The MAD AIR Study and its Impact on Building Science
The 1988 research paper Mechanical Air Distribution And Interacting Relationships (MAD AIR), authored by John Tooley and Neil Moyer, represents a seminal work in building science that fundamentally shifted the industry's understanding of how HVAC systems impact home performance. Moving beyond a singular focus on the building envelope, the MAD AIR study systematically demonstrated that forced-air mechanical systems are a dominant driving force behind air leakage and pressure imbalances in homes. The paper's core findings revealed that duct leakage outside the conditioned space and the closure of interior doors in homes with central returns create significant positive or negative pressures, leading to a cascade of negative consequences.
Tooley and Moyer were among the first to comprehensively connect these mechanically induced pressures to four critical areas: excessive energy consumption, poor thermal comfort, degradation of building materials through moisture and mold, and severe indoor air quality problems, including the backdrafting of combustion appliances. The research, based on a study of 371 homes in Florida, quantified these pressure dynamics and established foundational principles that are now standard in building science education.
The legacy of the MAD AIR study was immediate and profound. It served as a cornerstone for Joe Lstiburek's doctoral thesis, which further developed the concepts of building pressure fields and airflow through interstitial cavities. It catalyzed a debate and subsequent innovation in HVAC design, leading to two primary solutions for mitigating the problem: Tooley's advocacy for aggressively sealing ductwork, and Lstiburek's promotion of moving ducts inside the conditioned space via unvented attics. The study's principles continue to inform best practices for duct sealing, pressure balancing, and whole-house system design, underscoring the critical need to view a house not as a collection of parts, but as an integrated system.
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1. Historical Context of Building Pressure Testing
The groundbreaking work of Tooley and Moyer in the late 1980s built upon a decade of foundational research into building airtightness. The tools and initial concepts that enabled the MAD AIR study emerged in the late 1970s and early 1980s.
The Blower Door: First developed in Sweden in 1977, the blower door was introduced to the American market a few years later. By 1986, thirteen companies were manufacturing this tool, which became invaluable for quantifying a building's air leakage.
The "House Doctors": In the late 1970s, Gautam Dutt and his colleagues at Princeton University began combining blower door pressure testing with infrared imaging. Calling themselves the "House Doctors," they diagnosed home performance issues and prescribed air sealing remedies. Dutt is also credited with discovering the "thermal bypass," a phenomenon where insulation's effectiveness is compromised by air movement.
Initial Research Focus: During the 1980s, the primary focus of energy auditors and researchers was on air leakage through the building enclosure itself—the walls, ceiling, and floors. The goal was to find and seal holes that allowed conditioned air to escape or unconditioned air to enter. The significant role of the HVAC system's ductwork was not yet widely understood.
2. The Groundbreaking "MAD AIR" Paper (1988)
In 1988, John Tooley and Neil Moyer of Natural Florida Retrofit, Inc. published their findings from extensive testing in central Florida, a hot and humid climate where central air conditioning is nearly universal. Their paper, Mechanical Air Distribution And Interacting Relationships, or "MAD AIR," shifted the industry's focus to the powerful and often detrimental effects of the HVAC system on the building itself.
2.1. Study Details and Methodology
The study documented findings from 371 single-family homes in central Florida between May 1987 and May 1988. The homes varied in age and construction type.
Construction Type
Percentage of Homes
Concrete Block
76.9%
Frame
11.6%
Mobile Homes
11.6%
The researchers' methodology involved first determining the baseline infiltration rate of the homes and then observing what happened to the pressure dynamics when the mechanical system was activated or when homeowner behaviors, such as closing doors, were simulated. They found that pressure differences could range from neutral to as high as 60 Pascals (0.24" w.c.).
2.2. Core Findings
The MAD AIR paper established three fundamental principles regarding the interaction between duct systems and house pressures, which have since become cornerstones of building science diagnostics.
Return Leakage: Leaks in the return ductwork located outside the conditioned space (e.g., in an attic or crawlspace) cause the house pressure to become positive. This forces conditioned air out of the house through the building envelope. This principle is often summarized by the mnemonic: "Return leaks blow."
Supply Leakage: Leaks in the supply ductwork located outside the conditioned space cause the house pressure to become negative. This pulls unconditioned, often humid and contaminated air into the house from outside. This is summarized as: "Supply leaks suck."
Interior Door Closure: Closing doors to rooms (typically bedrooms) that receive supply air but do not have an adequate return air path causes the main body of the house to develop negative pressure while the closed-off rooms become positively pressurized.
The study found that in a subset of 101 homes, 100% of the 18 homes with positive pressure had return leaks, and 80% of the 17 homes with negative pressure had supply leaks. Furthermore, 100% of the 56 homes that registered negative pressure with interior doors closed had a single central return system.
2.3. The Four Interacting Factors
Tooley and Moyer concluded that the pressure imbalances were not caused by a single defect but by a combination of four factors:
Duct System Failure: Leaks in the ductwork, which were found in varying degrees in 100% of the 371 systems inspected.
Duct System Design: The common use of single central returns starves the system for air when interior doors are closed.
