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Almost 70% of the houses in South Carolina have crawl space foundations. We build more crawl space foundations than any other state in the country. Yet we continue to have problems with our crawl spaces. These problems include mold and decay, elevated radon levels, and termite and other pest concerns. We see condensation on ductwork, mold on joists, termite and wood boring beetle damage and cupped hardwood floors. Our current solution is to increase ventilation of the crawl space.

At an Affordable Comfort meeting that I recently attended, a speaker from Canada said that venting crawl spaces in the southeastern US was lunacy. I have to agree. In this paper, I will discuss some of the fallacies I see with our current practice of venting crawl space foundations, and provide guidelines for a higher performance crawl space.

Fallacy #1 – A research basis for current crawl space ventilation guidelines exists.

Supposedly we vent crawl spaces to help control moisture. Looking back through historical documents we find several documents that discuss venting crawl spaces. In 1939, the Forest Products Lab published “Use and Abuse of Wood in House Construction” which contains “Screened vents totaling 3 percent of the house are best, with a thoroughly insulated floor… One small ventilator in each wall is hardly enough in the damp South.”

In 1942, the Federal Housing Administration’s “Property Standards and Minimum Construction Guidelines” contained the first requirement for ventilation of crawl spaces in regulatory literature. It predates any known research on crawl space performance. These requirements state in part “Provide a sufficient number of foundation wall vents to assure a total ventilating area equivalent to 1/2 percent of the enclosed area plus 1/2 square foot for each linear feet of wall enclosing that area.”

In 1948, the Housing and Home Finance Agency (HHFA) published “Crawl Spaces: their effect on dwellings.” This document contains a discussion of some investigative work done by Britton on several housing complexes. Britton said “when ventilation to the extent of 1/1500 of the building area was cut into the crawl space walls, in conjunction with ventilation of approximately 1/500 of the building area in the loft space walls and the covering of the crawl space ground with 55# mineral surfaced roofing, all trouble was apparently eliminated.” An interesting note with this discussion was that Britton was investigating attic moisture problems.

Britton included the note “Where crawl space floors are covered with 55# mineral surfaced roll roofing in an effective manner, the specified wall ventilation may well be reduced as much as 90% for controlled construction.” The HHFA followed with another document that stated “Where a good cover is applied over the entire surface of the ground in the crawl space, very little ventilation [10% of formula] is needed.”

The next thing we see is updated code requirements. The Minimum Property Standards of 1958 state “At least 4 foundation wall ventilators shall be provided, one located close to each corner of the space, having an aggregate net free ventilating area not less than 1/150 of the area of the basementless spaces, or ground surface treatment in the form of a vapor barrier material…plus at least 2 foundation wall ventilators having an aggregate net free ventilating area not less than 1/1500 of the area of the basementless space.” The only difference I see between this 1958 code and the 2000 IRC code is that today we require a minimum of four vents at the 1/1500 ventilation level.

From my investigations and those of Bill Rose of the Building Research Council at the University of Illinois, research to support these recommendations and the code does not exist. What I can find in the literature appears to be limited to a field investigation with several moisture control steps happening at once. I do not see an evaluation of the effectiveness of each step. That is: When attic ventilation AND foundation ventilation AND a ground cover were added, the ATTIC moisture problem was fixed. These papers certainly contain good information, but I do not think it contains enough information to support our existing building codes and ventilation requirements.

In addition, nothing in the literature was found that scientifically supports partially covering the soil in a crawl space.

Fallacy #2 – We build houses the same today as when current crawl space ventilation guidelines were established.

Many things have changed in the houses we build today versus what we built back in the 1930’s-1950’s. We often build on wetter sites (because many of the high-and-dry ones are gone.) We also build houses deeper into the ground. (I cannot count the times I have crawled DOWN into a crawl space.) We build smaller overhangs without gutters and downspouts, and sometimes do not slope the land away from the foundation.

