Serious question, Waldo. Why only once a month? Is that the only time you are at your home? That would give any surviving rats three whole weeks to reproduce, which is plenty of time to keep the rat problem growing for years. But if you did the cat thing, I can tell you from personal experience of living out in the country, that feral cats can live anywhere. Out at the plant my husband works at, they even had a pack of feral cats living out there, at the plant! They had so many cats and new kittens all the time, they couldn't even work without tripping over them, and they were nasty cats, too, pooing all over their equipment, etc. They even hauled them off to a wooded area about five miles away, but some found their way back, so the guys started taking them home as pets. Once you start feeding a hungry wild animal, it is nearly impossible to get them to leave. When we lived out at our old place, I used to feed the cats that wondered up, my husband used to call me the Cat Lady, but out there, we needed as many as we could get to keep the field rat population away. lol. But they can be just as loving as any domestic cat, and you might find yourself being closer to there than you realize.
So, even if they were successful in ridding the rats, you would then have feral cats living in that space. We have had feral cats give birth under our shed inside our fenced backyard, that is only about six inches off the ground. Lucky for us, the mama moved on after all her babies left, but we never made an attempt to feed them. If we had, they would have hung around waiting for the easy meal. Male cats may move on when the call of the wild strikes them, but female cats can linger for their whole lives, wanted or not. Never had the heart to "do away" with them or call for outside help. There was a feral female cat that lived between our and the neighbor's yard, slept under their RV in the driveway, for about two years. Turns out the neighbor had fed her for a couple of months early on and then she wouldn't leave. Husband wouldn't let me feed her, we really just hoped she would move on, she did survive off small wildlife in the woods right next to us, but she always came "home" to sleep under that camper. We found her one day in the ditch a little ways up the road, she had been run over.
Anyway, good luck with your problem. Think really hard about introducing ferals into your life. They can be very bothersome if you don't want them. And much more demanding than the rats are. The foam insulation is great for after the rat problem is taken care of, though.
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