The Building Science of Floor Systems  

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.

Sources of Vapor in Crawl Spaces

A well-designed crawl space that is based on the description above can certainly have bulk water, moisture, or water vapor present. There are five fundamental ways that can commonly happen as follow.

• Material porosity: Moisture can simply pass through any of the common materials that make up a crawl space. This is true for the walls, particularly if they are made of concrete masonry units but also for concrete. It is also true for the floor assembly in cases where a vapor drive condition moves between the conditioned space and the crawl space.
• Ground evaporation: The soil under and around buildings is organic and living primarily because soil is a great carrier and repository of water. That water can naturally make its way to the surface and evaporate up into the atmosphere. If that atmosphere is enclosed by a crawl space, then the moisture will build up there.
• Poor surface drainage: We all know water runs downhill, but to look at some finished grading around buildings, it would appear this design detail is sometimes forgotten. If water drains toward a crawl space, then it will likely enter and stay there, adding a new source of water, often with little if any means for draining it away.
• Humidity: Vent openings in walls are intended to allow for air to pass into and out of the crawl space. In southern climates in particular but common everywhere, this air can contain high amounts of moisture (i.e., humid air). That can combine with any other moisture already present and increase the moisture content in the crawl space even higher, rather than the intended condition of lowering it.
• HVAC: Crawl spaces are very common places to locate heating and cooling equipment, ducts, piping, etc. Sometimes, these systems contribute directly to the moisture or vapor in the crawl space through water that is drained or released. Other times, they can influence the conditions in the crawl space and create condensation on the ductwork or piping, especially if they are not insulated properly.

There can certainly be other causes as well, but these are the primary sources that we will concern ourselves with going forward.

Photo courtesy of Huber Engineered Woods, LLC

Moisture in a crawl space can come from multiple locations and sources.

The Role of Temperature

Water in general is affected a great deal by temperature, and this is true with water vapor and moisture too. At the most fundamental level, the temperature of the air will determine how much water vapor can be contained in that air until it reaches saturation. The warmer the air, the more water vapor, which explains why it often feels so much more humid in summer than in winter. Nonetheless, the relative humidity (RH) at any point in time is expressed as a percentage of water vapor in the air, so the same quantity of water vapor will produce a higher RH in cool air than it will in warm air. The fact that warm air can hold more moisture lowers the RH and means it has more capacity to hold even more vapor and feel more humid.

The other key aspect related to temperature is the dew point. This is the temperature where water vapor in the air begins to move out of a purely airborne state and turn into droplets of water. This can happen if humid air is suddenly cooled, thus losing capacity to hold vapor and becoming saturated. It can also happen if moisture-laden air comes into contact with a surface that is cool enough to be at or below the dew point. In this case, water droplets will condense on that surface, whether it is the outside of a cool drink container, a glass window cooled by air-conditioning, or a floor assembly cooled by the earth below it. Of course, in any of these cases, temperature, RH, and dew point are all interrelated and quite variable across a single day or season. They can also be influenced by other factors, such as heating and cooling degree days, barometric pressure, and local site conditions. Despite all of these moving targets, whenever the conditions are right, the laws of physics will kick in, and condensation will indeed occur.

The Problem with Moisture

Water and moisture are good and necessary for life and in fact help contribute to our human comfort and well-being. That is also true for trees that need water to grow and sustain life. Once those trees are cut down, dried of most of their moisture, and used for building materials, however, things change. Water is no longer desirable because the dried lumber and wood sheet products will now absorb water, which is something their cells have always done. In a living condition in the forest, this is all good, but in a static condition in a building, it is not since the absorption will change the shape of the wood (i.e., swelling, buckling, elongating, etc.).

Since the wood is no longer using the water to sustain its own life, any water that penetrates or saturates it will sit there and have other influences. If the wood does not properly dry out, then decay and rot can occur since the water now breaks down the wood instead of nurtures it. This is evident in all manner of wood-framed assemblies where water in any form has built up and not dried out. The organic nature of the dry wood and water simply does what it would on the forest floor, where a dead tree is being broken down by water and other factors and is reabsorbed into the earth. As the wood deteriorates, its structural capability goes with it, meaning structural failure could occur.

