Gunpowder is a high energy molecule and it will degrade to become a low energy molecule.
From
http://www.dtic.mil/dtic/tr/fulltext/u2/783499.pdf
Quote:
Nitrocellulose-base propellants are essentially unstable materials
that decompose on aging with the evolution of oxides of nitrogen. The
decomposition is autocatalytic and can lead to failure of the ammunition
or disastrous explosions. Many substances have been used to stabilize
nitrocellulose-base propellants,but by far the most widely used substance
is diphenylamine (DPA).
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You can go to
www.dtic.mil and search under insensitive munitions, stabilizer, and diphenylamine and find all sorts of information on gunpowders and aging.
Heat is the greatest enemy of gunpowder as it increases the rate of deterioration. The military uses a fume test, they stick gunpowder in a 150 F oven and if it fumes within 30 days it is tested for stabilizer content. If the content of stabilizer is less than 20% the powder is scrapped.
Humidity is bad. Water is a polar molecule and it will interact in a negative way with gunpowder. As water lands and evaporates off gunpowder it hastens the breakdown of the double bonds on the nitrocellulose molecule. It also wicks nitroglycerin to the surface, changing the burn rate of double based powders. The military does not store ammunition in airconditioned buildings, it costs to much. Bunkers are cool, because of all the dirt on top, but they are dank. After decades moisture will become a problem. Combined with heat, that is one reason ammunition dumps go poof all the time.
Incidentally all ionic compounds degrade gunpowder. Rust from those old metal cans hastens the deterioration.
A rule of thumb for gunpowder safe storage is 45 years for single based 20 years for double based. Keeping the stuff cool and dry is better than hot and humid.
Your interior temperatures sound good, just keep the lids on.