Sealing gaps and cracks is an excellent way to retain heat or cool air

Applying caulk, sealants and foam in any of the envelope’s gaps or cracks is an excellent way to conserve heat or cool air within the building. Most importantly, seal the seams of your attic, crawlspace, unheated basement and garage air ducts with mastic or foil tape, and then wrap with insulation to maintain proper temperatures.

http://www.toolbase.org/Home-Building-Topics/Energy-Efficiency/air-sealing