Crawl Space Construction :
BUILDING CODES
                                                            Historic Origins
                 Construction guidelines from agencies like the National Bureau of Standards (1923) and the Federal Housing Administration (1935) contain the earliest known recommendations that crawl spaces be ventilated with outside air. Despite documented failure in such crawl spaces in the 1930s and a lack of technical justification, the
                  FHA turned these guidelines into requirements in the early 1940s. Since then, building regulations across the United States have required ventilation of crawl spaces with outside air. This history is documented by William Rose (Davis, Warren and Rose, 2002) and shows that there is no scientific basis for current crawl space ventilation requirements.
        Building research from the 1970s onward has documented that this ventilation may cause or contribute to moisture problems instead of preventing them, and more recent building codes like the 2000 and 2003 versions of the International Residential Code (IRC) have included language to allow the construction of crawl spaces without ventilation openings to the outside. Unfortunately, these more recent codes still require ventilation with outside air as the default, and allow crawl spaces to be approved without any ground vapor retarder.
                The exceptions that allow closed crawl spaces are vague or incomplete and as a result are nearly impossible to follow or enforce consistently. For example, IRC Section R408.2 Exception 5 allows a closed crawl space if there is a ground vapor retarder, perimeter wall insulation, and conditioned air, but it gives no guidance on how much conditioned air should be supplied or acceptable methods for injecting the air.
               Other sections of the code are problematic for technical reasons. Exception 5 requires that the perimeter wall insulation be installed in accordance with Section N1102.1.7. Section N1102.1.7 requires the perimeter wall insulation to extend downward from the subfloor to the finished grade level and then vertically and/or horizontally for at least an additional 24 inches (61 cm). This is often referred to as the “L-shaped” method of installing insulation. This method is not viable in the Southeast for two reasons:
• First, the risk of insect pest damage is too high in the Southeast to deprive the pest management industry of an inspection gap at the top of the masonry foundation wall and convenient access to the sill plate for inspection or treatment whenever possible.
Insulation in ground contact can increasethe risk of termite infestation.
• Second, the “L-shaped” installation method is extremely impractical in terms of real life construction sequences, access, inspections, and potential pest treatments given currently available insulation materials.

              The 2004 Supplement to the International Codes retains the problematic insulation requirement in the IRC (now section N1102.2.8) and in the International Energy Conservation Code (Section 402.2.8).
              However, the Supplement has significantly changed Section R408 Under Floor Spaces in the IRC. The exceptions noted above in Section R408.2 have been deleted. A new section, R408.3 Unvented Crawl Space, has been added. This new section allows construction of a crawl space with no ventilation openings to the outside if (1) there is a sealed ground vapor retarder, (2) the perimeter walls are insulated in accordance with Section N1102.2.8, and (3) the crawl space is designed as a plenum or it is designed with either an exhaust ventilation system with an air pathway to the common area or a conditioned air supply with a return air pathway to the common area. There is no apparent compliance pathway for a closed crawl space that is isolated from the living space or that utilizes only floor insulation.





                                               The Ventilation Myth:

             Why doesn’t ventilation with outside summer air dry out a southeastern crawl space?
             The traditional method of controlling moisture in crawl spaces is to provide passive ventilation with outside air. However, wall-vented crawl spaces routinely experience moisture problems like condensation, surface mold growth, high wood moisture content, or wood rot. The typical “cure” for such problems has been to add more ventilation, either in the form of additional ventilation openings to the outside or by installing a fan or multiple fans to intentionally move more outside air through the crawl space.
Research has confirmed over the long term what building scientists have measured repeatedly in previous short-term investigations of crawl space failure: the outside air contains more water vapor than the air in the crawl space during the warm seasons, and has no potential to dry the crawl space. Instead, the outside air ends up contributing water vapor to the crawl space.
          To understand why, first there is a need to define three properties of air: temperature, relative humidity, and dew point temperature.

Temperature is the most familiar property, and is simply a measure of the heat in the air.
But what exactly is relative humidity? At any moment, the air around you contains a certain amount of water vapor. At the current temperature, there is also a limit to how much water vapor that air can contain before the water vapor condenses into liquid water… better known to us as “condensation” or “rain”.

