A few months ago, I bought a house and had it painted. I’m trying to clean out the laundry room now, where the paint cans with the extra paint have been hanging out, and I was thinking about putting them under the house. (I have a crawlspace that’s quite tall in the back, since my lot has a good amount of slope to it.) Is that okay? It gets quite warm here in the summers. Is paint one of those things that can build up toxic fumes and explode?
Uh, yeah. Paint (and stain and varnish and paint thinner) gives off flammable vapors that can build up and combust if it’s warm enough and not ventilated properly.
This stuff needs to be stored in a ventilated area. If the crawlspace is sufficiently ventilated so vapors can’t build up, you’re probably okay.
I’m sure danceswithcats will be along.
Robin
Well, the crawlspace does have vents - more accurately, vents made out of the brickwork. Are there enough of them?
In many places, paint and other hazardous materials have to be stored in a paint locker, outside of the building.
Paint will for lack of a better term curdle if it freezes.
Thank you for the introduction, MsRobyn. 
First, what kind of paint is it? Latex has inherently zero hazard, alkyd and oil base paints have a slight hazard, so the only ones you need be concerned with are pigmented shellacs, lacquers, and a few rare mixes which contain a high percentage of VOCs.
A tightly closed container is unlikely to rupture within the ordinary temperature variation of a dwelling. In SC, you get a bit hotter than PA in the summer, but your winters aren’t as bad, so we’re comparable. Right now, I’ve got 50+ gallons of mixed latex, alkyd, stains, solvents, and so forth underneath me in the basement shop. Properly stored on metal shelves, it’s a non problem, and some of it has been there since I moved in 10 years ago. Yes, it wouldn’t be pretty if the place caught on fire, but I’m rather anal about that.
On the other hand, storing paint out of doors on the ground in a crawl space strikes me as a bad plan, owing to the accelerated deterioration of the container, leading to release of product which could pose a fume/ignition hazard, as well as fouling the aquifer.
My call is to save those products which you have a likelihood of using in the future indoors in a well ventilated area, so long as the containers are in good condition and are properly closed. Those which you will not use, or are in deteriorated containers should be properly disposed of. Contact your State or County regarding hazardous material disposal-many municipalities have dropoff points for such household products.
Sorry for the length of this-contractor/firefighter/hazmat tech answers kinda run together. 
It’s latex paint. So, the verdict is that in, say, a shed or something it would be okay, but under the house on the ground it’s a bad idea? Is the ground the bad part, or the lack of climate control? I do have a space that’s under the laundry room, not under the main house, that’s got a poured concrete floor, although it lacks the climate-control-by-association of under the house. I may in the future be building a garage, if the historical commission ever calls me back, that sounds like it would be okay for storage as well. Does it make a difference if we’re talking about, say, just a year?
Since it’s latex, there’s no real health hazard-so long as you don’t drink it or pour it on the ground. What will accelerate can failure is condensation, so within an air conditioned/heated space is best for container longevity. Outdoors, the cans will become cool at night, and humidity of the day will condense on the exterior, leading to rusting. On a slab, use spare stirrer strips to put the cans on, just to allow a little air action underneath to keep bottoms dry. Also, as noted-freeze/thaw isn’t good for latex.
Plus, once a can of latex paint freezes, it’s worthless. South Carolina still freezes sometimes, right? Any temp extreme, high or low, is bad for paint. Under the house is virtually the same as leaving them outside. Store them someplace else.
There’s more than one construction type person here, you know. 
I don’t think this is true. Under my house stays cool in the summer, and obviously warmer in the winter or my pipes would freeze.
If you have a dry under-house environment, and don’t store the cans on the ground, I don’t see what the problem would be. Definitely don’t want them sitting in wet ground or standing water if they are metal. I say, put some bricks down to make a surface for them, and put them under the house.
I wouldn’t do it. YMMV
Thing is with paint, if you want to use it again, you have to treat it as nicely as possible. Materials separate, mold grows, solids harden… Any place that isn’t temp and humidity controled is going to ruin the paint eventually. Why take a chance? If you’re going to use it again, protect it. If not, dispose of it (as per local regs, of course).
Still, it’s just my opinion.
Also, even paints stored under ideal conditions fail. Whenever I finish and leave paint for the homeowner, I pour it in as small a container as possible. If I actually have a full(ish) gallon left, I will leave that, tightly sealed and labeled. If I have 3/4s of a gallon or less left, I put aside a qt for the homeowner, in a qt can and take the rest use on the next job as undercoat (if possible). Tightly sealed and labeled. The less air space in the container, the better the chances of having a workable paint sometime in the near future.
Only with latex paints, btw. I never leave any alkyds or lacquers on site.
It can’t ever reach freezing point or it’s shot. Metal can react badly to cement also. Put them on a couple of non metal spacers to keep them off the floor. Plastic comtainers can set directly on the cement.
Reality check-isn’t it customary to read the posts in a thread prior to posting?
Actually, it’s mostly in those plastic 5-gallon deals. So - would optimal storage be, say, taking a quart’s worth out of each, putting it in clean cans, and getting rid of the rest? I hadn’t thought about that. I guess if I needed more than a quart, I might as well get it matched and paint the whole room anew…
I could impose on my dad, I think, and put some stuff in his climate controlled garage, too. Might have to pursue that route. As it is, I think my dog keeps peeing on the cans. Which can’t be the optimal environmental conditions for them. 
The painter was supposed to come back and do a couple more touch ups and paint a vanity for me, but I haven’t heard from him and it’s been a few months.
Most paints saved by a home owner/do it yourselfer will go ‘west’ before they get around to another job they might use them on. Best to rid of them when the job is done and buy fresh new paint for the next job/project!
On one hand you’re right, but OTOH, having a can with mix formulation makes life much easier for the next painting person. Seriously, I’ve taken cans of paint with dried crud in them, peeled a piece out and my MAB paint guy can scan the color, and then tweak it if necessary to achieve a nice match.