What can I do for fiberglass insulation in my basement ceiling that doesn't work very well?

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Answered by Tim Snyder, Guest Editor

Fiberglass insulation won’t do anything to stop chilly drafts from coming into the basement, so the first thing to do is air-seal the rim joist. This wood framing rests on top of the foundation walls and is a major source of air infiltration and conductive heat loss. For best results, go the extra mile and insulate the rim joist while it’s being air-sealed. This can be done by friction-fitting rectangles of rigid foam insulation between floor joists and against the rim joist, then spraying canned foam around the edges of the rigid foam to complete the air sealing work. As shown in the photo, it may be necessary to cut the rigid foam to fit around pipes and wiring that extend through the rim joist.

To insulate and air-seal the rim joist in the basement, rigid foam insulation is wedged between joists and against the rim joist that rests atop the foundation wall. Then spray foam is used to seal around the edges of the foam. Foil-faced rigid foam is used here, but unfaced rigid foam insulation can also be used.

The next priorities for improving comfort and energy efficiency in the basement are the basement windows and rigid foam wall insulation. Consider having old single-pane basement windows replaced with new vinyl windows that contain insulated glass. By installing rigid foam insulation against the basement’s foundation walls, you’ll be moving the thermal boundary from the basement ceiling to the foundation walls. When the entire basement gets warmer as a result, the upstairs living space will also benefit. No more chilly floors.

About the author: A journalist specializing in sustainability, energy efficiency and home improvement topics, Tim Snyder is a former executive editor of Fine Homebuilding and American Woodworker magazines. Tim's recent work includes a green remodeling manual for the Environmental Protection Agency, and a chapter on “Smart Shelter” for The Whole Green Catalog. Tim’s blogs and web content can be found at myHOMEscience.com, DrEnergySaver.com, BasementSystems.com, MotherEarthNews.com, Wisebread.com, Green-Energy-News.com, RealtyBiznews.com and other sites.

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