          Relative humidity is the ratio of actual water vapor in the air to the maximum amount of water vapor that the air can possibly hold at the current temperature. So, the relative humidity changes if either the amount of water vapor in the air changes or the temperature of the air changes. If the relative humidity in one location is 90% and the relative humidity in another location is 40%, you can’t tell which location has more water vapor unless you also know the air temperatures. Cold air can hold less water vapor than warm air, so if air gets cooler without losing water vapor, its relative humidity increases.
          Dew point temperature, often just called “dew point,” is a more direct indication of how much water vapor is in the air. If you change the temperature of air without adding or removing water vapor, the dew point stays the same. If you remove water vapor, the dew point goes down whether or not the air temperature changes. If you add water vapor, the dew point goes up whether or not the air temperature changes. As you might guess, the dew point measurement tells you the temperature at which the water vapor in the air will condense into liquid water, either because the air cools down to the dew point temperature, or because the air comes into contact with a surface that is cooler than its dew point temperature.
       Psychrometric charts and slide rules allow you to calculate the unknown third property of air if you know the other two, for example, calculating dew point when the air temperature and relative humidity are known. They can calculate the change in one property if another property is changed. You can generally obtain a psychrometric chart or slide rule from manufacturers of air conditioning or dehumidification equipment, and there are several on-line psychrometric calculators on the Internet.

        As an example, let’s look at the properties of air on a mild summer day in North Carolina. Let’s say it’s 85° F (29° C) outside and the relative
humidity is 60%. Conventional wisdom tells us that this air is warm and dry, so it should be great for ventilating a crawl space. Using a slide rule or chart, we determine that the dew point of this air is 70° F (21° C), which is relatively dry compared to typical conditions. Now let’s assume that air goes into the crawl space. A typical summertime temperature in a wall-vented crawl space is 73° F (23° C), so the outside air cools down. No water vapor has been removed, so the dew point stays at 70° F. The temperature has dropped, so we need to use the psychrometric chart to find the new relative humidity… and now it’s a whopping 90%! Furthermore, the temperature of the ductwork, plumbing, and bottom side of the insulation are below 70° F, so the water vapor will condense on those surfaces.
It may even condense on the crawl space floor or on the wood framing! It turns out that ventilating with the outside air adds moisture to the crawl space, it does not dry it out.


                                                  ROOF RUNOFF

            Use a system to direct roof runoff away from the house and prevent the runoff from entering the crawl space. This is often a gutter system, but could also be a system of foundation waterproofing, perimeter gravel bed and a drain. Another option is foundation “flashing,” for example, a layer of impermeable material like EPDM rubber that extends down and away from the foundation wall below grade for approximately 6 feet.
           If sub-surface drain pipes or gutter leaders are used to manage roof runoff, they must not be connected to the crawl space drain. This eliminates the chance that blockages or heavy rains will cause roof runoff to enter the crawl space.

             Exterior ground and surface water:

Provide site grading around the perimeter of the house with a minimum of 6 inches (152 mm) of fall over 10 feet (3048 mm) of run to direct ground surface water away from the house. Swales or drains may be used if lot lines, slopes, walls, or other barriers prohibit the required grading. Ensure that landscapers or property owners do not install flower beds, tree mounds, mulch piles or other landscaping features that prevent drainage away from the house. In-ground irrigation systems or yard sprinklers require special attention because they can easily cause a water problem in an adjacent crawl space. Make sure these systems do not put water onto the crawl space walls.

               Provide a foundation drain system whenever the exterior grade is 12 inches (305 mm) or more above interior crawl space grade. Keep foundation drain systems separate from crawl space drain systems.
Provide foundation damp-proofing or water-proofing when the exterior grade is above the interior crawl space grade to prevent the flow of water through the wall by capillary action or “wicking.”
Raise the crawl space grade above the exterior grade to eliminate the need for a foundation drain, damp-proofing or water-proofing.
Protect exterior crawl space access door(s) from roof runoff. For example, if there is no roof runoff system, locate the access door on a gable end wall. Whenever possible, build the bottom of the access at least 4 inches (102 mm) higher than the exterior grade. If raised access is not possible, provide a dam or gravel drain to prevent water entry. Use non-corroding access doors, especially in coastal communities, to prevent deterioration.

The entrance to a closed crawl space should be elevated well above exterior grade, reducing the likelihood of water entry in heavy rains or minor flooding.

                 Humid Air:

Seal all gaps between foundation wall andsill plate, sill plate and band joist, and band joist and subfloor. Seal penetrations through the crawl space wall for water service, electrical service, plumbing fixtures, ductwork, etc. Use solid blocking and sealants to seal gaps between the exterior wall opening and ductwork for outdoor packaged-unit heating and cooling equipment, if present.
Seal connections from the crawl space to areas under attached porches or decks, which are common sites of liquid water intrusion or entry of humid outside air.
Houses in an area prone to flooding or designated as a Special Flood Hazard Area (SFHA) by the Federal Emergency Management Agency (FEMA) or the National Flood Insurance Program (NFIP) must have FEMA/NFIP-compliant flood vents in the crawl space perimeter wall.
When required, choose flood vents that minimize stand-by air leakage.

