Whether you choose fiberglass or cellulose, your insulation will only block the movement of heat. It won’t block the movement of air – and that matters.
In a typical home, air infiltration is endemic. Outdoor air is constantly entering your home through gaps and cracks in the building envelope. At the same time, conditioned air is leaking out. Due to the stack effect, most of this leakage occurs at the top and bottom of your home – your attic and crawlspace.
Sure, there’s insulation in your attic and crawlspace. But there’s probably nothing to prevent air from moving past plumbing, air duct, and electrical penetrations. Or attic staircase hatches. Or whole house fans.
In spite of all their benefits, neither fiberglass nor cellulose insulation provides an adequate air seal. While some research suggests that cellulose resists and/or blocks air movement more effectively than fiberglass, these materials are and should only be installed to reduce the passage of heat – not the passage of air. Only foam insulation and certain caulks can block air movement in a way that truly improves comfort and boosts energy efficiency.
Any time you’re thinking about adding insulation to improve your home’s thermal performance, consider air sealing as part of the same project. Have your insulation contractor seal all visible gaps and cracks inside the space you’re planning to insulate. You’ll improve the overall performance of the new insulation you’re buying.
You’ll also be more comfortable and save energy.