The most significant change we have made in the last 50 years, in my opinion, is air conditioning. In many parts of the country, we make a standard practice of creating artificially cooler temperatures in our homes. Now we easily create temperatures that are near or even below the dew point temperature of the surrounding air. Condensation occurs on surfaces that never before experienced condensation. Air conditioning has upset the balance we used to experience, and the balance we were using when the ventilation codes were created.

Fallacy #3 – The 1/150 or 1/1500 ventilation area requirements mean something.

I used an ASHRAE Standard 51-1985 air flow test device to measure the air flow through foundation vents ranging from 24 square inches of net free area (NFA) to 75 sq. in of NFA. The large NFA vent had a larger flow at a given pressure, but the flow was about 1.75 times that of the small vent rather than 3 times the flow as would be expected from the size difference. A 65 NFA automatic vent has an air flow much closer to the 24 vent than the 75 vent. (This occurs because of the additional screen on the inside of the vent, which is not used in the calculation of NFA for the vent, but provides restriction to air flow.) Therefore, the actual air flow achieved when meeting the 1/150 requirement appears to depend on the NFA of each vent as well as on the total aggregate ventilation area. An equivalent net free area made up of smaller NFA vents will provide more air flow that fewer large NFA vents.

Next, I estimated the air changes per hour in a 3-foot tall crawl space of a 1500 square foot house using these same vents. At 1/150, we would need 60 of the 24 NFA vents. The 60 vents would yield an air change rate of about 6.4 Air Changes per Hour (ACH). In contrast, the larger 75 NFA vent would require only 20 vents and provide only 3.4 ACH. The relatively large 65 NFA thermostatically-controlled vent would only provide 2.6 ACH if the 1/150 ratio was observed.

If we added a complete ground cover as the code allows, we could reduce the ventilation requirement to 1/1500. The number of vents required drops to six for the small 24 sq inch vents, and four for the other vents. This drops the air change rate to 0.64 ACH for the 24 NFA vents, 0.45 ACH for the automatic vent and 0.70 for the large 75 NFA vent.

This investigation has shown that specifying a NFA for crawl space ventilation does not seem to indicate the amount of ventilation that can or will occur in a crawl space. Using smaller NFA vents will provide more ventilation than when using larger NFA vents. Thermostatically controlled vents do not provide flow corresponding to a similar-sized manually-operated vent.

 Fallacy #4 – Venting will reduce crawl space moisture levels.

In reality, venting will only help reduce crawl space moisture levels when the outside air is dryer than crawl space air, or when enough hot outside air enters and warms the crawl space. Outside air in the summer may actually contain more moisture than crawl space air, and may make the situation worse, not better. In winter, venting will help dry a crawl space, sometimes to a detrimental extreme.

From a psychrometric standpoint, venting a crawl space to remove moisture works when the outside air is dryer than the crawl space air. “Dryer” does not mean a lower relative humidity, but rather a lower absolute humidity. Relative humidity is a ratio of the amount of moisture in the air relative to the amount the air can hold at that temperature. Absolute humidity is the amount of moisture in an amount of air. Air at 85 degrees and 60% RH has the same absolute humidity as air at 70 degrees and 100% RH. So venting a 70F/100% RH crawl space with 85F/60% RH air will not remove moisture.

The dew point temperature is the temperature at which condensation forms as the air is cooled. At the dew point temperature, the air is saturated and any further cooling will result in condensation. In the above example, both the 70F/100% RH crawl space air and the 85F/60% outside air have the same dew point temperature: namely 70F. If we vent a crawl space with air that has a higher dew point temperature than the crawl space air, we will actually be adding moisture to the crawl space rather than removing it.

Here in South Carolina, we often have outside air dew point temperatures around 75F. With air conditioning, cool soils and cold ductwork in our crawl spaces, the dew point temperature in our crawl spaces is often below 75F. When we vent them, we get condensation problems. Floors rot, mold or swell because of excess moisture. Ductwork sweats and becomes saturated with water. Duct energy losses go way up because the insulation isn’t insulation when it is wet.