Build hollow-block masonry foundation walls with either a continuous top course of solid masonry or the top course of masonry grouted solid to prevent passage of air from the interior of the wall into the crawl space.

             Evaporation from the ground and perimeter walls:

Cover all crawl space ground with a minimum 6-mil polyethylene vapor retarder. Lap seams at least 12 inches (305 mm).
Cover the masonry perimeter walls with minimum 6-mil polyethylene vapor retarder, leaving at least 3 inches (76 mm) of exposed masonry at the top of the wall.
Mechanically attach the vapor retarder material and seal it to the wall with duct mastic. Common strategies for mechanical attachment include powder-driven nails, pins or masonry screws that hold the vapor retarder up behind a furring strip or wall insulation.

       Seal the ground vapor retarder to interior columns at least 4 inches (102 mm) above the crawl space floor. Install the ground and wall vapor retarders as a sealed liner by sealing all seams and connections to masonry with fiberglass mesh tape embedded in duct mastic. If you choose to use a tape product to seal seams, ensure that all surfaces are clean before applying the tape and do not subject tape joints to mechanical stress.
        If you use unreinforced 6-mil polyethylene (the material most commonly available at home improvement retail stores), protect it with an additional layer of durable material (for example, artificial turf, vinyl runners, or other carpet material) in storage areas or traffic areas, like a service path to mechanical equipment. Consider using thicker, reinforced vapor retarder materials for improved durability and puncture resistance, and to eliminate the need for additional protective coverings.
The ground vapor retarder must be secured to resist movement or tears.

Secure the ground vapor retarder material as necessary to resist movement due to anticipated traffic. Anchor 6-mil polyethylene with sod staples or galvanized spikes through washers or nailing tins on approximately 12 foot (3658 mm) centers. Use additional staples or spikes to secure the vapor retarder in steeply sloped areas or in heavy traffic areas like the crawl space entrance or service paths. Heavier vapor retarder materials in small crawl spaces or in large crawl spaces with intermediate piers may not need any staples or spikes.

             Moisture Management:

Facilitating Removal:
Please remember that moisture still gets into a properly closed crawl space. Ground moisture can wick up through masonry walls or support columns and evaporate into the crawl space.
Rain can wick through the perimeter masonry wall or sill plate and evaporate into the crawl space. Wind, duct leakage or other building pressures will inevitably force some amount of humid air through the perimeter wall, since it’s impossible to do a perfect air-sealing job.
Plumbing failures or floods put liquid water in the crawl space.
For all these reasons, a closed crawl space needs components that remove both liquid water and water vapor. Duct leakage and diffusion of water vapor to the house above may provide some drying potential for a closed crawl space, but an intentional drying mechanism is required to ensure adequate moisture management.
Some installers and researchers have theorized that the moisture control improvements of closed crawl spaces allow the mechanical contractor to down-size the installed heating and cooling equipment. However, there are few commonly available tools at the time of this writing to calculate the impact of a closed mcrawl space on the sizing of mechanical equipment for the home.

Grade the floor of the crawl space to one or more low points. Provide a drain or sump pump at each low point to remove liquid water from the crawl space in case of a plumbing leak or other flooding event. Use a backflow valve in crawl space drains and a check valve in sump pump out-flow pipes to prevent reverse flow of outside water into the crawl space and to reduce the chance of vermin entry. Floor drains with p-traps that connect to the wholehouse plumbing waste drain or to a municipal sewer system may allow entry of sewer gases if (when) the trap dries out and pose a risk of sewage backup.

 Terminate crawl space drains or sump pump discharges in ways that reduce the risk of damage or blockage. For example, surround the termination with gravel or shrubbery to reduce the risk of soil blockage or damage from lawn equipment.
Provide a mechanical drying system to remove water vapor.
Use conditioned air from the supply-side ductwork or stand-alone dehumidifiers to meet the requirement for a mechanical drying system.
Terminate appliance water discharge pipes (for example, water heater
temperature/pressure relief valves, air conditioner or dehumidifier condensate drains, or water softener discharges) to outside, to an interior pump, or to a crawl space drain.
Drain appliance discharge pipes directly to outside or to an interior pump, not to crawl space drains. If the backflow valve in the crawl space drain is installed out of level or if there is water in the drain pipe exerting pressure on the backflow valve, appliance discharges may build up significantly inside the crawl space before they can drain out.
Include relative humidity monitors or liquid water detectors in the design to
inform occupants of the performance of the system.
Install auxiliary condensate drain pans with float kill-switches under air handlers or dehumidifiers in a closed crawl space for added security against overflows due to blockage in the condensate line.

