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What are the minimum standards for a weatherized crawl space?
6 mil vapor barrier covering all of soil of the crawl space floor (100% coverage) and extending up the perimeter walls and sides of the columns/piers 6 to 8 inches (mechanically or permanently sealed); air sealed floor of the residence at all penetrations (plumbing, electrical, and mechanical); R-19 floor insulation in place; foundation vents operational and/or sealed; and seams of the vapor barrier sealed.

What are the benefits of a closed crawl space?
Closing a crawl space (which includes sealing the vents, insulating the walls, creating a vapor barrier and conditioning the air in the boundaries of the crawl space) is now the standard for LEED homes, NAHB Green homes and various others rating systems that are recently being integrated into statewide building codes. The latest studies on crawl space ventilation indicate that ventilation (open crawl space) is actually contributing to moisture and humidity in the crawl space. Some of the benefits of a closed crawl space include:

  • Stay dryer than vented/open crawl spaces
  • Protect pipes from freezing
  • Require less insulation than open-air/vented crawl spaces (since the area of the perimeter walls is less than the area of the crawl space ceiling)
  • Bring ducts within the conditioned envelope of the home — an improvement that usually results in energy savings compared to vented crawl spaces.

According to a recent study of vented and unvented crawl spaces in North Carolina, homes with sealed crawl spaces with insulated foundation walls use 18% less energy for heating and cooling than identical homes with open/vented crawl spaces with insulation between the floor joists.

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Why should I worry about the humidity levels in my crawl space?
Closed crawl spaces can maintain relative humidity below the 70% target and dramatically reduce the potential for condensation on surfaces in the crawl space. Crawl spaces should be sealed, insulated, and passively kept warm if mechanical equipment, ducts and plumbing are left there. A crawl space is less likely to have moisture problems if the crawl space floor is higher than the exterior grade. Moisture control provided by a closed crawl space can dramatically reduce the risk and associated liability of mold and moisture damage in the southeastern climate.

Are crawl space contractors licensed in the state of NC?
No, but there are specific standards that all crawl space contractors should follow.

What does “drain to daylight mean”?
Foundation, perimeter, or “footing drains” keep basements and crawl spaces dry from the outside weather conditions. “Draining to daylight” is a relatively new addition to building codes, for an extended time they were tied into the sewer system. Draining to daylight is the water that flows via gravity or pump to a drain on the outside (daylight) somewhere. Often foundation perimeter drains and downspouts must drain out onto the surface and not something underground like a sewer or septic system. Condensation from humidity or collective moisture can pool in a crawl space, and with the help of foundation drains, this water is extracted into the exterior of the crawl space. If the slope of the building site allows, foundation drains should connect to solid pipe that runs to daylight. The solid pipe should be sloped at a minimum pitch of 1/4 inch per foot, although a steeper slope is better. If there is more than 200 linear feet of foundation, add a second outlet or increase the size of the outlet pipe from 4 inches to 6 inches. When there isn’t enough area on the lot, the exterior drains should be connected to a sump pump in the crawl space via a 6-inch line that penetrates the footing near the sump location.

Will I void my termite contract if I close my crawl space?
Most PCOs (pest control operator) will offer a renewal or additional inspections for a specified period of time at an additional cost. These agreements may indicate re-treatment or damage repair. Such agreements, sometimes referred to as a contract or warranty, are not an assurance that termites will not return, but provide for corrective action to be taken as specified in the agreement. The purpose of periodic inspections following an initial treatment is to determine whether the treatment was effective and if termites are present inside the treatment barrier. Follow-up inspections are limited to all visible or accessible areas, including crawl spaces, and should be conducted as indicated in the agreement issued by the PCO.

Where can I find out more information on NC Building Codes for crawl spaces?
The North Carolina Building Code Council adopted the new crawl space code language in September 2004 and the state of North Carolina approved it in November of 2004. In some cases, local code officials may require or accept a stamped letter of approval from a registered professional engineer as an alternate path for permitting and inspection.
Some key additions to the updated code on closed crawl spaces include:

  • The crawl space shall be separated from adjoining basements, porches, and garages by permanent walls. All utility penetrations shall be sealed.
  • A minimum 6-mil polyethylene vapor retarder or equivalent shall cover 100% of exposed earth in the crawl space, with joints lapped at least 12 inches (305 mm)
  • Closed crawl spaces used as supply or return air plenums for distribution of heated or cooled air shall comply with the requirements of the N.C. Mechanical Code. Crawl space plenums shall not contain plumbing cleanouts, gas lines or other prohibited components. Foam plastic insulation located in a crawl space plenum shall be protected against ignition by an approved thermal barrier.
  • The thermal insulation in a closed crawl space may be located in the floor system or at the exterior walls, with the exception that insulation shall be placed at the walls when the closed crawl space is designed to be an intentionally heated or cooled, conditioned space.
  • At least one of the following methods of space moisture vapor control shall be provided, and combinations of multiple methods are allowed: Dehumidifier, Supply air, House air, Exhaust fan, Conditioned space

