The Mold Tree: We Can't Plead Ignorance Forever

Expert Contributors:

Despite advancements in the industry, it's perplexing that we continue to encounter the same mold and moisture issues related to PTACs that plagued us back in the 1980s. Building Forensics Group, leveraging their extensive expertise of over three decades in building forensics, has developed a comprehensive diagram that offers invaluable guidance in mitigating the potential risks associated with this catastrophic problem in rooms equipped with PTACs.

Mold Walls PTAC Unit

While not all buildings equipped with packaged terminal air conditioners (PTACs) have moisture and mold problems, some do have significant - and even occasionally catastrophic - problems. The buildings that have successfully used PTAC or fan coil units (FCUs) without experiencing any mold or moisture problems have managed to achieve the delicate balance between PTAC/FCU equipment sizing, outside air vent position, toilet exhaust air flows, correct exterior wall construction, and a variety of other factors such as being in a forgiving climate. Below is an example of a mold tree showcasing where one might expect to find mold occurring in a building based on several factors determined early on, such as continuous versus non-continuous toilet exhaust, make-up air or no make-up air, etc. This decision tree has been developed to help implement the best system design in order to avoid future problems.

The Mold Tree

Mold Tree Continuous Toilet Exhaust Chart
Mold Tree Noncontinuous Toilet Exhaust Chart

The mold tree is a simplified version of the decisions that are generally involved in the design of certain types of hotels. Avoiding mold problems is more complicated and involves making the correct decisions on items such as air quantities, ductwork distribution, DOAS unit selection, controls and interlocks, just to name a few.

Continuous Toilet Exhaust

The top part of the mold tree shows what can be expected when using continuous toilet exhaust. If there’s no make-up air provided to the guest room, using a PTAC or FCU with continuous toilet exhaust results in a high probability of mold in the walls, especially in exterior walls or even in interior walls where moisture is pulled down the demising wall between two guest rooms. If make-up air is added but not ducted to each guest room, the probability that mold will result decreases. If dehumidified make-up air is ducted to each guest room, mold is possible but unlikely.

Noncontinuous Toilet Exhaust

The bottom of the chart demonstrating the use of non-continuous toilet exhaust contains numerous bold areas showing a higher probability of no mold occurring. One primary reason is because toilet exhaust is not being pulled continuously through either the guest room or the PTAC, meaning less air is being brought in from outside. Use of non-continuous toilet exhaust without make-up air creates a possibility for mold. However, if either the guest room or the hotel has a dedicated make-up air unit (whether ducted to the room or not), the possibility of mold occurring in the guest room is greatly decreased. The moral of this story is to use non-continuous toilet exhaust, duct make-up air to every guest room, and try to keep the room as pressurized as possible with a limited amount of make-up air.

What Can the Industry Do to Reduce Risks?

Mold Wall PTAC

In our review of hotel and building moisture and mold problems over the past several decades, our experience has been that most designers assume that PTAC manufacturers’ stated outside air ventilation rate is a reliable number. This assumption results in the faulty belief that a dedicated make-up air unit is not needed for either building pressurization or ventilation, since the open vent PTAC unit fulfills both requirements. The absence of dedicated make-up air systems in combination with PTAC units that cannot meet the manufacturer’s outside air claims has consistently produced buildings that were either (or both) under-ventilated or insufficiently pressurized.

This scenario can easily lead to mold, as can be seen in the picture of a demising wall between units to the right. The toilet exhaust, which was not offset by PTAC unit ventilation, caused a negative pressure in the wall, causing humid air to cool down and mold to grow. Interestingly, there is proof that the mold is a result of the colder air in the room causing condensation. The square spots on the wall without mold are where pictures were hanging. The pictures changed the thermal cross section through the wall at that point, keeping the air from cooling; therefore, mold didn’t grow behind the pictures. Even if PTAC units could deliver the outside air volumes their manufacturers claim, they would still encounter problems in maintaining proper space humidity levels in some climates. Continuous ventilation or fan operation without continuous dehumidification (such as when intermittent cooling occurs) is always a bad idea in any climate that has warm, humid summers, which includes much of the eastern half of the United States.

What Should Designers and Contractors Do?

One of the first steps should be to confirm the adequacy of the projected ventilation airflow rates and vent door instructions, and determine how the test and balances will be verified. In the absence of these minimal requirements, it is critical to not rely on the equipment claims, but rather provide conditioned ventilation air via a primary air duct to each room or space, per the most recent ASHRAE/HVAC applications. If a makeup air system that is separate from the PTAC is providing pressurization and ventilation, the PTAC units should be dedicated to sensible heating and cooling only, rather than being relied upon additionally for pressurization and ventilation.