                  Terminate clothes dryer exhaust vents: to outside.
         Terminate all kitchen and bathroom exhaust vents to outside.

                                                      Pest Control

Two important goals of a closed crawl space design are to
1. Avoid increasing the risk of damage from subterranean termites or other insect pests
2. Provide the ability for pest management professionals to inspect the structure and provide treatment, when necessary.

When inspectors perform inspections for real estate transactions if the professional identifies such areas on the inspection report. The definition
of “hidden or obstructed” includes areas of the building that would require disassembly with tools or removal of pieces or parts of the building to allow for inspection.
For properties under a service contract or warranty, the pest management contractor may be held responsible for any and all damage that occurs to the property after the date ofm their treatment. This discourages them from contracting on properties with hidden or obstructed areas. In new construction projects, pest management contractors may be reluctant to provide soil pre-treatment, wood pretreatment, or an alternative pre-construction treatment unless they have sufficient information about the closed crawl space design to believe that they will have adequate access for the pre-treatment or for future inspections or treatments.

Closed crawl space designs must pay particular attention to materials applied on the perimeter walls, since this is a common path for wood-destroying insects from the ground to the structure. It is not acceptable to drape insulation or vapor retarder materials from the band joist or sill plate to the crawl space floor because this prevents inspection.
Provide a termite inspection gap of at least 3 inches between the top of any wall  vapor retarder material and/or perimeter wall insulation and the top of the perimeter masonry wall. Vapor retarder or insulation materials must not contact any wood framing.
Provide a clearance or wicking gap of at least 3 inches between the bottom of any perimeter wall insulation and the crawl space floor surface.
When the perimeter wall of a closed crawl space is insulated, the band joist must also be insulated.

      The use of rigid foam on the band joist will seriously impair visual or
physical inspection (“probing” or “sounding”) of the band by a pest management professional, because the foam may be damaged in the process or impossible to replace in its original condition. Foam insulations with low (< 1 perm) permeability to water vapor will reduce the ability of the band to dry to the crawl space, but would also reduce the potential for condensation on the band when the band is cold.
       Insulating the band joist with faced batt insulation, with the vapor retarder facing towards the inside of the crawl space, facilitates inspection or
treatment by a pest management professional since it is more easily removed and replaced without damage. This strategy allows some drying of the band joist to the crawl space during the cooling season, but may increase the chance of condensation on the band when the band is cold. The facing material may need to be fire-rated for direct exposure.
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                     Exhaust Fan Strategies Require Careful Design
Certain Editions of the residential code allows the use of an exhaust fan to take air from the crawl space and reject it to the outside as an accepted method for providing the required water vapor control, with the assumption that the makeup air comes from the conditioned volume of the house above.
       This method presents some risks.
First, to avoid the risk of backdrafting any combustion appliances, don’t consider using exhaust fans in the crawl space unless all combustion appliances in the crawl space are direct vent (“two-pipe”) models, with all combustion air piped directly from outside to the appliance and all combustion gases piped directly from the appliance to outside.
Second, there is no guarantee that the make-up air for the fan will come from the house as opposed to outside. The floor air sealing (which the code also requires) reduces the flow of makeup air from the house. House exhaust fans and “stack effect” – the natural action of warm air rising up and out of a home – can reduce air pressure in the house such that air will not flow to the crawl space. When this happens, the air removed from the crawl space by the exhaust fan is more likely be replaced by air from outside – exactly the situation the closed crawl space is designed to avoid.
       There are several scenarios which may require you to exhaust air from the crawl space, especially in existing homes. For example, you may need to remove radon or other harmful soil gases, ensure isolation of crawl space air from the living space when there are environmental hazards like mold or asbestos present, or simply prevent objectionable crawl space odors from entering the home. In these cases, provide a designed source of makeup air (one example could be a second fan that injects house air into the crawl space) to ensure that the crawl space exhaust fan won’t create a combustion hazard or water vapor load.




                             Basic Maintenance for Crawl Spaces
Most people don’t like to go into crawl spaces, but periodic inspection of any crawl space helps to ensure that any problems are caught before they cause damage. Property owners can perform these inspections and basic maintenance checks themselves or hire a private home inspector or other contractor to do it for them.
                                             Property owners should:
• Ensure that access doors are closed,
especially during warm weather.
• Ensure that there are no solvents, gasoline
or other potentially hazardous materials in
the crawl space.
• Inspect the crawl space regularly to:
? Identify vapor retarder damage or water
problems. Note that small water leaks in a
crawl space may not be caught by relative
humidity sensors.
? Ensure that no damage occurs when any
contractors work in the crawl space.
? Check and replace batteries as needed in
sensors or alarms.
• Ensure that any water intrusion, especially
flooding, is quickly drained or pumped out
of the crawl space.

