Vented Crawl Space Research
Background - A raised first floor is endemic to southern architecture. Elevating the living space above ground level provides a sense of security and aesthetic appeal in a climate where porches and decks can be used nearly year round. Additionally, an elevated space provides a buffer from ground-borne insects and moisture, as well as protection from periodic flooding in low-lying areas. This house style can be constructed using a number of foundation options, one of which is a masonry crawl space foundation and raised wood deck.
Historically, it has been widely accepted as good building practice to ventilate the area beneath a raised wood deck supported by a crawl space foundation. In fact, the International Residential Code (IRC 2006) requires a minimum area of under-floor foundation ventilation based on a fraction of the overall crawl space area. This can be problematic in the hot, humid Gulf Coast summer months where outside air, saturated with vapor, cools in the lower (than ambient) temperature of a shaded, ground coupled crawl space. Builders, academics, and occupants hold differing opinions on how to best address this problem.
Some stakeholders involved in reconstruction after Hurricane Katrina avoid this question entirely by building or advocating either conditioned crawlspaces or raised slabs, each of which presents a similar vexing set of what-happens-when questions. In the case of the conditioned crawlspace there is an energy penalty associated with the larger volume of cooled/heated space, as well as the difficulty of adequate air and thermal seal in locations requiring flood vents in the foundation and dealing with crawl space moisture in very wet locations. Or, the finished surface of the raised slab presents both a condensing surface for moist air that infiltrates the building envelope and a durability challenge in certain soil conditions.
Building Codes and Crawl Space Ventilation - In the city of Mobile, AL, the location of two houses in this study, the IRC 2000 is the adopted residential building code. Nationwide it is more common that a newer version of the IRC, either the 2003 or the 2006 version, be cited for building compliance. Table 1 indicates how each of the three building code versions treat crawl space ventilation requirements.
Table 1. IRC Under-Floor Space Ventilation Requirement A
| Section |
IRC 2000 and 2003 |
IRC 2006 |
| Minimum Sq. Ft. of Ventilation Per |
Under-floor Space Area (sq. ft.) |
Minimum Sq. Ft. of Ventilation Per |
Under-floor Space Area (sq. ft.) |
| R408.1 Ventilation |
1 |
150 |
1 |
150 |
| R408.2.6 - Exceptions (to R408.1) |
No Exceptions |
| R408.2.6.1B |
0C |
N/A |
| R408.2.6.2 |
1 |
1,500 with ground vapor retarder |
A This table covers minimum area of exterior vents versus foundation size, only. Other exceptions, which include elimination of exterior vents, are contained in each of the codes. Refer to the building code for complete coverage of the subject.
B As warranted by climatic conditions which are not defined.
C However, ventilation openings to the inside are required. The code does not address the quantity of openings required to the inside.
|
The 2006 IRC allows no trade off in ventilation area if the ground in the below-floor space is covered with a vapor retarder, as did the previous versions. The 2006 IRC has also introduced a new section, R408.3, covering unvented crawl space design. This section is named “unvented crawl space.” The code actually refers to a conditioned crawl space in this section as exterior ventilation can be eliminated if 1) mechanical ventilation of the crawlspace at a stated rate, a pathway to the conditioned space, and perimeter insulation are provided, or 2) conditioned supply at a stated rate and return and perimeter insulation is provided, or 3) the under floor space is used as a plenum complying with section M1601.4.
The Evaluation Plan - The NAHB Research Center has been working with B.C. Daniels, a Mobile, AL company in its fifth decade of constructing custom residential and light commercial buildings in the local metropolitan area. The builder has two homes in progress designed with vented crawl space foundations supporting raised wood decks. Both houses comprise approximately 2,000 square feet of crawl space area and volume equal to approximately 7,500 cubic feet. Foundation crawlspaces have been designed for natural ventilation at a ratio of 1 square foot of ventilation area for every 275 square feet of crawl space area, or more than two times the IRC 2000 requirement when a ground vapor retarder is included in the design.
Each of the crawl spaces in this study will be fitted with sensors fastened to the wood joists at regular spacing that will collect and transmit temperature, humidity and wood moisture content data, hourly. One crawl space will be vented naturally via stationary vents. The other crawl space will be vented with both stationary vents and SmartVent®. The SmartVent determines if the vapor pressure (or dewpoint) is higher inside or outside on a 20 minute basis. If the dewpoint is higher in the crawlspace, it begins exhausting air from the crawl, if the dewpoint is higher outside, the fan waits for 20 minutes and checks the vapor pressure again. The SmartVent is designed to only run when it actually removes moisture from the crawl. Ambient temperature and climate data will be provided by a nearby NOAA weather station. The resultant data will be analyzed and reported.
From the data we expect to be able to determine if natural ventilation is sufficient for this climate and compare the performance of the SmartVent to passive ventilation. This evaluation plan will provide the option to reduce the ventilation area to the code minimum (via temporary closure and sealing of vents) for a period of time in order to document the effects of ventilation area.
Figure 1. Typical Crawl Space Foundation for a Raised Wood Deck
