I received an interesting question from a reader recently. He had a crawl space that was ventilated, but was also heated and cooled. He wanted to know what the best way was to insulate that space: floor insulation or insulation on the crawl space walls.

There are actually a couple of theories on this, and they both make sense, depending on exactly what your crawl space conditions are and what you’re trying to achieve. So here’s a good opportunity to take a look at both of them.

First, it’s important to understand the concept of the term "building envelope." Basically, everything within the "envelope" of your home that you’re trying to heat or cool — what’s known as "conditioned space" — should be surrounded by insulation. That prevents heat loss or gain, which improves comfort and lowers utility bills. In general, anything outside the conditioned space doesn’t need to be insulated.

Probably the easiest way to visualize this is to look at a simple cross section of a house with an attic. The house is conditioned space; the attic isn’t. So you want insulation in the walls, between the interior and the exterior.

The attic is a ventilated space, which is done to remove moisture, so it’s close to the ambient temperature of the exterior. Insulation is placed at the ceiling, between the conditioned space inside the house and the unconditioned space in the attic. It obviously wouldn’t make sense to place the insulation along the underside of the roof, since you’d be allowing heated (or cooled) air to escape from the house into the attic, where it would be wasted. You’d also block the flow of needed ventilation in the attic.

Now we come to the crawl space, and this is where it can get a little more complicated. You need to make a decision as to whether the crawl space is a ventilated and unconditioned space similar to an attic, or whether it’s an unventilated and conditioned space, similar to a conventional basement. There are advantages and disadvantages to both, and differing opinions among the pros as well.

Ventilated crawl spaces

Many building codes require the crawl space to be ventilated, which is done to help remove any accumulated moisture. In that case, the crawl space becomes unconditioned space, so the insulation would be placed up against the underside of the floor. Proper installation would require that the insulation’s vapor barrier be placed up, against the floor. You also want to put down a 6-mil plastic vapor barrier over the soil.

Always remember that this is unconditioned space. That means that the temperatures down there are very close to what the outside temperatures are. So it’s critically important that any water pipes and duct pipes located in the crawl space be well insulated.

Unventilated crawl spaces

There’s a growing trend toward the use of unventilated and conditioned crawl spaces. As one insulation expert pointed out, "We don’t ventilate basements, and crawl spaces are just short basements."

Here’s what the U.S. Department of Energy has to say:

"Most building codes require vents to aid in removing moisture from the crawl space. However, many building professionals now recognize that building an unventilated crawl space (or closing vents after the crawl space dries out following construction) is the best option in homes using proper moisture control and exterior drainage techniques.

"If you have or will have an unventilated crawl space, the best approach is to seal and insulate the foundation walls rather than the floor between the crawl space and the house. This strategy has the advantage of keeping piping and ductwork within the conditioned volume of the house so these building components don’t require insulation for energy efficiency or protection against freezing. The downside of this strategy is that rodents, pests or water can damage the insulation, and the crawl space must be built airtight and the air barrier maintained."

So, if you have an unventilated crawl space, then you’d want to insulate the crawl space walls instead of the floor. But what you really need to keep in mind with unventilated crawl spaces is that the crawl space needs to be airtight, and it needs to remain that way. So here are a few tips:

  • Install a well-sealed 10-mil vapor barrier on the ground. Lap the seams, extend it up the walls, and seal it to the walls. As an alternative, consider putting down a concrete slab. You need to do everything possible to keep moisture out of the crawl space.
  • Seal the rim joist and any penetrations to prevent air and moisture from getting in. Make sure no rain or snow can get into the space.
  • Install ductwork to condition the space.
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