Closed Crawl Spaces Do Double Duty

CLOSED CRAWL SPACES DO DOUBLE DUTY

A field demonstration project in North Carolina shows that this robust moisture control method also offers significant energy savings.

BY CYRUS DASTUR AND BRUCE DAVIS

all-vented crawl spaces are widely used in building con- Wstruction throughout North America. Approximately 200,000 new homes are built on crawl spaces every year, and an estimated 26 million such homes are already in existence.They are cheap to build; functional in terms of pro- viding a level foundation for flooring on sloping sites; and popular as spaces in which to locate plumbing, ductwork, and heating and air conditioning systems. ENERGY ADVANCED Unfortunately, wall-vented crawl spaces can also host a variety of serious moisture problems. Closed (also called sealed or unvented) crawl spaces represent both a Closed crawl spaces represent both a new business opportunity and a risk management tool for many new business opportunity and a risk man- different stakeholders in the construction industry. agement tool for many different stakeholders in the construction industry,from pest management companies to building ious crawl space ventilation and insula- fingers crossed against the possibility that performance contractors and foundation tion strategies affect moisture levels and the moisture solutions might actually specialists.As complaints and legal action energy use in houses in the southeastern cause an increase in energy consump- related to mold growth in homes have United States.Another primary goal of tion.We were pleasantly surprised when increased, homeowners, tenants, and the this research has been to demonstrate measurements showed the techniques to construction industry have become much practical, easily transferable, and clearly be very beneficial for both moisture more aware of the need to control mois- understandable crawl space construction control and energy efficiency. ture in homes.This awareness is prompt- techniques that would solve a multitude ing a growing number of owners and of moisture problems and would be at Experimental Setup builders to invest the additional time and least energy neutral; at best, we were money required to install closed crawl hoping to reduce energy consumption The 12 homes studied in this project spaces in both new and existing homes. for space conditioning.When the project are located in the same development in In response to these growing con- began in the fall of 2001, we did not Princeville, North Carolina. Six houses cerns,Advanced Energy has undertaken know whether the interventions would are built, side-by-side, on each side of one a multiyear effort to document how var- show energy savings. Some of us had our street.All are the same size—1,040 ft2—

HOME ENERGY • 2005 SPECIAL ISSUE www.homeenergy.org 1 foundation wall, stopping 3 inches from the top of the masonry to provide a termite inspection gap. The seams of the liner are sealed with ﬁberglass mesh tape and mastic, and the edges are sealed with mastic and mesh tape to the foundation wall or intermediate piers.The liner is mechanically secured to the soil with turf staples and to the foundation wall with a furring strip. Mechanical drying in the closed crawl spaces is provided by a 4-inch duct that delivers 35 CFM of conditioned air to the crawl space from the supply plenum whenever the air handler is running. As

