INTRODUCTION Infiltration@ adventitious or incidental air leakage through building envelopes@ is a common phenomenon that affects both indoor air quality and building energy consumption. Infiltration can contribute significantly to the overall heating or cooling load of a building@ but the magnitude of the effect depends on a host of factors@ including environmental conditions@ building design and operation@ and construction quality. Typically infiltration accounts for one-third to one-half of the space conditioning load of a home. In addition to increasing the conditioning load of a building@ infiltration can bring unwanted constituents into the building or into the building envelope and cause building failures. For example@ infiltration of hot@ humid air in an air conditioned building in the summer (or exfiltration of indoor air in a heated building in the winter) can cause condensation in the building envelope leading to potential structural failure and mold growth. For these reasons reducing infiltration is desirable. Infiltration@ however@ serves a vital purpose in most existing homes: it is the dominant mechanism for providing ventilation. The purpose of ventilation is to provide fresh (or at least outdoor) air for comfort and to ensure healthy indoor air quality by diluting contaminants. Historically@ people ventilate buildings to provide source control for both combustion products and objectionable odors (Sherman 2004). Currently@ a wide range of ventilation technologies is available to provide ventilation in dwellings including both mechanical systems and more sustainable technologies. Most of the existing housing stock in the U.S. uses infiltration combined with window opening to provide ventilation. Sometimes this results in overventilation with subsequent energy loss or under-ventilation and poor indoor air quality. Recent residential construction methods have created tighter@ more energy-saving building envelopes that create a potential for under-ventilation (Sherman and Dickerhoff 1994@ Sherman and Matson 2002). McWilliams and Sherman (2005) have reviewed ventilation standards and related Infiltration in ASHRAE's Residential Ventilation Standards Max Sherman@ PhD Fellow ASHRAE Max Sherman is a senior scientist at the Lawrence Berkeley National Laboratory@ Berkeley@ CA@ where he runs the Energy Performance of Buildings Group in the Indoor Environment Department. LO-09-085 ? 2009@ American Society of Heating@ Refrigerating and Air-Conditioning Engineers@ Inc. (www.ashrae.org). Published in ASHRAE Transactions 2009@ vol. 115@ part 2. For personal use only. Additional reproduction@ distribution@ or transmission in either print or digital form is not permitted without ASHRAE's prior written permission. 888 ASHRAE Transactions factors. Infiltration rates in new homes average a half to a quarter of the rates in existing stock. As a result@ new homes often need mechanical ventilation systems to meet current ventilation standards. Unless buildings are built completely tight and fully mechanically ventilated@ infiltration will always contribute towards ventilation. Ignoring that contribution can lead to over-ventilation and unnecessary energy expense; over-estimating of that contribution can lead to poor indoor air quality. This report uses simulation methods to help determine how infiltration can and should be properly valued in the context of residential ventilation.