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New IRC Code, effective in 2018: 

Ventilation openings in under-floor spaces specified in Sections R408.1 and R408.2 shall not be required where the following items are provided:

  1. Exposed earth is covered with a continuous Class I vapor retarder. Joints of the vapor retarder shall overlap by 6 inches (152 mm) and shall be sealed or taped. The edges of the vapor retarder shall extend not less than 6 inches (152 mm) up the stem wall and shall be attached and sealed to the stem wall or insulation.
  2. One of the following is provided for the under-floor space:

(2A) Continuously operated mechanical exhaust ventilation at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of crawl space floor area, including an air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.11 of this code.
(2B) Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute (0.47 L/s) for each 50 square feet (4.7 m2) of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.11 of this code.

3. Plenum in existing structures complying with Section M1601.5, if under- floor space is used as a plenum.

4. Dehumidification sized to provide 70 pints (33 liters) of moisture removal per day for every 1,000 ft2 (93 m2) of crawl space floor area. 

Reason: Unvented crawlspaces are required by Section R408.3 to provide a method for moisture control. Typical conditioning measures involve suppling conditioned air from the occupied (conditioned) space of the building or exhausting air from the crawlspace with make up air provided from the occupied (conditioned) space of the building. This code change allows another means of conditioning and controlling moisture, specifically dehumidification.

Why are the quotes I am receiving so drastic in price? / What should I ask the contractor for when gathering quotes for closing my crawl space?
Most contractors are quoting their prices on square footage of the crawl space, physical installment, materials and resources used, if any drainage systems are required, and the evaluation of any damages or conditions the crawl space is in. Less expensive systems use a plastic liner of 6-mil (.006″), 8-mil, 10-mil or 12-mil thickness. More expensive systems use specially designed multi-layer 20-mil or 23-mil vapor barriers, and may also include a dehumidifier, a sump pump or drainage trenches.

Why are houses built with crawl spaces?
Over 76 million existing homes in the US are currently built on crawl spaces, and an upward 18% of all new homes in the US are being constructed with crawl spaces. Crawl spaces are inexpensive to build, functional in terms of providing a level foundation for flooring on sloping sites, and convenient spaces in which to locate plumbing, electrical lines, and ductwork for heating and air conditioning systems. There are basic structural benefits that are provided in the design of a crawl space that are functional for certain types of homes; for example, crawl spaces get a house up off the ground (anywhere from 1-4 feet), which is especially important when considering damp or termite-prone areas. Crawl spaces are also more cost effective than basements, and very comparable in price to a slab.

Can I close my crawl space myself?
Yes, but we highly recommend referring to the crawl space contractor directory provided on HomeEnergyNC. If the costs of closing the crawl space falls under general contracting law (project costing less than $30,000) than a permit is not required. Although, certain additions and changes within the crawl space require a permit such as replacing HVAC, any electrical rewiring or the addition of dehumidifier, etc. If there are any heating or natural gas appliances within the crawl space, the exhaust must be piped to the outdoors and requires a permit. For insulation, if your home has under-floor insulation than additional insulation within the foundation walls of the crawl space is not required. Check to see what insulation and other systems directly affect your crawl space.

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Should I be worried about regional specific (rural vs. coastal) issues in my crawl space? 
For Eastern North Carolina, crawl space foundations have operable vents designed to provide ventilation for moisture control. Unfortunately, venting a crawl space during hot summer months allows hot humid air to enter into the cooler environment inside the crawl space. Instead of drying the crawl space, the hot humid air that enters this area actually increases moisture content. Once there, moisture gets into the colder floor framing where moisture content of the wood can increase up to a point that is ideal for fungal growth/wood damage; this is the most common cause of fungal growth inside crawl spaces in Eastern North Carolina

North Carolina: East South Central Division (35% crawlspace)

What are plumes? 
Plumes are pollutants released into the ground can work their way down into the groundwater, leading to groundwater pollution. The resulting body of polluted water within an aquifer is called a plume, with its migrating edges called plume fronts. Plumes are used to locate, map, and measure water pollution within the aquifer’s total body of water, and plume fronts to determine directions and speed of the contamination’s spreading in it. The crawlspace contractor may be asked to determine whether vapor intrusion may pose a risk beyond the known plume boundaries for groundwater plumes that have not reached steady-state conditions. Information regarding the groundwater flow direction and gradient is often useful in determining the potential for contaminant migration. Groundwater characterization can entail groundwater flow direction; vertical and horizontal gradients, including as appropriate, seasonal variation, tidal influences, and the effects of groundwater withdrawal; the rate of groundwater flow movement; and the integrity of any confining units or other barriers to migration of groundwater. Groundwater movement and its associated impact upon distance from the source to the receptor should be considered. Three factors that affect contaminant levels in groundwater that in-turn affects contaminant levels in soil gas are dispersion, dilution, and recharge.