Our mortgage, pest control and home inspection industries flag crawl space wood moisture contents above 20% as a potential problem. At this wood moisture content, mold supposedly can grow. I see problem-free crawl spaces with wood moisture contents of 16%. A wood moisture content of 16% relates to air at about 80% RH. The dew point temperature of a crawl space at 75F/80% RH is about 68F. Why in the world would I want to vent this crawl space with air that has a dew point temperature close to 75F? The result will be condensation on all the cool surfaces in the crawl space.

Hardwood floors over crawl spaces often experience cupping problems in the summer. Wood expands when it gets wet. The typical scenario I see is that air conditioning is keeping the living space moisture lower than the crawl space moisture. This results in uneven moisture levels on the upper and lower surfaces of the wood. The lower, wetter surface expands and causes the boards to cup.

A common solution is to add ventilation to the crawl space, to reduce the moisture levels. Guess what happens in the winter? The boards cup the opposite way. Now our crawl space is vented with relatively dry air such that things in the crawl space really dry out. (As you heat air, its relative humidity drops.) The more ventilation we add to cure summer cupping, the worse the reverse winter cupping. We are slamming the wood moisture levels from one extreme to the other. Other moisture related wood movement such as swelling and shrinking of doors happens in the house as well.

Fallacy #5 – Venting a crawl space is not an energy issue.

From an energy standpoint, why would we want to vent a crawl space? In the winter, an unconditioned crawl space is warmer than outside. Bringing in additional cold outside air will only tend to make the crawl space colder, and increase heat loss. In fact, we often install automatic vents that close during the winter just for this reason. The opposite situation occurs in the summer: warm outside air will add heat to a crawl space and increase the cooling load. Since we so often install ducts in crawl spaces, venting increases the energy loss from ducts. Summer-time condensation in duct insulation can easily double the energy losses from ducts. 

Fallacy #6 – Increasing ventilation of a crawl space is a viable soil gas mitigation procedure.

A potential solution for addressing elevated radon levels in crawl space structures is to increase the ventilation rate of the crawl space. The general rule of thumb is to double the ventilation rate to reduce radon levels by half. My first question is: what’s the current ventilation rate? A common mitigation strategy is to add a powered fan to mechanically increase ventilation rates. If we assume a 1/150 ventilated crawl space using 24 sq inch vents and a constant 1 MPH wind, we would need a fan that could provide over 6 air changes per hour. That’s 6*4500/60 = 450 CFM, just to reduce the radon levels by 50%. But why increase the ventilation rate of a crawl space to solve a soil gas problem when the increase in ventilation can cause so many other potential problems and expenses?

To Summarize:

Fallacy #1 – a research basis for current crawl space ventilation guidelines exists, when in actuality it does not appear to exist.

Fallacy #2 – We build houses the same today as when today’s crawl space ventilation guidelines were established, when in fact our houses today are drastically different.

Fallacy #3 – The total net free area will provide adequate ventilation, when in reality the actual flow measurements of small NFA versus large NFA vents installed to the same NFA ratio are drastically different.

Fallacy #4 – Venting will reduce crawl space moisture levels. Venting will only help reduce crawl space moisture levels when the outside air is dryer than crawl space air, or when enough hot outside air enters and warms the crawl space. Outside air in the summer may actually contain more moisture than crawl space air, and may make the situation worse, not better. In winter, venting will help dry a crawl space, sometimes to a detrimental extreme.

Fallacy #5 – Venting a crawl space is not an energy issue, when in reality it can increase both the heating and cooling load.

Fallacy #6 – Increasing ventilation of a crawl space is a viable soil gas mitigation procedure. Ventilation to reduce soil gas (radon, moisture, etc.) has so many inherent problems as discussed in this paper that, in my opinion, it is not worth the expense or liability.

Un-Venting a Crawl Space

Since moisture is such an issue in crawl spaces, addressing moisture issues is the first priority in closing a crawl space. Exterior water must be directed away from the foundation with proper grading of the lot and proper handling of roof runoff. Crawl space soil should be completely covered with a vapor retarder. Capillary moisture movement should be restricted using either capillary breaks under piers and foundation walls, or by covering foundation walls and piers with a vapor retarder. Foundation walls can be insulated rather than floors over crawl spaces for enhanced thermal performance. In some instances, a dehumidifier will need to be added to the crawl space because of the complexity of home designs and the psychrometrics involved. Specific details for a sealed crawl space can be found on our sealed crawl space specifications page.

