Crawl Space and North Carolina Building Code

If you live in North Carolina, then there is a good chance that you have lived somewhere that has a crawl space. There are several different options that are suitable for crawl spaces and it can be confusing to know which method is the most effective and efficient. According to the North Carolina Residential Building Code, you have the choice to have a wall vented crawl space or a closed crawl space. Traditionally, wall vented crawl spaces are the more popular option, but closed crawl spaces are gaining in popularity.

As the name suggests, a wall vented crawl space has foundation vent openings on the exterior walls. Wall vented crawl spaces are covered in Section R408 of the NC Residential Code and includes other stipulations, such as foundation vent location, covering material, and insulation. Obviously, with a wall vented crawl space, you do not have the option of installing a crawl space dehumidifier.

On the other hand, the building code also presents the options for a closed crawl space in section R409. There are multiple requirements listed, such as combustion air, wall insulation, and space moisture vapor control. Space moisture vapor control is where a dehumidifier enters the picture since a mechanical drying device is required in a closed crawl space.

Closed Crawl Spaces vs. Wall Vented Crawl Space

Now that you know the options for your crawl space, how do you choose? Well, a research study by Advanced Energy compared the performance of a wall vented crawl space to a sealed crawl space.

Set Up

The experiment was conducted in Princeville, North Carolina and used 12 single family homes that were located on the same cul-de-sac. All of the homes were one story and had identical floor plans of 1,047 square feet. For the purpose of the study, the houses were divided into 3 groups- Control Group, Experiment One Group, Experiment Two Group.

Phase 1 (Duct Air Leakage)

The first phase of the experiment took place from June 2001 to May 2003. The moisture control in all of the crawl spaces was provided by leakage from duct air. For uniformity, the duct leakage in all of the homes was reduced to comparable levels (51 CFM25 to 68 CFM25) The crawl spaces of the 12 homes were set up as follows:

Control Group (vented crawl space)
• wall foundation vents
• ground covering with polyethylene film, stapled in place

Experiment One (closed not insulated)
• wall foundation vents that were blocked and sealed
• wall and ground covering with polyethylene film
• floor insulation was removed

Experiment Two (closed and insulated)
• wall foundation vents that were blocked and sealed
• wall and ground covering with polyethylene film
• kept floor insulation
• insulated foundation walls

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Phase 2 (Supply Air)

Phase Two of the experiment occurred between June 2003 and March 2004. The duct air leakage was stopped in all 12 houses with sealing work. This included existing duct work and floor penetrations. In addition, the two experiment groups had a supply air duct installed. The supply air initiated whenever the thermostat determined that the heat pump was needed to condition the living space.

Control Group:
• conditions remained the same

Experiment One:
• added fiberglass batts added between floor joists
• added HVAC supply air duct to crawl space (35 CFM)

Experiment Two:
• upgraded insulation to 2 in. of R-13 foam insulation
• added HVAC supply air duct to crawl space (35 CFM)

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During both phases of the experiment, every house was equipped with three data loggers to track results. One data logger was placed inside the home at the return grille and the remaining two were placed in the center of the crawl space. The data trackers measured the outside air temperature and moisture content every 15 minutes.

Once the results were analyzed, it was evident that closed crawl spaces reduced the moisture more than wall vented crawl spaces. During Phase 1 of the study, where duct air leakage was utilized, both the Experiment 1 Group and the Experiment 2 Group provided better moisture control than the Control Group (wall vented crawl spaces). During Phase 2, using supply air ducting, very similar results were found with both Experiment 1 Group and Experiment 2 Group outperforming the Control Group. During the humid, summer months, the relative humidity in the closed crawl spaces was typically below 60%, while in the open crawl spaces, the relative humidity was normally above 80%.

In addition to duct air leakage and supply air, a dehumidifier was also tested briefly in the experiment. During a 3 week period in Phase 1, small dehumidifiers were used in the crawl spaces of the Experiment Groups. The dehumidifiers were able to easily maintain the relative humidity at 40%.

As for wood moisture content, vented crawl spaces had a wide range from extremely high (approx. 15% WMC in August 2003) to much lower in the winter (approx. 11% WMC in Feb. 2004). Meanwhile, the closed crawlspaces maintained a more stable wood moisture content throughout the year (approx. 11% WMC in August 2003 and approx. 11% WMC in Feb 2004).

It was concluded from the study that when constructed properly with 100% vapor barrier on the ground, a vented crawl space has the potential to be able to limit wood rot, although not eliminate it completely. Furthermore, a wall vented crawl space does not prevent moisture condensation or surface mold growth, leading to the belief that a closed crawl space is the more viable option.

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Types of Closed Crawl Spaces

Now that you see the benefits of a closed crawl space, lets take a closer look at the requirements outlined in the building code. Since the crawl space will be completely closed in, there needs to be a mechanical method for drying out moisture levels. Section R409.5 of the Residential Building Code outlines a few methods:

Option #1- Dehumidifier: Install a permanent dehumidifier with a minimum capacity of 15 pints per day that has an electrical outlet. In addition, the condensate discharge must use an internal condensate pump or be drained outside.

Option #2- Supply Air: Duct supply air from the air conditioning system into the crawl space (1 cubic foot per minute per 30 square feet of crawl space floor).

Option #3- House Air: Air from the house can be blown into the crawl space (1 cubic foot per minute per 50 square feet of crawl space floor)

Option #4- Exhaust Air: Exhaust air from the crawl space outside using a fan (1 cubic foot per minute per 50 square feet of crawl space floor).

The code only requires one method of mechanical drying but they can be combined, if you prefer. Now as a dehumidifier manufacturer, it should come as no surprise that we recommend Option #1-installing a dehumidifier. There are several reasons to back up this thinking, however. A dehumidifier is capable of easily maintaining the moisture level in your crawl space. It doesn’t depend on the air conditioning or any other system, it simply runs according to your set point. This is especially important during the transitional Spring and Fall seasons when your HVAC may not be running.

While some may argue that a dehumidifier requires additional attention, a Watch Dog dehumidifier actually requires very little interaction once installed. Yes, it does need annual maintenance, but, other than that, set the humidity level and you’re good to go. All WatchDogs are equipped with auto restart so something like a power outage is no problem. In addition, WatchDogs feature hot gas bypass and temperature controlled defrost so freezing isn’t an issue.

Besides maintaining the proper humidity level, dehumidifiers can help with other issues. For instance, using a dehumidifier can alleviate health issues your family may be suffering from such as allergies or asthma. Furthermore, a dehumidifier is one of the most effective ways to prevent mold from developing in the crawl space. Dehumidifiers are also an excellent tool for pest control throughout your home.

While there may be several options for your crawl space, after examining the research, it becomes clear, that a closed crawl space is a more effective option. As for choosing between the different types of closed crawl space, a dehumidifier is the method with the most benefits.

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Dastur, Cyrus, and Bruce Davis. "Closed Crawlspaces Do Double Duty." Advanced Energy. Advanced Energy, 2005. Web. 20 Aug. 2016.

Davis, Bruce, and Cyrus Dastur. "Moisture Performance of Closed Crawlspaces and Their Impact on Home Cooling and Heating Energy in the Southeastern U.S." Advanced Energy. Advanced Energy, 2004. Web. 20 Aug. 2016.

North Carolina. Building Code Council. North Carolina State Residential Code. Raleigh, N.C. (1202 Mail Service Center, Raleigh 27699-1202): North Carolina Building Code Council and North Carolina Dept. of Insurance, 2012. Print.