Published on March 10th, 2015 | by Henri


Insulating a basement

Q.  My basement is basically unheated. Would it be advantageous to put insulation around the ductwork or to insulate that particular space between the floor joists where the ducts are? Also, I notice that above the foundation where the end cap 2×10 is for the floor joists, it seems colder than the rest of the basement. Can I or should I put some sort of insulation against this end cap? Could I use rigid foam?

A.    If your basement has any appliances such as the furnace or boiler, a clothes washer and dryer, or you use it as a workshop, it is best to insulate its walls, but do not do so lower than three feet below the outside grade unless you are sure that the foundation has a working drainage system and that it has been backfilled with coarse material for good drainage.

Insulating a basement or crawl space all the way to the floor is taking a risk that frost may go down deep and crack the walls unless the steps mentioned above are in place.

If your basement is not used at all, insulating between the floor joists is an option. You should, however, case the ducts and pipes into a chase and insulate the exterior of the chase so that heat from the first floor can keep the pipes and ducts warmer.

Regardless of what you decide to do, you should insulate the band joists (which you call the end cap), as there is a great deal of heat loss through them (one percent or more of the total house losses in such a small area, according to some authoritative studies.) You can do so with fiberglass batts with an integral vapor retarder, or unfaced batts that will need to be covered with a plastic vapor retarder.

Or you can use rigid foam. If you choose  foam, use boards that are two inches thick and caulk their perimeter once you have set them in place. If you choose fiberglass, cut the batts one inch longer than the depth of the joists (to ensure a tight fit), push them in against the band joists without squeezing them, and staple the edges of the integral vapor retarder or the plastic to the floor joists. Be aware that if you do not use a vapor retarder, moisture will migrate through the fiberglass and condense against the cold band joists; this will lead to potential rot.

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