RESEARCH UPDATE

L ast year we reported on closed crawl spaces as a technologythat can dramatically improve the control of moisture in crawl

spaces (PCT magazine, June 2003). Closed crawl spaces representboth a new business opportunity and a risk management tool for pestmanagement firms, as the increase in complaints and legal actionrelated to mold growth in homes has made homeowners, tenants andthe home construction industry much more aware of the need tocontrol moisture in homes. A growing number of homeowners andbuilders are investing the additional time and money required toinstall closed crawl spaces in both new and existing homes. In NorthCarolina, where we have a humid climate and plenty of termite risk,more than 16,000 homes are built on a crawl space every year.Nationally, more than 250,000 new homes per year are built on acrawl space, with an estimated 26 million crawl space homes existingnationwide. The latest findings from Advanced Energy’s Princeville

Closed crawl spaces do double duty as amoisture solution and as an energy saver,according to research out of North Carolina.By Cyrus Dastur and Bruce Davis

Figure 1 (top). This wall-insulated closed crawl spaceuses 2-inch thick, foil-faced polyisocyanurate insula-tion. The foam plugs in the band could be replaced bysections of R-19 batt insulation. A termite inspectiongap is visible at the top of the wall insulation.

Figure 2 (bottom). This floor-insulated closed crawlspace has a termite inspection gap at the top of thevapor retarder. Note that penetrations are thoroughlysealed with mastic.

Figure 3 (left). The butterfly damper of this crawlspace supply opens while the system is running. Theassembly is sealed with mastic and supported bystrapping to ensure long-term proper operation ofthe butterfly damper.

PG. 1

Crawl Space Research project point to an-other benefit of closed crawl spaces: a signifi-cant reduction in the amount of energy neededto heat and cool homes built on this im-proved foundation design.

The following is a review of this research,which may be helpful to pest control firms thatwork in or close crawl spaces for customers.

CRAWL SPACE RESEARCH. Building pro-fessionals have long recognized that crawlspaces are prone to moisture problems andalso may waste energy. In 1994, a nationalsymposium on crawl space moisture prob-lems concluded that crawl spaces are danger-ously wet. In 1998, William Hill, a researcherat Ball State University, reported on theenergy wasted in vented crawl spaces.

In the spring of 2001, Advanced Energybegan a research project in the eastern NorthCarolina town of Princeville to compare themoisture and energy performance of closedcrawl spaces to that of wall vented crawlspaces. Another main goal of the project is todemonstrate the practical application of closedcrawl spaces using straightforward processesand readily available tools and materials. Co-funded by the U.S. Department of Energyand Advanced Energy, the project is directedby Bruce Davis and managed by Cyrus Dasturof Advanced Energy in Raleigh, N.C. Theproject team includes Dr. Achilles Karagiozis,Oak Ridge National Laboratory; Dr. WayneThomann, Duke University; Dr. JohnStraube, University of Waterloo, Canada;Architect Bill Rose, University of Illinois atChampaign-Urbana; Dr. Joe Lstiburek,Building Science Corp.; Physicist AntonTenWolde, U.S. Forest Products Labora-tory; Environmental Scientist Terry Brennan,Camroden Associates; and Project Consult-ant Bill Warren, Bill Warren Energy Ser-vices.

The 12 homes studied in this project arelocated in the same development, with sixhouses built side-by-side on each side of onestreet. All are the same size at 1,040 squarefeet, with the same floor plan and windowschedule (see Figure 4 above). After seeing thepotential for energy savings during a billinganalysis in early 2003, we outfitted all 12houses with electric sub-meters to recordexact energy consumption by each home’spackage-unit heat pump system (see Figure 6on page 4).

We reduced duct leakage and house leak-age to comparable levels across all the houses.Insulation deficiencies were corrected in allhouses and heat pump refrigerant charge andsystem airflow were measured to be consis-tent among all houses. All the houses have a

fresh air ventilation intake integrated withthe HVAC ductwork. A 6-inch insulated flexduct from outside routs air through a 1-inchpleated media filter and then connects di-rectly to the return plenum. Whenever theHVAC system is operating, 40 cubic feet perminute (cfm) of filtered fresh air is mixed intothe return air stream, conditioned, then dis-tributed to the house.

