Crawl Space Insulation Myths

Friday, November 15th, 2013

Some crawl spaces have insulation in the floor above, and some do not. One reason for insulation in the floor above is that the crawlspace is so cold in the winter due to open vents. First things first, these vents should be closed permanently, which would make a huge difference in the temperature of the floor above. Here are few important notes about crawl space insulation:

First thing to understand is that fiberglass insulation only works in a closed cavity. Fiberglass insulation is loose and air passes right through it. When it is between the joists in a vented crawl space, with the paper or foil side up, and the unfaced side down, it’s not doing much at all.  Not to mention, paper-faced insulation is mold candy – as mold loves paper even more than wood!

Foil-faced insulation is supposed to be a radiant barrier. You put the foil face to the heated side. But you need to have at least a 1/2″ air space in front of foil radiant barriers for them to reflect the heat. Most of the insulation, if it has a radiant barrier, is jammed up against the floor sheathing with no air space between the radiant barrier and the sheathing. Therefore, that element of the insulation is not working. Besides, such an air space creates a thermal bypass where air can flow on both sides of the insulation, rendering the insulation useless.

Lastly, when fiberglass insulation is even a little damp, it loses a whole lot of its insulation value.

Insulation in a crawl space is still a good idea. if you do it, insulation should always be installed against the air boundary surface of the house. The wood floor system above is full of gaps, joints and chases for wires, pipes and ducts- so it’s not the air boundary – and it’s the wrong place to insulate in a crawl space with no vents.

By sealing all outside air leaks and insulating the walls the crawl space is part of the conditioned space of the house. Foam insulation is the best because if a plumbing leak happens, then the insulation won’t get ruined and support mold growth.