“Air infiltration can account for 30% or more of a home’s heating and cooling costs and contribute to problems with moisture, noise, dust, and entry of pollutants, insects and rodents.  Nearly 45% of this uncontrolled air infiltrates through openings in ceilings, walls, and floors, as well as plumbing penetrations.”

U.S. Department of Energy

A better sealed, ventilated & insulated building envelope by Applied Building Science will save you money on utility bills for years to come. And, by using less energy to heat & cool your home or business, you reduce the negative impacts on the environment caused by fossil fuel use.

Applied Building Science thoroughly seals cracks, crevices and holes in the interior and exterior of the home. This includes recessed lights that can account for between $5 and $30 each in annual energy loss due to air leakage, and dump 1/3 of a gallon of moisture into the attic.

Air Infiltration Basics
Airflows within buildings are either controlled or uncontrolled. Controlled airflow is generated by a mechanical device and is designed to help ventilate a building and/or distribute “conditioned “air through out the building (in other words, the area we want to intentionally heat and cool). Ventilation systems, bath fans, spot ventilators, make-up air and air conditioning or furnace flow are typical sources of controlled air flow.

Uncontrolled airflow, on the other hand, is any non-designed or unintended movement of air into, out of, or within a building. This air enters a structure uncontrollably through cracks, gaps and openings - called bypasses or penetrations. This can be caused by wind, the force of heat rising within the building (called stack effect), or by out-of-control fans. See Building Science Fundamentals.

Leaks within the ductwork or wall cavities as part of the heating or cooling system are also uncontrolled airflow. Very leaky houses and commercial buildings are uncomfortable and have high energy bills. Also, a leaky house that allows moldy, dusty crawlspace or attic air to enter is not healthy. Finding and curing infiltration problems is a high priority for weatherization professionals like Applied Building Science.

The rate of air infiltration in a home depends on many factors, the most important being the size and location of holes in the thermal envelope and the difference in temperature between inside and outside. Warm air inside a dwelling gives rise to stack-effect infiltration as it tries to escape from the top of the envelope, sucking in cold air at the bottom. Wind and leaks in duct systems can also have a major effect on infiltration during the heating season, as is stack (or chimney) ¬effect infiltration which is at its worst on coldest days.

Large homes or commercial buildings with multiple floors and high ceilings - or situated on high elevations - can experience significant air infiltration due to stack and wind effects. Stack pressure can triple for every additional floor in a structure, which amplifies the suction of air from the bottom to the top of the building.

Fire protection, especially in attics, is also compromised with increased air infiltration. Oxygen-rich air rushing through open chaseways or bypasses from the living space or basement to the attic can quickly turn a controllable thermal event into a devastating inferno.

Note that in the middle of the heated envelope there is a neutral pressure zone where neither infiltration nor exfiltration occurs due to stack effect. This explains why caulking and weather stripping in mid-envelope tends to save less energy than careful attention to the bottom and top of the envelope, where these natural driving forces are greater.

Blower door air infiltration testing has contributed significantly to understanding and measuring air leakage in and out of buildings. Prior to this technology, most air leakage was thought to occur toward the mid-height of the conditioned building envelope, primarily through doors and windows. Although these components do contribute to air leakage, airflow around these areas represents a relatively small percentage of heat losses in most dwellings. Their effects tend to be amplified by annoying high-velocity currents through relatively small gaps & cracks.

According to the Advanced Energy Corp, there is up to 4 times more air infiltration in homes built before the year 2000 than after. To be sure, the use of plywood sheathing, house wraps, better windows and upgraded building assembly techniques have collectively “moved the needle” on reducing air infiltration in more recent construction.

But owners of newer homes do have problems as well. Due to the segmented nature of residential construction, many builders often see truly effective air leakage control work as a challenge or even a nuisance. Although they routinely do the more visible types of weatherstripping and caulking, truly comprehensive and integrated air sealing protocols are not considered state of the practice among mainstream builders.