The 12 homes in the study are brokeninto three groups of four homes each: onecontrol group and two experimental groups.The four control houses have conventionallyvented crawl spaces, with 11 8-inch by 16-inch foundation vents. Although the build-ing code allows a reduction in the amount ofwall venting when a ground vapor retarder ispresent, all 11 foundation vents were re-tained (see Figure 5 on page 3). (CurrentNorth Carolina code would not require theground vapor retarder since these vents pro-vide the net free area to meet the 1:150ventilation requirement.) Each house has a 6-mil polyethylene ground cover that is me-chanically secured to the soil with turf staples.The seams are lapped approximately 6 inchesbut are not sealed. The ground cover extendscompletely to the foundation wall and inter-mediate piers, covering 100 percent of thesoil. The floors of the control houses are in-sulated with R-19 Kraft-faced fiberglass batts.

The crawl space vents of the experimenthomes were sealed with rigid polystyrenefoam and mastic or spray foam. Each of theseclosed crawl space houses has a sealed, 6-milpolyethylene liner covering the floor andextending up the foundation wall to a height3 inches from the top of the masonry. Theseams of the liner are sealed with fiberglassmesh tape and mastic and the edges are sealedwith mastic to the foundation wall or inter-mediate piers. The liner is mechanically se-cured to the soil with turf staples and to thefoundation wall with a furring strip.

Moisture control in the closed crawl spacesis provided by a small duct that provides 35cfm of air to the crawl space from the supplyplenum whenever the air handler is running(see Figure 3 on page 1). No fan-timing or fan-cycling controls are used in the mechanicalsystem and no stand-alone dehumidifiers areused for moisture control. A balancing damperpermits adjustment of the flow and a back-flow butterfly damper prevents movement ofair from the crawl space into the supplyplenum when the system is off.

Four of the closed crawl spaces are insu-lated with R-19 Kraft-faced fiberglass batts(see Figure 2 on page 1) in the floor above thecrawl space and the other four are insulatedwith 2 inches of R-13 foil-faced poly-isocyanurate foam (see Figure 1 on page 1) onthe perimeter wall and on the band joist. Thefoam was installed such that there is a 3-inchgap between the top of the foam and thebottom of the sill plate to allow for monitor-ing of termite activity and there is a secondgap at the bottom of the foam insulation toprevent ground contact and wicking of mois-ture into the foam insulation. The groundvapor retarder is attached to the inside surfaceof the foam insulation.

The crawl space experiment has beenmonitored for nearly three years. Ongoingresults clearly indicate that the closed crawlspaces consistently outperform the wall ventedcrawl spaces in terms of both moisture con-trol and energy use.

ENERGY SAVINGS. Total energy used forheating and/or cooling for the houses in eachgroup during the four seasons and the year asa whole is shown in the “Seasonal and AnnualEnergy Comparison” chart (on page 4). Thepercentage of savings as compared to thecontrol houses is reported in this table.

Going beyond the expectations of somein the building science industry, the closedcrawl space homes exhibit clear energy sav-ings over the control houses. This is true evenfor the four closed crawl space houses withwall insulation where we provided a termiteinspection gap and did not continue theinsulation down 24 inches below grade orout 24 inches horizontally onto the crawlspace floor, as is typically recommended.

For the 12 months displayed, the floor-insulated closed crawl space houses have usedan average of 15% less energy for spaceconditioning than the control houses, whichrepresents a savings of approximately 870kWh (or roughly $87) per year for eachhousehold.

The wall-insulated closed crawl spacehouses have used on average 18% less energy

Figure 4. One of 12 identical homes on thesame street in the Princeville crawl spaceproject.

RESEARCH UPDATE

PG. 2

than the control houses over the same 12-month period, which represents a savings ofapproximately 1025 kWh (or roughly $103)per year for each household.

The energy performance for both theexperiment groups has been substantiallybetter than was expected. Reviewing the data,we hypothesize three possible reasons for theenergy performance improvement in theclosed crawl space houses. First, the controlof moisture levels in the crawl space results ina reduction of the latent load on the house;second, the whole house as a system im-proved by virtue of the closed crawl space;and third, the houses are more closely coupledto the ground temperature.

We have looked carefully at the perfor-mance of the heat pumps on each house toensure that individual variations are not skew-ing the energy results. One variable thatremains to be reconciled is the effect ofhomeowner operation: indoor temperaturesvary from 1°F to 2°F among the groups, withthe residents of the control houses keepingtheir homes slightly cooler in the summerand slightly warmer in the winter. We areanalyzing this data in depth to determine itsimpact on the overall efficiency results andexpect a slight reduction in the savings thatwe have seen thus far.

MOISTURE RESULTS. Despite 2003 be-ing the wettest year in recorded history inmuch of North Carolina, the closed crawlspaces continued to demonstrate excellentmoisture control. Daily average relative hu-midity remained below 70 percent for theentire summer, actually getting lower andlower from June through August. Dew pointsin the closed crawl spaces were controlledbelow 60°F all year long, while the controlcrawl spaces experienced relative humiditynear 90 percent and dew points higher than70°F for most of the summer.

