What Are Crawl Spaces?

 

Many homes built on crawl space foundations in Western North Carolina suffer from poor moisture management. Some of the common symptoms of a crawl space moisture problem are:

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• Mold or moisture damage in the crawl space or living area

• Musty odors in the living area

• Condensation ("sweating") on air conditioning duct work or equipment

• Condensation on insulation, water pipes or truss plates in the crawl space

• Buckled hardwood floors

• High humidity in the living area

• Insect infestations

• Rot in wooden framing members

 

These symptoms are most often noticed in the humid spring and summer seasons but can occur at any time of the year. Often, the heating and air conditioning contractor is the first person the residents call to deal with the problem. Typically though, the problem is not due to a failure of the air conditioning system; it results from poor moisture control in the crawl space.

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For many decades, building codes and conventional wisdom have prescribed ventilation with outside air as the primary method of moisture control in crawl spaces. In the humid Southeast, however, ventilation with outside air only makes moisture problems worse.

 

1. Seal exterior wall penetrations and mating surfaces at top and bottom of sill plate and at top and bottom of band joist. Crawl space access panel(s) or door(s) must be air-sealed.

 

2. No open foundation vents are allowed in exterior walls. Openings to ventilated porch foundations must be air-sealed with an access panel or permanent materials. Install flood vents per local residential code where required.

 

3. Slope finished grade away from building per local residential code or for 6-inch drop over 10 feet. Provide a method to transport roof runoff away from the house. Gutters and downspouts are one such method.

 

4. Damp proof or waterproof the exterior wall surface when the crawl space floor is below exterior grade.

 

5. It is not necessary to provide a capillary break between the footings and foundation walls or interior columns.

 

6. Provide foundation drain to daylight per local residential code requirements.

 

7. Seal all plumbing, electrical, duct, cable, and other penetrations through the sub-floor with fire-stop materials and sealants. Fiberglass or rock-wool insulation alone are not sufficient.

 

8. Insulate floor joist cavities. Place insulation in full contact with the sub-floor and ensure that it is secured in place. Use R-value required by local residential code and install without gaps, voids, or compression. Leave a minimum 3” termite inspection gap between the top of the wall vapor retarder and the top of the masonry wall.

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9. Seal the top of the vapor retarder to the wall with duct mastic or equivalent sealant. Optionally, apply a light colored paint or coating over the inspection gap to improve inspect ability by pest control professionals.

 

10. Air seal all heating and cooling duct work with a mastic system. Install all ductwork located in the crawl space with R-value per local code requirement.

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11. Control moisture vapor in the crawl space with supply air from the house air-conditioning system. Set supply air volume per local residential code requirement. Adjust as needed to control relative humidity in crawl space to desired level. Provide a back flow damper and either a balancing damper or constant airflow regulator to control airflow. Multiple supply vents may be used to achieve the desired airflow and/or desired distribution of air. No return air vent is allowed in the crawl space.

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12. Terminate water heater drains, temperature/pressure relief pipes, and A/C condensate drains to outdoors or to an interior pump that discharges to a drain or outdoors. Terminate all kitchen, bathroom and clothes dryer vents to outdoors.

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13. Any fuel-fired furnaces, water heaters, or other appliance in a closed crawl space should be of a “direct vent” or "two pipe" design, meaning that all air for combustion is piped directly from outside to the appliance and all combustion exhaust gases are piped directly from the appliance to outside.

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14. Cover 100% of the crawl space floor with a minimum 6-mil vapor retarder. Install vapor retarder material on the inside wall surfaces, and mechanically fasten and seal it to the top of the walls, leaving the required inspection gap. Extend the material up the interior columns at least 4 inches above the crawl space floor. Seal all seams and edges with fiberglass mesh tape and mastic or equivalent. Mechanically secure the vapor retarder to the ground as necessary.

 

15. Grade the crawl space floor to one or more low points. Provide crawl space drain(s) or sump pump(s) at lowest point(s). Slope drains to daylight and include an accessible back flow valve and 1/4-inch rodent screening. Gutter drains and foundation drains (interior or exterior) must not be connected to the crawl space drain.

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Conventional wisdom on crawl spaces over the years has ranged from always vent to always seal. As with many things, it’s not that simple. It really depends on local conditions. Properly designed and installed closed crawl spaces can be made to work well almost everywhere. On the other hand, vented crawl spaces are not suited for hot-humid regions where there is a risk of condensation. Closed crawls also come with some risks.

 

Without venting there is a smaller dilution factor and the possibility of higher pollutant concentrations. If you power-vent a crawl space the fan must be used and maintained. In heating-dominated climates, there may be an increased heating load in a closed crawl.

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When working with a crawl space, always consider local conditions. Evaluate what has worked in your community and remember that the best crawl space design won’t make up for poor water management on the site.

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So does all of the stuff you just read make you feel dizzy with not really understanding which type is best for my home, business or company! Contact Bill Sargent - SGT Structural Repairs. Set up an appointment and allow us to visit you and examine your situation and then provide you with information about the findings so you can make an educated decision on what best for your situation.