New residential energy efficiency building codes are trying to tackle the challenge of how the home’s flow of energy, heat, air, and moisture will affect the indoor environment, including attics and crawlspaces. These codes are requiring homes to meet specific air tightness and indoor air quality standards, both of which are driving the need for dedicated dehumidification and mechanical ventilation.
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The code item which addresses the tightness of the building sets ACH 50 standards based on location. Climate Zones 1 and 2 must achieve ≤5 ach50, and in Zones 3 – 8 ≤3 ach50. What does this mean? According to Kimberly Llewellyn, consultant at Austin based building science firm, Positive Energy, an ACH50 number is a standardized score which describes how tight a home is. ACH stands for Air Changes per Hour, which is the number of times in an hour that the volume of air in a house is replaced at the test pressure 50 Pascals, which is about equivalent to the conditions created by a 20 mph wind. The lower the score, the tighter the building, which means less “accidental air” gets in by uncontrolled means of infiltration or exfiltration (See Climate zone map above).
Build it Tight. Ventilate Right.
Reducing the amount of air that leaks in and out of the home through insulation, air sealing, and new windows and doors are some of the first steps when it comes to cutting heating and cooling costs. Such energy saving strategies can result in unintended consequences. If effective mechanical ventilation and moisture control methods are not implemented, the result could lead to uncomfortable living conditions, homeowner health issues, and in extreme cases, significant structural damage.
This is why if you live in an area that has adopted the 2012 IECC code (and there will be 20 states that have by the end of 2015), you must now also make sure that the home has a whole-house mechanical ventilation system to bring in the right amount of fresh filtered air to dilute indoor pollutants, and replenish oxygen.
R403.5 Mechanical ventilation (Mandatory).
The building should be provided with ventilation that meets the requirements of the International Residential Code, or International Mechanical Code, as applicable, or with other approved means of ventilation. Air intakes and exhausts should have automatic or gravity dampers that close when the ventilation system is not operating.
It is also important to know that even if your residential building code currently does not mandate a required amount of mechanical fresh air, there are recommended standards that help ensure a home’s occupants are getting the fresh air needed. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has developed the standard 62.2: The Standards for Ventilation and Indoor Air Quality.
This standard is used to calculate the MINIMUM ventilation requirement using ASHRAE 62.2-2013.
ASHRAE Airflow in CFM = [House Area in Sq. Ft. x 0.03] + [(Number of Bedrooms +1) x 7.5]
2500 sq. ft. house with 3 bedrooms, 4 occupants = [2500 X 0.03] + [(3+1) X 7.5] = 105 CFM
The ASHRAE 62.2 2010 standard would have required 55 cfm of fresh air for the same exact house. This is due to the assumption that a leaky house had some natural ventilation that helped dilute pollutants.
As you can see, our houses are getting tighter, and as a result we need to bring in more fresh air. Bringing in filtered fresh air from a known location into a tightly air-sealed home to dilute indoor air contaminants is absolutely necessary. Depending on what region you live in, you will also need to think about how to remove the humidity that you are bringing into the home. Dedicated dehumidification will almost certainly be necessary to maintain <50%RH in a house that meets these new building codes in green grass climates (areas where dew points reach above 60 degrees F).
Traditional practice left it up to the A/C system to remove moisture in an attempt to keep it at an acceptable level. However, tightening houses limits the air conditioner running time, and oversized cooling systems can result in poor dehumidification. Also, the energy efficient, higher SEER A/C units do not remove as much moisture as the older, lower SEER systems. Keep in mind that an A/C system is only removing moisture when it is running. The shoulder seasons tend to be when dew points are highest, and temperatures are mild, resulting in high interior humidity levels with little to no moisture removal.
According to building science experts, in humid climates it is best to use a supply ventilation system to provide a slight positive pressure on the home in order to avoid the wet outside air being sucked into the home through the walls. During certain parts of the year (shoulder seasons), relative humidity will need to be controlled with a dehumidifier or the A/C system.