Moisture Management■ A roof runoff system to direct water away

from the house■ Site grading and landscaping that directs

ground surface water away from the house■ Foundation drains and foundation damp-

proofing or water-proofing to protect thestructure from sub-surface water

■ Air sealing of the access door, perimeter walland perimeter framing to prevent the entry ofmoisture-laden outside air and to separate thecrawl space from areas under porches or decks

■ An access door that is protected from roofrunoff, at least 4" higher than the exteriorsoil grade and made of a non-corrodingmaterial, especially in coastal communities

■ A fully sealed vapor retarder on the floor andperimeter walls to reduce evaporation ofwater into the crawl space

■ Appliance discharge pipes and exhaustsfrom kitchens, bathrooms and clothes dryersthat terminate outside the crawl space

■ A mechanical drying system to reducehumidity (for example, a supply ofconditioned air or a dehumidifier)

■ Drains with backflow valves or sump pumpsto remove liquid water from the crawl spaceif necessary

■ Flood vents that minimize standby airleakage, when required

Pest Control ■ A termite inspection gap at the top of the

perimeter wall to facilitate detection■ Batt insulation in the band joist area of wall-

insulated closed crawl spaces to facilitateinspection or treatment

Combustion Safety■ Specification of direct-vent (“two-pipe”)

combustion appliances to ensure adequatecombustion air for fuel-fired appliances, ifapplicable. Some manufacturers have direct-vent cabinets for use with non-direct-ventmodels. Alternate means of providingcombustion air must be approved by theappliance manufacturer and local residentialcode requirements.

■ For homes on slabs, basements or crawlspaces, specification of carbon monoxidemonitors or alarms if the structure has an attached garage or any combustionappliances

■ For homes on basements or crawl spaces,optional specification of an appropriatelyplaced raw-gas leak alarm if fuel-firedappliances or fuel lines are in the home

Fire Safety■ Air sealing of all penetrations in the sub-

floor with non-porous materials■ Documentation of fire-rating to allow

installation of exposed foam insulationwithout a thermal barrier, if applicable

■ Documentation of fire-rating to allowinstallation of exposed facing or backingmaterials on batt insulation, if applicable

Thermal Insulation■ Insulation at the sub-floor or at the perimeter

wall to provide the R-value required by thelocal residential code. Note that perimeterinsulation may be located on the interiorsurface, exterior surface or inside theperimeter wall, or the perimeter wall itselfmay provide the required R-value

■ Insulation at the sub-floor installed withoutgaps or compression and in full contact withthe sub-floor to achieve nominal R-value

■ Non-porous insulation if the perimeter wallis insulated

■ Insulation on the crawl space access(es) ofminimum R-2

Radon Control■ In areas where radon is a risk or where the

local residential code requires control ofradon or other soil gases, houses withclosed crawl space foundations can betested, monitored and, if necessary,mitigated with the same techniques used forhouses with a slab or basement foundationin the same region.

■ The U.S. Environmental Protection Agencyand the Surgeon General recommend testingall homes for radon

A Quick Reference on Closed Crawl Spaces www.crawlspaces.org

Getting StartedBoth scientific research projects and real-worldinstallations demonstrate that properly closed crawlspaces can provide much better moisture control thanconventional, wall-vented crawl spaces in temperate-humid climates. Homes with closed crawl spaces(often also called “sealed,” “unvented” or“conditioned” crawl spaces) also can savesignificantly on energy when compared to homes withwall-vented crawl spaces.

This sheet summarizes key issues that builders, codeofficials and consumers should keep in mind whendeciding how to design or install closed crawl spacesin new construction.

The second sheet provides two sample designs thathave been field-tested and extensively monitored.

For more details and information on improving wall-vented crawl spaces in existing homes, visitwww.crawlspaces.org.

DESIGNING CLOSED CRAWL SPACESThe recommended components of a good design fall into six major categories:

H I G H - P E R F O R M A N C E H O M E S

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Recommended Strategies for Avoiding ProblemsQuality installers of closed crawl spaces knowthat managing moisture during the process ofconstruction is critical for success and will useseveral strategies to avoid problems:■ Grading the crawl space floor surface to one

or more low spots and installing a drain orother method to remove water that enters thecrawl space prior to dry-in

■ Grading around the foundation as soon aspossible to reduce the amount of roof runoffor ground surface water that enters the crawlspace

■ Minimizing the amount of rainwater that canenter the crawl space

■ Covering the crawl space floor with atemporary vapor retarder by the time thestructure is dried-in. This temporary covermust be removed before installation of thefinished ground vapor retarder.

■ Puncturing holes at low spots in thetemporary ground vapor retarder (if the groundis not saturated) to allow small puddles todrain into the ground

■ Closing the access door(s) and running adehumidifier to remove accumulated moistureafter the structure is dried-in

■ Coordinating with the construction site crew toensure that the crawl space door is closed asmuch as possible, that all drains remainunblocked and that any dehumidifiers have apower source, run long enough to provide thenecessary drying and are protected from theft

Additional Recommendations■ Coordinate with your pest management

professional to ensure that the closed crawl space system does not interfere withtheir treatment or affect their warranty, if applicable

■ Mechanically fasten the liner material to theperimeter wall in addition to sealing it to thewall. Adhesives, sealants or tapes alone maybe insufficient to support the material.

