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The Envelope of a Home

Simply speaking, the materials of a house that separate the outside from the inside are considered the home's envelope. It is the barrier that keeps us from experiencing the extremes of the outside environment and keeps the inside of our home comfortable, dry, and energy efficient.

Home tightening 

Air infiltration - Outside air can enter your house through cracks in you foundation wall, sliding, or around windows and doors. Inside (conditioned) air can escape around chimneys, vents, pipes, and wires that penetrate walls, ceilings, and attics.

Every home needs a certain amount of fresh air for the furnace and appliances that burn fuel, for getting rid of excess moisture, and for reducing odors and stuffiness. When this air exchange is controlled, it’s called ventilation. When it seeps uncontrolled through cracks and holes in the envelope of our home, it is called air infiltration. Air that leaks through the ceiling, walls, foundation, and other areas are significant sources of heating and cooling losses in a home—up to one third in a typical house. Stopping air leaks is the best way to conserve energy, save money, and increase comfort. The bottom line: If you don’t tighten up your home first, other improvements (like windows, insulation, furnaces) may not be as effective.

Infiltration through air leaks can occur in three ways:

  • Wind-driven infiltration happens during cold weather months when the wind blows cold, dry air into a house through cracks and openings and forces warm air out. During warmer weather, the wind blows in warm, humid air, forcing cooler air out.
  • Stack-effect infiltration takes place during the natural process of convection. As warm air rises and escapes through cracks at the top of the house, it pulls cold air into the lower areas.
  • Negative air pressure infiltration starts when appliances use air for combustion or when ventilation fans are operated. Outdoor air enters through any available openings to equalize the pressure inside a home. This can be a problem if adequate fresh air for combustion has not been provided. It can lead to a potentially dangerous carbon monoxide risk as fresh air is pulled down chimneys or vents, interfering with their proper operation.

Typically, air infiltration causes drafts and a chilly feeling near windows and doors and in basements. Adjusting your thermostat will not stop the drafts, but sealing hidden cracks and openings will. By stopping air leaks at their source, you’ll stay warmer in the winter and cooler in the summer, use less fuel, and reduce your utility bills.

Where do you start?

Fortunately, air infiltration is one of the easiest forms of energy loss to correct. The process requires only a careful inspection of your home and some inexpensive weather-stripping, caulking, and filler materials. 

Most people know they should caulk and weather-strip around the exterior of their homes to protect it from the elements. However, it is equally important to protect your home from interior air leaks. Moist interior air can enter the walls and ceiling through cracks and holes, and condensation buildup in those locations can damage or destroy insulation, wiring, wood, or other building materials.

Look for air leaks

Before research and building science demonstrated the role air leaks play in energy loss, most people assumed that insulation was enough to stop heat flow through a building. Although insulation slows heat transfer, it is easily compromised by air flow, whether driven by outside wind conditions or convection currents within the building. The only way to stop this air movement—and the associated heat loss—is by eliminating the holes and pathways between the inside of the house and the outside.

The first step to tightening up your home’s envelope is performing a detailed inspection for air leaks. This should be part of every energy assessment, and the inspector should be able to show you locations where air is leaking and needs to be sealed. A good rule of thumb is to seal the high air leaks first; in other words, start by plugging holes and leaks in the attic; then move to the exterior walls, and look for smaller leaks around doors, windows, and electrical switches and outlets; finally check out your basement.


Attic air leaks: more common than you'd guess 

Most existing homes have many unsealed penetrations into the attic for wires, chimneys, vents, etc. Sealing will prevent heat loss and damage to building materials.

Hatches and doors to the attic 

Masonry chimney seal - Cut "L"-shaped pieces of aluminum or sheet metal to fit around chimney. Seal gap between metal and chimney with red fire-stop caulk (above). Then make metal dams to hold the insulation away from the chimney at least two inches (below). Seal joints with metal foil tape.
  • Weather-strip the edges of the access hole and insulate the back of the attic hatch/door. Don’t forget doors to unheated knee wall areas.

Holes in attic floor

  • Seal all openings around electrical wires, pipes, ducts, and vents with a good general-purpose caulk or spray foam. You may need to use a filler material for larger holes.

Recessed lights and bathroom fans

  • These fixtures can poke into the attic insulation and create a pathway for air leaks. Caulk around them from below with flexible, high-temperature caulk. Install an airtight box sealed over the top of them in the attic, not only to stop air leakage but to reduce fire hazard from insulation lying against the fixture. When replacing, use only air-tight fixtures.

Chase for plumbing vent stack(s)

  • This channel may run inside the walls of your home, from the basement to the attic, with openings at each floor where the lines branch off. If the chase isn’t much larger than the vent, seal with expanding foam. For larger chases, use drywall, wood, or rigid foam—and caulk or foam around all edges. 

Fireplace chimney and furnace/water heater flues

  • Close the gap between house framing and the chimney and vent flues with metal; seal with red fire-stop caulk.

Knee-wall storage drawers

  • If storage drawers are recessed into the attic space, build an airtight, insulated box around the backside of the drawers. Don’t forget to seal all joints with caulk or sheathing tape. (Additional information on knee walls is located in the "Attic" section under insulation.)

Other holes

  • Using the appropriate materials, seal all other holes between the heated space in your house and the attic.