Most of us don’t think much about our basements, especially if we have one that isn’t particularly pleasant. But the basement is part of the house, and what goes on there affects our fuel bill and how comfortable we feel upstairs.
Even if you don’t deliberately heat your basement, you are probably at least partially heating it. Your heating and hot water equipment and a bunch of ducts or pipes are down there. It usually doesn’t make sense to put insulation in the basement ceiling. It’s almost always better to put it on the basement walls. The only major exception to this is when the basement has no equipment or heating distribution in it, there’s no danger of freezing pipes, and you don’t care how cold it gets down there.
You shouldn’t insulate a basement that is wet. It’s very important to do anything you can to resolve any water issues before you insulate. How to do that is beyond the scope of this article, but if you have a wet basement, fix the water problem first.
In terms of performance, it’s best to insulate on the outside of the foundation walls, but in most existing homes that’s difficult because of the digging that would be required and the stuff (porch, deck, driveway, steps) in the way. We’ll assume for the purposes of this article that you are insulating the basement walls on the inside.
You should avoid fibrous insulations (cellulose, fiberglass) below the first floor unless all moisture is carefully managed, and there is an effective air barrier layer on the inside of the insulation. If you want to avoid moisture issues, use a foam insulation (spray or board) and install it to be airtight, so the basement air can’t get outside of it. This holds true for the main basement walls and the rim (or band) joist.
The rim (or band) joist refers to the perimeter of the first-floor deck framing in modern stud-frame construction. It’s immediately on top of the foundation wall and is usually 8-12 inches high. At this location, there is typically only about two inches of wood between the basement and outside. The best way for a homeowner to insulate this is with blocks of foil-faced foam, glued in place with construction adhesive, and sealed around using a caulking gun or foam gun. Professionals usually use spray foam here because it’s quicker.
The basement walls are also usually best done with foam. For concrete block or poured concrete, you can use the same foil-faced foam glued to the wall. With rough stone foundations you are limited to spray foam, which requires specialized equipment and training.
The Vermont energy code doesn’t require that you insulate your existing basement walls, but it does say that if you choose to, you need to do at least R-15. This is about 2 ¼ to 3 inches of foam, depending on the product.
Building codes require foam insulation to be covered by a fire protection layer. There’s an exception for the band joist, but all other areas must be covered. This is often ½” drywall, which is approved for this purpose. Other options include an intumescent paint (good choice for spray foam), and at least one board foam (sold under the trade name “Thermax”) that has a factory-applied aluminum layer that meets the protection requirements.
Don’t forget the bulkhead door, if you have one. If the only door is the steel bulkhead cover itself, you need to frame out the opening in the basement wall and install a door. An inexpensive insulated steel pre-hung door (36”, no window) works fine. If you have a rough door of planks or plywood, you can replace it or install a layer of rigid foam to the outside surface and weather strip it, so it doesn’t leak much air.
Next time we’ll talk about attics.
Dave Keefe is a fifth-generation Vermonter who has worked for over 35 years as a contractor, consultant and teacher to improve the performance of existing homes.