A couple weeks ago I went under the house with my father-in-law to do a periodic inspection. While I was under there I noticed that while the crawlspace vents were closed there was a significant amount of light bleeding through them. It was bad enough that when we emerged from the crawlspace I had to double check to make sure they were actually closed. With the recent below freezing temperatures and a major snowstorm in the forecast (that ended up dumping over a foot of snow on us) I was concerned about the possibility of freezing pipes.
On my way home the following afternoon I stopped by Lowe’s and picked up a 4’x8’ piece of foam insulation board for about $12. (The link doesn’t show it was available but my local store at least had it in stock.) From there I cut 7”x16” pieces to fit each vent and wedged them in there.
It’s worth noting that while we’d had similar cold temperatures before without an issue I didn’t want to take any chances. It’s cheap insurance against a potential bigger problem.