The following is a guest post written by Joshua Rhodes and Brent Stephens, PhD students at The University of Texas at Austin. As a part of their research, they work on two different aspects of buildings, with Josh focusing on energy use and efficiency and Brent focusing on indoor air quality. Their post compliments David Wogan’s previous piece on building energy audits, by taking a quantitative look at home energy retrofits and what can happen if you go too far.
Buildings use a lot of energy – approximately 40% of the energy consumed in the US, compared to 30% each for the industrial and transportation sectors. Residential buildings consume a little more than half of the total energy consumed by buildings, or about 22% of the total . Rightfully so, energy efficiency has been a priority of the Obama administration, which has been encouraging energy efficient new construction and energy retrofits for existing homes.
Two examples of federal energy efficiency efforts are the EPA and DOE’s ENERGY STAR Qualified New Homes program, which aims to build homes that are 20-30% more efficient than standard homes , and the HOME PERFORMANCE WITH ENERGY STAR program, which retrofits existing homes and aims for savings of 25-35% of their previous energy usage .
Tightening the Building Envelope
Typical home energy improvements include more efficient heating, ventilation and air-conditioning (HVAC) equipment, increased insulation, better windows, more efficient appliances, HVAC maintenance, and envelope tightening by air sealing. Considering the need for cooling during the summer time (it is 107 °F [42 °C] in Austin, TX as we are writing this post), Equation 1 shows how the bulk of heat gets inside a home. The same equation holds for wintertime, but the solar and interior terms are negative (they actually help you heat your home).
In this equation, QHVAC is the rate at which your air-conditioner must remove energy (or in the winter must add), in Watts. Qconduction is the conduction of heat through your walls, ceiling, floor, and windows, Qsolar is the amount of solar gain through your windows, Qinterior is the amount of heat from computers, people, and appliances, and Qinfiltration is the amount of heat that comes in as outside air seeps through cracks in the home envelope. Thus, considering air-conditioning in a cooling climate, reducing any of the terms on the right side of Equation 1 will reduce the amount of heat that must be removed by the HVAC system, and thus reduce energy consumption.
But Not Too Far – Air Quality Concerns
While there is a point of diminishing returns on energy and cost savings for most home improvements (i.e., reducing Qconduction and Qsolar in cooling climates), excessive envelope tightening (i.e., reducing Qinfiltration) can inadvertently introduce other potentially serious problems . Because most homes in the US do not provide fresh air with dedicated mechanical ventilation, they are typically only vented by natural infiltration, or by manually opening windows or operating exhaust fans. So, all things being equal, a reduction in air leakage in attempts to save energy will also reduce the amount of fresh air flowing into a home.
Let’s first look at the energy requirements of air infiltration, described by Equation 2.
In this equation, ρ is the density of air, cp is the specific heat capacity of air, ΔT is the temperature difference between the inside and outside, V is the volume of the house, and AER is the air exchange rate, or the fraction of the inside air that is replaced by outdoor air every hour. Typical air exchange rates in residences are about 0.5 per hour for existing homes  and 0.2-0.3 for new construction , meaning that indoor air is replaced by outdoor air every 2 to 5 hours, on average, for older and new homes, respectively. Given an indoor temperature set point of 78°F (26°C), an outdoor temperature of 100°F (38°C), the excess instantaneous cooling load in typical 1500 ft2 one-story home (V = 12000 ft3) would be about 700 W for a conventional home and as low as 300 W for a relatively new, or tightened, home; nearly a 60% reduction in the infiltration load.
But tightening the building envelope too much can inadvertently lead to the accumulation of indoor air pollutants that can be hazardous to occupant’s health and well being. Equation 3 is a simplified mass balance equation that is often used to represent the concentration of many indoor air pollutants.
In this equation, Cin is the inside concentration of the given pollutant, P is the fractional efficiency of the building envelope for filtering the pollutant from outdoors (through deposition or reaction with materials in the envelope), Co is the outside concentration of the same pollutant in question, E is the total emission rate of said pollutant, V is the volume of the home, and AER is the air exchange rate. As AER decreases, the contribution of indoor sources increases.
What can be seen from Equations 2 and 3 is that decreasing AER by tightening a home’s envelope can reduce the cooling load of the home, but can also increase indoor pollutant levels. This is especially important for pollutants that are emitted or generated indoors. Some of these pollutants include tobacco smoke, formaldehyde (from insulation, furniture foams, and pressed wood products), carbon monoxide (from indoor combustion and gas range stoves), para-dichlorobenzene (from toilet bowl deodorizers), and naphthalene (used in nearly pure form as moth balls), among many others. The potential health effects of these pollutants range widely from respiratory irritation to cancer .