Other natural processes kick in if the temperature and moisture in the wood are in the range where mold and mildew can form. This not only can expedite the deterioration of the wood, but it can also contribute to indoor air quality (IAQ) issues, which can lead to health-related issues for people who are living inside. In the case of crawl spaces where HVAC systems are located, breaches of ductwork or openings around air filters, etc. can distribute some of these harmful conditions throughout the rest of the building.

Photos courtesy of Huber Engineered Woods, LLC

Moisture trapped in a floor assembly can deteriorate and damage wood and insulation to the point
of decreased performance or failure.

Other building products can become problematic too when moisture infiltrates them. Certain types of insulation are susceptible to the same types of deterioration in performance as wood. While some may not physically deteriorate, insulation manufacturers routinely point out that R-values will not be maintained if their product gets wet. Wet insulation can also provide fertile breeding areas for mold and mildew with the same negative impacts on the building and people as noted for wood.

The issue with moisture in crawl spaces then is that it can interact with building materials to create real problems. These include everything from minor nuisances to serious structural issues and human health problems. None of these should be taken lightly since they pose real threats to human safety and welfare, not to mention professional liability.

Code Considerations for Crawl Spaces

When it comes to the design of crawl spaces, most architects will defer to code requirements. However, there are actually a number of variations and exceptions, particularly in the International Residential Code (IRC), that are not always taken into account during design. We will be referencing the 2018 version of the IRC here, but many of the provisions are the same in earlier versions too.

First, it is important to be clear on how the code defines these spaces. The IRC addresses crawl space foundations in Section R408 as “under-floor space.” This generic term covers crawl spaces, flood-zone-created spaces, or spaces of any height that are “between the bottom of the floor joists and the earth under any building,” (R408.1). However, it specifically states, “except space occupied by a basement.” A basement is a term defined in Section 202 of the IRC as “a story that is not a story above grade plane.” There are further definitions of story and grade plane, but the point is that there are separate provisions for a basement than for a crawl space.

Some of the standard provisions applicable to crawl spaces are summarized as follows.

Vented Crawl Spaces

Section R408.1 of the IRC first addresses ventilation. It requires 1 square foot of ventilation for each 150 square feet of under-floor area. If a Class I vapor barrier is used on the floor (i.e., 0.1 perm or less, such as polyethylene film, foil-faced insulation, aluminum foil, etc.), the ventilation requirement is reduced by a factor of 10—down to 1 square foot of ventilation for each 1,500 square feet of under-floor area. This can be interpreted to acknowledge the importance of creating a barrier between vapor evaporating from the ground and the space. In either case, there is a requirement stating that “one such ventilating opening shall be within 3 feet of each corner of the building.”

Section R408.2 goes on to describe six types of acceptable materials and their thicknesses that can be used to cover the ventilation openings, including sheet metal plates, grill/grating, bricks, hardware cloth, and corrosion-resistant wire mesh. It also clarifies the exception for the 1:1,500 square feet ventilation when a vapor barrier is used, indicating that the required openings must provide cross ventilation of the space. It also allows the use of operable louvers. These provisions make it clear how to design and specify crawl space ventilation to comply with the code. Of course, like most code requirements, these are minimum provisions that rely on the design professional to create the most appropriate design for a building. These provisions also assume that the ventilation is a solution applicable to all locations without taking into account that warm, moist air enters through the ventilation openings and may condense on cooler surfaces in the space.

Unvented Crawl spaces

The code does not end here. In fact, it allows for unvented crawl spaces to be constructed in R408.3 when certain conditions are met. The first requirement here is that “exposed earth is covered with a continuous Class I vapor retarder” as required for reducing the ventilation-opening area above. However, for unvented crawl spaces, the requirements are increased in that “joints of the vapor retarder shall overlap by 6 inches and shall be sealed or taped.” This is a straight-forward detail that makes a notable difference in stopping moisture. The requirement continues, stating, “The edges of the vapor retarder shall extend not less than 6 inches up the stem wall and shall be attached and sealed to the stem wall or insulation.” This provision assures the edges and all penetrations of the vapor retarder are addressed to provide fully continuous protection against ground moisture.

Photo courtesy of Huber Engineered Woods, LLC

Class I vapor retarders in unvented crawl spaces need to have seams and edges taped, and the retarder needs to extend at least 6 inches up all walls, piers, and other protrusions.

An unvented crawl space must also comply with an additional requirement beyond the Class I vapor retarder described above. The IRC allows the designer to pick any one of the following options.

