Caulk, in its soft, clay-like form, fits easily into fissures, holes and cracks to seal them off. With exposure to air, caulk expands and creates an airtight seal. Placing caulk around an air conditioner helps prevent outside air from leaking into a home around the unit. It also prevents cool air generated by the air conditioner from leaking out of the cracks around the unit. Recommended caulk types depend upon your needs.
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Consider your needs before purchasing caulk. Numerous factors affect the performance of caulking material. Some caulks only perform well indoors while others work equally well inside and out. Paint adheres to some types of caulk but not to others. Some types of caulk last for as long as 50 years while others dissolve when exposed to excessive moisture. Some caulks may suit your needs perfectly but contain environmentally-unfriendly chemicals or produce noxious fumes. Consider your caulking needs to decide which type to purchase for sealing around your air conditioner.
Recommended General Caulks
The Virginia Department of Minerals, Mines and Energy recommends acrylic latex caulk for use as an all-around caulking material. Latex caulk is relatively inexpensive, easy to apply and maintain, and bonds well with wood and metal. Paint adheres well to acrylic latex caulk, and the material lasts as long as 10 to 20 years. Pure silicone, one of the most expensive caulks, may last as long as 50 years, though you can’t paint most varieties of this caulking material. Butyl rubber caulk adheres well to metal and masonry material, such as brick, mortar and concrete. While it is resistant to water upon settling, "Fine Home Making" magazine reports that butyl rubber caulk adheres poorly to wet surfaces in its clay-like form and is difficult to work with.
The Sierra Club Green Home program recommends choosing water-based rather than solvent-based caulking material for all of your home needs. The organization notes that water-based caulks are easier to work with and clean than solvent-based caulks, which may release noxious fumes in their clay-like form. Silicone, polyurethane, butyl rubber, oil and resin constitute common types of solvent-based caulks. According to an article in “Fine Home Building Magazine,” water-based caulk requires a few days to set, making it hard to apply outdoors in adverse weather. Solvent-based caulks don't have this problem.
Sealants and More
There are various alternatives to caulk, including sealants and sprays. Water-based foams and polyurethane come aerosolized in cans. When applied to cracks, crevices or other openings, these sprays dry to form a seal. Some sprays form rigid surfaces when dry while others produce spongy, soft material, similar to caulk. Water-based caulk-like sprays do not exhibit the same degree of water resistance as caulks. Polyurethane sprays, meanwhile, contain potentially hazardous chemicals. Sealant works like glue, creating a bond between two surfaces. However, it should not be used around air conditioners because it essentially binds the unit to its surroundings, preventing air from circulating and preventing you from easily removing the unit.