So what’s the indoor air quality penalty for this ~60% reduction in infiltration cooling loads? Given an indoor pollutant with no outdoor sources (like formaldehyde in most areas), reducing a home’s air exchange rate from 0.5 per hour to 0.2 per hour would directly increase indoor concentrations by a factor of 2.5.
Was it worth it?
Well, perhaps yes. If special attention is paid to limiting indoor sources of pollutants, maybe home retrofits can actually achieve both energy and health benefits. For example, increased air sealing and new construction practices might be changing the value of P in homes (the ability for outdoor pollutants to penetrate through building envelopes). At the University of Texas at Austin, one of the authors has recently developed a method to measure the penetration efficiency for outdoor ozone, and found that P for ozone actually appears to be lower in newer homes than in older homes. 
Ozone is important because it is harmful on its own, and can also form some harmful indoor air pollutants via indoor chemistry . For example, ozone reacts with d-limonene – the ubiquitous chemical responsible for citrus scents found in everything from cleaners to candles – to produce a whole host of harmful chemicals, including formaldehyde, a known carcinogen. If building envelopes can be designed to buffer from outdoor ozone, we may actually observe a range of health benefits.
Tighten Up Your House, But not Too Much…
Historically, homes have been built such that they had high air exchange rates and indoor air pollutant concentrations were diluted to the levels that they exist outdoors. However, as building envelopes become tighter, the concentrations of indoor air pollutants will increase. There are promising new filtration products, such as impregnated activated carbon filters, that can remove some chemical pollutants from the indoor air, but they generally require additional energy to operate. There is obviously a trade off between energy efficiency and indoor air quality, and while we must increase the energy efficiency of our residential building stock, we should not do so at the cost of our health. All in all, as building envelopes become tighter, the indoor pollutant source mix is more important. Practical behavioral practices can often easily reduce the amount of indoor air pollution that one is exposed to. Discontinuing the use of strongly fragranced cleaners and air fresheners, ozone generators sold as air cleaners (this is always a bad idea  and deserves its own blog post), unsealed pressed wood products, and moth balls can go a long way in reducing the amount of indoor air pollution to which you are exposed.
 Belzer, D., G. Mosey, P. Plympton, and L. Dagher., 2007. Home Performance with ENERGY STAR: Utility Bill July 2007 Analysis on Homes Participating in Austin Energy’s Program. Rep. no. NREL/TP-640-41903. Golden, CO: National Renewable Energy Laboratory.
 Manuel, J., 2011. "Avoiding Health Pitfalls of Home Energy-Efficiency Retrofits." Environmental Health Perspectives 119:a76-a79
 Murray, D.M and D.E. Burmaster., 1995. “Residential Air Exchange Rates in the United States: Empirical and Estimated Parametric Distributions by Season and Climatic Region.” Risk Analysis 15(4):459-465.
 Offermann, F.J., 2009. “Ventilation and Indoor Air Quality in New Homes.” California Energy Commission Report CEC-500-2009-085. http://www.arb.ca.gov/research/apr/past/04-310.pdf
 Weschler, C., 2009. "Changes in Indoor Pollutants since the 1950s." Atmospheric Environment 43(1):153-69.
 Stephens, B. Academic Homepage: “Ozone penetration into buildings.” https://sites.google.com/site/stephensbrent/living-dissertation#TOC-Ozone-penetration-into-buildings.
 Weschler, C.J., 2000. "Ozone in Indoor Environments: Concentration and Chemistry." Indoor Air 10(4):269-88.
 Siegel, J.A., M.S. Waring, X. Yu, and R.L. Corsi, 2006. "Indoor air quality implications of portable ion generators." Proceedings of the A&WMA Specialty Conference on Indoor Environmental Quality – Problems, Research, and Solutions, Research Triangle Park, North Carolina (July 2006) 1:1-12.
2. Photo of Blower Door was found on Wikipedia and exists in the public domain.
About the Authors:
Joshua Rhodes is a Ph.D. candidate in Civil, Architectural, and Environmental Engineering at the University of Texas at Austin. His current research is in the area of residential smart grid applications, including system-level applications of energy efficiency and distributed generation. He enjoys mountain biking and rock climbing, and will get up early to make it happen before class. (http://www.webberenergygroup.com/people)
Brent Stephens is a PhD candidate in Civil, Architectural and Environmental Engineering at the University of Texas at Austin, where he is working to develop novel methods for measuring the fate and transport of indoor pollutants in buildings. He received his B.S. in Civil Engineering from Tennessee Technological University in 2007 and his M.S. in Environmental and Water Resources Engineering from UT-Austin in 2009. He is currently a member of a National Science Foundation IGERT program in Indoor Environmental Science and Engineering (http://www.caee.utexas.edu/igert/).