Procedures

WATER VAPOR PERMEANCE

Figure 16. Sample ready for water vapor permeance testing.

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Method B; however, two manufacturers did not

14 shows a schematic of the modified-cup appa-

specify the method, only that E96 was the stan-

ratus for water vapor permeance testing. This

dard used. Since agreement should not be ex-

method combines the features of both the ASTM

pected between results obtained by different meth-

dry and wet cup methods (Schwartz et al. 1989).

ods, it makes sense to specify a method and require

The modified test method creates the maximum

all membranes to be tested accordingly.

vapor pressure difference possible at the test tem-

perature of 84F (29C) that we used. This modi-

There are several critical aspects of the ASTM

E96 procedure that can adversely affect the per-

fied method eliminates the need to maintain a 50%

meance results. Toas (1989) reported on a 1985

RH environment on one side of the sample. That

round-robin test series using the E96 procedure.

saves a lot of effort in setting up and maintaining

He stated that in performing the ASTM E96 test,

a fixed relative humidity in the test chamber. Fig-

test operators must seal the test sample perfectly

ure 15 shows the modified-cup apparatus used for

to the test dish, carefully weigh the sealed speci-

permeance testing. The apparatus is held tightly

men with a balance having the proper sensitivity,

together by two aluminum plates bolted together.

and maintain the proper atmosphere for the test.

The assembly is placed in the test chamber and

Low permeance materials are difficult to measure,

periodically disassembled so that the water cup,

as the weight gain or loss is usually quite small

specimen, and desiccant cup portions can be

and the potential for error is much greater. Conse-

weighed. Then it is reassembled and the test con-

quently three material specimens are required, and

a "dummy" should be used when testing materi-

We compared the results of the modified-cup

als with a permeance less than 0.05 perm. Statisti-

to a variation of the standard water test method

using a test temperature of 84F (rather than 73.4F

cally significant results require at least four rep-

[23C]) and an RH of 16% (rather than 50%). We

etitions of each membrane. These results usually

are determined using a least squares regression

also poked a hole through three of the membranes

analysis of the weight loss or gain, modified by

and re-tested these membranes using the water-

the dummy specimen as a function of time to ob-

tain the water vapor transmission. We did not at-

tempt a statistical study as this would have been

Sample preparation

too costly. We decided instead to proof test an al-

Figure 16 shows a membrane sample ready for

ternative method and compare it to a variation of

testing. The membrane samples were adhered to

a standard method.

a stainless steel mounting plate using two tech-

niques. The first technique used pressure-sensi-

tive adhesives on five of the six membranes tested.

We investigated a "Modified-Cup Method" as

A mounting jig was used to center these mem-

a result of our review and conversations with oth-

branes on their mask, and then the steel plate was

ers familiar with the E96 test procedures. Figure

pressed onto the membrane (after the release sheet

Table 5. Manufacturers' data for the membranes tested, sorted from lowest to highest permeance.

(gr/hr ft2 in. Hg)

Product tested

Sopralene Flam Antirock

10AN Easy Pave ER

AC Bridge and Deck Seal

Western Governors University

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