Tree Limbing and Felling Workshops at ShelterJuly 11, 2013
FB Post #3October 15, 2020
Isaac Asimov wrote “Science gathers knowledge faster than society gathers wisdom.” We have seen this play out countless times in residential construction: double pane windows with seals that failed within a few years creating ‘obscurred glass’, vapor barriers installed on the wrong side of the wall assembly causing rot to form, solar collectors on the roof that froze in winter, flexible polybutylene piping with only two fittings per line that still failed at the fitting, to name a few. We are seeing it today in the form of airtight construction with little or no ventilation. Science has told us that building airtight homes will dramatically decrease heating and cooling costs, saving thousands over the life of a building. Compelling argument. The trouble is that many builders are not making the corollary adjustment: providing adequate planned ventilation.
The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) first published Standard 62.2:Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings in 2003. The standard was published again in 2004 and every three years since then. It is a very readable and informative document that contains not only the recommendations of the Standard, but also a good description of ventilation and its importance in residential buildings. The Standard addresses (and describes) three primary requirements for ventilation. 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 quality air; and finally “source control”. This last one is somewhat different than the prior two since it attempts to limit the sources of air contaminants rather than remove them once in the air. The intent of ASHRAE is to have the standard adopted by building codes, not used as an educational tool. You may find it interesting to read their companion ASHRAE Guideline 24-2008 if you are looking for more explanation.
Both the 2004 and 2007 editions contained few changes, but the 2013 edition has a fairly significant change. The standard, acknowledging that homes are now being built universally tighter than in 2004, has eliminated an assumed air leakage rate of 2 cubic feet per minute (cfm) per 100 square feet of living space. To compensate for this loss of leakiness, the Standard increased the recommended ventilation rate from 7.5 cfm per person plus .01cfm per square foot to 7.5 cfm per person plus .03 cfm per square foot. The effect is to roughly double the recommended ventilation rate! For example, consider a family of four living in a 2500 square foot home. The old recommendation for ventilation was
7.5 cfm/person * 4 people + .01 cfm * 2500 ft2 = 55 cfm.
The new recommendation is
7.5 cfm/person * 4 people + .03 cfm * 2500 ft2 = 105 cfm.
This near doubling is seemingly in response to elimination of a 2 cfm leak assumption. In the example above, 2cfm of unintended ventilation were eliminated and 50 cfm were added. That’s a big jump! This should be of particular note to people who are planning on using the standard to determine AMOUNT of ventilation and using negative pressure (exhaust only) as the METHOD of ventilation. It will be hard to meet the standard with exhaust only; you will likely be kept awake at night by the whistling of the make up air screaming through the small leaks in your building envelope. You will likely have such a large negative pressure that you may not be able to open your outward swinging doors; small birds may get sucked against the weather stripping of your windows as they try to fly by; you may raise the water table in your community…………..I could go on with the hyperbole, but you probably get the point: you will have to maintain a large negative pressure to reach the recommended flow rate (CFM).
The new ventilation recommendations are a good argument for a balanced ventilation system, with a controlled inlet and outlet. A great way to achieve this is with a Heat Recovery Ventilator (HRV) or Energy Recovery Ventilator (ERV), formerly known as air to air heat exchangers. This type of system, coupled with kitchen and bath fans for spot ventilation, will easily be able to meet the standard and provide fresh air from a reliable location outside the house.
ASHRAEs Standard 62.2 is not yet a part of the building code (the International Residential Code has not adopted it), so remains just a recommendation for now. However, inadequate ventilation results in poor indoor air quality (IAQ) where contaminants are allowed to reach unhealthy concentrations and relative humidity levels soar allowing mold to thrive. A thorough approach to ventilation examines the size and occupancy of the building to establish an acceptable flow rate as well as monitoring indoor air quality such as relative humidity and carbon monoxide to measure adequacy.