One of the difficulties that come into play is the rapid-fire succession of jobs that are done by the various subcontracting trades on the job site. For instance, as soon as the drywall is finished, the carpet is laid and the baseboard is installed, leaving a potentially large (and uncomfortable) perimeter air leak around the exterior wall.

Cold living spaces (bonus rooms) over garages and overhanging floors, for example, will continue to nag unsuspecting new home buyers until building performance guidelines with air sealing are enacted. At the same time, buyers should not be ignoring the energy efficiency and comfort advantages of a well-sealed structure in favor of other “must have” features. In truth, maximizing energy efficiency is a buyer’s decision, not a building code requirement.

What is an Air Barrier
The ceilings, walls and floors that separate the inside heated space from the outside unheated space form the air barrier and the insulation barrier for a house. These two barriers differ by the materials used. For most homes, the sheet goods that form the ceiling, walls, and floor (such as drywall, wood sheathing, and floor decking) are effective at stopping air leakage. However, there are often cracks where walls meet floors and ceilings. This is especially true in older houses when lumber shrinks above the foundation walls. Also, ceilings, walls, and floors have had holes cut into them during the construction process to provide access for wiring, dropped ceilings, recessed lights, chimneys and piping. While the best time to plug these holes and cracks is during the initial construction of a house, this often is not done.

Most Insulation Products Do Not Block Air Flow
Some people confuse insulation with air sealing. By definition, insulation’s primary function is to provide thermal resistance to radiant or conductive heat flow. Certain types of cellular-structured insulation, such as dense-packed cellulose and rigid or close-cell spray-on polyurethane foams, can be effective at reducing air flow as well as heat flow. However, the most common type of insulation—low-density spun batt or loose-blown fiberglass—does not stop air leakage. Dirty fiberglass insulation is a telltale sign of air movement (it collects dirt like a filter and sometimes referred to as “filter-glass”). See the Colorado Study comparing the airflow resistance performance of fiberglass and cellulose insulation.

While fiberglass insulation works well to block conductive heat loss (the flow of heat through solid materials) they do not work well to block convective heat loss (the flow of heat through a fluid substance-air or water). This is why porous insulation works at its peak efficiency when air flow (pressure) through and around the material is minimized.

What Are The Priorities For Air Sealing?
As a result of the widespread use of blower doors, building performance professionals like Applied Building Science increasingly seal leak paths  –called major bypasses or penetrations - in attics, basements and crawl spaces where most significant air infiltration and ex-filtration takes place. These holes include:

• Kneewall cavities
• Mid-roof rafter chases
• Open wall cavities connecting the attic to the living area, basement or crawl space (signs of dirty fiberglass)
• Party walls between condominiums units and apartments
• “Bonus” openings from poor construction practices
• Attic hatches and fold-down stairs
• Plumbing vents
• Dropped ceilings
• Open Soffits (the box above the kitchen cabinets)
• Recessed lights
• Living spaces over unheated garages and overhanging floors/walls
• Unvented exhaust fans
• Whole house fans
• Chimney flue
• Chaseways containing heat and cooling ductwork in the attic
• Basement or crawl space rim joists (where the foundation meets the wood framing
• Baseboard molding on exterior walls (signs of dirty carpet)
• Exterior crawl space hatches
• Hopper-type basement windows
• See Signs of An Inefficient Home or Office for air-leakage related symptoms

How We Fix It

Applied Building Science uses number of different products and strategies to reduce air infiltration in and out of a home or commercial building. However, the 1st order of business is to locate the high-value bypasses and penetration that are hidden from the naked eye. For example, if the attic already has some level of insulation, it must be either pulled back or removed to locate hidden holes.

Applied Building Science uses different air sealing products to achieve the desired results.  They include:

• Close-cell spray-on polyurethane foam (one and two part)
• Rigid foam board
• Densepack cellulose insulation
• Plywood barriers and hatches
• Pressure-treated crawl space access hatches
• Sheet metal
• Fire-resistant and flexible caulks
• Air-tight assembly products
• High –performance weatherstripping
• High-performance air sealing tape and mastic 
• Best-practice finish carpentry techniques