A commonly used storage container is a corrugated carton or a wooden box. Unless
they are wax-coated or resin-treated, water vapor is transmitted through them quite readily.
Standard practice is to line them with a plastic film, such as polyethylene. While the plastic
film will hold liquid water, water vapor may pass through it. For instance, 1-mil polyethylene
will pass water vapor at the rate of 1.5 grams/sq. ft./ 24 hrs, while 4-mil polyethylene
will only pass about 0.05 grams/sq. ft./24 hrs at 75° F and 100% RH. We normally recommend
that you use a minimum of 4-mil polyethylene, or something equivalent to it in water
vapor transmission rate.
For short term storage - two to four weeks - we suggest that you use eight ounces of DRIERITE for every 20 sq. ft. of 4-mil polyethylene, plus sufficient DRIERITE to remove the water in the contents of the bag or container. For medium term storage - one to three months - use eight ounces for each 10 sq. ft. of vapor barrier. For long term storage - three months or longer - use eight ounces of DRIERITE for every five square feet of vapor barrier. Enough extra DRIERITE will have to be added in the last two cases to remove the water in the contents of the container.
The quantities of DRIERITE recommended above are based on relatively stable storage conditions. If there are large daily fluctuations in temperature or if extremely high temperatures and high humidity are to be encountered, the amount of DRIERITE should be doubled.
There are four sources of water contamination in a closed container or package: the
water vapor in the air inside the package, the water adsorbed in the materials inside the
package, the water adsorbed on the walls of the container, and the permeation of water
vapor into the container. In order to protect the contents of a container or package from the
destructive effects of water, enough DRIERITE must be used to remove all the water from
A hermetically sealed container, closed in a normal ambient atmosphere, would require approximately four ounces of DRIERITE for every 10 cubic feet of free volume to dry the air in the container. At more extreme conditions (100° F and 100% R.H.) eight ounces of DRIERITE would be required. Anything in the container that has water adsorbed in it would require extra DRIERITE. Wood and paper products can have as much as 10% by weight adsorbed water. Styrofoam and some rubber products may have as little as 0.5% adsorbed water. Once the amount of water in the material in the container has been estimated, you can calculate the amount of extra DRIERITE required by dividing the weight of the adsorbed water by 0.06. The total amount of DRIERITE required to protect a hermetically sealed container would be the sum of the amount of DRIERITE required to dry the air volume plus the amount of DRIERITE required to dry the materials in the container.
If a well-sealed container, such as a 55 gallon drum with a gasketed lid, is being used, the amount of DRIERITE required is calculated the same as for a hermetically sealed container, except that an additional four ounces of DRIERITE per ten cubic feet of volume is used to compensate for moisture permeation through the seal. To protect an enclosure such as a safe, tool box or a slip cover can, where there is no gasket, requires about eight ounces of DRIERITE per five cubic feet of volume plus the DRIERITE required to remove the water from the contents of the enclosure. The quantities of DRIERITE suggested above are predicated on the assumption that the ambient storage conditions are rather stable, and there are not large temperature fluctuations daily. If an unsealed container were stored in a area where the temperature could fluctuate 30° F daily, the container may possibly breathe 200% of its free volume per month. If this were the case, additional DRIERITE would be needed to absorb the water entering due to the breathing of the container.