INDUSTRY RESPONSE. The traditional“improvement” for a vented crawl space hasbeen to add more ventilation, either in theform of additional ventilation openings or byinstalling a fan to intentionally move moreair through the crawl space. The Princevilleresearch has confirmed over the long term

what Advanced Energy and other buildingscience specialists have measured in manyinvestigations of crawl space failure: In manyregions, the outside air contains more mois-ture than the air in the crawl space during thewarm seasons and has no potential to dry thecrawl space. In fact, it often ends up contrib-uting moisture. Consider this: The averagedew point of the outside air at Princevilleduring the summer of 2003 was 73°F. Thiscorresponds to relatively moderate conditionsof 88 degrees and 60 percent relative humid-ity. When that air goes into the crawl spaceand encounters anything cooler than 73 de-grees, the relative humidity peaks at 100percent and the water vapor in the air con-denses on that object just like it would on acool drink set out on the porch railing. Supplyducts, water pipes, water tanks and even thefloor of the crawl space and the wood framingabove can experience this condensation, es-

pecially if the homeowners like to conditiontheir house to very cool temperatures. If con-ditions aren’t bad enough for condensation,the relative humidity of the air entering thecrawl space will still easily reach levels of 90percent or higher for prolonged periods of time.

Besides the increased risk of moisturedamage and mold growth that results fromthis high relative humidity and condensa-tion, the heat that is transferred from thewater vapor to a supply duct when the vaporcondenses on the duct is fighting the effortsof the air conditioner to remove heat fromthe house. Poorly insulated ductwork onlymakes these problems worse. The closed crawlspaces don’t experience any of this condensa-tion, even in the most humid times of theyear. As another benefit, when the air in thecrawl space stays dry, the wood in the crawlspace stays dry. Average wood moisture con-tent in the closed crawl space houses (measuredin 10 different locations in each house every 60days) remains below 12 percent year round.

Crawl space air can move easily throughthe numerous typical holes in the subfloorbetween the crawl space and the house andduct leakage often drives this flow when theduct system is located in the crawl space.Humid air entering the house from the crawlspace results in more work for the air condi-

tioner as well as homeowner complaints ofdiscomfort and odors.

At a minimum, every crawl space shouldhave a ground vapor retarder covering 100percent of the soil. Adding ventilation open-ings or crawl space ventilation fans is muchmore likely to hamper moisture control andincrease energy use unless you are in a verydry climate like the Southwest.

NEXT STEPS. The findings of this studywould transfer well to houses of similar ge-ometry and geography to the study homes.However, there are many other types ofhouses in other locations. Given the experi-ment design we expect considerable transferof results for both moisture control andenergy savings, but we will not know howwell the moisture and energy performancecan be replicated in mainstream productionhouses until a number are constructed andevaluated.

Currently the energy benefits of closedcrawl spaces are not correctly predicted by

popular energy analysis software, so it may besome time before closed crawl spaces get theirdue respect when builders choose house speci-fications aimed at achieving a certified mini-mum energy rating. We believe that ourresearch findings will spur new refinementsin the analysis tools and that in the meantimethe data will reinforce the argument thatconsumers can improve their homes by build-

RESEARCH UPDATE

Ongoing results clearly indicatethat the closed crawl spacesconsistently outperform thewall vented crawl spaces in

terms of both moisture controland energy use.

Going beyond the expectationsof some in the building science

industry, the closed crawlspace homes exhibit clear

energy savings over thecontrol houses.

Figure 5. The wall-vented control houses inthis study are much better than the typicalcrawl space we encounter in the field. Theyhave well-installed insulation and 100 percentground vapor retarder coverage of the soil,and no problems with intrusion of liquid waterfrom outside.

In many regions, the outsideair contains more moisture

than the air in the crawl spaceduring the warm seasons and

has no potential to dry thecrawl space.

PG. 3

ing or retrofitting a properly closed crawlspace. Planning is in process for expandingthe current research into additional locationsto determine whether closed crawl spaces willproduce similar energy savings in other cli-mate zones. Advanced Energy is already work-ing on two new crawl space research projects;both are targeted at further understandingthe impacts of wall vented and closed crawlspaces on indoor air quality and one specifi-cally seeks to identify and explore any humanhealth impacts that could be correlated to thetype of construction used.

