Heat does not really "ruin" whisky directly, the way sunlight will breakdown a bottles flavors into blandness.
However, temperature swings can do one very damaging thing to a bottle of older whisky, and is the cause
of most evaporation....
As the room temurature rises, the contents of the bottle will swell and expand, forcing air out of any cap
or cork that is not 100% air-tight...hence the "angels share" that is missing from many an older bottle. By
itself, evaporation alone does a small amount of harm: Usually lowering the proof of your favorite swig
ever so slightly. (Alcohol itself will evaporate before the the water-content does.)
The big concern is in the temperature drop that happens afterwards - As the contents start to cool, they
will shrink in size and thus cause a small "vacuum" effect with in the bottle. To equalize the air pressure
inside the bottle, new air enters by the same methods in which the angels confiscated their share.
***Doesn't sound like much at first though...but what is not always initiall realixed is that the "new air" is
contaminated with particles of the dust/mold/bacteria that we breath every day: Unfortunately, even
though it may be high proofed alcohol, it doesn't have an immune system of it's own...and what happens
over time, is faster evaporation effect from the bateria eating away the remaining sugars, as well as a
thickening (almost to the point of syrup) of the contents.
Best rule of thumb for any old "unopened" bottle, is the "Neck-Check":
1) If the contents are below the neck, expect a bit of change in taste - often 'rusty/metallic' to the tongue .
(This is usually just oxidation, and a bit of airation can fix most of that),
2) If it's below the shoulder approach with considerable caution - Likely spoilage has already taken effect.
Cheers, and healthy tippling !!!
Walter C Hurst