In a good crawl space, with good moisture control in and around the foundation, moisture problems won’t exist. Interior moisture levels will be more stable. Hardwood floors and other interior wood will be more stable and less prone to shrinkage and warping. Energy costs will be lower and duct condensation will be eliminated.


ASHRAE. 2001. 2001 ASHRAE Handbook Fundamentals, pp. 6.4 6.9, Atlanta: American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc.

Boercker, F., 1984, “Technical Review of a Residential Conservation Service Measure: Insulation of Crawl Spaces”, ORNL, Oak Ridge Tenn 37830

BRAB. 1962, “Ground Cover of Crawl Spaces”, Report No. 15A to the Federal Housing Administration, Publication no. 998, Building Research Advisory Board, National Academy of Sciences-National Research Council, Washington, DC.

Britton, R.R. 1948. “Crawl Spaces”, HHFA Technical Bulletin No. 2, January 1948. Washington, DC.

DeWitt, C. A., 1991, “Calibration of Foundation Vent Flow Rates”, Report to the Clemson University Housing Institute, Clemson, SC.

DeWitt, C. A., 1993, “Ventilation of Crawl Spaces in a Warm, Humid Climate”, Dissertation, Clemson University, Clemson, SC.

ICBO. (1998). International One and Two Family Dwelling Code. Falls Church, VA: The International Code Council, Inc.

Small Homes Council. 1980. “F4.4 Crawl Space Houses,” Vol 4, no 2. University of Illinois. Champaign, IL.

Verrall, A.F. and Amburgey, T.L. 1975. Prevention and Control of Decay in Homes, USDA Forest Service, and Department of Housing and Urban Development, US Government Printing Office, 001 0

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Successful moisture control in some crawl spaces requires complete coverage of the soil with a ground cover and the elimination of ventilation. One of the major concerns with sealing a crawl space is the lack of ability to inspect for and treat termite infestations. In a vented crawl space with floor insulation, foundation walls are more accessible, and supposedly easier to treat and inspect. In a sealed, unvented crawl space, ground covers and/or insulation often cover the inside of the foundation walls, supposedly rendering inspection and treatment more difficult.

Treatment and inspection of termites in South Carolina are controlled by several issues: 1) SC Pest Regulations, 2) Termiticide labels, 3) EPA Pesticide rules (PR-Notice 96-7), and 4) Pest Control Company liability and insurance issues. This paper will discuss the ramifications of these issues when dealing with sealed crawl spaces.

Dealing with 100% Ground Cover

The first step in controlling moisture in a crawl space, whether sealed or vented, should be a ground cover over 100% of the soil. (Of course, exterior surface and rain water need to be dealt with correctly, as well.) Since significant moisture can enter a crawl space through that part of the foundation wall below grade, RLC Engineering, LLC’s recommendation is to continue the ground cover up the foundation wall to at least the level of the exterior soil. (In a retrofit situation, this ground cover can stop six to eight inches below the mud sill to allow for a termite inspection strip.)

When a ground cover is installed to or even up the foundation wall, the customary trench-and-flood application of termiticide is difficult. To perform this application, the ground cover must be moved. PCO’s are reluctant to perform this function because of the economics involved as well as liabilities associated with disturbing this system.

An alternative approach to applying the termiticide is to inject the chemical though the ground cover, similar to applying the chemical through a concrete slab. This can be performed by applying a “basement” classification to the crawl space. The termiticide chemical can be injected through the ground cover as if the ground cover was a slab. I.e. if the chemical label calls for injection at no more than 12 inch spacing, use the same spacing to inject through the ground cover. With a short nozzle, the nozzle can be pushed through the ground cover material.

Resulting holes in the ground cover do not need to be filled, because the crawl space is not a “commonly occupied space.” The holes also do not need to be filled from a moisture stand point, because an insignificant amount of moisture will migrate up through the holes.

