Comparing Ventilation Strategies for Hot, Humid Climates

The first thing to know about ventilating a home is 1 CFM in = 1 CFM out.  Meaning that the house will try to equalize when air enters or leaves the home.  Depending on your climate zone and the time of the year, it is important to control which way the air will push in order to equalize.  According to building science experts, in hot, humid climates it is best to provide a slight positive pressure on in the home to avoid the wet outside air being sucked into the home through the walls.

There are three basic approaches to consider when contemplating how to implement a fresh air ventilation strategy while we consider control of moisture within a structure. Including on rainy days when it is 70° outside:

• Exhaust ventilation – blowing air out of a home
• Supply ventilation – blowing air into a home
• Balanced ventilation – blowing air both into and out of a home

Each approach has advantages and disadvantages which vary based on the climate zone.

An exhaust ventilation system is easy and inexpensive to install.

But for hot/humid climates exhaust ventilation is not recommended because of concerns that air entering the structure could be humid and will cool off as it works through the structure on its way into the home, potentially resulting in condensation within the wall cavity. Additionally, since the fresh air comes in from all over, there is no way to filter, dry, heat or cool the outside air before it is introduced into the home.

Balanced ventilation is a better ventilation strategy in hot/humid climates since it avoids depressurizing the home.

Most balanced systems use a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV). These systems blow air out of and into the home at the same time, and put the two airstreams through a core that exchanges heat (HRV) or heat and moisture (ERV). These systems help reduce the cost of ventilation because the incoming air is tempered by the outgoing air. On the downside, these units are expensive and lack the ability to mechanically remove moisture. In an ERV, the ability to remove moisture is solely dependent on the difference in dryness between one air stream and the other. If both have the same moisture content, there is no moisture exchange. It is important

Most balanced systems use a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV).

These systems blow air out of and into the home at the same time and put the two airstreams through a core that exchanges heat (HRV) or heat and moisture (ERV). These systems help reduce the cost of ventilation because the incoming air is tempered by the outgoing air. On the downside, these units are expensive and lack the ability to mechanically remove moisture. In an ERV, the ability to remove moisture is solely dependent on the difference in dryness between one air stream and the other. If both have the same moisture content, there is no moisture exchange. It is important
to understand that anytime the outdoor moisture level exceeds the indoor, it is adding latent load to the structure. It is then left up to the A/C system to remove this moisture to keep it at an acceptable level.

Supply ventilation is considered the best approach for ventilation in a hot humid climate because it keeps air moving from the inside of the home toward the outside, removing the concern about internal condensation.

Supply ventilation systems provide make-up air for spot ventilation fans such as bathroom exhaust fans and range hoods. The pressurization of the home also reduces the amount of total ventilation a home will experience due to other pressures on the home, such as wind. A simplified supply ventilation system can be inexpensive to install.

Since air is coming into the home from a duct to the outside, a supply ventilation approach also provides the ability to filter, dry, heat or cool outside air before it is introduced. The main concern with fresh air ventilation in the south is that the hot, humid air is introduced into the home, increasing both latent and sensible cooling loads. For this reason, EPA’s Building America program lists the use of supplemental dehumidification systems in hot/humid climates as a best practice.

Unlike a typical HRV or ERV used to implement a balanced ventilation strategy, a supply ventilation strategy with dehumidification, such as an Ultra-Aire whole house ventilating dehumidifier.

This provides the ability to mechanically remove water from the ventilated air until a specific set-point is reached. Although more expensive to install than a simplified supply ventilation set-up, a supply ventilation system with supplemental dehumidification can provide fresh air ventilation along with year-round humidity control. This results in an indoor environment that is healthy, comfortable and provides the ideal conditions conducive to protecting and preserving the building and its contents.

For the ultimate in indoor air quality – especially in today’s tight, well-insulated homes in hot humid climates – consider a supply ventilation system with supplemental dehumidification.

The best of these systems combine independent control of ventilation and dehumidification, high-efficiency filtration for incoming air, and energy efficient dehumidification systems (look for the Energy Star® label). This is especially important for humid climates. Ultra-Aire originated the category of whole-house ventilating dehumidifiers and offers the most energy efficient units on the market. The company offers multiple sizes and configurations to ensure there is a unit for every application.