• Mechanical exhaust ventilation: This must operate continuously, connect to the conditioned space via a duct or transfer grille, and move air at the rate of 1 cubic foot per minute for each 50 square feet of crawl space area. In addition, the walls (not the ceiling) of the crawl space must be insulated.
• Conditioned air supply: Instead of simply exhausting air, conditioned air can be supplied and exhausted by design following all of the same parameters as for mechanical exhaust ventilation above, including the insulation requirements. This essentially creates an enclosed, conditioned space under the floor.
• Plenum: If the under-floor space is used as an air plenum for the building, this is allowed, as long as all of the requirements for a plenum are met. Section M1601.5.1 covers these requirements, and the first thing it points out is that such under-floor plenums are no longer allowed in new construction. The provisions are for modifications or repairs to such plenums in existing buildings only.
• Dehumidification: A new provision in the 2018 code allows for the dehumidification of the space (i.e., removing the moisture) at the rate of 70 pints (8.75 gallons) per day for every 1,000 square feet of space. Of course, this water needs a place to drain away to and must be part of the design.

Taking the unvented crawl space approach is clearly allowed, but it moves the line of the building enclosure from the crawl space ceiling (floor construction above) to the perimeter walls and the ground line. IECC 2018 is also okay with this approach with requirements that mesh directly with those above. Section R402.2.11 of IECC 2018 identifies the requirements of eliminating the ventilation, permanently attaching the insulation, and providing a Class I vapor retarder in the same manner as called for in unvented crawl spaces. Essentially, this approach treats an unvented crawl space similarly to a basement in terms of thermal performance and moisture control, while addressing some of the common differences in construction between the two.

The Role of Flooring

With an understanding of the relevant requirements and options for the crawl space itself, we now turn our attention to the flooring assembly above it. The common construction is to use wood floor joists (i.e., dimensioned lumber, engineered wood joists, floor trusses, etc.) to span the distances between the perimeter of the crawl space and any intermediary piers or supports. On top of this are placed panels of plywood or engineered wood that is glued and nailed per generally accepted standards. Then the final layer is placed in the form of finish flooring of whatever type is selected. Underneath, insulation is added between the joists if it is a vented crawl space, or the space is left open if it is an unvented crawl space.

This approach to floor assembly construction over a space below has been in use for generations and is fairly common. The issue we are concerned about here is the way that moisture and temperature operate in these assemblies. The fundamental physics of the situation is that the floor assembly creates a barrier between the space above and the space below. When the temperature and humidity levels are different on the two sides, nature will do what it always does: seek to equalize the differences. Heat always flows toward colder areas, and air with more humidity spreads into dryer air. The wood portions of the assembly will allow both heat and humidity to transfer based on the physical properties of the wood used. We use insulation to further interrupt or slow the transfer of heat to keep buildings more comfortable and use less heating/cooling energy in the process. Humidity or vapor transfer is restricted either intentionally or accidentally also by the materials that are used in the assembly.

Images courtesy of Huber Engineered Woods, LLC

Different types of flooring will react to moisture in different ways and will have different rates of permeability. Improper moisture management can show up as floor problems, but the real issue is below.

When vapor moves by itself through a material, this is referred to as “vapor drive,” which is not necessarily induced by the air but rather the materials. So if moisture is trapped next to or inside a material (like wood), it is vapor drive that will move it outward, allowing the material to dry out. The determination of how much vapor will transfer into or through a material, and at what rate, is the permeability (perm rate) of the materials used. We have already noted that wood is porous and fairly permeable, such that floor joists can allow for moisture to pass through. Wood subflooring is made from wood, but the glues and resins used in some engineered wood panels reduce its permeability, which can be a good thing. In fact, within the wood subflooring sector, there are ranges of quality that are simply referred to as good, better, and best. The permeability of the subflooring tends to decrease as the quality increases, thus helping to restrict vapor drive. Insulation comes in different types and can range from impermeable (i.e., closed-cell spray foam) to very permeable (i.e., fiberglass batts) and places in between, all depending on the material selected. Finally, the finish flooring can range widely based on materials used. For example, carpet and hardwood flooring are very permeable. On the other hand, many resilient floorings, including the increasingly popular LVT and LVP, are quite impermeable, meaning that they do not allow much if any vapor or moisture to pass through them.

Image courtesy of Huber Engineered Woods, LLC

Different types of insulation have different perm ratings indicating their vapor permeance.

