Temperature: Its Role in Restorative Drying
2003 CleanFax Article
by Ken Larsen
One of the most overlooked and misused factors in restorative drying is temperature. Most of us with formal restoration
training are familiar with the three sections of the drying pie. However, we often focus on just airflow and humidity and
ignore the role of temperature. This leaves out a huge element of the science of psychrometry and therefore that of rapid
drying.
A technician who clearly understands psychrometry and the dynamics of moisture movement through structural materials
(moisture content) will be able to produce a balanced drying strategy that includes all three elements of the drying pie.
This will enable them to meet their timeline targets and accomplish their drying goals.
The Drying Pie Revisited
Today's IICRC-trained restorative drying professional is well aware of the three factors affecting the speed of drying wet
surfaces. The "Restorative Drying Pie" includes these important factors (see figure 1):
- Humidity
- Airflow
- Temperature
A strategy that implements all three factors is essential to complete and rapid drying. Unfortunately, restoration technicians aren't always the first "restorer" on the job!
It is not uncommon for a homeowner to call a restorer and explain in detail how they have had a water intrusion in their
home and employed a restorative drying system of their own. Homeowners usually create systems that address only one or two
of the principles illustrated in the "restorative drying pie." It is not unusual that this unbalanced drying system results
in odors, mold contamination, or secondary damages. How often have we heard the homeowner describe their drying procedure this way?
"I had water flood my home and I
- Opened my windows." Or;
- I put my convenience fan on the wet area." Or;
- I turned up my HVAC heat."
"Now I have an odor—and I don't know what to do!"
What went wrong? Each of these well meaning efforts failed to address at least one section of the "Restorative Drying Pie".
Actions by our clients are never the only challenges that the restoration professional will face.
Other scenarios that you will likely recognize as a challenge to many jobs are listed below:
"I am restoring my client's property and it isn't drying the way I want it to because
- I can't get air to the wet areas" Or;
- I can't introduce airflow to the structure for fear that I will spread spores or contamination." Or;
- The house is over 100°F and now my dehumidifiers aren't working correctly." Or;
- The Moisture Content (MC) in the plywood floor or concrete just isn't moving. Why?" Or;
- I am trying to dry a crawlspace or an attic and I can't get any drying equipment into the area." Or;
- The environment is too cold and the structure is drying too slowly."
This list of challenges is virtually endless. It is precisely this challenge that attracts many restorers to this
industry! Each project is unique and requires tremendously innovative thought and practice to successfully restore
these wet environments. This innovative thought may force the restorer to "think outside of the box." Several of the
challenges mentioned above can be overcome through a clear understanding of the role of temperature on the drying job.
However, as homeowner experiences have demonstrated, employing only a single principle from the Restorative Drying Pie can
have detrimental consequences. So how can a restorer determine an acceptable temperature range?
How Hot is Too Hot?
Many people wish to increase the "thirst" of the air by raising the temperature of the drying environment. Although relative
humidity decreases as the temperature rises, we know that specific humidity does not change. For some, it is very tempting to
simply raise the temperature of the environment in order to drop the relative humidity and increase the thirst of the air.
But it is possible to cause undesirable secondary damage to both contents and structure if the temperature becomes too high.
So how hot is TOO HOT?
You should consider several factors when determining maximum temperatures inside the structure:
- Obviously, hot indoor temperatures are most undesirable when clients remain in the structure during the drying process.
- Elevated indoor temperatures that significantly alter the moisture content of unaffected hygroscopic materials are undesirable.
Think about how kiln-dried lumber is produced. Does wood change in size as it absorbs or loses moisture? Absolutely!
- Psychrometry teaches us that if the specific humidity or moisture content stays the same, a change in temperature will not affect
the dew point. Surprising to some—it IS possible to have condensation issues even when employing the use of added heat!
- Most refrigerant dehumidification systems found in our industry drop off in performance significantly as the temperature rises above approximately 95°F (34°C). Desiccant technology always works best when the wet air is cooler.
There are several other factors that may be added to this list, as the project's unique needs are determined. The point here is that
there are upper limits to the amount of heat that we should add to the structure.
What upper limits are reasonable in these situations? Well, in the case of your client's physical comfort, 70°F to 75°F is
reasonable to most individuals. In the case of refrigerant dehumidifier performance, 90°F to 95°F is the upper limit of
acceptability. The other issues mentioned above require a decision based upon surrounding factors like ambient conditions in
adjacent unaffected areas. An educated understanding of how different hygroscopic materials react to physical moisture temperature
and humidity are also required.
How Low is Too Low?
