Will it harm a baby grand piano (other than simply going out of tune) to store it in our garage for a couple months while I try to sell my other piano to make space for the new one? I'm in Tennessee and currently the humidity is quite high.

No.

It depends of the humidity level, as strings are not nickel plated and wood is like a sponge.

Possibly then enclosing the piano in a film and using one or 2 bags of sillicate would be cautious.

Small grand pianos are less prone to get soundboard cracks but high moisture cannot be good for the action, soundboard and strings

pure wool blanket as an humidity barrier, plus film, plus sillicate bags if available. Why would you want you piano to suffer when it can be avoid ?
It also depends of the quality of the action indeed.


PS while in the garage, ask the mechanics to change the oil and verify the brakes

Pianos in garages and screen porches here often end up with rusty strings. I would keep the garage door shut and seal the piano with plastic as much as you can.

I would try to find another place to store it. Or you could put the vertical in the garage.

Most people that I know of start out with the intention of storing for a brief period and often wind up storing it for longer periods of time. Garages are not my first choice for storing something as nice as a piano... Storing a piano with no humidity control is not a good thing either...

If the garage is climate controlled, then it should be ok. If it isn't, you're taking a gamble. Why not store it on its side in your house? That would be ideal.

The wooden parts are gaining moisture and it's will warp. Metal parts are rusting. A piano slowly dies

Every word of caution here is spot on. Especially in a humid climate, you are risking rust and excessive moisture absorption by the wood.

I second B986; store it in your house.

The title of your post says "storing piano vertically" so I'm assuming the piano is going to be on its side. Here are my recommendations:

Storing a grand on its side for an extended period can lead to gravity causing hammers to drift out of alignment. Have a technician secure the hammers before storing the piano on its side.

Have a qualified technician install a dampp-chaser system: 2 dehumidifier bars and a humidistat - leave out the humidifier part.

Cover the piano well with blankets. I wouldn't completely covering the piano with plastic film as it would be better for the piano to breath. Mildew is more likely to develop with stagnant air. The dampp-chaser will keep air dry and moving, and the blankets will give it somewhere to go.

Ryan, I agree a little about the piano "breathing" but for a few months, an enclosing together with moisture adbsorbing bags will be secure, in my opinion.
Warming with 2 dampp chaser will not suffice if the conditions are very bad. no correction of the air in cavity, for instance, passive systems as sillicate cristals are very efficient if there are enough of them (they can be find as an absorbant material for pets "facilities", if you cannot find sealed bags ready to use)

I heard of a small Pleyel grand (mod F 1.64m , from the 1930 era) that was enclosed for 40 years in a silk cloth plus wool cover,and plastic (or rubber impregnated cloth) film all around, and from what was said to me the piano was like new (it was a piano baught for a boy who deceased shortly after, hence the packing of the piano that stay unused until sold.

Yes it is better to secure the hammer shanks, but mostly if the piano will be in a truck on the road;
WIth along period on its side eventually the cloth at the left of the action will compress a little so the action position can be checked once the piano on its legs, but, after a few months I would be ready to find 0.2 mm compression, not much, that cloth is yet compressed by the action return spring and is very firm usually.

of course verify the flange screws tightening is good (at any moment)
with some extra moisture they will be more tight, if the moisture is high the wood will compress and when back to normal conditions the screw will need some tightening.

I think that the piano must to breath

I believe there is a chance that the piano may turn into a car if stored in the garage.

I live in TN also,and as a tuner technician, I say no. Unless the garage is climate controlled.
I have recently tuned and repairred several pianos that had been stored in garages and they were very "sick". When a piano is stored, like Jerry says, it is usually longer than anticipated and can harm the piano. Rusty strings, moldy parts, dust, mice, rats, and any other kind of vermin that we have in TN. My suggestion would be somewhere on its side in the house if possible.
Another thought would be a climate controlled storage unit, even though it may seem expensive, it could save you money in the long run.

Jay's Piano Tuning Service

www.jays-piano-tuning.com

"...Rusty strings, moldy parts, dust, mice, rats, and any other kind of vermin that we have in TN..."

Don't forget roaches. Then there are squirrels, which will be overwintering soon. Or you could open it up to find you have an ant farm, a hornet nest, or a beehive. Try and find a tech who will work on that. And I won't even mention the effect on your nice piano of fumes from automotive products, old cans of paint, weed killer, gas leaked from the lawnmower, dripping rain. If it will take the polish off your wife's nails, it will do the same for your grand's finish.

It is a poor idea to put a piano in a garage; I have seen the inside of garages. You are kidding yourself, and no one else, if you think this is a remotely good place to put a piano. Get rid of something else, if you're so short on room. The TV, the Lazy-Boy, the couch--- out, out, out--- all breeders of future heart attacks. You will thank yourself later, when you're visiting your high school buddies, who are in the nursing home, and you aren't. If you can't part with these treasures, stow them in the garage--- make yourself a man cave, like the rest of us.

Something tells me you will not listen to this good advice, but I will try one last time. Rent a climate-controlled storage for the grand, telling yourself that it will only be for a month or two. After the rent eats up whatever you sold your old upright for, that will be plenty of time to change your mind.

You won't hurt it; you'll ruin it.

Probably your piano will not be hurt --particularly if it is during the winter months. Without protection like a Dampp Chaser system, it is far harder on pianos to be inside than out. (The assumption, is that you have a space that is truly sealed to the elements and vermin, such as the typical attached 2-3 car garage with concrete floor and functioning doors.)

BTW I'm not talking theory, I have serviced pianos that have been literally outside as well as in unheated spaces.

I would not use plastic as that doesn't function as people think. Moisture can get through anything and then it remains trapped. (i.e. If it is 100% humidity in an environment and you seal something in plastic, eventually it will be 100% there, too). Rather, use blankets and then canvas, or perhaps a plastic tarp that is loosely draped over the blanketed piano. I'd use wool or synthetic blankets, not cotton-filled movers pads.

You'll get lots of well-meaning advice to the contrary from "knowledge" that has apparent validity because it has been oft repeated. That doesn't make it true. There can be many factors, so it depends. As has been mentioned, squirrels can make a mess that mice can't even dream of.

Under no circumstances should your piano be where there is liquid water or salt.

I never did it. I think as need to do so. A piano need conserve. Piano or parts of it that can be removed should be covered with plastic 3-5 mm thick film wrapped container. Should get a tight bag with solder ends Then use a home vacuum pump to pump the air. In the vacuum of our piano will not be subject to the negative impact

This is often the best recommendation. Have professional PIANO MOVERS wrap it with moving pads, strap it to a grand piano skid and store flat against a wall in your home. They'll remove the legs and lyre. Store these in separate bags or wrappings in an indoor closet. It won't take up much room at all. [Be sure it is not stored over or under heating or air registers, air return, or in direct sunlight.]

Hmmmm. If it will turn into a BMW or some such auto, I'd be willing to give it a try. I just hope a title comes with it.

Store your piano in a friend's house. I got my start in piano when I was little and our neighbors didn't have anywhere to put their unused huge upgright piano so they let me use it for over fifteen years. They did take it back when we moved because they considered it a family heirloom. We maintained it so it was in better condition when it was returned. Why let a piano site for several years when someone who doesn't have access to a piano could be using it?