One of the most important – but least glamorous – of the features of Canada’s Greenest Home (and most natural and sustainable buildings) is the vapour permeability of the walls. It doesn’t sound like a big deal, but it’s a major difference between conventional building and so-called alternative building and represents a very different way of thinking about building performance that can have important performance ramifications. What follows is a simplified explanation of this difference. For more detailed information, I suggest the excellent material available for free at BuildingScience.com.
To understand the difference, one must first know that moisture will always move from areas of high concentration to areas of low concentration (a variation of the “nature abhors a vacuum” principle). For the majority of the heating season, this means that moisture is trying to move from our warm, moist interior spaces into the outdoors, where it is cooler and drier. If there are leaks or holes in the building enclosure, this warm moist air will move quickly. But even if there are no leaks or holes, this moisture will still migrate to the exterior by diffusion – a molecular movement of moisture through the materials of the building. A material’s ability to resist this diffusion is known as its permeability. Materials with low permeability ratings allow very little moisture through, and materials with high perm ratings can allow quite a bit of moisture through.
Okay, that’s a lot of words to say that there is a natural vapour drive through the enclosure of a building.
For the past few decades, mainstream buildings in northern climates have relied on a vapour barrier – basically a thick plastic sheeting – to prevent air leaks from inside to outside and to prevent the diffusion of moisture into the wall. This practice arose from the failures of many early “air tight” homes, in which moisture was able to accumulate in the wall cavities and resulted in rot and mold issues. The solution was to use a vapour barrier membrane on the interior of the walls to prevent this from happening.
In the natural/sustainable building world, we have always preferred to use wall assemblies that are vapour permeable. This acknowledges the fact that the vapour drive in buildings is inevitable, relentless, and not necessarily a problem unless materials are introduced that do not allow for this vapour to pass through at a reasonable rate. A good example of such a material is OSB (chip board) or plywood, the two most common exterior sheathing materials in conventional construction. These materials will resist the migration of moisture such that it can accumulate and condense on the interior side of the sheathing and begin to cause problems. If moisture can’t pass through the exterior sheathing, it must be prevented from entering from the interior side… hence the plastic vapour barrier.
The plastered straw bale walls (both the lime-cement prefab walls and the clay-plastered site-baled walls) are examples of permeable walls. The plasters on the interior and exterior as well as the straw insulation are very capable of allowing the movement of moisture through the materials in either direction. In this way, problems arising from moisture accumulation are prevented, and the walls have an ability to dry out in either direction should there be times of high moisture loading.
Canada’s Greenest Home also incorporates some double stud frame wall sections, including the entire south wall and the window openings in the prefab bale walls. Since we are not using plasters on these walls (we could have… but wanted to demonstrate some more sustainable, conventional approaches), a sheet barrier of some kind is required. We definitely didn’t want to give up on a vapour permeable strategy…
Our first step was to choose an exterior sheathing that is quite permeable. For this we used DensGlass, a gypsum board product (union made in Ontario, Canada and fully recycleable) with a high perm rating. Should we have moisture movement through these walls, it will be able to dry through the DensGlass at a rate similar to our plastered walls.
The second step was to find a sheet barrier that meets the current code requirements for a vapour barrier (BuildingScience.com argues for the term “vapour control layer” which is a much more accurate term for what’s required) and yet doesn’t completely blow our desire for vapour permeability.
The answer seems to have come in the form of a “smart” vapour barrier, as suggested by Ross Elliott at HomeSol Building Solutions (our excellent energy auditor/advisor). In our case, we used a product called “Membrain” (insert groan here) from CertainTeed. This product (and similar versions from other companies) offers the vapour resistance of conventional plastic sheeting, but with a composition that allows for drying back through the membrane should conditions on the backside be more humid. While it lacks the low embodied energy and friendliness of the plasters we used elsewhere, these products allow conventional builders to achieve some of the same benefits at a very low cost and without having to switch building techniques.
You can read more about these “smart” barriers and how they work at BuildingGreen.com.
While we would always make plasters and natural insulations our first choice, the combination of recycled drywall, smart membrane, dense-packed cellulose insulation and permeable sheathing is a way to embrace “permeable” thinking within a mainstream paradigm.
Canada’s Greenest Home is an attempt to blend more “radical” natural building strategies with those that can work for mainstream builders. We’d like to see houses like this one be replicated by conventional builders, as well as natural builders. While builders at each end of this spectrum may choose one strategy/material over another, we think there is value in both approaches and are trying to demonstrate both in this project.