The goal in successful floor assembly design is to allow for the opportunity for the floor assembly to dry out if vapor or moisture does get into it. In the days when no insulation was used in the floors and the assembly was primarily wood, this was pretty easy, as any moisture or vapor passed easily through to either side of the assembly. Building science now requires us to design for the drying potential of assemblies to avoid creating a condition that will prevent proper drying. In the case of floors over a vented crawl space, the general recommended strategy is to increase the permeability from the bottom up to allow drying to occur toward the interior if needed. The assumption here is that a vented crawl space is being used that has little drying potential toward the ground.

The design and construction issue for floor assemblies over crawl spaces then becomes how to achieve a vapor drive through the assembly upward into the interior. The answer lies in the selection of the materials used. If permeable flooring is used, (i.e., carpet, hardwood), the permeability of the insulation and subflooring is less critical since the flooring will allow drying toward the interior. However, if impermeable flooring is used, (i.e., LVT, LVP, or other non-permeable resilient flooring), the permeability of the insulation and subflooring is critical. In these cases, it may be necessary to look at ways to reverse the vapor drive to the under-floor space or prevent it from entering altogether. In this latter case, it is important to pay attention to the details of all the materials. It is not advisable to create a “double barrier” that has both a top layer (flooring) and a bottom layer (insulation) that can trap moisture between them because they are both impermeable.

Subfloor Options

One of the key differences in flooring assemblies that plays directly into everything we have discussed thus far is the selection of the subflooring material. While individual boards were commonly used before World War II, plywood became the subflooring of choice in the mid-20th century. The American Plywood Association (APA), now known as the Engineered Wood Association, developed testing for subflooring panels known as PS-1 and PS-2 criteria for the structural performance of the plywood and the floor joists acting together as a system. In the 21^st century, engineered wood is the more common alternative to plywood, which typically means a type of oriented strand board (OSB). When used for subfloor panels, it still must demonstrate, through third-party verification, the structural performance required by PS-2 testing, which has been updated through the U.S. Department of Commerce (DOC).

Both plywood and OSB subflooring come in different grades, essentially rated good, better, and best. The best category is also referred to as high-performance subflooring and brings some desirable characteristics related to the factors we have been discussing. High-performance OSB subflooring has been tested and compared to other products. It has been shown to outperform commodity OSB (good grade) and most plywood in terms of moisture absorption and vapor permeability. This is good new because it means there is less likelihood of deterioration and mold growth if the high-performance OSB does get “wet” from moisture or vapor. It also means that its permeability needs to be factored into the assembly for drying potential. It is worth noting that some LVT and LVP manufacturers recommend using a better-quality OSB if not high-performance OSB to help ensure long-term performance of their finished flooring material. Because of its impermeable nature, the use of LVT and LVP means that the drying potential to the interior is limited. To ensure that a high-performance OSB or engineered wood panel is used, one suggestion is to specify a product that has been reviewed by the International Code Council Evaluation Services. One such reviewed panel was the first engineered wood product introduced to the market to receive such an evaluation that documents higher-performance design values over PS-2 requirements.

Image courtesy of Huber Engineered Woods, LLC

One high-performance engineered wood subflooring has documented greater resistance against moisture absorption than other types of OSB and plywood panels.

When other, more permeable flooring options are used, a high-performance engineered wood product can also be an ideal choice. It has been tested to show that it can resist absorption and deterioration better than the traditional alternatives and can be consistent with the pattern of drying to the interior since the flooring will be more permeable than the subfloor. The high-performance engineered wood product may also be more permeable than the insulation, which will help maintain the proper vapor drive to ensure drying to the inside.

Best Practices and Solutions

The variety of interrelated issues and the options for construction that we have reviewed have been studied by building scientists for some time. In particular, the well-known Building Science Corporation (BSC) cofounded by the engineer Joseph Lstiburek has investigated this issue in some depth and published a number of articles and other free documents available at www.buildingscience.com/documents. Some of the recommendations are included in our review of best practices for crawl spaces outlined as follows.

