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The Barriers - A definition

The word enclosure replaces the more dated term envelope and represents the man-made separation between differently conditioned spaces and also between conditioned spaces and the great outdoors.  It is at the heart of building science and represents my passion for at least the last decade.  It represents the name of the project, which will hopefully be a beacon of best practices in terms of enclosure design and construction.

Broken down, the enclosure represents a series of barriers including (in order of importance in terms of building integrity); water shedding surface, water resistant barrier, air barrier, vapour barrier, and thermal barrier.  Below I will enplane the purpose of each barrier from the perspective of a heating dominated climate (only climate I have thoroughly studied)

The water shedding surface (WSS) represents the part of the enclosure you can physically see from the outside of the building.  The cladding (siding), windows & doors, and roofing all represent the primary water shedding surfaces.  The purpose is to deflect the vast majority of storm water away from the building as the first plane of protection.

The Water Resistant Barrier (WRB) primary purpose is to be a final barrier to keep the storm water out.  No bulk water should flow past this barrier, and any materials outboard of this barrier should be able to handle occasional wetting.  The barrier protects all inboard materials that cannot take regular wetting (like wall studs, floor assemblies, drywall, engineered beams, etc).  In the building code, this barrier is called the second plane of protection.  It is a very good idea to ensure that while being a barrier to liquid water, it is permeable to moisture in the vapour form (think Gore-Tex coat).  This will allow the assembly to dry to the low pressure side, which in a heating dominated climate, is the outdoors.

The next barrier in terms of importance is the Air Barrier (AB).  While an air barrier leads to a more comfortable home (prevents drafts), and reduces energy use (air that you have conditioned is not just leaking out of house and being replaced by outside air that has to be re-conditioned), its primary purpose if to prevent moisture laden air from flowing through any of your enclosure’s exterior assemblies, whether that be your walls, foundation, below grade floor slab, or roof.  If moisture laden air can flow through an assembly, there is a risk it could condense on colder surfaces within the assembly, which in turn could lead to fungi growth and rot of the assembly’s components.  Air infiltrating to the interior of the dwelling can also bring with it outdoor or even indoor contaminates (think of all the nasties you have seen in your crawlspace and ask your self if you really want the air you are breathing to filter through that area).   The air barrier can be placed at any layer of the assembly and it will stop air flow through the assembly.

The purpose of a Vapour Barrier (VB) is to prevent direct transfer of moisture, by means of physical contact, through the assembly.  So in heating dominated climates, it prevents the moisture present in our interior air, from diffusing into the wall and roof assemblies.

The final barrier is the Thermal Barrier.  Most people know this as insulation and its primary purpose is to prevent heat flow through an assembly, weather it be the heat in conditioned air moving to the colder exterior, or the summer heat moving from the exterior inside the home.  While it is the most important barrier from the point of view to keep the occupants comfortable, it is the least important from a buildings durability, and in fact, adding insulation to a building puts the building at high risk of break down if all the other layers above are not first detailed correctly.  In fact, it is the increasing  insulation levels since the early 80’s that has led to the vast majority of building issues over the last 30 years.  This is because, the more of the interior heat you block from entering a wall or roof, the colder the outboard ends of the assemblies become and the higher the risk that moisture in the air within the assembly can condense.  By blocking the heat, you also prevent the drying out of wood components that may have already become wet.

Of all of the barriers, the air barrier is the hardest to detail and has generally been a complete failure in residential construction.  To this day, there is no defined measurable goal set out in the Canadian building code, and as a result, builders have not been held to any standard (apparently this will finally change in the next code cycle).

Seldom is the air barrier a unique components in the assembly and is typically riding on the coat tails of one of the other barriers. In BC, this barrier has primary been combined with the vapour barrier to disastrous results.  9 out of 10 houses in BC utilize poly as the vapour barrier with a crude attempt to seal that poly to also act as the air barrier.  I inspected hundreds of homes and can advise I never once found an intact air barrier when the vapour barrier poly was being used to create it.  It is just too difficult to seal a loose plastic sheet around the literary hundreds if not thousands of penetrations of your typical dwelling (electrical outlets, switch boxes, cable & telephone outlets, windows and doors, partition walls to ceiling, drilled holes to attic for piping, wires, vents, the list goes on). Also, that sheet typically does not have secure back support (resting on compressible insulation if anything at all) and so taped joints can pull apart the first time a door is opened or the wind blows onto the building.

On, we are combining the WRB and AB by using a liquid tight but vapour open barrier that is also air tight.  The barrier we have chosen is the Delta Vent SA (for self adhered).  The benefits to using a self adhered barrier on the sheathing as the air barrier are as follows:

  • easy to detail, there are very few penetrations on the outside of a wall or roof
  • durable, when adhered to plywood and then buried beneath insulation, the barrier is well supported on both sides
  • inspectable, the WRB/AB can be fully detailed and air door tested prior to installing the insulation and cladding.

There is a lot more to say on this subject, but consider this the Cole’s Notes version.

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