The Building Science of Floor Systems

Vapor drive can be just as important in crawl space wood floor assemblies as it is in wall designs
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Sponsored by AdvanTech® subfloor assembly
By Peter J. Arsenault, FAIA, NCARB, LEED AP

Learning Objectives:

  1. Identify and recognize the health and safety issues of moisture and vapor drive in wood-framed floor assemblies, particularly over crawl space foundations.
  2. Investigate the building code requirements and exceptions related to crawl space design, ventilation, and protection of the structure.
  3. Assess the characteristics of subflooring and finish flooring materials that can contribute to moisture and vapor issues in a wood floor assembly.
  4. Review best practices and identify solutions as found in building projects that incorporate the principles presented for successful, safe floor construction.


AAA 1 Structured Learning Hour
This course can be self-reported to the AANB, as per their CE Guidelines
AAPEI 1 Structured Learning Hour
MAA 1 Structured Learning Hour
This course can be self-reported to the NLAA.
This course can be self-reported to the NSAA
NWTAA 1 Structured Learning Hour
OAA 1 Learning Hour
SAA 1 Hour of Core Learning
This course can be self-reported to the AIBC, as per their CE Guidelines.
This course is approved as a Structured Course
This course can be self-reported to the AANB, as per their CE Guidelines
Approved for structured learning
Approved for Core Learning
This course can be self-reported to the NLAA
Course may qualify for Learning Hours with NWTAA
Course eligible for OAA Learning Hours
This course is approved as a core course
This course can be self-reported for Learning Units to the Architectural Institute of British Columbia
This test is no longer available for credit

Building science in wood-framed buildings has been widely used to address issues related to moisture migration, air leakage, and vapor diffusion, most commonly in wall and roof assemblies. However, all of these same dynamics take place in floor assemblies too, especially when a crawl space is used for the foundation type below the floor. It is further exacerbated in warm and humid climate zones, which seem to be the areas where crawl spaces are most common. In some cases, whether by design or by chance, moisture buildup in the floor assembly has not been a significant problem. However, the recent popularity of resilient flooring being used over wood subfloors, particularly luxury vinyl tile (LVT) or luxury vinyl plank (LVP), has been discovered to create some risk. Such vapor-impermeable flooring does not allow a floor assembly to dry to the interior if it does absorb moisture, causing deterioration, mold, and mildew issues that have routinely been so widely addressed in walls and roofs. This course will take a closer look at the building science of vapor drive that can lead to moisture issues in wood floor assemblies when proper sequencing of materials to allow drying is not accounted for in design. It will address the use of some of the most common construction and material options in today’s wood-framed flooring assemblies. It will also look at some guidelines and best practices to help reduce the likelihood of developing problems over the life of the building.

Photo courtesy of Huber Engineered Woods, LLC

Floor assembly systems are concealed once construction is complete. When they are over crawl spaces, the goal is not to also conceal moisture problems.

The Issue: Vapor in Crawl Spaces

The plane of the building enclosure between occupied, conditioned space and nonconditioned crawl space is the floor assembly. As such, it warrants the same amount of attention as other parts of the building enclosure, such as exterior wall assemblies and roof assemblies.

Typical Crawl Space Construction

Let us start with a clear understanding of the common ways crawl spaces are constructed. The most typical condition is that concrete or masonry walls are used to raise up the lowest floor of a building from the ground plane. However, the crawl space is not dug out the same way a basement would be. Rather, the footings for the crawl space walls are placed at an appropriate depth depending on local practice and frost lines. In the southern United States, the footing depths are much shallower than in the north, making crawl spaces a viable and less expensive option than excavating for full basements.

The space between the ground and the bottom of the floor above is usually quite shallow, hence the moniker of “crawl” space, which is not typically tall enough for a person to stand but rather requires squatting or crawling around. This distance is shorter than the constructed walls, which extend below grade to the footings and vary widely from literally inches to several feet.

The floor assembly above the crawl space (i.e., the “ceiling” of the crawl space) is most commonly wood framed in a conventional manner. In older buildings, it was not common to put insulation in the floor construction, but energy codes do currently require insulation. For buildings covered under the latest residential portions of the International Energy Conservation Code 2018 (IECC 2018), the prescriptive insulation value for floors that form part of the building enclosure is R-13 to R-38, depending on climate zone (Table R402.1.2). The commercial requirement is R-30 in all climate zones for wood-based joist/framing construction (Table C402.1.3). Based on this, insulation in floors above crawl spaces has become routine. (More on this later.)

Image courtesy of Huber Engineered Woods, LLC

Typical crawl space construction includes vented sidewalls, an insulated floor, and the potential for vapor to move from moist to dryer conditions.

The addition of insulation has also introduced other considerations. To help prevent the buildup of moisture, crawl spaces are commonly vented to allow moisture-laden air to be carried away. This is the common practice for ventilating insulated attics, so the thinking has simply been transferred to crawl spaces. Since ventilation is being relied upon, there are very few cases where an interior vapor retarder is placed in the floor the way it might be in a wall assembly. While all of this appears logical on the surface, the reality is that there are some building scientists who take issue with this approach and recommend unvented crawl spaces. (More on this to come too.)

There is a variation on crawl space construction that has become widespread wherever flooding is a concern, such as coastal areas in the United States. In To qualify for flood insurance, the floor needs to be at least 1 foot higher than a designated flood level for a geographic flood zone. In many cases, this has meant that the homes are raised up on piers or stilts such that the space below is open and can be the equivalent of a full story or more. Often that area is used for parking, storage, or a covered outdoor area but is not conditioned space. Indeed, it needs to allow flood waters to pass through unimpeded so as not to cause structural damage to the home above. Since the floor assembly above this area is also part of the building enclosure, the discussion of crawl space design and the concepts we will cover here apply to this raised, stilted condition as well.

Photos courtesy of Huber Engineered Woods, LLC

Crawl spaces in flood-prone areas are more commonly built on piers with open sides to allow the free passage of water and air below the floor assembly.


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Originally published in Architectural Record
Originally published in May 2020