material with low vapor permeability and then carefully taping the seams. We have used CDX plywood for that purpose and minutely sealed all the seams with ZIP flashing tape. Plywood has a vapor permeability of less than .5 perms, meaning very little vapor will permeate into and through the sheets. If properly taped, or sealed, and ideally painted, it can function as a vapor retardant.


If you want to totally geek out, here is a great threat on the pros and cons of using plywood as a vapor retardant:


The next option is to use the drywall as an interior air barrier by running the entire surface of the ceiling in one consecutive installation prior to framing the interior walls. This approach minimizes penetrations in the ceiling, and paired with a high quality primer and paint can drastically reduce the permeability of your ceiling. Depending on the paint and primer used, this method can function as an effective air barrier, and vapor retardant. Considering the airtight nature of oil-based and latex-based paints, this method results in fairly airtight walls and ceilings. It is important to consider potential moisture intrusion point in this installation, seeing as whatever moisture does enter this airtight space won’t have anywhere to go, and without ventilation is unlikely to dry.


A highly efficient and technically advanced method are one-directional (or variable permeability) vapor membranes. A high-tech product, these synthetic membranes are attached to the studs, ideally prior to any interior walls being framed, and stapled to the framing members. The membranes are one directional, meaning they let moisture out of the wall, but don’t let any vapor into the wall. Compared to the use of painted drywall and zipped plywood, this method allows the walls to exhale moisture and have a certain degree of ventilation while preventing moist air to travel into the walls.


That’s it for today, thanks for geeking out with us.





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