Why Fiberglass Under Your Floor Fails

The floor isn't making cold.

What's happening is heat is moving from the warmer part of your house toward the colder crawl space underneath it.

That's called conduction.

Heat moves from warm toward cold as long as there's a temperature difference.

So if your living room is 70 degrees and your crawl space is 45, heat is going to keep moving through that floor assembly toward the colder crawl space.

And here's something I always try to explain to homeowners.

Even after you fix the crawl space, your floor probably isn't going to feel warm unless you've got something like radiant floor heat installed.

The surface of your feet is warmer than the floor, so your body is still giving up heat to it.

What we're trying to do is make the floor warmer, not make it feel like a heated floor.

In the winter, if we can raise that floor surface temperature from say a low 50 to 60 degrees into a range closer to your thermostat setting, your feet are losing heat to it at a much slower pace.

When you are able to do this, the room feels more comfortable even though the floor itself isn't actually warm.

Now you'd think, well, I've got insulation under there, so I should be fine.

But having insulation and having insulation that's actually performing the way it's supposed to are two different things.

Fiberglass only works the way it's supposed to when it's installed correctly.

If it's hanging loose because there aren't enough insulation supports...

Failing because of moisture from high humidity or a plumbing leak, compressed, cut badly around plumbing or electrical lines, or missing in sections, the real performance can be a lot lower than what it says on the label.

I see this constantly.

A floor that's insulated and a floor that's performing the way it's supposed to are not the same thing.

And most homeowners have no idea because homeowners never go in their crawl space.

And even if every piece of fiberglass is installed perfectly, there's still another path for heat to move through, and that's the floor joists themselves.

The fiberglass is installed between the joists, but the wood framing is still exposed to the crawlspace.

wood has some insulating value but not nearly as much as a cavity filled with insulation if your floor joists are 16 inches on center the joists themselves represent roughly nine percent of the repeating floor area and that's before we even count beams blocking double joists or other framing so on a thousand square foot floor You're talking about roughly 90 square feet of wood framing where you don't have the full R-value of that fiberglass bat.

That's called thermal bridging.