Final answer:
A backer rod is designed to seal joints from air infiltration (option c) , support sealant application, and ensure a durable seal. It's different from a fiberglass batt that is used for thermal insulation as it's specifically used in joints rather than for overall insulation of walls and ceilings.
Step-by-step explanation:
The main function of a backer rod is to seal joints from air infiltration. A backer rod is a flexible foam rod that is inserted into a joint before it is sealed with caulking or sealant. Its purpose is to fill the depth of the joint so that the sealant can be applied at the correct depth, which is typically between a quarter to a half of the width of the joint.
By doing so, the backer rod helps to support the sealant, allows for proper sealant tooling, and ensures a strong bond between the sealant and the joint surfaces. Additionally, backer rods can help to conserve sealant by reducing the amount of sealant needed to fill a joint and also prevent three-sided adhesion, which can lead to sealant failure.
While fiberglass batt, the subject is used for thermal insulation and to prevent heat transfer between the inside and outside of a building, a backer rod's function is specifically focused on joint treatment rather than structural or thermal performance. Unlike fiberglass batt, which is used in walls and ceilings, a backer rod is inserted into gaps and joints in various construction elements, such as around windows and doors, to ensure an airtight and watertight seal.