Consider a few common reasons that a restorer may choose to lower the temperature in their drying strategy, along with
their consequences:
- In an effort to reduce the potential of mold growth in indoor environments, some restorers reduce the heat in the drying environment to levels below 65°F. A common misunderstanding is that mold will not grow at temperatures below the "mesophylic" range (68°F to 86°F). This is a mistake, as there are several categories of molds that grow at temperatures far outside this narrow temperature range. More than one restorer has been surprised when they discover that this strategy fails!
- Just as most refrigerant dehumidifiers suffer a decrease in performance at temperatures ABOVE 90°F, they also drastically decrease in performance at temperatures BELOW 70°F. Although it is true that many styles of refrigerant dehumidifiers employ technology that allow them to continue operating down to 33°F, the actual physical moisture that they remove becomes less and less as the temperature nears freezing. The capacity of the cold air to hold moisture is so small that the dehumidifier's full moisture removal cannot be used to full advantage.
Below is a scenario that we can use to help us see that temperature plays a key role in most drying decisions.
Scenario
Date: January 10, 2003
Your client's fresh water line to the icemaker in the refrigerator failed. The water line broke 12 hours ago and you are able
to address the water damage immediately. Your investigation of the water flow within the structure reveals that there is a
crawlspace beneath the house and you inspect further. A rough finished, unheated concrete crawlspace with several vents
around the perimeter has collected enough water to cover the crawlspace with several inches of water. To add to this
restoration challenge, your inspection reveals that the wood subfloor is dripping wet. Insulation stuffed between the
joists is saturated and falling down. It is clear that this environment, although not inhabited, is in need of quick
attention in order to maintain the health of the structure.
Extracting and/or pumping the water from the crawlspace is no challenge for your company. Your real challenge is determining
how to dry the wood floor joists and the plywood in such a cold unheated environment. To complicate the restoration, the
temperatures in the crawlspace are COLD! 40°F @ 80%Rh (29 grains per pound). Outside temperatures are 34°F @ 40%Rh
(11 grains per pound).
Considerations:
- You decide to remove the saturated insulation from the structure. It is easier to replace than to dry.
- You are unable to place refrigerant dehumidifiers into the crawlspace. Even if you could, it would not remove significant
moisture from the environment because of the low temperature. The dehumidifiers need heat in this situation.
- Air movement would be nice—but how do you control the resulting humidity? We could pump the air out of the crawlspace
through one of the vents surrounding the structure, but that would just draw in 34°F air from outside! The outside air is
not at a quality temperature for drying. We need heat!
- Pumping desiccated air into the crawl space is possible with the air escaping out of the perimeter vents. However,
this process requires more CFM than most portable desiccants can produce. Your unit only produces about 120 CFM of extremely
dry air. Your crawlspace is almost saturated and yet presently has only 29 gpp! It will take far too long to evaporate the
moisture from the substrates with this tool. We need heat!
- But wait! Notice the specific humidity in the air outside the house! It is at 11 grains per pound (gpp)! Why, this is
almost as good as the air that your desiccant dehumidifier produces! Plus, this 11-grain air is virtually limitless! You could
pump hundreds or even thousands of cubic feet per minute into that crawlspace if only you had… HEAT!
Those in the restoration industry who clearly understand this science of psychrometry are now employing the use of this outside
low-grain air with a minor adjustment to a single element of the Restorative Drying Pie: Temperature!
Indirect fired diesel furnaces are now being produced that can increase temperature by more than 80 degrees Fahrenheit! This is
done without introducing water—the undesirable byproduct of combustion found on other furnaces. Let's do some mathematics on this...
Let's assume you own an indirect fired diesel furnace that produces a temperature increase of 80°F @ 800 CFM at the outlet.
How would that assist in the drying of this crawlspace?
If we take this 34°F, 11-grain air from outside and heat it up 80°F, we would have 114°F air coming out of the furnace. Air
at this temperature has the potential to hold up to 467 gpp (11gpp @ 34°F/40%Rh heated up to 114°F). This air is now extremely
thirsty AND the relative humidity is down to about 2.5%Rh! Now we have extremely thirsty air and lots of it @ 800 CFM!
Of course, I am not suggesting that you should heat the crawlspace's ambient conditions to such extremely high temperatures. As
stated earlier, temperatures at this level may cause undesirable results. You would temper the running limits of this furnace
with a thermostat and set it for something more reasonable, perhaps 70°F. With all the vents available in the crawlspace, the
humidity produced will easily escape. We may assist this "dumping" of humidity with the installation of an air mover to exhaust
the warm humid air.
It is imperative that we remember that this cold outdoor air is what provides this incredible drying opportunity. This would be
impossible to produce in the summertime humidity and temperature conditions found in many states.
Finally, it is important to remember that as we alter temperatures and humidity levels in the crawlspace, we will have to keep
a close eye on dew point temperatures that are found on many of the exterior uninsulated surfaces. This leads us into next month's
issue that will address this very topic.
|
|