To prevent splash-back of the chemical through the holes, use a 4-way nozzle that distributes the liquid chemical in a more horizontal direction. In certain circumstances, the termiticide can be applied as a foam to ensure complete coverage of the area between the injection holes, and to reduce the likelihood of splash-back. With a thin ground cover such as 6-mil polyethylene, injecting foam will provide better coverage than injecting through a slab because of the ability to determine when the cavity under the ground cover is filled.

Dealing with Concealed Foundation Walls

From an energy standpoint, insulating foundation walls is more energy efficient and economical than insulating the underside of the floor above the crawl space. (With open web floor trusses, insulating the floor is often very ineffective because of the inability to create a continuous thermal barrier.) When ducts are located in a crawl space, insulating the foundation wall is even more cost effective. Therefore, the second part of sealing a crawl space is insulating the foundation walls.

When the foundation walls are insulated, inspecting for termite activity is more difficult and can be nearly impossible. At the same time though, inspecting an insulated crawl space wall is no different than inspecting the foundation wall of a finished basement.

The specifications for sealing a crawl space published by RLC Engineering, LLC include the installation of a metal termite shield that protrudes to the inside past any foundation wall coverings. With the use of this termite shield, inspecting a crawl space with covered foundation walls is actually easier and more effective than inspecting a finished basement wall.

From a physical standpoint, a sealed crawl space with insulated foundation walls is no different than a finished basement. A 6-mil ground cover is no different than a thin concrete slab. SC Pest Regulatory Standards, chemical manufacturer labels, and EPA requirements do not prevent treating a sealed crawl space differently than a finished basement. Insurance issues should not be a concern for the same reason, as long as due diligence and proper procedures are followed. RLC Engineering, LLC will provide support in treating a sealed crawl space as a finished basement.

Treatment alternatives

A couple treatment alternatives exist for dealing with sealed crawl spaces. The Sentricon bait system can be used in lieu of soil treatment with appropriate use of SC Pest Regulatory Waivers and Disclosures. If accepted by EPA, new labeling of Termidor will allow treatment of only the exterior perimeter of a foundation. These systems would remove the requirement to treat the inside of the crawl space perimeter.

Some spray-applied cellulose insulation is treated with a borate additive that performs as a termiticide as well. If this material is used to insulate the interior of a crawl space foundation wall, the potential for termite access via a “finished” foundation wall is significantly reduced.


PR-Notice 96-7

Section D.2.a on post-construction treatment of accessible crawl spaces says “…..When soil type and/or conditions make trenching prohibitive, rodding may be used……”

Section D.2.B on inaccessible crawl spaces says you can drill into the crawl space and spray.

Section F on foamed termiticides says “…Applications may be made … into block voids or structural voids …or to the soil in crawlspaces”, and other similar voids.

Section H on plugging holes says “…”All holes in commonly occupied areas into which material has been applied must be plugged….”


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A sealed (unvented) crawl space will out-perform a vented crawl space in terms of both moisture and thermal issues in South Carolina’s climate. The following new-construction specifications will provide a high-performance sealed/unvented crawl space foundation. (Note: In retrofit situations, additional steps such as mechanical humidity control may be necessary to ensure adequate moisture control. Under no circumstances should a wet crawl space be sealed without the use of a mechanical dehumidification system.)

