Isaac Asimov wrote"Science gathers knowledge faster than society gathers wisdom." Wehave seen this play out countless times in residential construction: doublepane windows with seals that failed within a few years creating 'obscurredglass', vapor barriers installed on the wrong side of the wall assembly causingrot to form, solar collectors on the roof that froze in winter, flexiblepolybutylene piping with only two fittings per line that still failed at thefitting, to name a few. We are seeing ittoday in the form of airtight construction with little or no ventilation.Science has told us that building airtight homes will dramatically decreaseheating and cooling costs, saving thousands over the life of a building.Compelling argument. The trouble is that many builders are not making thecorollary adjustment: providing adequate planned ventilation.The American Societyof Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) firstpublished Standard 62.2:Ventilation and Acceptable Indoor Air Quality inLow-Rise Residential Buildings in 2003. The standard was published again in2004 and every three years since then. It is a very readable and informativedocument that contains not only the recommendations of the Standard, but also agood description of ventilation and its importance in residential buildings.The Standard addresses (and describes) three primary requirements forventilation. These are "whole-building", or dilution, ventilation;spot ventilation for removal of moisture and contaminants from specific rooms(bathroom, kitchen) or areas that tend to generate or contain problem qualityair; and finally "source control". This last one is somewhatdifferent than the prior two since it attempts to limit the sources of air contaminants rather than remove them oncein the air. The intent of ASHRAE is to have the standard adopted by buildingcodes, not used as an educational tool. You may find it interesting to readtheir companion ASHRAE Guideline 24-2008 if you are looking for moreexplanation.

Both the 2004 and2007 editions contained few changes, but the 2013 edition has a fairlysignificant change. The standard, acknowledging that homes are now being builtuniversally tighter than in 2004, has eliminated an assumed air leakage rate of2 cubic feet per minute (cfm) per 100 square feet of living space. To compensatefor this loss of leakiness, the Standard increased the recommended ventilationrate from 7.5 cfm per person plus .01cfm per square foot to 7.5 cfm per personplus .03 cfm per square foot. Theeffect is to roughly double the recommended ventilation rate! For example,consider a family of four living in a 2500 square foot home. The oldrecommendation for ventilation was

7.5 cfm/person * 4 people + .01 cfm * 2500 ft²= 55 cfm.

The newrecommendation is

7.5 cfm/person * 4 people + .03 cfm * 2500 ft²= 105 cfm.

This near doublingis seemingly in response to elimination of a 2 cfm leak assumption. In theexample above, 2cfm of unintended ventilation were eliminated and 50 cfm wereadded. That's a big jump! This should be of particular note to people who areplanning on using the standard to determine AMOUNT of ventilation and usingnegative pressure (exhaust only) as the METHOD of ventilation. It will be hardto meet the standard with exhaust only; you will likely be kept awake at nightby the whistling of the make up air screaming through the small leaks in yourbuilding envelope. You will likely have such a large negative pressure that youmay not be able to open your outward swinging doors; small birds may get suckedagainst the weather stripping of your windows as they try to fly by; you mayraise the water table in your community..............I could go on with thehyperbole, but you probably get the point: you will have to maintain a largenegative pressure to reach the recommended flow rate (CFM).

The new ventilationrecommendations are a good argument for a balanced ventilation system, with a controlledinlet and outlet. A great way to achieve this is with a Heat RecoveryVentilator (HRV) or Energy Recovery Ventilator (ERV), formerly known as air toair heat exchangers. This type of system, coupled with kitchen and bath fansfor spot ventilation, will easily be able to meet the standard and providefresh air from a reliable location outside the house.

ASHRAEs Standard62.2 is not yet a part of the building code (the International Residential Codehas not adopted it), so remains just a recommendation for now. However, inadequateventilation results in poor indoor air quality (IAQ) where contaminants areallowed to reach unhealthy concentrations and relative humidity levels soarallowing mold to thrive. A thorough approach to ventilation examines the sizeand occupancy of the building to establish an acceptable flow rate as well asmonitoring indoor air quality such as relative humidity and carbon monoxide tomeasure adequacy.