■ Use duct mastic with embedded fiberglassmesh tape to seal seams in crawl space vaporretarder materials. If tapes or sealants are tobe used, ensure that the product is acceptedfor use by the manufacturer of the material and will provide long-term performance.Regardless of the method, a quality installerwill have a process in place to ensure properinstallation.

■ Protect 6-mil un-reinforced polyethyleneground vapor retarders to avoid damage inareas that will be accessed for servicing ofmechanical equipment. Thicker, reinforcedpolyethylene materials are more durable.

Quality Assurance OptionsExperienced installers care about the long-termperformance of their system and will usuallyoffer one or more of the following qualityassurance options with their closed crawl spacesystem:■ A monitoring system to inform the homeowner

of relative humidity levels in the crawl space■ A water alarm to inform the homeowner of a

buildup of liquid water in the crawl space■ A posted sign informing anyone entering the

crawl space of the system componentsmaintaining the integrity of the system

■ A repair kit for fixing small amounts ofdamage to the vapor retarder after the house is occupied

■ An annual monitoring service to replacebatteries in monitoring systems and check thatthe crawl space system is in good workingorder. This may include measurements ofwood moisture content or air tightness, ordownload and analysis of data loggingequipment

■ A guarantee that relative humidity will bemaintained below an agreed-upon target(typically 70% or lower) over specifiedperiods of time with exceptions for water leaksor intrusion outside their control

■ References to independent third-parties forquality assurance

Choosing a design for a closed crawl space goeshand-in-hand with the first steps required toinstall the closed crawl space: coordinating withcode officials to ensure that your design meetslocal residential code requirements and obtainingthe required permits. At the time of this writing,the North Carolina Building Code Council hasadopted and the state of North Carolina hasapproved new code language governing bothwall-vented and closed crawl spaces. This newcode is available for reference as of January 1,2005, as an alternate to the existing printed code,and the new code will be enforced in 2006 withthe release of the updated North CarolinaResidential Code. In some cases, local codeofficials may require or accept a stamped letter ofapproval from a registered professional engineeras an alternate path for permitting and inspection.

Quality installers ensure that their employees aretrained in the use of the tools and materialsrequired to install their closed crawl spacesystem and generally provide and/or require theuse of respirators and protective clothing for thecomfort and safety of their employees. Qualityinstallers of closed crawl spaces will havedocumented processes for coordinating with thegeneral contractor and other building trades toensure proper scheduling and to reduce the riskof damage to the closed crawl space systemduring construction. They also will explain anyimpact on pricing or scheduling of variablefactors like the number of piers in a crawlspace, height above or below grade orcomplexity of the foundation footprint.

A Quick Reference on Closed Crawl Spaces

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INSTALLING CLOSED CRAWL SPACES

www.crawlspaces.org

H I G H - P E R F O R M A N C E H O M E S

A Quick Reference on Closed Crawl Spaces

1. Seal exterior wall penetrations andmating surfaces at top and bottom of sillplate and at top and bottom of bandjoist. Crawl space access panel(s) ordoor(s) must be air-sealed.

2. No open foundation vents are allowed inexterior walls. Openings to ventilatedporch foundations must be air-sealedwith an access panel or permanentmaterials. Install flood vents per localresidential code where required.

3. Slope finished grade away from buildingper local residential code or for 6-inchdrop over 10 feet. Provide a method totransport roof runoff away from thehouse. Gutters and downspouts are onesuch method.

4. Dampproof or waterproof the exteriorwall surface when the crawl space flooris below exterior grade.

5. It is not necessary to provide a capillarybreak between the footings andfoundation walls or interior columns.

6. Provide foundation drain to daylight perlocal 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 rockwoolinsulation alone are not sufficient.

8. Insulate floor joist cavities. Placeinsulation in full contact with the sub-floor and ensure that it is secured inplace. Use R-value required by localresidential code and install without gaps,voids, or compression.

9. Leave a minimum 3” termite inspectiongap between the top of the wall vaporretarder and the top of the masonry wall.Seal the top of the vapor retarder to thewall with duct mastic or equivalentsealant. Optionally, apply a light coloredpaint or coating over the inspection gapto improve inspectability by pest controlprofessionals.

10. Air seal all heating and cooling ductworkwith a mastic system. Install all ductworklocated in the crawl space with R-valueper local code requirement.

11. Control moisture vapor in the crawl spacewith supply air from the house air-conditioning system. Set supply airvolume per local residential coderequirement. Adjust as needed to controlrelative humidity in crawl space to desiredlevel. Provide a backflow damper andeither a balancing damper or constantairflow regulator to control airflow.Multiple supply vents may be used toachieve the desired airflow and/or desireddistribution of air. No return air vent isallowed in the crawl space.

