Attic Venting

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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;
• stack effect; and
• forced-air HVAC equipment, such as the central air handler. 

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

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:

• any openings and cracks where two walls meet;
• any openings and cracks where the wall meets the ceiling;
• any openings and cracks near interior door frames;
• gaps around electrical receptacle outlets, switches, and recessed light fixtures;
• gaps behind recessed cabinets;
• false ceilings and soffits in the kitchen;
• behind bathtubs and shower stall units;
• through floor cavities of finished attics adjacent to unconditioned attic spaces;
• utility chaseways for ducts, pipes and wires; and
• plumbing and electrical wire penetrations.

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:

• all joints, seams and penetrations, including utility penetrations;
• site-built windows, doors and skylights;
• openings between window and door assemblies, and their respective jambs and framing;
• knee walls;
• drop ceilings and chases adjacent to the thermal envelope;
• wall cavities and chases that extend into unconditioned space;
• walls and the ceiling separating an attached garage from conditioned space;
• openings behind tub and shower enclosures on exterior walls;
• common walls between dwelling units; and
• other sources of air leakage.

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:

• vertical mechanical chases;
• attic access hatches and pull-down ladders;
• floor overhangs;
• openings behind tub and shower enclosures;
• plumbing stack penetrations;
• utility penetrations in walls; and
• any exposed wall cavities that open into an adjacent attic space.

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:

• high-quality caulks;
• construction adhesives;
• spray polyurethane foam;
• gaskets;
• sill sealers;
• tapes; and
• a number of specialty products, such as gasketed electrical receptacles, switch boxes, and ceiling light-fixture boxes.