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.
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