Why are dehumidifiers important?

A dehumidifier helps maintain an ideal relative humidity (RH) level in your home. If the problem is long-term because of high humidity then you might want to consider installing a permanent crawlspace dehumidifier. There are different sized dehumidifiers that work best for each space. Some benefits of having a dehumidifier in your crawlspace include structural protection, increased comfort within your own home, increase efficiency, improved health and air quality, and pest protection. 

Should I be concerned about Radon? 
In crawl spaces with unavoidable or naturally occurring air pollutants (for example, radon), contaminated air can be removed from the crawl space via air-sealing and depressurizing the space using an exhaust fan that is vented to the outdoors. If the source of contaminated air is the soil (which is the case for radon), it is more effective to depressurize beneath the ground cover (Install a ground cover to reduce moisture entry from the soil: Cover the soil floor in the crawlspace with durable plastic sheeting (such as 6 mil black plastic). Overlap edges of the sheeting approximately 12 inches and anchor with boards, gravel, bricks, etc.). The installed fan should use less than 90 watts, have a sound rating of less than 2 sones and should not move more than the amount of air it would take to meet the ventilation requirements of the home. Crawl spaces that have naturally-vented combustion appliances (such as a furnace) should not be depressurized by this method as it may lead to back drafting of these appliances. Often, but not always, those vapors originate from an outside source, such as from the volatilization of chemicals in soil or groundwater that first migrate upward into a basement or crawl space and then into the space above where residents may live (ex. the dwelling space). Most of the early efforts to model indoor vapor intrusion focused on the radioactive gas radon, which has been a concern in the United States for decades.

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What if my crawl space is in a flood zone? 
If flood vent openings do not comply with National Flood Insurance Program requirements, the floor of the crawlspace, attached garage, or other enclosure becomes the “lowest floor” of the structure. This may result in significantly higher flood insurance premiums, which would be based on the elevation of the enclosure floor relative to the 100-year flood elevation. Crawl space ventilation vents are designed to facilitate the flow of air and generally do not meet the requirements for flood vents. However, in some circumstances it is possible to design permanent openings that satisfy the required criteria for flood openings as well as ventilation. Windows, doors, and garage doors are not considered openings for flood venting purposes. However, openings that meet the flood vent standards can be installed in doors and windows. Flood vents not meeting the above criteria are allowed only if a registered professional engineer or architect certifies that the design meets the requirement to automatically equalize hydrostatic forces on the exterior walls by allowing for the entry and exit of floodwaters.


What if there are combustion appliances in my crawl space?
If there is a furnace or water heater in a crawl space that requires air to burn for combustion of gas or oil, you may need to allow air to enter the crawl space for that purpose. This air, once used to burn fuel, goes up the chimney. The idea is if you create a vacuum in your crawl space because an enclosed space has been sealed tightly, so then you will not have a draft to take exhaust gases up the chimney from your furnace or water heater. You must not depressurize a space where combustion appliances are located. One easy way to provide combustion air is to install two vents in the crawl space ceiling to the first floor. This way air can be drawn down into the crawl space if the combustion appliances need it. If you have combustion appliances in the crawl space, it’s better than the alternative, which is to leave a vent to the outside open where unconditioned (hot, cold, wet) outside air can enter. For new homes, or when replacing your water heater or HVAC unit, you should use “direct vent” appliances, instead of “atmospherically vented” ones. Direct vent appliances have a place where you can attach a dedicated combustion air supply duct to the outside. It is important to have a (CO) carbon monoxide alarm if you have combustion appliances or attached garages.


What questions should I be asking my crawl space Contractor? 

  • A clear outline of scope of work and how the items listed are addressing any problems
  • What kind of labor & material warranty the contractor offers.
  • Will the work be done according to NC code?
  • As needed the crawl space door is replaced; and in some situations a dehumidifier and/or a sump pump is installed. These “closing” systems can also be used in a basement with a dirt floor, one with uneven walls and floors, or one with a large rock or other unmovable object.
  • The crawl space floor should be cleaned (no rocks or other debris to rip the plastic) and graded. When installed, the encapsulation system should be completely airtight. All seams should be overlapped by at least a foot, and sealed shut with special tape. Every potential gap or seam should be sealed, as should any vents.
  • If possible, get estimates from several companies with specific information about the materials used and the work to be done. For example, some materials such as dehumidifiers and polyethylene plastics are more expensive than others but provide the same service.