DACDENERGY ADVANCED designed, the extra air simply exfiltrates through the crawl space perimeter wall. No fan-timing or fan-cycling controls are used in the mechanical system, and no We outfitted all 12 houses with electricity submeters to record exact energy consumption by each stand-alone dehumidifiers are used for home’s package unit heat pump system. moisture control.A balancing damper per- mits adjustment of the flow, and a back- flow butterfly gravity damper with a with the same floor plan and window of filtered fresh air is mixed into the nonmetallic hinge prevents movement of schedule.The development was built on return airstream, conditioned, and then air from the crawl space into the supply controlled-fill soil to elevate it above the distributed to the house. No fan-timing plenum when the system is off. 100-year floodplain, which added to the or fan-cycling controls are used in the Four of the closed crawl spaces are uniformity of the site soil conditions, and mechanical system. insulated with R-19 kraft-faced fiberglass each home site was graded to provide The four control houses have conven- batts in the floor, and the other four are proper drainage. tionally vented crawl spaces, with 11 8 insulated with 2 inches of R-13 foil-faced The study homes are broken into inch x 16 inch foundation vents. Each polyisocyanurate foam on the perimeter three groups of four homes each: one house has a 6-mil polyethylene ground wall and on the band joist.This closed- control group and two experimental cover that is mechanically secured to the cell foam was installed with a 3-inch gap groups. We reduced duct leakage and soil with turf staples. The seams are between the top of the foam and the bot- house leakage to comparable levels across lapped approximately 6 inches but are tom of the sill plate, to allow for monitor- all the groups.Average duct leakage varies not sealed. The ground cover extends ing of termite activity, and there is a from 51 CFM25 to 68 CFM25 for these completely to the foundation wall and second gap at the bottom of the foam groups, which represents rates of 5%–7% intermediate piers, covering 100% of the insulation to prevent ground contact and CFM25 per square foot of floor area. soil.Although the building code allows a wicking of moisture into the foam insu- Dividing the leakage by the conditioned reduction in the amount of wall venting lation.This foam meets the ASTM E84 floor area lets us use a consistent target when a ground vapor retarder is present, and Factory Mutual FM 4880 require- across many different sizes of home.Aver- all 11 foundation vents were retained. ments of the International Residential age house leakage varies from 0.22 to (Note that 2000 North Carolina code Code for use without a thermal barrier. 0.27 CFM50 per square foot of envelope does not require the ground vapor The ground vapor retarder is attached to area. Insulation deficiencies were cor- retarder, since these vents provide the net the inside surface of the foam insulation rected in all houses, and heat pump free area to meet the 1:150 ventilation with fiberglass mesh tape and mastic.We refrigerant charge and system air flow area to crawl space area requirement.) specifically did not install the wall insula- were measured and corrected as needed The floors of the control houses are insu- tion in the typically recommended form, in all houses. lated with well-installed R-19 Kraft- which specifies wall insulation to 24 All the houses have a fresh-air venti- faced fiberglass batts. inches below outside grade or horizon- lation intake integrated with the HVAC The crawl space vents of the experi- tally on top of the soil in from the foun- ductwork.A 6-inch insulated flex duct ment homes were sealed with rigid dation wall for 24 inches. Instead, the from outside routs air through a 1-inch polystyrene foam and mastic or spray bottom edge of the crawl space wall insu- pleated media filter and then connects foam. Each of these closed crawl spaces lation extends only 3 to 6 inches below directly to the return plenum.Whenever has a sealed, 6-mil polyethylene liner outside soil grade level. the HVAC system is operating, 40 CFM covering the floor and extending up the