12. Terminate water heater drains,temperature/pressure relief pipes, andA/C condensate drains to outdoors or toan interior pump that discharges to adrain or outdoors. Terminate all kitchen,bathroom and clothes dryer vents tooutdoors.

13. Any fuel-fired furnaces, water heaters, orother appliance in a closed crawl spaceshould be of a “direct vent” or "two pipe"design, meaning that all air forcombustion is piped directly fromoutside to the appliance and all

combustion exhaust gases are pipeddirectly from the appliance to outside.

14. Cover 100% of the crawl space floor witha minimum 6-mil vapor retarder. Installvapor retarder material on the inside wallsurfaces, and mechanically fasten andseal it to the top of the walls, leaving therequired inspection gap. Extend thematerial up the interior columns at least4 inches above the crawl space floor.Seal all seams and edges with fiberglassmesh tape and mastic or equivalent.Mechanically secure the vapor retarder tothe ground as necessary.

15. Grade the crawl space floor to one ormore low points. Provide crawl spacedrain(s) or sump pump(s) at lowestpoint(s). Slope drains to daylight andinclude an accessible backflow valve and1/4-inch rodent screening. Gutter drainsand foundation drains (interior or exterior)must not be connected to the crawl space drain.

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DESIGN SAMPLE 1A closed crawl space with supply air and floor insulation

www.crawlspaces.orgA Quick Reference on Closed Crawl Spaces

1. Seal exterior wall penetrations and matingsurfaces at top and bottom of sill plateand 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 inexterior walls. Openings to ventilatedporch foundations must be air-sealedwith an access panel or permanentmaterials. Install flood vents per localresidential code where required.

3. Slope finished grade away from buildingper local residential code or for 6-inch dropover 10 feet. Provide a method to transportroof runoff away from the house. Guttersand downspouts are one such method.

4. Dampproof or waterproof the exterior wallsurface when the crawl space floor isbelow exterior grade.

5. It is not necessary to provide a capillarybreak between the footings andfoundation walls or interior columns.

6. Provide foundation drain to daylight perlocal residential code requirements.

7. Seal all plumbing, electrical, duct, cable, and other penetrations through thesub-floor with fire-stop materials and sealants. Fiberglass or rockwoolinsulation alone are not sufficient.

8. Insulate the crawl space wall over the wallvapor retarder material with rigid foam orother non-porous insulation material.Leave a minimum 3” termite inspectiongap between the top of the wall insulationand the top of the masonry wall. Leave a3” wicking gap between the bottom of the

wall insulation and the crawl space floorsurface. Obtain R-value from localresidential code. Ensure that theinsulation complies with local residentialcode requirements for installation withouta thermal barrier or ignition barrier.Insulate the band joist with batt insulationto facilitate removal and reinsertion duringpest control inspections. Ensure that battfacings comply with local firerequirements.

9. Leave a minimum 3” termite inspectiongap between the top of the wall vaporretarder and the top of the masonry wall.Seal the top of the vapor retarder to thewall with duct mastic or equivalentsealant. Optionally, apply a light coloredpaint or coating over the inspection gapto improve inspectability by pest controlprofessionals.

10. Air seal all heating and cooling ductworkwith a mastic system. Install all ductworklocated in the crawl space with R-valueper local code requirement.

11. Control moisture vapor in the crawl spacewith supply air from the house air-conditioning system. Set supply air volumeper local residential code requirement.Adjust as needed to control relativehumidity in crawl space to desired level.Provide a backflow damper and either abalancing damper or constant airflowregulator to control airflow. Multiple supplyvents may be used to achieve the desiredairflow and/or desired distribution of air. Noreturn air vent is allowed in the crawl space.

12. Terminate water heater drains,temperature/pressure relief pipes, and

A/C condensate drains to outdoors or toan interior pump that discharges to adrain or outdoors. Terminate all kitchen,bathroom, and clothes dryer vents to outdoors.

13. Any fuel-fired furnaces, water heaters, orother appliance in a closed crawl spaceshould be of a “direct vent” or “two pipe”design, meaning that all air forcombustion is piped directly from outsideto the appliance and all combustionexhaust gases are piped directly from theappliance to outside.

14. Cover 100% of the crawl space floor witha minimum 6-mil vapor retarder. Installvapor retarder material on the inside wallsurfaces, and mechanically fasten andseal it to the top of the walls, leaving therequired inspection gap. Extend the

material up the interior columns at least 4 inches above the crawl space floor. Sealall seams and edges with fiberglass meshtape and mastic or equivalent.Mechanically secure the vapor retarder tothe ground as necessary.

15. Grade the crawl space floor to one or morelow points. Provide crawl space drain(s) orsump pump(s) at lowest point(s). Slopedrains to daylight and include anaccessible backflow valve and 1/4-inchrodent screening. Gutter drains andfoundation drains (interior or exterior) mustnot be connected to the crawl space drain.

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DESIGN SAMPLE 2A closed crawl space with supply air and wall insulation

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