On September the 26th, UDC hosted a Green Building seminar on air barriers. This was the last in a series of seminars organized by DCRA. If you are interested in attending any, they will resume in the spring of 2019. These seminars are free and open to the public so they are usually well attended and presented by people who are passionate about their subject.
Air barriers are defined as a “material or materials assembled and joined together to provide a barrier to air leakage through the building envelope. An air barrier may be a single material or a combination of materials”.
Air barriers stop the uncontrolled flow of air through a building’s exterior envelope. Airflow is caused by air moving from high pressure to low pressure. This moving air carries both heat and moisture. It can carry 50 to 100 times more moisture than diffusion alone. This is a huge consideration in a climate zone with high humidity such as ours; here in DC an air barrier becomes an even more essential component of any wall assembly.
Air leakage through a building’s envelope (walls, slab and roof) is generally caused by one of the following three phenomena:
a) Wind – on the exterior of a building creates positive pressure on the windward side and negative pressure on the leeward side which draws air through the building.
b) Stack Effect – warm air rises and cool air sinks.
c) Mechanical Pressure – caused by supply and exhaust imbalances.
We’ve all had the experience of standing beside a window and feeling cold. This is a common air barrier failure, and is an example of infiltration which is often caused by poorly sealed joints between adjacent building components. An example of exfiltration happens when a positively pressurized building has holes in the wall assembly. The pressure forces warm air through the insulation and into the wall cavity which cools causing condensation within the wall cavity.
High pitched roofs allow for passive ventilation (through the stack effect). Attic vents, dormer windows, vented soffits create heat differentials that allow for warm, moisture-laden air to rise and exit the building and cool air to enter.
Historically, buildings were not constructed to be airtight. They were drafty and poorly insulated but that also meant they had a lot of drying potential. But construction has changed, moving from masonry walls to the modern cavity wall. Interior finishes became more sensitive to relative humidity, residential HVAC proliferated increasing the need for more airtight homes. In response, the International Residential Code (IRC) adopted requirements for a continuous vapor barrier in 2009 following the International Building Code’s 2006 requirement.
In traditional masonry buildings, the air barrier is the wall itself. The thickness of the brick, paint on plaster, taped drywall or just paint on a brick wall all act as air barriers. Still, air leakage is quite common in these buildings. It is very difficult to make these layers continuous around the entire perimeter of the building envelope. When retrofitting older masonry buildings a lot of extra attention and detailing are required to eliminate gaps or leaks within the air barrier.
With the addition of air barriers, vapor barriers and proper thermal insulation, it becomes very important to take into account the unique hydrothermal properties of masonry walls. Misapplications can result in harmful effects like spalling since brick is designed to dry to both sides. Also the historical character of the building or neighborhood should be taken into consideration in any retrofit of a masonry building since it could result in changes to the exterior of the building.
The most important thing to remember about air barriers is continuity! A simple rule of thumb is on a drawing you should be able to put your finger on the air barrier and trace it around the entirety of the building perimeter without picking your finger up.
Air barrier products may take several forms:
Mechanically-attached membranes (also known as housewraps),
self adhered membranes (peel and stick).
fluid applied membranes
and spray applied foam.
There are several methods that are commonly used to measure the air-tightness of houses and buildings today. Blower door tests are used to help identify problem areas. A blower door is a powerful fan that mounts into the frame of an exterior door. The fan pulls air out of the house, lowering the air pressure inside. The higher outside air pressure then flows in through all unsealed cracks and openings. These tests determine the air infiltration rate of a building.
Blower Door Test
Thermal Imaging
Infra-red cameras can give real-time thermal images of the whole or parts of your home. When the blower door test is paired with a thermographic inspection, they can show exactly where cold air is flowing into your home. These tests must performed by professional energy auditors. It is important to employ these measures intermittently during the construction process since these tools are the best way to determine whether the assemblies are air-tight. They need to be carried out during construction while the building envelope components are still accessible (so that problem areas/leaks can be identified and fixed).
If you are renovating, building an addition to your existing house, or building from the ground up, you should familiarize yourself with Table 402.4.1.1 in the International Energy Code(IECC). In DC the Green Building reviewers at DCRA will check to make sure that this table and the components of the air barrier are properly identified in the drawings when a building permit is under review.
Air sealing also makes a big difference to indoor comfort. If you have an older leaky home and don’t want to spend the money on expensive retrofits you might want to spend some time on simple air-sealing efforts like caulking around window frame perimeters and electrical outlets and checking wall/ceiling joints for leaks. Ensuring that windows properly lock and seal and adding door sweeps also make a big difference. Weather-stripping, the process of sealing around doors, and windows, is another simple and cheap way to mitigate leakage without breaking the bank.
In single family homes, leakage areas are typically found in these areas:
· Recessed lights
· Duct connections
· Attic connection between top-plate and drywall in the interior of building.
· Corners
· Rim joists
· Attic Hatches
· Chimneys (without dampers)
· Poorly built additions
· Ill-fitting door and window frames (in older homes)
With an airtight house you will have lower heating bills, fewer drafts (i.e. more comfort), reduced chance of mold and rot because of moisture, and a better performing ventilation system. Now that’s a wrap!