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Building Science: Understanding Air Flow

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Building Science: Understanding Air Flow

As reported in the January 2005 issue of PB, building science is a systems approach to home building that considers relationships between a home's components and its environment. The goal of building science: to optimize occupant health, comfort and safety; maximize energy efficiency and structural durability and reduce builder and contractor callbacks.

By Stan Gatland, Manager, Building Science March 31, 2005
This article first appeared in the PB April 2005 issue of Pro Builder.

As reported in the January 2005 issue of PB, building science is a systems approach to home building that considers relationships between a home's components and its environment.

The goal of building science: to optimize occupant health, comfort and safety; maximize energy efficiency and structural durability and reduce builder and contractor callbacks.

Introduction to Air Flow

When talking about air flow, what is actually being discussed is air leakage in a building. Air moves in and out of a house intentionally or unintentionally. There are many benefits of controlling air flow, some of which include:

  • Increasing energy efficiency, a concern for many homeowners
  • Improving the comfort of the occupants and avoid drafts
  • Controlling moisture in the home

During the winter, air flow can introduce moisture into wall, ceiling and roof assemblies from inside the house through the walls and attic. In the summer, the added humidity comes into the house, affecting comfort and energy efficiency.

Controlling air flow ultimately improves overall homeowner satisfaction.

Building Air Leakage: Problems and Solutions

This article will examine some common air flow problems and their solutions, starting at the roof and ceiling system, moving down the walls, rooms adjacent to an attached garage, the basement and crawlspace ceiling and lastly, HVAC systems.

Roof/Ceiling System


Problems associated with roof/ceiling systems can include:

  • Heat loss or gain in the ceiling
  • Air flow can cause surface condensation and staining, such as in recessed lights


Solutions for fixing problems associated with roof/ceiling systems include:

  • Make certain that recessed lights are properly sealed with foam or caulking. Remember, IC-rated lighting fixtures are always recommended. Some lights come with a gasket that will air seal the fixture.
  • Make sure all penetrations are sealed. Use caulk or foam to create a seal.
  • Any spot ventilation, such as a fan in the bathroom or kitchen, needs to be sealed properly.
  • The attic hatch should also be sealed.
  • If there is an air distribution system going through the attic space — common in the South — it should be sealed against air leakage.


Problems associated with walls can include:

  • Occupant discomfort caused by surface temperature and drafts
  • Air that infiltrates through a wall can cause surface staining


Solutions for fixing problems associated with walls include:

  • House wraps to seal in air
  • Interior caulking
  • Adhered sheathings. For exterior insulating systems, the product is glued and fastened to the outside of the structure to act as an air barrier. The same thing can be done on the interior of the house. Drywall can be glued and screwed into place and then caulked to prevent air flow.
  • If there are unusual details where the walls and ceilings interface, make sure they are properly sealed as well. If the space is too wide to caulk, fill it with a piece of gypsum board or insulating sheathing.
  • Any kind of plumbing or electrical penetration that goes horizontally and between floors should be sealed as well
  • All windows and doors should be sealed as well. If using vinyl windows, make sure to use a low-expanding foam so that the window isn't damaged. Be sure to check with the window manufacturer for the product they recommend.
Rooms Adjacent To Attached Garage


Problems associated with rooms adjacent to an attached garage include:

  • Uncomfortable temperatures
  • Surface staining
  • Poor indoor air quality, especially if the rooms is attached to a garage where a car or chemicals are stored
  • Pest infiltration, such as spiders or other insects. If there are air holes, pests can get into interior spaces.


Solutions for fixing problems related to rooms adjacent to an attached garage include:

  • Sheath between connecting spaces.
  • If there is room, seal the area next to the garage — code permitting. It's better to seal this area with an insulated sheathing rather than plywood or drywall. Sealing the area this way provides some thermal value as well.
Basement and Crawlspace Ceiling


Problems associated with basements and crawlspace ceilings include:

  • Adjacent rooms can be uncomfortable if the air flow is not restricted
  • Energy efficiency can be compromised


Solutions for fixing problems associated basement and crawlspace ceilings include:

  • Be sure to seal all penetrations from plumbing that comes through the basement or the crawlspace from the outdoors
  • Seal ductwork and plumbing chases. These are generally large openings between floors and are a direct path from a drafty basement right to the tub.
  • Air seal rim joists where the foundation and the wood sill meet. Use a caulk and seal package or compressible foam that lays on the concrete slab between the wood and the concrete.
HVAC System Problems

Heating, ventilating and air conditioning systems all present their own challenges.


Problems associated with HVAC systems include:

  • Homeowner comfort
  • Energy efficiency of the home


Solutions for fixing problems related HVAC systems include:

  • Make sure the ductwork is well-sealed with mastic. Duct tape can degrade over time.
  • Use return ducts rather than wall or floor cavities for improved efficiency
Pay Attention to Details

In addition to the solutions listed above, the following tips can help builders ensure proper air flow:

TIP 1. Pay attention to details such as knee wall sheathing, which can be used as an air barrier.

TIP 2. Seal soffits and chases as they provide open spaces to the outside that let in air.

TIP 3. Be sure to align air and thermal barriers around difficult architectural details that are common in today's homes.

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