  1. Combustion devices located within the crawl space shall be sealed combustion. Under no circumstances shall a naturally vented combustion device be installed in a sealed crawl space.
  2. No passive ventilation openings shall be allowed between the exterior and the interior of the crawl space.
  3. The top of foundation walls/piers shall be no less than 4-block (32″) higher than the highest soil within the crawl space.
  4. A capillary break/barrier (6-mil poly or equivalent) shall be installed between interior footings and piers. Footings shall be formed with keyway or protruding rebar to restrict horizontal movement of pier (per local code.)
  5. The top course on all foundation walls and interior piers shall be solid: I.e. either poured concrete walls, solid cap blocks, bond beams or filled hollow block cores.
  6. Preservative-treated 2×8 or larger mud sills shall be used between foundation wall/pier and framing material.
  7. Termite protection shall be provided by pre-treatment and by incorporation of corrosion-resistant metallic termite shield between the top of foundation walls/piers and the mud sill. The metallic termite shield shall extend to the exterior at least 1 inch past the finish wall material and to the inside at least one (1) inch past the expected width of foundation wall insulation.
  8. Joints in the termite shield shall be permanently fused with solder (or equivalent) or overlapped a minimum of 6 inches and sealed with a rubberized asphalt sheet membrane at least 6 inches wide. Penetrations through the termite shield for anchor bolts, etc. shall be sealed with a minimum 6 inch square of rubberized asphalt sheet material between the termite shield and the mud sill. Rubberized asphalt material shall be a minimum of 35 mils thick with adhesive surfaces on both sides. (Example: MFM Building Products “Double Bond” material)
  9. Caulk or sill sealer shall be installed between the top of the foundation wall and the termite shield, and between the termite shield and the mud sill. The joint between the mud sill and the band joist shall be caulked or similarly sealed.
  10. A soil vapor retarder (vapor barrier) membrane (6-mil poly or equivalent) shall be installed to completely (100%) cover exposed soil in the crawl space. Seams should be overlapped a minimum of one foot.
  11. Soil vapor retarder membrane shall turn up the inside of foundation walls to at least the height of exterior soil level. Where the outside of the foundation wall is not damp-proofed, the soil vapor retarder membrane shall extend to the top of the foundation wall.
  12. Foundation walls shall be insulated on the interior surface with R-6 or higher insulation (duct insulation, rigid foam insulation, spray-in-place expanding urethane foam or equivalent, per local code). Band joists to be insulated as well. (1998 IECC)
  13. Floors over crawl spaces shall not to be insulated.
  14. All penetrations through subfloor and foundation walls shall be sealed with approved draft-stop material.
  15. The exterior soil shall slope away from the foundation at least 6 inches in the first 10 feet completely around the perimeter. If used, gutters and downspouts should drain at least 10 feet from the foundation.
  16. Where there is a probability of standing water in the crawl space, subsurface drains, French drains, sump pumps, sloped soil and other similar measures shall be used to prevent standing water.
  17. In areas where radon may be encountered, a passive radon mitigation system shall be installed.
  18. Clothes dryers, bathroom exhaust fans, AC condensate drains and similar moisture-containing exhausts or vents shall not drain into the crawl space.
  19. Conditioned air from the house HVAC system shall be provided to the crawl space through supply register(s) at a rate of one (1) to two (2) cubic feet per minute per 100 square feet of crawl space soil surface area (house footprint area). Exception: When a mechanical dehumidification system or active sub-membrane radon mitigation system is installed in the crawl space, conditioned air shall not be supplied to the crawl space.
  20. The insulated, vapor-retarded foundation wall shall be essentially in vertical alignment with the insulated above-grade walls of the living area. Where a deck, porch or similar entity exists within the footprint of the primary foundation wall such that it creates an un-insulated interface between the crawl space and the exterior, an interior air-tight, vapor-retarded, insulated pseudo-foundation wall will be added under the above-grade walls of the adjoining living area. (I.E. The insulated foundation wall is to be directly underneath the insulated wall of the living space directly above it.)

Rationale for a sealed/unvented crawl space

South Carolina’s warm, humid climate creates a situation where surfaces in an air conditioned house are often below the dew point or condensation temperature of the outside air. Where condensation or very high relative humidity occurs, mold and decay can occur.

In a crawl space, the soil temperature, living space temperature and duct temperature all contribute to lowering the temperature within the crawl space. Outside air entering this area can often be cooled to the point where condensation occurs, either on structural members or on ductwork. The solution to prevent condensation is to either warm the surfaces, or lower the moisture levels. Raising surface temperatures is not likely due to our climate, occupant desires and the laws of physics. The other option to reducing high moisture levels and condensation is to lower moisture levels within the crawl space.

Sources of moisture are primarily evaporation from the soil, diffusion through foundation walls, and with humid air entering the crawl space. When a soil vapor retarder (vapor barrier) is used to reduce evaporation from the soil and diffusions through foundation walls, the remaining major source of moisture is from ventilation air.

