In hot and dry climates, evaporative cooling is an effective cooling method. An evaporative cooler is made up of a blower fan, a pump and pads that are kept wet. Hot, dry air from outdoors moves through soaked pads and blows indoors. The indoor air temperature becomes cooler and the humidity increases. The efficiency of an evaporative cooling system depends on relative humidity, ambient temperature, blower speed, quality of the water, and the moisture holding area and configuration of the pad.


Evaporative coolers in general fall into two categories. One is MasterCool, a specific brand with a rigid pad system made from cellulose. Other evaporative coolers encompass many brands. What they share is the use of a pad called a straw pad that is not actually made of straw. Straw pads are instead made of aspen excelsior.


Rigid MasterCool pads are made of a type of cellulose that can be kept wet without deterioration. The corrugated cellulose sheets are bonded together. Cells meet at angles. The sheet thickness, about 15 cm, is designed to maximize each cell's contact with air and the surface area available to achieve evaporation. The sheets are then molded into a pad that can be 8 inches thick for the larger coolers. The thick MasterCool system pad, which may be sold in sections that when installed will abut one another, is placed against one wall of the evaporative cooler.

An aspen pad evaporative cooler pulls air from the outside into the metal side panels of a four-sided unit. Excelsior strands make up the pads. Water is pumped above the pad and drips down onto it. Air blows through the pad and the cooled air blows into the house.

The strands in aspen pads are formed and shaped into rectangular or square pads about 1 inch thick. They are inserted into gridlike vertical holders that keep the pads from sagging. Aspen wood has desirable characteristics for this use. It has no odor, is inert chemically, is highly absorbent and can be kept wet, although it will eventually degrade.


MasterCool pads have a longer expected life than aspen pads. Aspen pads last one evaporative cooling season. Their cost is usually is the $10 range per pad and a cooler may need four pads. It is likely to cost in the $100 range for a MasterCool replacement pad.


The special MasterCool cooling pad, thanks to its unique components and design, achieves a cooling efficiency of approximately 90 percent. The pad retains a large volume of water and that enables it to cool efficiently and quickly. Aspen pads are thinner, retain less water and are about 85 percent efficient.


Warranties on aspen pads vary by manufacturer but are not likely to exceed one evaporative cooling season. MasterCool pads promise a longer life and often have a five-year warranty.