Live in Vegas, My music room is in a building with no temperature control, what can I do to keep my instruments from ruining in this hot, hot dry temp? Guitars are hanging on wall. Will a regular room humidifier work?
For wooden instruments humidity is much more important than temperature. If the humidity goes below about 30% then your guitars are liable to crack. You should invest in a hygrometer and a humidifier (get a good one, cheap ones are often useless). The ideal humidity range for your guitars is about 40-60%.
Not really relevant for Las Vegas: if the humidity goes too high (over 80%) then your guitars are liable to suffer from mildew and mould.
High heat can affect the glue on some guitars causing it to lose it's bonding ability to some degree and considering that a glued in neck is under tension from the strings this could possibly cause problems for the action and neck of a guitar. However, low humidity is usually the more common cause of problems in dry climates. Many dry climates have evaporative cooling systems that help create highly humid interiors in buildings, and these can help preserve the health of your instruments, but if your room is without any cooling system at all, you can find humidifiers at reasonable prices online that will keep the moisture level at a safe level for your instruments. Some humidifiers operate using evaporation principals and can cool the air slightly in the process. There are also portable evaporative room coolers that cool more but need ventilation to the outside to be effective. Used correctly, this kind of unit can keep your music room suitable to keep your guitars in. This is probably only necessary from May through September in Vegas.
Unhelpfully the answer will be, “it depends.”
I have a dilruba (an instrument generally used in Indian classical music) and it changes pitch in extreme heat but with some retuning and slight adjustments to the moveable frets plays fine. It is, however, made of much thicker wood than a guitar.
My classical guitar does not like heat - as others have set out any glue may become less solid which could damage the instrument. My son has an electric guitar with a predominantly ‘plastic’ body and a cheap wooden neck but it seems to remain in tune even in very low temperatures.
Best to avoid extreme temperatures (high or low) when storing or playing instruments though!