A psychrometric chart can be used to show that for much of the year, South Carolina’s climate is such that venting a crawl space actually makes the crawl space wetter. A sealed crawl space has several advantages. First, the crawl space is dryer, decreasing the likelihood of mold and decay. In addition, wood is stronger when it is dryer, therefore the structure performs better. A decrease in moisture in the crawl space allows air conditioners to run more efficiently, since they have to remove less moisture from the air. Duct insulation also stays dry, maintaining the integrity of the insulation. Humidity levels within the crawl space and house are more uniform from season to season, creating less movement of hardwood floors, interior wood trim and cabinetry.

Attic moisture problems are often reduced, since many attic moisture problems are related to high crawl space moisture levels.

From thermal, moisture, structural, occupant health, financial and operational standpoints, sealed crawl space foundations perform better in my opinion than vented crawl spaces. Essentially all other building science experts around the country also hold this opinion. (Ref: Energy and Environment Building Association)


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What are Ground covers?

A ground cover, also known as a soil cover, is a sheet material installed on the soil in a crawl space to keep moisture from moving upward through the soil into the air where it can cause problems with the house. Soil moisture can lead to decay of joists, flooring, roofing and other parts of the house as well as contribute to problems with painted surfaces, hardwood floor and other wooden items, and cause mold-related indoor air quality problems.

Ground covers are sometimes referred to as vapor retarders, vapor barriers, moisture barriers or ground covers. The most common type is 6-mil polyethylene sheets; however roll roofing and EPDM rubber can sometimes be used.

Why are ground covers needed?

Years ago, most houses didn’t need ground covers in the crawl space area. They were built on high, dry sites with plenty of ventilation under the house. With high and dry sites becoming scarce, builders are resorting to less suitable land. Houses also didn’t have insulation that prevents moisture from escaping rapidly, or air conditioning that creates artificially cool surfaces.

The combination of significantly reduced ventilation, lower foundations and cooled surfaces results in ideal conditions for mold growth and wood decay.

As much as twelve gallons of moisture vapor can be released from the soil into the crawl space of a 1000 square foot house in a single day. A ground cover reduces the amount of moisture added to the crawl space, thereby reducing the potential for mold and decay. The first and simplest line of defense is a complete ground cover in the crawl space.

Before installing a ground cover

Check site drainage. No water should stand under the house. If it does, divert the water away from the house by ditching, sub-surface drainage methods, or a sump pump. Be sure that the land slopes away from the house for 10 feet all the way around the house. Add extensions so that downspouts drain water at least 5 feet from the foundation.

Prepare the crawl space soil. Soil under the house should be crowned to prevent standing water in the crawl space. Remove wood scraps and other debris. Smooth the soil to prevent hills and valleys.

Above all, solve your drainage problems before installing ground covers. Otherwise, you may do more harm than good: if standing water is still present in the crawl space, the ground cover with prevent the water from seeping into the ground.

How to install a ground cover:

  1. Remove all debris from the crawl space, including wood, cans, bottles and rocks from the crawl space area.
  2. Unroll the ground cover. Spread the ground cover over the entire surface of the crawl space. Trim around piers. Lap the ground cover up foundation walls if the outside ground level is above the crawl space soil level. Leave at least 8 inches between the ground cover and any wood framing or flooring members to prevent hidden termite pathways.
  3. Overlap joints approximately 12 inches. Anchor the ground cover in place with bricks, sand or long nails.

Condensation and liquid water are often present under a ground cover. This is normal, and indicates that the ground cover is performing its job. Do not fold back the ground cover to dry out this water.

If water puddles on top of the ground cover, determine and solve the source of the water. Correct plumbing leaks, site drainage and duct condensation problems. If necessary use a knife or nail to poke holes or small slits in the ground cover to let the water seep down through the ground cover. (These small holes will let gravity push water down through the holes, but not allow a significant amount of water vapor from moving up through the holes.)

