Wednesday, November 23, 2011

Bake out vs. Flush out

In the last month or so, a client of mine told me that occupants of a property he manages had been complaining about apparent elevated levels of Volatile Organic Compounds (VOC) following some roof work.  I went to the site and spotted some less than optimum situations and recommended some remedies.  A week or two later I received a call from my client saying that his client had decided to do a bake out.  I was a bit surprised since it had been my understanding that this procedure had fallen into disuse in favor of the flush out.  This being the case, I decided to do a bit of research to better educate myself on the two techniques.  In the event that others might be interested, I decided to share what I found through my blog.

It was my understanding that a bake out was simply the use of heat to cause VOCs to off gas and that there was no particular use of ventilation.  A flush out as I understood it was introducing a large amount of outside air to a building for a period of time in order to increase the off gassing of VOCs and to disperse these gases via outside air.

I found these definitions to be fairly accurate, but I did find one definition of bake out that included increased ventilation to exhaust the emissions.  That additional item of the definition makes a big difference it the potential use of this methodology.
From the information I have been able to gather, there appear to be some problems with both techniques and a middle ground might be preferable.
Problems with the bake out are certainly obvious if significant outside air isn’t introduced, because without sufficient air change, VOCs are likely to become reabsorbed by porous and semi-porous materials.  Other problems concern the inability to uniformly heat up various building materials.  This might cause warping due to some connected materials expanding more quickly than others.  For example, concrete slabs heat up more slowly than many floor coverings, vinyl or wooden floors could warp as a result.  According to the Healthy House Institute – – “Researchers also found that some chemicals were released into the air that aren’t released at normal room temperatures, and that the post-bake-out out gassing rates were hardly lower than the pre-bake-out emissions.  Bake-outs seem to have little effect on formaldehyde levels in a building, probably because formaldehyde-containing materials, such as particle board, are thick enough to have a substantial reservoir of formaldehyde in them.  It’s been suggested that part of the reduction in emissions that has been seen is due to the drying of concrete during a bake-out.  This results in a lower relative humidity indoors and lower out gassing rates.”
A flush out procedure can produce problems of its own, at least in hot, humid locales.  In moist climates, or even during rainy periods or those of particularly high humidity, the flush out could be problematic.  According to Environmental Observer, the newsletter of The Associated General Contractors of America, “Building startup procedures to meet LEED® credits include a credit flush-out of indoor containments using increased outdoor air either at the end of construction or during the initial occupancy period….  The amount of air needed to meet the flush-out requirements places a building at increased risk because of the moisture introduced with the increased outdoor air.  LEED®  requirements are that a minimum of 14,000 cubic feet per square foot of floor area is required for flush out.  This presents multiple problems: most HVAC systems are not designed to dehumidify that amount of outdoor air which, in a 100,000 square foot building, is 1,400 cubic feet of outside air.  Depending on the outside conditions at the time of the flush-out as much as 240,000 gallons of water can be added to a 100,000 square foot building.  This added moisture will be absorbed into building materials, finishes and furnishings, increasing the risk of mold growth.”
My conclusion after research on bake out vs. flush out, is that the use of either of these methods must involve a substantial degree of observation and common sense.  Often rote following of a rule or applying a specific procedure to a wide variety of situations can result in unforeseen problems. 
I seriously doubt that using a bake out without also increasing ventilation would be particularly effective and could present problems with materials inside the building.  At times combining an increase in temperature with the flush out might make sense, but perhaps not pushing the temperature up into the 90’s. 
In terms of the flush out, the outdoor conditions must be taken into consideration.  From an indoor air quality viewpoint, I tend to favor substantial outside air for dilution of contaminants, but I realize that in any endeavor analysis of costs and benefits are necessary.  Often with LEED® buildings and construction schedules getting behind, there is a rush to get the building occupied.  The reduced flush out along with an overall increase of outside air provided via the HVAC system with the building occupied could be a solution in a lot of cases, but outdoor conditions still need to be taken into consideration especially in terms of humidity.  In any case, it seems to me that studies need to be done for the purpose of developing proper flush out procedures based on a variety of outdoor temperatures and humidity.  The result of such studies could result in guidelines for proper procedures to achieve acceptable indoor air quality by application of flush out/bake out based upon outdoor temperatures and humidity at the time these activities are undertaken.
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