Air leakage through building assemblies can move large quantities of water vapor and is a major factor in many vapor-related moisture problems. Building envelopes should be designed and constructed to reduce air leakage from inside to outside in cold climates, and from outside to inside in hot-humid climates. To achieve this objective, the big air leaks in the building’s envelope must be sealed. Most insulation will not stop air leaks. In addition, a suitable air-barrier system should be carefully considered and installed during the construction of the home.
Any one or a combination of the following drives air leakage:
Wind and stack effect-driven air leakage is best handled by the use of air barriers. HVAC equipment should be properly installed, and seams and connections in the distribution piping should be sealed airtight.
There are a few precautions worth mentioning when the building envelope is “tightened.”
First, the use of air barriers and air-leakage sealing practices can reduce the supply of combustion air for fuel-fired equipment (oil or gas furnaces, gas water heaters, gas dryers, etc.) located within the conditioned space. This can result in negative pressure and back-drafting of combustion products. The operation of spot exhaust fans (kitchen or bath), whole-house exhaust ventilation, and even the stack effect can also cause depressurization of the indoor space near combustion equipment and lead to back-drafting and the introduction of combustion products into the home, such as carbon monoxide. Because of these health and safety concerns, sealed combustion equipment is often installed when the house is “tight.”
Second, mechanical ventilation may be required or recommended to address other consequences of tightening the building envelope, such as IAQ (indoor air quality) and humidity control. For example, modern residential building codes still permit the use of operable windows as a means of providing fresh-air ventilation, though this has been contested in recent years. It may be risky to rely solely on the behavior of the occupant to provide adequate ventilation in this manner in the absence of higher levels of natural ventilation.
As a final precaution, air-barrier materials must also be considered in terms of their impact on vapor movement and water shedding. For instance, if an air barrier is used on the exterior of the wall as a weather barrier underneath cladding or housewrap, it must have adequate water-resistant qualities. And if an air barrier is used on the inside of a wall in a hot-humid climate, it needs to be a permeable material and not one that will prevent vapor from drying to the inside.
Air sealing is important because air carries both moisture and energy, usually in the direction that the homeowner does not want. Air leaks can carry hot, humid outdoor air into the house in the summertime. Air leaks can carry warm, moist air from a bathroom into the cool attic in the winter.
Your client will probably already know that air can leak in and out of their house through small openings around doors and windows, and through a fireplace chimney. Air can leak into the house from unconditioned spaces, such as the attic, basement and crawlspace. What your client might not be fully aware of are the other pathways for air leakage, including:
Air sealing is an essential first step. It is important to stop the air leakage prior to adding insulation in these particular areas because the insulation might make the pathways hidden and difficult to access.
Because these leakage pathways exist due to the tendency of warm air to rise and cool air to fall, the attic is often the best place to find air leaks and seal them up. Usually, adding more insulation at the attic floor area will not stop leaks because the air will flow through and around the insulation. Most insulation will not stop air leaks.
Look for Big Air Leaks
To ensure that an air barrier functions as intended, leaks in the building envelope and air barrier system must be reasonably controlled. The methods are generally low-tech and common-sense oriented.
Current building codes (such as the IRC 2006) require air sealing around the following areas:
Sealing materials include acceptable air-barrier materials and durable caulks, weatherstripping, sealants, tapes and gaskets, as appropriate. The material could be a suitable film or solid material.
The above list is exhaustive. All obvious air-leakage pathways should be sealed. Yet, practicality suggests that the major focus should be on the big leaks and big holes.
Big leakage points that should be air-sealed include:
Major leakage points in a house are illustrated. Property inspectors should look here.
Air Sealing from the Attic
It is important to seal up the air leakage pathways between the living space and the attic space, especially before your client adds any insulation in the attic.
The materials for sealing air-leakage pathways should be products that are durable and compatible with the joined materials, especially around hot surfaces. Examples include:
This is a content preview space you can use to get your audience interested in what you have to say so they can’t wait to learn and read more. Pull out the most interesting detail that appears on the page and write it here.