2 www.homeenergy.org 2005 SPECIAL ISSUE • HOME ENERGY Instrumentation Crawl Space and Outdoor Relative Humidity and Data Collection 100 Outside 90 Wall vented 80 Closed We have been recording outside air temperature and moisture content using 70 three battery-operated data loggers dis- 60 tributed across the development in loca- 50 tions shielded from rain and direct sun. We used the same type of logger to 40 Relative humidity (%) Relative record conditions inside each house and 30 ENERGY ADVANCED inside each crawl space. Measurements 20 were recorded at 15-minute intervals. 06/03 07/03 08/03 09/03 10/03 11/03 12/03 01/04 02/04 03/04 04/04 The house data logger was placed at the center of the house in the HVAC return Figure 1. The closed crawl spaces performed better than the vented ones with regard to RH. closet, and two loggers (one extra for redundancy) were located together in the center of the crawl space on the support beam for the floor joists. We measured Average Wood Moisture Content wood moisture content on a 60-day 18 16 Vented + Closed + Closed + interval at ten locations in each crawl R-19 floor R-19 floor R-13 wall space, including sill plate, band joist, floor 14 joist, center beam, and subfloor readings. After seeing the potential for energy 12 savings during a billing analysis in early 10 Wood MC (%) Wood 2003, we outfitted all 12 houses with 8 ENERGY ADVANCED electricity submeters to record exact 6 energy consumption by each home’s 09/03 10/03 12/04 02/04 04/04 06/04 09/04 package unit heat pump system. The whole-house meter and the submeters Figure 2. Closed crawl spaces were also better off than vented ones with regard to wood moisture. are read monthly. The crawl space experiment has been monitored for more than three years as of this writing. Ongoing mea- Table 1. Summer (June–August) Relative is notable, not only surements clearly indicate that the Humidity Summary because it reduces the closed crawl spaces consistently outper- 2002 2003 likelihood of surface form the wall-vented crawl spaces in RH Threshold Vented Closed Vented Closed mold growth but also terms of both moisture control and Above 90% 1 00% 100% 123% 100% because dry lumber is energy use.What follows is a cross sec- Above 80% 1 39% 100% 186% 100% less attractive to ter- tion of our findings from a one-year Above 70% 1 79% 100% 198% 105% mites and very inhos- Above 60% 1 94% 100% 100% 164% period ranging from June of 2003 Above 50% 100% 100% 100% 100% pitable to through May of 2004. Monitoring of Southeastern species these crawl spaces will continue until of wood-boring beetle March 2005. the germination of mold spores, along pests. with high relative humidity (RH) for As fate would have it, 2003 was the Moisture Performance extended periods of time, makes a wettest year in recorded history in most vented crawl space in the Southeast an of North Carolina, while 2002 was a Typical problems we see regularly in ideal location for mold growth and record-setting drought year. In the sum- wall-vented crawl spaces include con- wood decay to take hold. mer of 2003, the RH in the vented densation on air conditioning ducts, The closed crawl spaces in our pro- crawl spaces stayed above 70% almost all water lines, and insulation; standing ject perform notably better than the the time, while the closed crawl spaces water on top of the ground vapor vented crawl spaces with regard to RH reached similar humidity levels only 5% retarder; stained walls from water pene- and wood moisture content (see Figures of the time (see Table 1). tration and efflorescence; and water-sat- 1 and 2).The fact that the framing lum- Clearly,the closed crawl spaces pro- urated soil inside the crawl space. The ber in the closed crawl spaces stays vided far better performance under the presence of liquid water, which supports below the 12% wood moisture threshold harshest conditions than the vented

HOME ENERGY • 2005 SPECIAL ISSUE www.homeenergy.org 3 retarder installation and maintenance in general construction. During any warm season here in Princeville, the outside air contains more moisture than the air in the vented crawl spaces and instead of pro- viding drying potential, contributes moisture. Consider this: The average dew point of the outside air at Princeville during the summer of 2003 was 73°F.This corresponds to relatively moderate conditions of 88°F and 60% RH.When that air goes into the crawl space and encounters any object that is cooler than 73°F, the RH peaks at 100% and the water vapor in the air

DACDENERGY ADVANCED will condense on that object just as it would on a cool drink set out on the porch railing. Supply ducts (55°F- 65°F), water pipes and tanks (55°F- The wall-vented control houses in this study have much better crawl spaces than the typical crawl 65°F), and even the floor of the crawl space we encounter in the field. The control houses have well-installed insulation and 100% ground space (65°F-70°F) and the wood fram- vapor retarder coverage of the soil, and no problems with intrusion of liquid water from outside. ing above (70°F-78°F) can experience this condensation, especially if the homeowner likes to condition the house to temperatures below 72°F. Even if conditions aren’t bad enough for condensation, the RH of the air entering the crawl space will still easily reach levels of 90% or higher for pro- longed periods of time. From our dew point measurements, we observed that the closed crawl spaces stay moister than the vented crawl spaces in winter. Limiting the moisture swing seen by the house over the course of the year reduces the likelihood of common cosmetic problems like shrinking and swelling of hard- wood floors and trim carpentry and cracking or nail pops in drywall. DACDENERGY ADVANCED Energy Performance Going beyond our expectations, the A more typical vented crawl space in North Carolina exhibits water leakage, poor drainage, a low- closed-crawl space houses exhibit clear quality vapor retarder, if any, and various complications such as attached under-porch areas. energy savings over the control houses (see Figure 3).This is true even for the four closed-crawl space houses with crawl spaces did under even the mildest extensively wall-vented crawl spaces wall insulation where we provided a ter- conditions. These results highlight the without water intrusion and with a mite inspection gap and did not install fact that even a carefully installed and 100% vapor retarder ground cover may the insulation either down 24 inches maintained ground vapor retarder cov- prevent wood rot in crawl spaces, but below grade or 24 inches horizontally ering 100% of the soil is not sufficient moisture control would be even worse onto the crawl space floor, as is typically to control crawl space moisture levels in in typical vented crawl spaces, given the recommended in energy codes and a vented crawl space.Well-constructed, usual poor quality of ground vapor design guidelines.