Homeowner Interaction: Occupants closing doors for privacy or closing registers to unused rooms directly creates pressure imbalances.
System Cleanliness: Dirty filters, blowers, or evaporator coils restrict airflow and can exacerbate pressure differences.
2.4. Critical Implications of "MAD AIR"
The paper's most significant contribution was its clear-eyed assessment of the consequences of these pressure dynamics. Tooley and Moyer explicitly stated that these factors were "major contributors to (1) excessive energy consumption, (2) poor thermal comfort, (3) degradation of building materials and (4) indoor air quality problems (i.e. homeowner health to the possible extent of illness, grave sickness and even death)."
They detailed how mechanically induced infiltration pulls hot, humid air from attics (where temperatures can exceed 130°F) into the conditioned space, leading to comfort issues and moisture problems like mold. Crucially, they identified the risk of negative pressures causing backdrafting of combustion appliances (furnaces, water heaters, fireplaces), pulling dangerous flue gases like carbon monoxide into the home.
3. The Legacy and Expansion of MAD AIR Principles
The MAD AIR paper served as a catalyst, validating the observations of other researchers and providing a framework for future work that shaped the modern home performance industry.
3.1. Joe Lstiburek and the "Building as a System" Concept
Dr. Joe Lstiburek, a prominent building scientist, credits the MAD AIR paper as a vital part of his doctoral research.
PhD Thesis: His thesis, Toward an Understanding and Prediction of Air Flow in Buildings, expanded on Tooley and Moyer's work. He argued that buildings must be understood as "complex three dimensional air flow networks driven by complex air pressure relationships."
Interstitial Cavities: Lstiburek heavily focused on the role of interstitial cavities—the spaces within building assemblies like joist bays and wall cavities. He explained how duct leakage pressurizes these spaces, turning them into pathways for air leakage between the conditioned space and the outside.
The Great Debate: Fix Ducts vs. Move Ducts: The MAD AIR findings spurred a debate on the best solution.
John Tooley's Approach: Argued for fixing the problem directly by making ducts airtight. He championed the use of mastic for sealing and duct testing for verification, working with figures like Gary Nelson of The Energy Conservatory, who developed the necessary tools like the Duct Blaster®. This approach proved it was possible to reduce duct leakage from a typical 20% down to 3-5%.
Joe Lstiburek's Approach: Initially skeptical that the trades could achieve sufficient airtightness, he argued the only foolproof solution was to move the ducts inside the conditioned envelope. He became a major proponent of unvented, conditioned attics, where the thermal and air barriers are moved to the roof deck.
3.2. Quantifying the Energy Penalties
Lstiburek and others later quantified the energy impact of different duct placement strategies, confirming the severity of the problem identified by MAD AIR.
Duct Configuration Scenario
Cooling & Heating Energy Penalty (Compared to Base Case)
Base Case (Best): Airtight ducts and air handler inside conditioned space; vented attic over an airtight ceiling.
0%
Typical (Worst): Leaky (~20%) ducts and air handler in a vented attic.
30% or more
Tooley's Solution: Airtight (3-5% leakage) ducts in a vented attic.
~10%
Lstiburek's Solution: Conditioned (unvented) attic with ducts inside.
~10% (due to increased building surface area)
Buried Ducts: Airtight ducts in a vented attic, insulated with spray foam and buried in insulation.
Less than 5%
3.3. Corroborating Research
Subsequent studies reinforced the MAD AIR findings. A 1991 investigation by James B. Cummings at the Florida Solar Energy Center on 160 homes found:
Infiltration rates were three times greater when the air handler was operating.
Repairing duct leaks reduced average cooling energy use by 17.2%, for an estimated annual savings of $110, with a simple payback of less than two years.
Duct repairs could reduce winter peak electrical demand by an estimated 1.6 kW per house.
4. Practical Applications and Related Misconceptions
The principles of MAD AIR directly challenge several common but misguided home energy strategies, particularly those related to attics.
Powered Attic Ventilators (PAVs): The concept of "MAD AIR" explains precisely why PAVs are a bad idea. A 1996 paper by John Tooley and Bruce Davis concluded they "should not be used."
PAVs depressurize the attic, pulling large amounts of conditioned air from the house through ceiling leaks (e.g., recessed lights, top plates).
Research found that, on average, PAVs draw 231 cfm of conditioned air out of the house.
When accounting for their own energy use, they are net energy losers.
They can create enough negative pressure to backdraft combustion appliances.
Passive Attic Ventilation: Many homeowners believe adding more vents will cool the house. However, research by Professor Bill Rose shows that for well-insulated attics (R-25 or greater), ventilation has a negligible effect on cooling loads. The proper solution for a hot attic is to ensure it is well-insulated and air-sealed from the house below.
Radiant Barriers: Research from Oak Ridge National Laboratory shows that the effectiveness of radiant barriers is minimal in an attic that already has proper levels of insulation (R-30 or higher). This is why major Arizona utilities do not offer rebates for radiant barriers but do for insulation, duct sealing, and air sealing—strategies that address the core issues identified by MAD AIR.