Ventilation: Fresh Air or Hot Air?

by Moses D. F. Ling, PE, RA

 

At this time, all municipalities in Pennsylvania have either opted in or opted out of the administration responsibilities of the Uniform Building Code. The code being enforced is the International Building Code with a number of the associated codes. The intention is to have building designs be code compliant regardless of their location within Pennsylvania. Some locations such as Centre Region will see very little impact while more rural areas will be facing some of the code provisions for the first time.

 

One of the sections that is sure to impact the building industry, as implementation becomes more widespread, is Chapter 4 of the International Mechanical Code – Ventilation. The code requires that outside air be either naturally or mechanically introduce to occupied spaces.

 

Natural Ventilation can be satisfied by providing operable windows, door and louvers with area equal to or exceeding 4% of the floor area. Mechanical ventilation rates are specified in Table 403.3. This appears rather straight forward. However, the impact of this quantity of outdoor air if not properly treated can negatively impact the conditions of the occupied space.

 

The psychrometric properties of the outside air vary as the weather condition changes throughout each day. It can be very desirable at one point and unacceptable later the same day. The psychrometric conditions can be divided into several general categories. The impact of three of these categories can cause significant problems.

 

• Cold and Dry – In the winter, outside air contains very little moisture (no thunderstorms in the winter). When low humidity cold air is brought into the building and heated, the resultant relative humidity drops drastically. Low humidity indoor condition of less than 20% is common.
• Hot and Humid – In the summer, hot and muggy conditions demands that the cooling system be capable of reducing the temperature and wringing out the humidity. It is often assumed that the air-conditioning system can achieve the task. It may, if the ratio of sensible to latent load matches the system performance or the system design and controls have the sophistication to match the varying conditions. Most systems cannot provide an exact match. If comfort is the only objective, some variation in indoor humidity may be acceptable. High humidity, however, will foster the growth of mold and other micro-organisms.
• Cool and Humid – If dehumidification is achieved by the cooling effect of the air-conditioning system, then how is dehumidification accomplished when the temperature of the air does not demand cooling? It is NOT, with most of the conventional commercial HVAC system. Systems with better performance and controls can be designed to handle these conditions. One common technique is reheat.

 

So Moses, what does this have to do with architecture?

1. This brings the discussion back to several points in the initial presentation of Thinking Inside the Box. Is the project set up for failure? Is adequate attention paid to these issues? Is the project design and construction budget allocated properly?
2. Uncontrolled humidity can wreak havoc on architectural woodwork. Dry conditions will cause the joints to come apart. Humid conditions will cause swelling. If the swelling is excessive, buckling can occur. Don’t look at the carpenter. Look at how design decisions were made and budget allocated.