Crawlspace - vent or seal?

[1918ColonialRevival]

Sealed crawlspaces in the Southeast are not a good thing. The humidity makes it an environment that enables wood rot and mold growth. I've been in several houses in the South that have had significant structural damage in the main sills, support beams and joists, and they all had one thing in common - the crawlspace was sealed.

I would close up the sides with some good quality lattice (not the thin stuff that the big box stores sell) set into treated wood frames. It'll keep the varmints out and still allow the air to circulate under the house. Also, be sure to address any standing water issues around the foundation. If the stray cats keep showing up, mix some ammonia and water in a pump sprayer and spray it along the areas where they frequent.

[Manalto]

Sealed crawlspaces in the Southeast are not a good thing. The humidity makes it an environment that enables wood rot and mold growth. I've been in several houses in the South that have had significant structural damage in the main sills, support beams and joists, and they all had one thing in common - the crawlspace was sealed.

I would close up the sides with some good quality lattice (not the thin stuff that the big box stores sell) set into treated wood frames. It'll keep the varmints out and still allow the air to circulate under the house. Also, be sure to address any standing water issues around the foundation. If the stray cats keep showing up, mix some ammonia and water in a pump sprayer and spray it along the areas where they frequent.

Thank you for confirming my decision and for the feline repellent suggestion. Unfortunately, until I'm there permanently, cats will find refuge on my property but at least they won't be under the house. There is no standing water at that address.

[Willa]

Are there any cat rescue groups in the area ? If you could TNR (TrapNeuterReturn) the cats, it will cut down on stinky tomcat spray, kittens (and pregnant mums looking for a place to have them), and gradually decrease the number of strays.

Most cat rescue groups are constantly overwhelmed, so if you demonstrate your goodwill - whether by donation, or help with trapping, transportation, etc. - that would go a long way to getting THEIR attention. They are the least responsive to people who want them to come in to solve their problem for $ 0.00, esp. folks who are demanding, rude or act like this is a great emergency.

Citrus peels are a good non toxic cat repellant. Chop up lemon, lime, orange peels and put them in the areas the cats frequent. Alternately you can also grate the peels and sprinkle that on the ground.

50/50 white vinegar and water in a spray bottle will neutralize tomcat spray smells, as will plain hydrogen peroxide.

Definitely enclose the crawlspace with lattice, etc. You really do not want the misery of a deceased critter under there in an inaccessible area...

[GibsonGM]

Sealed crawlspaces in the Southeast are not a good thing. The humidity makes it an environment that enables wood rot and mold growth. I've been in several houses in the South that have had significant structural damage in the main sills, support beams and joists, and they all had one thing in common - the crawlspace was sealed.

I would close up the sides with some good quality lattice (not the thin stuff that the big box stores sell) set into treated wood frames. It'll keep the varmints out and still allow the air to circulate under the house. Also, be sure to address any standing water issues around the foundation. If the stray cats keep showing up, mix some ammonia and water in a pump sprayer and spray it along the areas where they frequent.

If it's sealed, it's sealed, and won't have humidity issues. That is a science in and of itself, though! It would be extremely difficult for a homeowner to achieve SEALED on their own. They put down a thick rubber membrane and seal it to the foundation walls, and around any pipes exiting or what have you, often times by spray-foaming the walls. Air infiltration is minimal...you will only have humidity in the space if there is a major air leak, opening to the ground surface in the membrane, or standing water getting in either from the exterior or a leak of some sort. The ones you saw with damage weren't sealed, obviously. They just THOUGHT they were. I've seen a bunch of that, too. And also in spaces with too few and too small vent openings...where they try BOTH methods together (which is a fail).

Most people have a dehumidifier in there too, or at LEAST are able to monitor the humidity in the space if it's sealed. I wouldn't leave my BASEMENT alone with out some means to do that, ha ha.

There's nothing wrong with an open space, but it MUST ventilate completely. So, no insulation in the floor. No 'tiny vents'...gotta get that air IN there and thru it. I like the lattice idea! The dew point and RH must remain the same under the structure as outside it. Not a problem if enough air flow. If you EVER have wood reaching the dew point temp, and moist air coming from outside, you deposit water on the framing beneath.

[1918ColonialRevival]

...That is a science in and of itself, though! It would be extremely difficult for a homeowner to achieve SEALED on their own....

And that was my point . In the Southeast, particularly the closer you get to the coasts, the difficulty to achieve this goes way up. The wood is continually expanding and contracting with the changes in humidity. And if one were to achieve it, it would require constant monitoring.

[GibsonGM]

Absolutely, Revival. I don't know how people can sleep at night who DON'T monitor things like that, ha ha. Getting it done right can cost some big bucks.

No choice up here; you must have things enclosed. There are a lot of places that have been damaged by improper design, unfortunately. Looking at them now and then would have caught it before it was too late!

