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Insulating Your Basement or Crawlspace

BandJoist-Air-LeakageUntil recently, crawlspaces were required to be ventilated to stop moisture problems, and basements were considered cold, dark caves where we didn’t spend much time. Over the last several years, the building codes have finally caught up with the science that has told us that the area under our houses would be better if we treated it like part of the house.

With that in mind, the basement or crawlspace should be airtight and insulated just like the rest of your house. The only concern might be if there were combustion appliances down there – heaters and water heaters – that need air for combustion, and in that case, there are tests we run to make sure that sealing these areas will not cause a problem.

The first thing to do is to make sure you don’t have a water problem. Checking gutter, downspouts, and ground grading is part of our audit. Installing vapor and water barrier plastic on the floor of a crawlspace, or installing sump pumps may be necessary to make sure that these areas are dry.

Air sealing is straightforward and is done at the exterior walls. Proper materials are important, and experience and equipment helps guide the process.

The walls can then be insulated and the most common practice locally for basements is to use a wide, continuous insulation blanket attached to the wall. This gives good insulation values quickly and cheaply.

In crawlspaces, installing rigid insulation board on the walls after vapor barriers are installed on the floor is the most cost-effective approach.

If the interior of the basement is to be finished, there are a couple of options. Firring strips can be added to the inside of the exterior walls and rigid foam panels installed between them. Drywall or any other interior surface can then be installed. The exterior walls can also be studded with 2 X 4’s and then fiberglass batts installed before an interior finish is applied. Consider a sheet of plastic against the foundation wall to keep any moisture out of the back of the batts.

The most important thing to remember is that insulation does not work if it does not physically touch what you are trying to insulate. So any foundation insulation must be in full contact with the foundation wall.

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