4 www.homeenergy.org 2005 SPECIAL ISSUE • HOME ENERGY For the 12 months analyzed, the floor- Seasonal and Annual Space Conditioning Energy Use insulated closed-crawl space houses have 25 used an average of 15% less energy for Savings Closed with Closed with space conditioning than the control R-19 floor R-13 wall houses. This represents a savings of 20 Summer -22% -36% approximately 870 kWh, or roughly $87, Fall -05% -10% per year for each household. The wall- Winter -10% + 4% insulated closed-crawl space houses have 15 used, on average, 18% less energy than the Spring -19% -28% control houses over the same 12-month Annual -15% -18% period. This represents a savings of kWh 10 approximately 1,030 kWh, or roughly $100, per year for each household. By the luck of the draw, the control 5 houses ended up with higher occupancy numbers and more children than the experiment houses.While we could and 0 did control for the variables of climate, site Summer Fall Winter Spring Annual drainage, architecture, insulation, shell (06–08/03) (09–11/03) (12/03–02/04) (03–05/04) ENERGY ADVANCED leakage, duct leakage, and mechanical Wall-vented Closed + R-19 floor Closed + R-13 wall equipment performance, there remain variations in baseload consumption and occupant thermostat settings among the Figure 3. The submetered heating and cooling energy use for the period June 2003 through May 2004 groups that may be significant due to the indicates significant energy savings. (The bars represent the average per house usage for each group.) small sample size. We did not submeter the appliance, lighting, water heating, or exhaust fan loads, but noted that the total baseload regard to these differences in thermostat use in the control houses was signifi- setpoint, we graphed average temperature Implications cantly higher—10%-20% in any given difference between outside and inside for This project is scheduled to month—than that in the experiment each house in each month and plotted conclude after the winter of 2004-05, houses over the entire year. The extra that against the kWh consumption for when we will test one final configura- occupants and baseload in the controls space conditioning for that house in the tion to assess the energy impact of 24 would theoretically increase the need same month. The trendlines for each inches of foam insulation installed hor- for cooling in the summer and decrease group of houses indicate the same sea- izontally on the ground of the wall- the need for heating in the winter.The sonal patterns of performance that we insulated, closed-crawl space houses. difference in baseload use between the observed in the raw submeter data. Final results and additional analysis will controls and the floor-insulated experi- We have not administered a formal be posted on the project Web site, ment houses is about the same in both survey of occupant comfort, but we www.crawl spaces.org. summer and winter, which suggests that recently had an experience that indicated We’re very excited about the degree of the surpluses offset each other in terms that the crawl space’s moisture perfor- performance improvement shown by the of heat pump energy used and saved in mance may also affect the occupant’s closed crawl spaces, especially considering the control houses to compensate for thermostat settings. In June of this year, that our control houses represent the best the difference. However, there is more we modified the vented control crawl possible performance of wall-vented of a difference in baseload consumption spaces to test a new version of a closed crawl spaces.The vast majority of newly between the controls and the wall-insu- crawl space. We installed a crawl space built vented crawl spaces are not installed lated experiment houses in the summer supply duct to provide supplemental dry- or maintained to the standards used for than there is in the winter, which makes ing, as was done in the other closed crawl this project.We believe that the findings the summertime wall-experiment house spaces.When we returned to the site four of this study will transfer well to houses performance look better. days later, one resident (who rarely adjusts whose geometry and geography are sim- A review of the interior house data her thermostat) excitedly told us that the ilar to those of the study homes. How- indicates that the control houses were day after we improved her crawl space, ever, additional consideration and study operated 1°F –2°F cooler than the she had to turn up her thermostat are required for houses in other locations experiment houses in the summer and because she felt too cold in the house! or with different geometry.The energy 1°F–2°F warmer than the experiment results seem to indicate that wall-insulated houses in the winter.To normalize the closed crawl spaces will perform best in heat pump energy consumption with cooling-based climates; it seems likely that