[Manalto]

Thank you everyone for this discussion. I had been advised, with a tone of urgency no less, to embark on the expensive and difficult mission of creating a sealed space by someone who has been helpful in the past so I know his intentions were good, however misguided. It has become abundantly clear, as I continue with the project of fixing up an old house, that bad advice is epidemic. Believing what you're advising sometimes bears little resemblance to knowing what you're doing - but I'm preaching to the choir here.

It's a simple fix to exclude animals larger than the mesh or lattice I put around the base of the house. I still haven't figured that out aesthetically, but I'm leaning toward some version of lattice (not diagonal)painted the dark green (almost-black) that I've used on the metal casements and screen doors.

[mjt]

We custom built lattice for our back porch (see bottom photo here). It works well to keep the bunny-sized critters from living under the porch while still allowing for air circulation.

[phil]

letting cats in here may help with mouse issues. I'd try about 1/4 inch screen to keep mice out but leave an access for cats that you can open even if you need to borrow a neighbor's kitty, or a rat dog or something. If you own your own cat you can chuck her down here for an hour once in a while just be careful with poisons and traps and such. I have doors to my attic space and kitty is more than happy to go in and check things out. If there is anything at all in there I'll know right away because she'll then want to scope it out all day long.

my brother just went up to our summer place. we have a cabin that he stayed in but the basement of the house is closed off , shared with out shop space is the crawlspace for the house which goes un-monitored so we have them and they are hard to stop.

mice get in there and you know how hard they are to stop permanently. what he did was let the motorized ( gas powered) golf cart run overnight in the garage so that the fumes would kill anything living in there. meanwhile he slept in a separate cabin. Maybe a ozone generator could work for that too. they tend to kill anything living and so you can do treatments and that may kill bugs, spores, spiders etc as well. ( and housplants) If it's open to the air , if you are there you might keep mice out but if you are away , they will likely find some way in. we use mouse poison but they tend to eat it or pack it away or they can become immune to it. we make sure to put any poison where pets can't get into it.. the warferin works somewhat. I had some stuff that is like sesame seeds with stronger poison. arsenic I think.. I'd keep that away from cats. I dont think a cat would eat the warfarin. it's a blood thinner so I think it takes a certain amount, after all they give it to people as a blood thinner. with dogs I wouldn't trust them because they will eat pretty weird stuff. good policy to put it all in a box only a mouse or a rat could get into. I tried using the clam traps for rats, I had a few around before I got my cat and they worked but now I have a cat I wouldn't care to use one and I've never seen a rat around my place. if you catch a rat in a clam trap you will have a stink if you don't police the trap. I used my granddad's trick you put the trap in a box and burn some paper, that is so the trap doesn't smell like a human. evidently rats are smart that way and they have a good sense of smell. If you catch a pet in a clam trap you will forever be cursed for your own bad judgement. That would be horrible so if you use those be very careful where you use them but they do work for rats if you are so unlucky to have them in, I use a stack of lumber, rats love lumber piles and i arranged the lumber so no one's pet could get near the trap and made sure not to forget about it. there are other traps that are more pet friendly like a stick balanced over a water bucket for mice , or or a wooden box trap where the sides fall , for rats. (depending on the size of the pet) It wont catch a big dog but some dogs are rat sized. some might prefer to set them free and some dont give a rat's ass about a rat so you may follow your conscience on that.

last year we had a rat that got down the toilet vent. it found it could find shelter in the basin of the toilet which was drained. the seat was down so it couldn't get out of the toilet. we have had experiences where they get into walls. talk about a stink! we also get woodpeckers in the walls. they eat through the cedar siding. I was a bit amazed the rat could not only get down the vent pipe to the toilet but it obviously could get out for food and water too. It would have been worse if the seat was open, or a light plastic one. if it had squeezed under the seat and got out. it probably couldn't get back out of the house unless it chewed some other way out.. so maybe a door that could let an animal like that out but not in would be beneficial? . a one way door of some sort might let any pest out that did manage to enter some other way? It probably isn't hard to prevent a rat from entering the toilet vent but it isn't a common issue since normally they would only reach the water in the bowl. we drain it so it doesn't freeze.
mice are hard to stop, they can get through pretty tiny gaps but I doubt they can get through a 1/4" screen... chicken wire is way too big. rats can chew through wood if they think they want in bad enough. they have tough teeth. some suggested mixing steel wool and silicone because it's not that fun for critters to chew through. not much can go through tin , expanded metal might work ok.

coyotes are natural mouse predators too. maybe if you leave it open then they would do their thing. Ive heard that their pee will chase a rat away but I always wondered how you'd ever get their pee

i had raccoons crapping down the side of my house on a regular basis. every evening. I was on their route. I tried putting stinky rags from turpentine. It might have worked. last I tried peeing in a bucket and put that where they go thinking they might not like the smell of my own pee much ..

[1918ColonialRevival]

The lattice in mjt's picture is aesthetically correct for an early 20th Century house. My next door neighbor's house (built 1916) has it around their porch.

Also, for smaller varmints, I have a minor alteration to Phil's solution. Get some bronze screen and apply it on the inside of the frames you build, behind the lattice. Bronze screen will last for decades provided nothing punctures it.