Want a Healthier Home? Let Your Walls Breathe
Take a minute to learn about vapor retarders
Whether you're finishing off the basement or attic space, putting on an addition or building new, healthy-minded homeowners almost inevitably fret over their insulation selection. While different insulation solutions do have their pros and cons most do not negatively impact the overall health of the home. In fact, they help it.
Fiberglass, one of the most common insulation materials on the market, provides high thermal resistance, fire resistance and sound absorbency creating a comfortable and relaxing environment. Cotton batts (aka blue jeans), cellulose, and spray foam offer similar properties.
The potential for improving the overall health of your home comes during what is often an afterthought in wall construction, the vapor retarder.
What is a vapor retarder? How can it affect the health of my home?
Often sold in rolls, a vapor retarder, or sometimes called a vapor barrier, prevents moisture from diffusing into the wall, ceiling or floor during a cold winter. When moisture vapor penetrates the wall cavity, it can condense on cold surfaces and accumulate in the building envelope. This trapped moisture can be problematic to your home and your health.
If moisture in the wall is left unattended it can cause many serious problems:
- Rot. When trapped inside a wall cavity for an extended period of time, moisture can cause building materials, such as wood, drywall and steel, to deteriorate or corrode.
- Diminished efficiency. If moisture penetrates the insulation, the insulation will lose R-Value, or its capacity to resist heat flow, which makes your home less energy efficient.
- Mold growth. When exposed to moisture for an extended amount of time, wood- and paper-based materials are a prime food source for mold. This mold growth can lower the indoor air quality of your home and can have a negative impact on the comfort and respiratory health of everyone inside.
If this bit of knowledge wasn't passed down to you from an earlier generation it may be because moisture build up in walls is a relatively new problem. A typical 1970s home experienced enough air leakage to equal an 11.4" diameter hole in the wall. While not energy efficient, all these leaks did keep things dry.
As construction methods have gotten tighter, our homes have gotten more efficient and comfortable but we've also increased the possibility of moisture getting trapped in the wall. It is impossible to stop water vapor from penetrating the home's exterior so without leaks the wall is not be able to dry out as efficiently. This means new adaptable solutions, such as smart vapor retarders, are needed.
Not all walls need vapor retarders. Answer these three questions to know if yours does.
Benefits and Limitations of Traditional Materials
Vapor retarders have traditionally fallen into two categories, class I and class II. Both class I and II are fairly vapor tight and serve to stop the majority of moisture from getting in.
These materials work well in areas with cold winters but moderate, dry summers. This is because traditional vapor retarders are good at stopping moisture from entering the wall when the moisture is coming from outside the house (cold months). Where they struggle is when moisture levels are higher on the inside (warm months).
You can see the difference in permeance illustrated in the chart below. Polyethylene, a common class I vapor retarder, maintains the same level of permeance across all levels of humidity while MemBrain, a smart vapor retarder, is able to adapt to changing conditions.
Not all regions have enough summer-time moisture to be affected by this, but a large portion of North America is considered mixed-climate, where the moisture-drive direction is balanced between the winter and summer seasons. In these regions, homes using traditional polyethylene vapor retarders may successfully keep moisture out of the wall when it's cold out, but trap it there during summer when the moisture drive reverses.
The exterior of your home is another factor that can affect the risk of moisture building up in the walls. According to the U.S. Census Bureau 2009 statistics, more than half of all new homes are clad with absorptive materials such as brick, stucco, wood, fiber cement or stone. These materials release moisture into the building cavity which can exacerbate this issue especially in summer months.
Smart Solutions Adapt to Changing Conditions
Smart vapor retarders have solved this problem by letting the wall “breathe." In other words, they have the unique ability to be able to react to changes in relative humidity by altering their physical structure. During the winter, when relative humidity is low, smart vapor retarders are able to provide resistance to vapor penetration from the interior. However when the relative humidity increases to 60% or above its permeance dramatically increases, thus allowing the water vapor to pass through, facilitating the drying of wet building systems.
Since it's nearly impossible to prevent moisture from entering 100% of the time, using a smart vapor retarder allows water to escape from building assemblies as water vapor and the wall to stay dry.
"Our walls are wetter than they have ever been. This is not because the physics have changed, but because new construction practices are not easily allowing the moisture that's getting in, to get out," explains CertainTeed Building Scientist Ted Winslow. "We have a growing need for increased R-values in our homes and buildings along with a steep reduction in air movement through the wall. While these efforts are great from an energy-efficiency standpoint, the lower rate of drying is now presenting our homes with new challenges that cannot be neglected."
Once these walls get wet, they are staying wet. While, construction practices get smarter, our materials need to too.
Learn more about Membrain, CertainTeed's continuous air barrier and smart vapor retarder.
Ask a Building Scientist: What is the difference between a vapor retarder and a vapor barrier?
It is literally semantics. There are no barriers in this world so we've stopped using the word barrier in regards to buildings. A barrier implies nothing gets passed. Water vapor gets past everything to some degree and so how resistant you are is what it's all about. Today we call them retarders and it's how much moisture the material can impede. They have different classifications, class I and class II and class III based on that exact thing - how much do you block and how much don't you block? So we went from barrier to retarder in terms of vocabulary to represent that it's actually a spectrum of performance and not black or white.