HOME ENERGY • 2005 SPECIAL ISSUE www.homeenergy.org 5 floor-insulated closed crawl spaces the potential indoor air quality (IAQ) will perform best in heating-based and health impacts of closed-crawl climates. Of course, these homes have space construction, which could indi- shallow foundations, and we have not cate a whole new class of benefits tested crawl space foundations with from this technology. The future deeper footings, such as may be could include insurance premium found farther north.A wall insulation savings for houses built on certified strategy may prove to perform best in closed crawl spaces. such houses. We won’t know with One specific measure of IAQ that any certainty how well the improve- we have addressed in this project is ments in moisture and energy perfor- radon risk. Long term radon moni- mance will transfer to houses in other toring was performed in all the crawl climates until a number of such spaces and conditioned areas. Slightly houses are actually constructed and higher average concentrations were monitored.We are now starting up a found in the closed crawl spaces, with project to gather those data in multi- measurements averaging 0.5 pic- ple climate zones. This project will ocuries per liter (pCi/l) in the vented

also demonstrate the ability of the ENERGY ADVANCED crawl spaces and 1.1 pCi/l in the production housing market to incor- closed crawl spaces.The radon mea- porate closed crawl space technology surements in the conditioned space into their construction processes. do not show any difference correlated Currently we find that the energy This floor-insulated crawl space has a termite inspection gap with the type of foundation they are benefits of closed crawl spaces are at the top of the vapor retarder. Note that penetrations are on; all houses average approximately not completely predicted by popular thoroughly sealed with mastic. 0.5 pCi/l with a maximum reading energy analysis software packages, so of 0.7 pCi/l in any house.The com- it may be some time before closed monly accepted action level for crawl spaces get their due respect radon remediation is 4 pCi/l. We when builders choose house specifi- believe that a closed crawl space cations aimed at achieving a certified should be thought of as a short base- minimum energy rating. We hope ment when it comes to radon miti- that our research findings will spur gation, since similar measures are refinements in the analysis tools, and applicable for assessment and control that in the meantime the data will of risk where needed. reinforce the argument that con- Professionals seeking to install sumers can improve their homes by closed crawl spaces face a formidable building or retrofitting a properly learning curve to be successful. As closed crawl space. Data from a sub- with all aspects of the construction set of two highly instrumented industry,choosing the correct materi- houses in the project are being used als, tools, and techniques is only half to validate a hygrothermal modeling the battle; training and quality assur- tool under development at Oak ance are also critical to ensure that the Ridge National Laboratory. right work is performed the right way Initial construction costs associated at the right time.A good closed crawl

with building closed crawl spaces will ENERGY ADVANCED space must be designed properly with likely be more than for traditional regard to control of internal and wall-vented construction.As the new external water sources from the very construction methods are evaluated start of construction.You must choose by both builders and researchers, it This wall-insulated crawl space uses 2-inch-thick foil-faced a drying mechanism—conditioned will be important to factor in the polyisocyanurate insulation. The foam plugs in the band air supply or a dehumidifier, for value of reduced callbacks for mois- could be replaced by sections of batt insulation. A termite example—for long term, active ture and mold complaints, the percep- inspection gap is visible at the top of the wall insulation. moisture control. Other important tion of enhanced value by the design issues include pest manage- consumer, and the resulting improvement, combustion safety, fire safety, ment in sales price and volume, and proper insulation, and radon control, reduced legal action. Reduced mainte- enhance the value of closed-crawl space if applicable. nance; a reduction in costly, long-term construction to the consumer. Other Implementation requires close coor- repairs; and significant energy savings will Advanced Energy research is focused on dination with building officials, since the

6 www.homeenergy.org 2005 SPECIAL ISSUE • HOME ENERGY FOR MORE INFORMATION: We are by no means the ﬁrst to investi- gate the moisture performance of wall- vented crawl spaces. Previous work includes a review of crawl space inves- tigation and regulation through history (Rose 1994) and a review of many of the issues associated with wall vented crawl space construction (Rose & Ten- Wolde 1994). These reviews, along with several others, are included in Recom- mended Practices for Controlling Mois- ture in Crawl Spaces, ASHRAE Technical