Crawl space moisture is a problem in many homes in the United States. . Water underneath your house – in the form of condensation, water vapor, or liquid – is a recipe for rot as well as unhealthy mold that can be sucked up into your home.

A dirt, vented or unconditioned concrete crawl space under a home is a very bad idea. The earth has very high humidity in the soil. This water vapor moves easily into the crawl space and upwards into the house environment.

Damp environments are unhealthy and destructive... a place where mold thrives and spreads by producing millions of airborne spores – a rare few of which are even toxic – and heating and cooling costs are higher, especially if you have a vented dirt crawl space. Insects and critters of all kinds love damp environments and wet materials. Rot and decay occur in damp environments causing structural damage.

Crawl spaces do not have to be wet or flooded to be extremely unhealthy.

The natural airflow in a house is from bottom to top. This sucks the moist air and everything in it up into the living areas of the home. Mold spores, odors, humidity and critters create a very unhealthy environment for people. Many people are allergic to these things, and experience a host of symptoms and don't realize that their dirt crawl space is affecting their health.  



What the Code Says:

The under-floor space between the bottom of the floor joists and the earth under any building (except space occupied by a basement or cellar) shall be provided with ventilation openings through foundation walls or exterior walls. The minimum net area of ventilation openings shall not be less than 1 square foot for each 150 square feet (0.67 m ² for each 100 m ² ) of under-floor space area. One such ventilating opening shall be within 3 feet (914 mm) of each corner of said building.

Openings for under-floor ventilation.

The minimum net area of ventilation openings shall not be less than 1 square foot (0.0929 m ² ) for each 150 square feet (100 m ² ) of underfloor space area. One such ventilating opening shall be within 3 feet (914 mm) of each corner of the building. Ventilation openings shall be covered for their height and width with any of the following materials provided that the least dimension of the covering shall not exceed 1/4 inch (6.4 mm):

1. Perforated sheet metal plates not less than 0.070 inch (1.8 mm) thick.

2. Expanded sheet metal plates not less than 0.047 inch (1.2 mm) thick.

3. Cast iron grills or grating.

4. Extruded load-bearing brick vents.

5. Hardware cloth of 0.035 inch (0.89 mm) wire or heavier.

6. Corrosion-resistant wire mesh, with the least dimension being 1/8 inch (3.2 mm).

Exceptions:

1. Where warranted by climatic conditions, ventilation openings to the outdoors are not required if ventilation openings to the interior are provided.

2. The total area of ventilation openings may be reduced to 1/1500 of the under-floor area where the ground surface is treated with an approved vapor retarder material and the required openings are placed so as to provide cross-ventilation of the space. The installation of operable louvers shall not be prohibited.

3. Under-floor spaces used as supply plenums for distribution of heated and cooled air shall comply with the requirements of Section M1601.4

4. Ventilation openings are not required where continuously operated mechanical ventilation is provided at a rate of 1.0 cfm (10 m ² ) for each 50 square feet (1.02 L/s) of underfloor space floor area and ground surface is covered with an approved vapor retarder material.

5. Ventilation openings are not required when the ground surface is covered with an approved vapor retarder material, the space is supplied with conditioned air and the perimeter walls are insulated in accordance with local codes.

 Access.

Access shall be provided to all under-floor spaces. Access openings through the floor shall be a minimum of 18 inches by 24 inches (457 mm x 610 mm). Openings through a perimeter wall shall be 16 inches by 24 inches (407 mm x 610 mm). When any portion of the through wall access is below grade, an areaway of not less than 16 inches by 24 inches shall be provided. The bottom of the areaway shall be below the threshold of the access opening. Through wall access openings shall not be located under a door to the residence. Refer to mechanical code for access requirements where mechanical equipment is located under floors.

 Removal of debris.

The under-floor grade shall be cleaned of all vegetation and organic material. All wood forms used for placing concrete shall be removed before a building is occupied or used for any purpose. All construction materials shall be removed before a building is occupied or used for any purpose.

 Finished grade.

The finished grade of under-floor surface may be located at the bottom of the footings; however, where there is evidence that the groundwater table can rise to within 6 inches (152 mm) of the finished floor at the building perimeter or where there is evidence that the surface water does not readily drain from the building site, the grade in the underfloor space shall be as high as the outside finished grade, unless an approved drainage system is provided.