INFORMATION SERIES:  Ventilation

Considerable amounts of moisture are generated in most homes in the course of a normal day from cooking and bathing, as well as normal respiration. A moderate humidity level is desirable for occupant comfort and to prevent furniture damage. In fact, low moisture levels induce many people to install humidifiers during the heating season, especially if they have forced warm air heating. Once installed, they must be maintained on a regular basis to ensure that proper humidification takes place. It is not unusual for these units to either not operate at all or to pump in excessive amounts of moisture.

Excessive humidity can manifest itself in the form of mildew growth or condensation on windows or cold areas of walls.  Adequate ventilation of unconditioned spaces such as attics and crawl spaces is also essential to avoid excessive heat or moisture build-up. In warm weather, excessive heat build-up in the attic can shorten the service life of the roofing and increase living area temperatures. In many areas, summer heat also means high humidity. Excessive moisture trapped in an attic or crawl space can lead to mildew or fungi growth, odors, and in severe or chronic cases, wood decay. Excessive moisture also serves to attract wood destroying insects.

The HouseMaster Technical Staff advises that heat infiltration into an attic or crawl space from the living area during the heating season is a concern due to the energy loss and associated costs. Of greatest concern, however, may be the fact that as the warm air contacts cold surfaces, such as the roof sheathing or joists in a crawl space, condensation may occur. Excessive condensation over a long period of time may eventually cause decay of the affected elements. This condition is especially common with cathedral ceilings and inadequately vented attics with insulation between the rafters.

Cold weather condensation concerns are generally greatest on the north side of the house in a cathedral ceiling and/or whenever there is an inadequate vapor barrier or an improper exhaust vent discharge into the attic. A warm un-vented attic is also often the cause of ice dam formation at the eaves.

The detrimental conditions caused by excessive heat or moisture buildup in attics or crawl spaces or contamination of the living areas can be prevented by providing adequate ventilation. In attics, it is generally recommended that there be one square foot of free vent area for each 150 square feet of attic floor area. Additional provisions will be required if there is no vapor barrier or duct to the peculiarity of the particular house style or location.

In an existing house, actual conditions can be the determining factor. It the attic is 10 to 20' above outside air temperature on a warm day, or there is any odor, condensation or signs of mold growth, additional ventilation is required. The ideal arrangement in most cases is to provide soffit vents and vents near the roof peak (preferably ridge vents). In lieu of this arrangement, several roof and/or gable vents can be used. These vents should be left open year round; with proper insulation there should not be a significant energy loss concern.

While it is essential to provide adequate ventilation by passive means, in some cases forced venting may be advantageous. A wind turbine ventilator is often added for a low cost boost to air flow Electric attic exhaust fans, however, are often more effective at replacing the hot humid attic air with cool outside air. Thermostatic controls are provided for these units; in certain situations a humidistat may also be required. It an exhaust fan is present, it is important to ensure that adequate intake air provisions are provided, otherwise the unit will not operate effectively. Also, the fan should be checked periodically to determine that it is operational. Normal fan thermostat settings are 100''-110".

Some manufacturers have issued recall notices regarding possibly defective motors. While most of these units have either failed or have been replaced already, as a precaution record the model number and contact the manufacturer. Whole house fans are also used for ventilation purposes. Generally these are primarily used to remove excess heat build-up from the living area. As with any mechanical device periodic maintenance will be required to prevent malfunction.

Below-house crawl spaces should also have provisions for ventilation.  In some cases, heating units or distribution systems are located in the crawl spaces and indirectly heat it. In these cases, dampered vents, either automatic or manual, should be provided. Care must also be taken to prevent any pipe freeze-up or excessive air infiltration. Insulation should be installed between floor joists or on the crawl space walls, depending on the method of venting used.  

The ideal vent placement is near the foundation corners and in such a way that cross ventilation exists. The minimum venting should provide at least one square foot of free vent area for every 1500 square feet: more is required in most cases. In addition to the vents, to help reduce excessive moisture, adequate foundation grading and drainage should be provided and a vapor barrier should be placed over any exposed dirt floor areas.

All fuel burning appliances (i.e. furnaces, boilers and water heaters) should also be vented, according to manufacturer recommendations. These appliances should also be serviced periodically and exhaust provisions should be checked for proper operation to reduce possible air quality concerns.  

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