Why your floors went cold so fast
Cold floors that show up over a day or two point almost straight at the crawlspace. The insulation under your floor is the buffer between your feet and the outside air, and when a chunk of it fails, the floor above it follows within hours.
The usual culprit is fiberglass batts that fell out of the joist bays. They were held in by friction, humidity loosened them, and gravity did the rest.
The second most common cause is water. Wet fiberglass loses nearly all its R-value, so even if the batt is technically still up there, it's not doing much.
The good news: this is one of the more fixable insulation problems. crawling under the house, you can often solve it in an afternoon for around $100 in materials.
What's actually going on down there
When Techs get called for sudden cold floors, I head straight under the house and look under the coldest room. The problem is almost always sitting right there in plain sight.
Fallen or sagging batts are the number one find. Original installers often skipped the support wires, so the batts just perch in the bays until they don't. You'll see them draped on the dirt or hanging half out.
Wet insulation is next. Dark, matted, heavy fiberglass tells the story. Something is leaking or the crawlspace humidity is too high, and the batt gave up.
Then there's the open-vent surprise. Older homes have manual vents meant to close in winter, and one left open funnels cold air right across your subfloor.
Rodents round out the list. Mice and squirrels shred insulation for bedding, and a nest-sized gap in a batt is enough to chill the floor over it.
Missing vapor barrier is the slow burn. Bare dirt keeps everything damp, which quietly undermines the insulation and the wood over time.