Special considerations for older homes:

Some older homes have existed with such high moisture levels that installation of a complete ground cover causes excessive drying, shrinking and warping of flooring, cabinets and trim. In most cases, the excessive drying is a result of abnormally high ventilation rates of the house. These high ventilation rates result in high indoor moisture levels in the summer and low humidity levels in the winter. The best solution then, is to find and correct the leaks to reduce ventilation rates. In very rare cases, it may be necessary to uncover some of the soil to increase moisture levels. If this procedure is necessary, simply roll back a foot or so of the ground cover around the foundation wall.

Note: Other changes to a house such as new windows, doors, and heating and cooling systems can cause moisture levels to rise. If this type of change is made to a house, complete ground coverage is warranted. 

CAUTION: If signs of decay are evident in the crawl space, complete ground coverage is necessary even if excessive drying is noted inside the house. Once started, less moisture is necessary to continue the decay process. Stop the decay, then deal with low indoor humidity levels some other way.

Crawl space ventilation

Current building codes normally require crawl space ventilation. With proper site drainage and a complete ground cover, building scientists agree that crawl space ventilation is not necessary and even detrimental in warm, humid climates. RLC Engineering, LLC’s recommendations are to design and operate crawl space foundations as sealed, unvented foundations. Design specifications are available from RLC Engineering, LLC describing construction details. These specifications have been accepted by several building code entities.


One type of wood moisture meter. The pins are pressed into the wood to obtain a reading.

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Wood Moisture Content (WMC) is often used as an indicator of decay problems in houses. This document describes the meaning and use of readings from a wood moisture meter.

The moisture content of wooden substructure members is routinely being disclosed in the closing process of real estate transactions. These WMC readings are usually provided by pest control operators, and are included in the “termite letter” (form CL-100 Official South Carolina Wood Infestation Report). Wide-spread and often extensive moisture damage occurs in residences across the state of South Carolina and elsewhere in the Southeast due to the region’s warm, humid climate.

Wood Moisture Content is the weight of water in a piece of wood expressed as a percentage of oven dry weight of wood. Fresh cut trees can have a wood moisture content over 200%, while completely dried wood will have a wood moisture content of 0%. Wood in buildings usually has a wood moisture content of 5% to 15%.

WMC Scale’s Meanings:

Below 12% – Readings in this range are common to kiln or oven dried woods and furniture grades of wood, and represent dry conditions. Most interior wood is in this range.

12% – 16% – Readings in this range are common to lumber during construction, air dried lumber and “healthy” residential substructures (beneath first floor in crawl spaces). These are typical readings for exterior wood.

16% – 20% – Readings in this range indicate a possible elevated level of wood moisture. Such readings should alert the homeowner to look for a source of excess moisture. The excess moisture source should be corrected if found.

20% – 28% – Readings in this range indicate that conditions are border-line for decay. Surface molds may develop. The excess moisture source should be corrected immediately, and monitored until the WMC returns to the 12-16 range.

28% and above – Readings in this range are often accompanied by decay damage. Substructures with WMC in this range may show decay or rot in floor joist, sills, and subflooring. Repair is often required when WMC readings are in this range.

Wood moisture content between 0% and about 28% is dependent upon the relative humidity (RH) of the air. As the air’s RH increases, so does the moisture content of any wood exposed to the air. Wood exposed to air with a RH of about 90% will reach a Wood Moisture Content of about 20%. Above 90% RH or 20% WMC, mold can grow on the wood.

Decay fungi need liquid water to grow. Once wood is dried below about 28% WMC, water is not available to support decay, unless the wood is exposed to liquid water. This water may come from condensation, roof leaks, plumbing leaks, or contact with the soil. If decay or WMC readings over 28% are present, find and fix the sources of liquid water quickly.

WMC of framing members in a crawl space will usually be lowest in late winter and highest in late summer. If low WMC readings were obtained during the winter, and other signs of high moisture levels are present, obtain additional readings during the summer.

Other signs of high moisture readings include surface mold, evidence of water flows in the crawl space, evidence of water stains or evaporation from foundation walls and columns, evidence of condensation on ducts and evidence of water drops impacting the soil under ducts.