DECAY AND PERMEAMILITY TESTS CU SHEET
MATERIALS USED AS SOIL COVERS IN
EASEMENTLIESS MUSES
May 1955
C
raILV
No. 2007
UNITED STATESLDEPARTMENT OF AGRICULTURE
LOREST SERVICE
FOREST PRODUCTS LABORATORY
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin
•
DECAY AND PERMEABILITY TESTS OF SHEET MATERIALS
USED AS SOIL COVERS IN BASEMENTLESS HOUSES
By
C.'S. MOSES, Pathologist
1 2 Forest Service
Forest Products Laboratory,-,-U. S. Department of Agriculture
Summary
Eleven samples of sheet materials used or proposed for use as soil covers
in basementless houses were tested for decay resistance. Only asbestosfibered specimens were free from fungus attack,but on the basis of the
test and experience, asphalt roll roofings of other fibers and in
weights of 45 pounds and over were also judged adequate to serve as soil
covers.
The water vapor transmission of artificially and naturally infected samples of roll roofing was tested by conventional and simplified techniques.' No important increases in permeance were found to result from
fungus attack.
Introduction
The use of 55-pound roll roofing on the soil under basementless houses
has been recommended as one means of keeping wood subfloor structures
Maintained at Madison, Wis., in cooperation with the University of
Wisconsin.
2
-Part of the research on this project was performed under the Housing
Research Program of the Office of the Administrator, Housing and Home
Finance Agency.
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Report No. 2007
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Agriculture-Madison
•
too dry to decay.2 In this use, the roll roofing acts as a vapor barrier, preventing soil moisture in the form of water vapor from invading
the crawl space where it can condense on and be absorbed by the wood.
Wood with a moisture content of 20 percent or less will not decay.
Other materials have also been proposed and used as soil covers, and
records areavailable concerning their efficiency over a 5-year
period.1
Roll roofing and other sheet materials used as soil covers necessarily
undergo severe exposure to attack of fungi and other micro-organisms
during service. Any resultant loss of strength might be detrimental to
the use of this material, especially if it were to be crawled over consistently or subjected to forces tending to dislodge it. Of even
greater importance is the knowledge of whether such fungus attack might
increase its permeability to water vapor. However, while a water vapor
transmission of 1 perm or less has been suggested for vapor barriers
apparently no standard for the permeance of soil covers has been
proposed.1
The trials reported herein are attempts to compare some properties of
sheet materials as measured by accelerated laboratory tests. These experiments were intended to supplement the information continuing to be
supplied by the service tests. Some of the research repor t ed here has
been used as the basis of an article in "Housing Research"- but is included in brief form in order to bring together all work in this field.
It should be noted that these data apply only to sheet materials used
as soil covers and that no attempt has been made to interpret the results of using such materials as membranes under slab-on-ground
construction.
'Diller, Jesse D. Reduction of decay hazard in basementless houses on
wet sites. Forest Pathology Special Release No. 30. 1950.
Diller, Jesse D. Soil cover reduces decay hazard of basementless
-houses. Forest Pathology Special Release No. 38. 1953.
2Britton, Ralph R., and Robert C. Reichel. Water vapor transmission of
building materials using four different testing methods. HUFA Tech.
Bull. No. 12: 13-15. 1950.
6
-Russell, William A. Durability of moisture-resistant membrane materials in contact with the ground. H&BTA Housing Research No. 4: 23-28.
1952.
Report No. 2007
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I. Resistance to Fungus Attack
Method of Obtaining Data
A total of 11 samples of soil cover materials were tested for fungus resistance by a modified soil-block method./ From 3 to 10 replicates of
each sample were exposed to various fungi for a period of 3 months in
individual containers incubated at 80° F. and 70 percent relative humidity. Deterioration due to fungus attack was measured by the loss in
weight, but for a number of roll roofing samples the loss in tensile
strength also was measured. Specimens used for the tensile tests were
4 by 1/2 inch with the fiber direction parallel to the long dimension
while the remainder were 4-1/2 inches in diameter. The latter size was
selected so as to compare with those used in the permeability test described later.
Loss in weight was computed from equilibrium weights of the specimens
placed, before and after exposure to the test fungi, in rooms adjusted
for constant temperature and relative humidity (one test was at 73° F.
and 50 percent relative humidity, the other at 80° F. and 65 percent
relative humidity). The tensile values were determined on a Schopper
Tensile Tester according to TAPPI standard method T 404 M 47. In all
cases, values were adjusted according to reference specimens exposed
in uninoculated containers.
Results and Conclusions
The results of the tests of soil cover materials are given in table 1.
All fibers except the asbestos were subject to deterioration by fungi.
While the weight losses of the roll roofing samples were relatively low
because of the small proportion of fiber in them, the strength losses
were considerable. However, there were no great differences in this respect between roofings of various weights. Since experience with 45and 55-pound asphalt roll roofing has been favorable, it would appear
that these and roofings containing asbestos fibers were sufficiently resistant to fungi to serve as soil covers.
The weight losses for the asphalt laminate-specimens were high, and in
most cases the paper faces of the sheets were completely disintegrated.
The tarred felt sample was resistant to some fungi but not to all; it
'Duncan, Catherine G. Soil-block and agar-block techniques for evaluation of oil-type wood preservatives: creosote, copper naphthenate and
pentachiorophenol. Forest Pathology Special Release No. 37. 1953.
•
Report No. 2007 -3-
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also tended to become brittle after fungus exposure and probably would
be easily punctured and torn if used as a soil cover.
Permeability to Water Vapor of
II.