Professionals seeking to install closed crawlspaces face a formidable learning curve to besuccessful. As with all aspects of the construc-tion industry, choosing the correct materials,tools and techniques is only half the battle;training and quality assurance are also criticalto ensure that the right work is performed theright way at the right time. Besides properpest management, a good closed crawl spaceneeds to be designed properly with regard tocontrol of internal and external water sourcesfrom the very start of construction. A dryingmechanism (e.g. conditioned air supply, adehumidifier, etc.) must be chosen for long-term, active moisture control. Other impor-tant design issues include combustion safety,fire safety, proper insulation and radon con-trol if applicable.

Implementation requires close coordina-tion with building officials, since buildingcodes are lagging behind this technology andmany current codes provide only a tortuous

compliance path for closed crawl spaces.During our work to set up the houses in thisstudy the scattered and conflicting nature ofdifferent building code elements governingclosed crawl spaces became evident. For closedcrawl spaces to be practical for both buildersand code enforcement officials we are recom-mending a separate section in the code that is

RESEARCH UPDATE

Figure 6. The sub-meter that measures electricity used by this heat pump is visible to the leftof the duct cowling.

Source: Advanced Energy Princeville Crawl Space Research Project

SEASONAL AND ANNUALENERGY COMPARISONS

Wall-VentedClosed withR-19 Floor

Closed withR-13 Wall

Summer(Jun - Aug ’03)

Fall(Sep - Nov ’03)

Winter(Dec ’03 - Feb ’04)

Spring(Mar - May ’04)

Annual

25

20

15

10

5

0

Closed withR-19 Floor

-22%

-5%

-10%

-19%

-15%

Closed withR-13 Wall

-36%

-10%

+4%

-28%

-18%

Savings

Summer

Fall

Winter

Spring

Annual

kilo

wat

t-ho

urs

specifically dedicated to these constructionmethods. We have helped the N.C. BuildingCode Council and code services staff to draftnew code language for closed crawl spaceswith the assistance of the North CarolinaStructural Pest Control Board, the NationalPest Management Association and severalinstallers of closed crawl spaces across thestate. The draft language is currently underreview for adoption by the Council.

Pricing contracts and managing the safetyand training of your employees is especiallyimportant when developing and offering anynew service. In closed crawl space work,coordination with other trades becomes aneven greater factor since they will be literallywalking all over your work to do their own.As one example of pricing, closed crawl spacesin one North Carolina market (assuming a2,000-square-foot house) range from $4,200for a simple, new-construction project to$6,700 for a complex retrofit project. Includ-ing wall insulation adds $1,500 to $3,000 tothe sale for these examples. Complexities thatincrease the price over the “simple” baselineinclude the extra length of perimeter forhouses with lots of angles, the number ofsupport columns that break up the floor ofthe crawl space and sloping grade, whichrequires additional material and fitting onthe crawl space walls. Retrofit complicationscan also include existing water problems,contaminated or damaged materials and de-bris that must be removed prior to workcommencing.

PG. 4

Closed crawl spaces represent a new busi-ness opportunity and risk management ser-vice for the pest management industry. Pestmanagement firms that rise to the challengeof implementing specifications and proce-dures for installing closed crawl spaces as amoisture control technique will be able totake satisfaction in the knowledge that theyare also providing their customers with thebenefit of energy savings.All photos are courtesy of Advanced Energy.

Cyrus Dastur is Princeville crawl space projectmanager and Bruce Davis is research director.

Advanced Energy is a private, non-profit corporationlocated in Raleigh, N.C., that serves as a state and

national resource to help utility, industrial and residen-tial customers improve the return on their energyinvestment. Its mission is to create economic andenvironmental benefits through innovative approachesto energy. The crawl space research project is supportedby the U.S. Department of Energy under contractnumber DE-FC26-00NT40995. The government re-serves for itself and others acting on its behalf a royaltyfree, nonexclusive, irrevocable, worldwide license forgovernmental purposes to publish, distribute, translate,duplicate, exhibit and perform this copyrighted paper.

Resources:Issues in Crawl Space Design and Construction, Ameri-can Society of Heating, Refrigeration and Air-Condi-tioning Engineers Inc., winter meeting, January 1994.

“Measured Energy Penalties from Crawl Space Venti-lation.” In Proceedings of the 1998 Summer Study onEnergy Efficiency in Buildings, Vol. 1. Washington,D.C., American Council for an Energy EfficientEconomy.

“Moisture Solution Becomes Efficiency Bonanza inSoutheastern United States.” In Proceedings of the 2004Summer Study on Energy Efficiency in Buildings, Vol. 1.Washington, D.C., American Council for an EnergyEfficient Economy.

Reprinted with the permission of PCT magazine, October 2004.

RESEARCH UPDATE

909 Capability Drive, Suite 2100Raleigh, NC 27606-3870

919-857-9000www.crawlspaces.org

PG. 5