Consider Unvented Crawl Spaces

Based on their own testing, the people at BSC clearly state that they are not a fan of vented crawl spaces. They highly prefer the use of unvented crawl spaces that are constructed to the code-required level or better. In other words, make them mini-basements with all surfaces properly addressed. This means a continuous, overlapping, taped, and sealed Class I vapor retarder on the floor extending up the perimeter walls and any penetrations, such as piers or other items. Even better, a poured concrete floor with a vapor barrier below the concrete is preferred. The walls of the crawl spaces should be insulated and air sealed the same way other walls of the building enclosure are addressed. In this case, BSC points out that the structural walls in contact with the earth will be cooler than the typical summertime air temperature, such that any moisture that makes its way between the insulation and the walls may condense. Therefore, the insulation should allow for drying to the interior.

The floor assembly above unvented, sealed, and insulated crawl spaces should not be insulated, but rather it should be treated like other uninsulated framed floors above conditioned space. This is because BSC also recommends conditioning the air in the crawl space just like the indoor air above it is conditioned. Hence, the temperature and humidity of the crawl space are close to, if not the same as, the temperature and humidity of the space above it. As such, there is a balance or equilibrium so vapor movement becomes less prevalent and less of an issue. The crawl space is more accurately described as a conditioned crawl space rather than an unvented one. The recommendation is that airflows of 50 cubic feet per minute for each 1,000 square feet of crawl space be maintained.

In terms of flooring, conditioned crawl spaces allow the use of any type of flooring, including LVT and LVP, without many of the worries associated with vented crawl spaces. With fully conditioned spaces on both sides of the flooring, moisture or vapor should be able to dry readily on either side. Still, in this less-moisture-prone environment, a high-performance engineered wood subflooring product with ERS-1785 documented values is a better subflooring specification option. The additional performance benefits in this Evaluation Service Report include higher strength, stiffness, and fastener-holding design values than required for PS-2 panels. This type of underlayment adds the additional value of staying flat, smooth, and sturdy for long-term durability under finished flooring.

If Vented Crawl Spaces Are Used

If an unvented or conditioned crawl space is not feasible for any reason, the design of vented crawl spaces needs to be undertaken very carefully to create a design that is better than code minimum based on the following points.

• Material porosity: It is critical to be sure that moisture is addressed at all of the sources we discussed earlier. Begin by looking at all of the materials that are being used to surround the crawl space and understand their porosity and permeability. Waterproofing or at least damproofing of the walls will always be a good idea, just as it is in basement construction. Further, BSC recommends the use of “capillary breaks” (a non-permeable barrier) at key locations, such as between the footing and the crawl space wall, and at the top of this wall and the framing. This will help stop moisture from wicking up into the concrete or masonry and making its way where it is not wanted.
  

  Images courtesy of Huber Engineered Woods, LLC

  When looking at materials in a crawl space, it is important to allow for the permeance ability to increase, not decrease, as the assembly moves upward.

• Ground evaporation: Recognizing that a great deal of moisture can come from ground evaporation, it is wisest to simply plan on installing a continuous, sealed, Class I vapor retarder with the edges extended up at least 6 inches. The retarder is required anyway, so the incremental difference to seal the overlaps and extend it up is relatively small in the grand scheme of things. Note that 6-mil polyethylene, while minimally acceptable under the code, is not known for holding up very well when left uncovered. Therefore, a heavier material with some internal strength (e.g., some have fiberglass reinforcing) will be more effective and able to withstand people working on top of it, both during construction and the building life cycle.
• Surface drainage: Yes, the ground around the building should slope away from the crawl space. But other details like the entry or access door into the crawl space from the outside need to have a raised sill to keep water from running in. The code requires this in Section R408.4 of the IRC, stating, “The bottom of the areaway shall be below the threshold of the access opening,” although there is no prescriptive dimension for this. Further, BSC and others note that the presence of a roof overhang and gutters will help keep rainwater from falling and building up along the side of the crawl space by draining it away from the walls.
• Humidity: If a vented crawl space is being used, the air that enters in is going to be as humid as the air outside. Even though that may cause condensation on the cooler surfaces, such as the perimeter walls and the ground cover, those materials are not usually as prone to deterioration as wood and insulation. One effective means to protect the wood as recommended by BSC is to use foam insulation that covers the wood. This can be done in a few different ways.
  • First, semi-permeable spray-foam insulation can be sprayed not only between the floor joists but also over and around them, thus fully encasing them in protective and insulating foam. This will work great as long as the drying toward the interior rule is still followed. That is easily accomplished with semi-permeable, high-performance subflooring and wood or carpet flooring. It is not necessarily accomplished by using LVT or LVP or other non-permeable flooring, so their use should be considered carefully in this case.
    