Artificially Infected Specimens
The data in this section were the basis of the article in Housing
Research that was previously mentioned.6
Method of Obtaining Data
Three replicate specimens of six types of roll roofing 4-1/2 inches in
diameter were testd for permeance at the National Bureau of Standards
by the dry method.– These specimens, together with their aluminum
holders, then were exposed to the action , of 3 decay fungi in pure culture according to a modified soil-block technique for a period of 3
months. They were returned to the Bureau of Standards and tested for
permeance again. The original permeance value of these specimens, their
weight loss due to fungus exposure, and permeance value after this exposure are shown in table 2.
Results and Conclusions
No important changes in permeance occurred as a result of exposure to
fungus attack although measurable weight losses were incurred by some
specimens, and the fungi were observed to grow through a number of them
(table 2). Except for the 15-pound asphalt saturated felt which was
too badly deteriorated for permeance tests to be made, all the samples
would appear to have adequate durability and vapor barrier efficiency
for soil covers.
III.
Permeability to Water Vapor of
Naturally Infected Specimens
Permeance tests were made on 2 samples of 45-pound roll roofing that
had been used as a soil cover for 2 years and thus exposed to the
-The tests of water-vapor permeance were carried out under the direction of E. R. Oglio.
Report No. 2007
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deterioration induced by natural infection and handling. Samples of new
roll roofing were tested for comparison.
Method of Obtaining Data
The used specimens of soil cover were of two sorts -- those that showed
evidence of fungus infection , in the form of mycelial mats (fig. 1) and
those that appeared noninfected in a visual examination. The. defects
in the specimens included both the deterioration due to fungus infection
and that caused' by handling and crawling on the roll roofing while it
was installed and used as a soil cover. Such obvious defects as tears
and cracks were eliminated but the specimens tested had undergone considerable rubbing, flexing, and a slight amount of pitting from contact
wItth the soil. The new specimens were selected from a roll of roofing
which had been kept at room temperature.
The test was run in an incubator maintained at 95° F. and 90 percent
relative humidity. There was no provision for mechanically circulating
the air. The test dishes used to hold the specimens were the bottom
parts of 95 millimeter (3-3/4 inches in diameter) Petri plates having
ground edges. These plates allowed the specimens an exposed• area of
9.95 square inches or 0.00641 square meter. All tests were by the dry
method, and the desiccant used was anhydrous calcium chloride, about
23 grams to a dish. A scale accurate to 0.05 gram was used to obtain
successive weights of the units.
Three kinds of sealers were used. The edges of the roofing specimens
were given two coats of aluminum varnish; The samples were fastened
and sealed to the Petri plates with a coating of a solvent-base, synthetic rubber cement,and then melted wax (a mixture of amorphous and
paraffin wax prepared for the purpose) was run around the outside of the
plate at the junction of the specimen.
The units (fig. 2) were placed in the incubator with the specimen down
so that the calcium chloride was in direct contact with the roll roofing. Wood sticker strips 1/2 inch thick, placed outside the test area,
elevated the units above the wire shelves of the incubator so that
passage of air over the sample was not restricted. Each unit was kept
in the same relative position during the test. The units were weighed
at intervals of a few days until it became apparent that gain in weight
would be slow. The intervals were then lengthened and observations continued for about 6 months.
The permeability of the specimens was calculated from the constant rate
of weight change. This was conveniently found by-plotting successive
weights against time and using the data where at least three of these
111
Report No. 2007 -5-
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points lay on a straight line. Water vapor transmission in grams per 24
hours per square meter was calculated according to the method of the •
American Society for Testing Materials Committee on Water Vapor Transmission, dated January 1, 1951.
W.V.T. =
ELX_01
txa
where is the gain in grams, t is the time in hours during which gain
was noted, and a is the exposed area of the specimen in square meters.
Water vapor transmission may also be expressed in grains per hour per
square foot, which is grams per 24 hours per square meter multiplied by
the factor 0.0597.
When the water vapor transmission in grains per hour per square foot is
adjusted to 1 inch of mercury vapor pressure differential, the result
is given in perms.a This is done by dividing the values in grains per
hour per square foot by the vapor pressure difference in inches of
mercury during the test; in this case the calculated pressure was 1.49
inches of mercury.
Results and Conclusions
The results of the permeability tests are given in table 3. The values
obtained suggest that any handling of the roll roofing material tends
to produce an increase in permeance and that the presence of obvious
decay definitely causes an increase. However, since all water vapor
transmission was below the limit of 1 perm these differences are of
little practical importance. The soil cover from which these samples
were taken had given complete protection against condensation moisture
in the crawl spaces of 2 test houses for 2 seasons. Whether the infection can proceed to a stage where permeance is significantly affected
seems doubtful in view of the reports of vapor barrier efficiency after
9 years of service.
There was no easy way to evaluate the sensitivity and efficiency of the
simplified water vapor transmission test used in this experiment. When
uniform conditions of temperature and humidity were maintained in the
incubator, plots of successive weights occurred along a straight line
reasonably well; average variation in rate of weight change in a given
test ranged from 0.0006 to 0.008 gram per day. One of the test units
failed because of a faulty seal. Thus, it appears that this technique
in conjunction with an ordinary laboratory incubator was sufficiently
precise to reveal important differences in the water vapor transmission
of such low permeability material as roll roofing.
Report No. 2007
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C11
Report No. 2007
0
41 41
el •r1
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a)
•
Figure 1.--Mycelial mat over portion of roll roofing
from which specimens were selected.
Z
M 8,178 F
•
•
Figure 2.--The unit used for the water vapor
permeability test showing the Petri plate
test dish, the calcium chloride within used
as the desiccant, outer portion of the roll
roofing specimen, the edge seal of aluminum
varnish, and the wax sealer.
Z M 89251 F
•