ASHRAE Journal - November 2010 - 58

iaq applications	
Study Shows Mechanical Outdoor Ventilation Necessary

iaq in airtight Homes
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By Francis (Bud) J. offermann, p.E.,	Member	ASHRAE

omes	are	being	constructed	and	retrofitted	to	make	them	more	airtight	 for	energy	conservation	purposes.	In	addition,	many	homeowners	re-

port	never	opening	windows	for	ventilation.	As	a	result,	these	airtight	homes	

with	closed	windows	have	very	low	outdoor	air	exchange	rates,	which	can	 result	 in	 elevated	 concentrations	 of	 indoor	 air	 contaminants	 with	 indoor	 sources.	In	recognition	of	this	trend,	ASHRAE	Standard	62.2,	Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings,	has	required	 mechanical	outdoor	air	ventilation	in	residences	since	2003.
To address this concern regarding residential ventilation, a multi-season field study of ventilation and indoor air quality was conducted in 108 new singlefamily, detached homes in California.1 While most new homes are built without a mechanical outdoor air system, this study searched and recruited 26 homes with a mechanical outdoor air system. In this study the field teams measured indoor and outdoor air concentrations of 22 volatile organic compounds (VOCs), including formaldehyde and acetaldehyde, PM2.5 particulate matter, nitrogen dioxide, carbon monoxide, and carbon dioxide over 24-hour periods. The outdoor air ventilation rates were determined concurrent with the air contaminant measurements using passive perfluorocarbon tracer (PFT) gas measurements. The final report1 describes the details of the home selection, measurement methods, results, and conclusions. Here we report the findings related to the performance of the mechanical outdoor air systems. Most homes were single or two-story, slab-on-grade, stucco exterior, with an attached garage, and a forced air heating/ cooling unit (FAU) in the attic. Two types
58	 ASHRAE	Journal	

of mechanical outdoor air systems were encountered in this field study: ducted outdoor air to the return side of the home FAU (DOA systems) and heat recovery ventilators (HRV systems). The HRVs were independent systems that exhausted air from the bathrooms and utility room and supplied outdoor air, pre-cooled/heated by the exhaust air in an air-to-air heat exchanger, to the hallway near the return for the FAU. There were a total of 26 homes with mechanical outdoor air systems: 17 DOA systems and nine HRV systems. The blower door measurements of the airtightness of these new homes indicated that the homes were relatively airtight. The median ACH50 was 4.8, and ranged from 2.8 to 8.4. A total of 32% of the homes did not use their windows during the 24-hour Test Day, and 15% of the homes did not use their windows during the entire preceding week. The median 24-hour outdoor air exchange rate was 0.26 air changes per hour (ach); 57% of the homes were below 0.30 air changes per hour and 25% were less than 0.18 ach. Of the indoor air contaminants measured, only formaldehyde and PM2.5
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particulate matter exceeded recommended non-cancer and non-reproductive exposure guidelines. For formaldehyde (24-hour measurements), 98% of the homes exceeded the California Office of Environmental Health Hazard Assessment (OEHHA) Chronic and 8-hour reference exposure levels (RELs) of 9 µg/m3, and 28% exceeded the OEHHA Acute REL of 55 µg/m3. For PM2.5, only one home, with an indoor concentration of 36 µg/m3, exceeded the EPA PM2.5 24-hour ambient air quality standard of 35 µg/m3. Most new homes had indoor concentrations of formaldehyde that exceeded recommended non-cancer irritant guidelines. The primary source of formaldehyde in homes is composite wood products; particleboard was used in 99% of the kitchen and bathroom cabinetry, as well as many pieces of furniture. Other sources of composite wood include plywood and oriented strand board in walls, subfloors, and attics, and medium density fiberboard in baseboards, window shades, interior doors, and window/door trims. Figure 1 is a scatter plot of the indoor concentrations of formaldehyde and the outdoor air exchange rates, depicting a significant (p <0.0001) inverse correlation (i.e., higher indoor formaldehyde concentrations correlated with lower outdoor air exchange rates). The only other significant correlation between indoor formaldehyde concentrations was indoor air temperature, which was found to correlate positively (i.e., higher indoor air temperatures correlated with higher indoor formaldehyde concentrations). As depicted in Figure 1, homes with low air exchange rates have a higher frequency of exceeding the formaldehyde acute one hour irritant guideline of 55 µg/m3. The median Standard 62.2 recommended minimum outdoor air exchange rate for the homes in this study was 0.30 ach (i.e.,
	 November	 2010
ASHRAE Journal - November 2010

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