    Image courtesy of Huber Engineered Woods, LLC

    Closed-cell spray insulation can be used that does not cover the entire floor joist in locations where space below the floor is open to the air.

  • Second, rigid, foil-faced foam boards can be installed continuously along the bottom of the floor joists and taped or otherwise sealed along the edges and joints. This creates a very non-permeable condition, even though it may be physically a bit more difficult to install in a shallow crawl space. The foil is an excellent vapor barrier as long as the tape or sealant is also equally excellent. Additional cavity insulation can be installed above the rigid boards (i.e., in between the floor joists) for additional insulating value. In this case, BSC recommends keeping a space between the top of the insulation and the subflooring. The rationale is that this will keep all of the wood the same temperature as the conditioned space above and therefore avoid condensation. In this non-permeable, fairly airtight, insulation-on-the-bottom scenario, any type of flooring can be considered fine to use. This includes LVT and LVP since they are now more permeable than the foil-faced insulation below and drying, if ever needed, can move to the interior.
    

    Image courtesy of Huber Engineered Woods, LLC

    Rigid polyiso insulation with a foil facing and taped edges/seams creates an impermeable condition that provides little opportunity for vapor intrusion, allowing virtually any type of flooring to be used.

  • Third, if a flood-resistant condition is being designed with the building raised up on piers or stilts, it is likely that the sides are much more open on this type of under-floor space. The underside of the floor is quite well ventilated in this case, meaning that there is no cooling from the earth that can otherwise cause condensation in the summer. Here, insulation in the floor is still needed, perhaps more so to prevent an air-conditioned main floor from causing condensation on the underside of the floor. However, it likely does not need to fully encase the floor joists as described above. Instead, spray-foam or similar insulation can be installed between the joists as long as it also seals completely to the joists. Flooring guidelines as in the first option above apply here as well.
• HVAC: The purpose of a heating/cooling system in a building is to change the temperature (and humidity) on the inside compared to the outside. When the means to do that include installing equipment, ductwork, or other components in the crawl space, their impact on the crawl space needs to be considered. If the walls of the crawl space are not insulated, the ductwork or piping needs to be insulated. This prevents condensation from forming and adding to the moisture in the crawl space. The same could be said for plumbing, both hot and cold lines, to conserve energy and prevent condensation respectively. Relatedly, the occupants or users of the building can play a role by keeping the indoor air-conditioning temperature at a reasonable level. Setting the thermostat at a very low level will cool the building materials, making them more susceptible to cause condensation in assemblies, including crawl space floor assemblies.
• Maintaining subfloor integrity: When the floor assembly is being constructed, the integrity of all the materials needs to be maintained. The subfloor is often left exposed during construction until the very end when the finish flooring is installed. If the building is not yet closed in and it rains, standing water may form on the subfloor. This is another reason to use high-performance subflooring since it can resist damage and swelling (notably along the panel edges) much better than lower grades due to its superior ability to resist absorbing moisture. However, some construction crews are prone to look for a quick solution to drain away water in this case, so they will drill small holes into the subfloor and allow the water to fall into the crawl space. This should never be allowed since it can cause considerable problems with both the crawl space and the subflooring. Rather, rainwater should be swept off the subfloor and out of a door opening onto the ground.

Following these recommendations and best practices can help avoid the problems associated with crawl spaces, particularly when non-permeable flooring is used.

Conclusion

The design of a crawl space foundations needs to be coordinated based on whether or not it is vented, the quality of materials used in the wood floor assembly, and the permeability of the finish flooring. This is true for all projects but particularly in more humid climates. Recommendations for vented crawl spaces are based on increasing the permeability from the bottom up, allowing for drying to occur toward the interior. This may mean avoiding the use of impermeable flooring, such as LVT or LVP. Wherever possible, however, properly designed and constructed unvented or conditioned crawl spaces are preferred since they eliminate many of the problems associated with vented crawl spaces and can allow the use of any type of flooring. In all cases, using high-performance materials for the subfloor can help with overall design strategies, guard against some potential problems, and contribute to a successful project for all involved.  

Peter J. Arsenault, FAIA, NCARB, LEED AP, is a nationally known architect, consultant, continuing education presenter, and prolific author advancing building performance through better design. www.linkedin.com/in/pjaarch, www.pjaarch.com