as* FOREST RESEARCH CENTE

NORTHE"STERN FOREST EXPERI~EN~~GB~~Tc-1 pile 

-1, b- I 

Upper Darby, Pennsylvar .i~~*. . -* -. , , 

a 

No. 85 

1958 

MOISTURE-BARRIER PERFORMANCE 

OF GROUND COVERS IN BASEhENTLESS HOMES 

The use of ground covers in crawl-space under houses 

is now widely accepted by builders, maintenance men, and 

home owners. Such covers are practical for reducing decay 

in subfloor timbers. They also effectively prevent soil 

moisture from moving upward into the house, and thus aid in 

preventing swelling and buckling of floors. 

Since 1946, the U.S. Forest Service has experimented 

in the Washington, D.C., area with various covers in home 

crawl-spaces where winter condensation on sills and joists 

had been a problem. Tests in 30 basementless homes have 

proved the effectiveness of some barriers in reducing underfloor moisture. The study has also provided a means of 

comparing the relative costs of materials, and the installation time required for each cover. 

Those moisture barriers tested include: roll roofing 

(15-, 30-, 45-, 55-, 90-, and 110-pound weights), reinforced 

asphalt-laminated building paper, asphalt (soap) emulsion 

applied as a spray over the soil surface, a 2.75-inch layer 

of slag or pea-size gravel, polyethylene film (0.004-inch), 

and unmounted aluminum foil (0.00035-, 0.001-, 0.0015-, and 

0.002-inch thickness). 

Average wood-moisture readings of sills and joists 

were taken before the ground covers were placed. Subsequent 

readings were made at 3- to 4-month intervals. An electrical-resistance type meter was used to determine the moisture 

content at the center and corners of the crawl space. 

The relative effectiveness of nine ground covers for 

nine typical houses is shown in figure 1. With the exception of polyethylene film, tested for only 3 years, all 

covers have been in place for 9 or 10 years. The graphs in 

figure 1 are numbered in the order of the material's effectiveness in reducing wood-moisture content below the 20-percent level, and in preventing visible condensation moisture 

from occurring. 
PEA-SI ZE GRAVEL 

(2 3-1 NCH LAYER) 

UNMOUNTED 

(0.002 INCH) 

I 5- POUND FELT 

PAPER . 

15-POUND FELT 

a 

YEARS UNDER TEST 

Figure 1.--Relative effectiveness of nine different ground 

covers in reducing moisture content of wooden sills and 

joists under basementless buildings. Each point on the 

graphs represents the average of the readings taken 3 to 

4 times yearly. Graphs are arranged in the order of the 

material's general effectiveness. 

Except for the last two covers, all nine shown in the 

figure effectively prevented visible condensation moisture. 

Still, the margin of safety decreases in the last four 

covers. The asphalt laminated paper even deteriorated. 

This first became apparent during the eighth year, and for 

the particular house shown (FP-30), the effects of deteriorat ion are apparent. 

Table 1 lists the cost (late 1957) of the ground 

covers, man-hours for installation, and gross installation 

cost for each of the nine covers. These figures apply to 

buildings in the vicinity of Washington, D.C., with a crawlspace area of about 1,000 square feet. 
On the basis of gross cost and moisture-barrier performance of the nine ground covers for the 9- to 10-year 

period, the most desirable cover is two layers of asphaltsaturated felt. Installed, this barrier costs only $24.00, 

and gives nearly as good protectionasthe first four covers, 

which range in cost from $34.00 to $61.25 for a coverage of 

1,000 square feet of crawl-space area. 

Table 1.--Cost and time required to install crawl-space ground covers 

in basementless houses in Washington, D.C., 1957 

 he $1-an-hour wage chosen to represent labor costs is an arbitrary figure, and 

is low for many areas. 

However, for crawl spaces where the ground cover is 

subjected to physical wear, such as by people or animals 

crawling over it, durability of the material should also be 

considered. It might be desirable to make an initial investment for a heavier material, such as 45-pound roll roofing, or a heavier grade of asphalt-saturated felt. 

1 

Gross 

cost1 

Kind of 

ground cover 

--JESSE D. DILLER 

Forest Pathologist 

Virginia Pine-Hardwood Research Center 

Northeastern Forest Experiment station 

Forest Service, U.S. Dept. Agriculture 

Dollars Man-hours Dollars 

Roll roofing (90 lbs. wt.) 43,OO 4 47.00 

Roll roofing (55 lbs. wt.) 36.00 3% 39.50 

Pea-size gravel (2-3/4" layer) 41.25 20 61.25 

Unmounted aluminum foil (.00211) 30,OO 4 34.00 

Asphalt felt (15 lbs., 2 layers) 20.00 4 24.00 

Asphalt-laminated paper 17.90 3% 21.40 

Polyethylene film (.00411) 22.00 2 24.00 

Asphalt (soap) emulsion spray 10.00 3 13.00 

Asphalt felt (15 lbs., 1 layer) 10.00 3 13.00 

AGRICULTURE-FOREST SERVICE-UPPER DARBY 

3 

Ground cover cost 

per 1,000 sq. ft. 

Time for 

installation