Section II. CONTROLLED HUMIDITY STORAGE

TM 38-260

(1) IPE should be placed in the type of storage affording the highest degree of protection that can be

economically provided.

(2) Paragraphs 5-6 through 5-13 provide the instructions for placing the above storage conditions into effect.

Section II. CONTROLLED HUMIDITY STORAGE

5-6. General

a. Controlled humidity storage is that type of storage where the atmosphere in the storage area is maintained at a

relative humidity of 50 percent or less. This type of storage will protect properly cleaned and preserved IPE for an

indefinite period.

b. Controlled humidity storage is achieved by removing water vapor from the storage area until the desired

relative humidity has been reached.

c. When water vapor is removed from a storage space, a vapor pressure differential is established between the

dehumidified space and the variable vapor pressure in the outside air. The pressure differential, or force, attempts to

equalize itself by diffusing water vapor through all porous hygroscopic wall materials and also through window and door

openings, open ventilators, and cracks in walls. This diffusion effect is called water vapor transmission. As the wind

blows against the building, air moves into the enclosed space. This is called infiltration, and it carries wet air in and dry

air out. A good vapor barrier must be provided to exclude this air and vapor transfer. This requirement is generally

achieved by the erection of a vapor barrier, which makes the storage area relatively air-tight and moisture-proof.

d. A vapor barrier may be provided by the following methods:

(1) By sealing the storage building against water seepage, water vapor transmission, and air leakage

(controlled humidity buildings).

(2) By the erection of a polyethylene hutment (indoor plastic film controlled humidity structure).

(3) By the erection of a hutment designed specifically to prevent infiltration of water vapor and outside air

(indoor metal hutment or commercially available humidity controlled structure).

e. Dehumidification equipment.

(1) There is not a set formula which can accurately predict the size of the dehumidification unit required for an

individual storage site. Dehumidification unit size is based on the number of cubic feet to be dehumidified; however, the

transmission rate of outside air and moisture from the floor, etc. , directly affect the amount of water a dehumidification

unit must remove to keep the relative humidity below 50 percent. The sizes of dehumidification units

5-4