Mold & Mildew: relative vs. absolute humidity
Lew Harriman
potential mold growth in a school in Tampa, Florida):
> My question is: When dealing with mold and mildew growth, is relative
> humidity the only relevant factor or is absolute humidity (grains of
> moisture) equally, if not more, relevant?
John,
I spent some time investigating mold and mildew in hotels a few years
ago. The American Hotel & Motel Association published the results of a
survey I did for them with that same title. Most of what I learned about
fungal growth was from Dr. Jim Kimbrough of the Plant Pathology
Department of the University of Florida in Gainesville. He and Dr,
Virginia Peart have published very brief and comprehensive information
about mold and mildew in buildings and HVAC systems, and the information
is designed with the Florida circumstances in mind. So you might want to
contact either of those two great people for good information on the
subject.
But briefly, Jim Kimbrough explains that mold and mildew do not care
about *either* absolute or relative humidity. What they care about is
the amount of moisture in their food source. Either high relative or
high absolute humidity can get that moisture into their food source.
The spores land in dirt (or some other carbon-based material such as
books, jet fuel, tree trunks, etc). To grow, the spores need food. They
get that by secreting digestive enzymes out through their cell wall. If
the material they landed on has enough moisture in it, the enzymes can
dissolve that material. (Ever see purple stains on wall covering in
hotels? That's the enzymes at work, dissolving the pigments in the wall
covering while digesting the wall covering and its adhesive)
The spore pulls the nutrient solution back into the cell through a
difference in osmotic pressure. With the liquified nutrient, the spore
can grow more rapidly. After a while, fungal metabolism supplies enough
water from the digestion of nutrients to really accelerate the process
without external moisture being added to the process.
So in fact, relative humidity and absolute humidity both influence mold
growth. Books absorb moisture according to the relative humidity in the
surrounding air. The more moisture they absorb, the more rapidly they
will grow mold and mildew. Conversely, if the relative humidity is kept
low, the books will grow mold at a much slower rate (less water
available) In that respect, your newly-trained Energy Police are
correct.
But there are two problems with shutting off the AC units, in spite of
the fact that the rh drops when the temperature rises.
a. The absolute moisture is rising, as you pointed out. Water vapor
leaks into the building through humid air leaks, rain leaks, wind
pressure, internal suction and, to a much lesser extent, by vapor
diffusion.
Then, when the AC system comes back on and starts to cool down the
space, the relative humidity goes way up. The system seldom has enough
capacity to remove the increased absolute amount of moisture that
collected in the space while the system was off. The temperature falls,
but the absolute moisture stays constant, so the relative humidity
rises. At high relative humidity, the books gobble moisture.
b. Books don't give up moisture easily.
A recent ASHRAE research project from Canada showed that after books
pick up moisture, they don't give it up unless the air stays well below
30% rh for extended periods. Even then the moisture release is very
slow. So what happens in a library is that the books soak up moisture
during periods of high humidity, fail to give it up when the humidity
drops only to 40-50%. The books never give up moisture, so it
accummulates, eventually collecting enough moisture to grow mold and
mildew regardless of the relative humidity at the center of the room.
Once the accellerated growth process gets going it's very difficult to
stop. Anti-fungal wiping of each page, drying in a very dry environment,
etc.
Your Energy Cops might do well to take a simple sniff test in different
rooms around the school. If they smell musty odors, they can be sure
that their treasured "energy savings" have been converted into mold and
mildew. Then they might want to calculating the cost of removing the
mold, (or replacing the material) vs. the cost of the "saved energy" to
see if the decision to shut off the dehumidification system was really
cost-effective.
(We don't even want to think about the immunosuppressant effects of some
molds, that make it likely for building occupants to have colds and
"flu-like" symptoms in water-damaged buildings)
There are many other problems relating to mold and mildew in duct work
and in building cavities, and in humidity-related damage to gym floors.
Basically, it's a lousy idea to shut off dehumidification equipment in
Florida. If the only DH equipment you have is the air cooler, then run
it at a slow air speed to reduce the cooling and increase its
dehumidification.
A far better idea is to install a dedicated dehumidification system
controlled by a humidistat instead of a thermostat. Then temperature and
humidity are controlled independently, and don't "get in each other's
way" as they do when only a thermostat controls the mechanical system.
That strategy actually *does* save energy, because the temperature can
be allowed to float way up without the risk of humidity damage. But that
approach costs money and usually involves new equipment.
Hope this is useful,
Lew
---------------------------
Lew Harriman
Mason-Grant Consulting
P.O. Box 6547
Portsmouth, NH 03802 USA
(603) 431-0635
www.MasonGrant.com
LewHa...@MasonGrant.com
---------------------------
Murdentech
Terriffic information there. I've heard a lot about the problems with mold
growth and "sick buildings" in Florida and other tropical regions. We seem
to have some similar problems in the Cincinnati area, but on a much smaller
scale.
It would seem logical in this case to have equipment designed with a
secondary condenser coil inside the indoor air flow. This would allow the
equipment to become one great dehumidifier when cooling is not needed but
dehumidification would be a tremendous benefit to "dry out" the building
during unoccupied periods. Not to mention that even when the cooling
equipment is in an off cycle during occupied mode, the fan is still sucking
in tons of damp air from the outdoors.
Do you know of much work being done with rechargable dessicants in this
environment and has it helped the humidity problems there?
--
Jeff Murden, SMA
www.murdentech.com
Lew Harriman <LewHa...@MasonGrant.com> wrote in message
news:380B51...@MasonGrant.com...
Pete & Sandi Groenewold
one. In fact, several of the manufacturers of refrigerant-based
dehumidifiers/air conditioners do just that. Our (Desert Aire's) own "QS"
series has an eight-row evaporator coupled with a hot gas reheat coil, which
will maintain supply air temperatures within a degree or so of set point. In
fact, we can automatically maintain both the temperature and the humidity of
your conditioned space.
However, I realize this NG is not for advertising products. And as Lew
Harriman (a moisture removal guru, in my opinion!) will certainly suggest,
there are many ways to economically cool and dehumidify a space. One of the
methods I really like is a hybrid refrigerant/energy recovery wheel system. If
you're in the Southeast and wheel defrost is not an issue, such systems can
save a building owner a nice chunk of change over the operating life of the
equipment.
One manufacturer of these systems is Aaon, in Tulsa, OK. I think they build a
decent unit for the money. BTW, I'd love to hear from others regarding your
experiences with hybrid DX/heat wheel; DX/heat pipe, or DX/plate-type systems.
Thanks in advance for your comments!
Peter Groenewold
Vernon O
humidity.
Pete & Sandi Groenewold <gr...@execpc.com> wrote in message
news:380bee84$0$48...@news.execpc.com...