DACDENERGY ADVANCED Data Bulletin, volume 10, number 3. Davis, Bruce and William E. Warren. “Field Study Pilot Report: A Field Study Comparison of the Energy and The butterfly damper of this crawl space supply opens while the system is running. The Moisture Performance Characteristics assembly is sealed with mastic and supported by strapping to ensure that the butterfly of Ventilated Versus Sealed Crawl damper continues to operate properly. spaces in the South,” Raleigh, N.C.: Advanced Energy, 2002. building codes are lagging behind this from $4,200 for a simple, new-construc- Davis, Bruce, William E. Warren, and technology,and many current codes pro- tion project to $6,700 for a complex William B. Rose. “Technology vide only a tortuous compliance path for retrofit project. Including wall insulation Assessment Report: A Field Study Com- parison of the Energy and Moisture Per- closed crawl spaces. During our work to adds $1,500–$3,000 to the sale for these formance Characteristics of Ventilated set up the houses in this study,the scat- examples, minus the cost of floor insula- Versus Sealed Crawl spaces in the tered and conflicting nature of different tion if it is a new construction project. South,” Raleigh, N.C.: Advanced Energy, building code elements governing closed Complexities that increase the price over 2002. crawl spaces became evident. For closed the simple baseline include the extra crawl spaces to be practical for both length of perimeter for houses with lots Davis, Bruce, and Cyrus K. Dastur. builders and code enforcement officials, of angles, the number of support “Moisture Solution Becomes Efficiency we are recommending a separate section columns that break up the floor of the Bonanza in the Southeastern United in the code that is specifically dedicated crawl space, and a sloping grade that States,” in Proceedings of the 2004 to these construction methods.We have requires additional material and fitting Summer Study on Energy Efficiency in Buildings, Vol. 1. Washington, D.C. helped the North Carolina Building on the crawl space walls. Retrofit com- American Council for an Energy Code Council and code services staff to plications can also include existing water Efficient Economy, 2004. draft new code language for closed crawl problems, contaminated or damaged spaces with the assistance of the North materials, and debris that must be Hill, William W. “Measured Energy Carolina Structural Pest Control Board, removed before the work starts. Penalties from Crawl Space the National Pest Management Associa- Closed crawl spaces clearly represent Ventilation,” in Proceedings of the tion, and several installers of closed crawl both a new business opportunity and a 1998 Summer Study on Energy spaces across the state.The draft language risk management service for the shelter Efficiency in Buildings, Vol. 1. has been approved by the Building Code industry.Firms that rise to the challenge Washington, D.C. American Council for Council and is now under final review of implementing specifications and pro- an Energy Efficient Economy, 1998. by state agencies before it becomes avail- cedures for installing closed crawl spaces Rose, W. B. “A Review of the Regulatory able for use. as a moisture control technique will be and Technical Literature Related to Pricing contracts, and managing the able to profit from the knowledge that Crawl space Moisture Control,” in safety and training of employees, are both they are also giving their customers the ASHRAE Transactions, Vol. 100 Pt. 1. especially important when one is devel- benefit of energy savings. Atlanta, Ga. American Society of Heat- oping and offering any new service. In ing, Refrigerating and Air-Conditioning closed-crawl space work, coordination Cyrus Dastur is a building science associate Engineers, 1994. with other trades becomes an even and Bruce Davis is research director at Rose, W. B. and A. TenWolde. “Issues in greater factor, since they will be literally Advanced Energy,in Raleigh, North Carolina. Crawl Space Design and Construction — walking all over that work to do their a Symposium Summary.” ASHRAE Tech- own.As one example of pricing, closed The project described in this article is cofunded nical Data Bulletin: Recommended crawl spaces in one North Carolina mar- by DOE and Advanced Energy. Practices for Controlling Moisture in ket, assuming a 2,000 ft2 house, range Crawl spaces 10 (3), 1994.

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