I keep them in my basement which stays between 55-65 and I run a dehumidifier. Never had a problem with bugs or rodents. I periodically go down and eat a small handful to see if they going stale. I usually go through the malt faster then this but so far so good. I'm not really worried about it going bad.
According to site/stores that specialize in bulk foods and storage, that's a great comination to keep food fresh for a long time. I read on one of these sites that for every 6 degrees F you go above or below 70F, you double or half the shelf life of a product. For example, if you bought a big bag of grain that has a 6 months shelf life, it'll be 12 months at 63F, 24 months at 57F, but only 3 months at 76F.
Keeping out mosture (as you are), critters (again, not an issue for you) help a lot too, becuase these will result in poor food quality almost instantly. The last piece of the puzze is O2, which can be eliminated/reduced with O2 absorbers, or CO2 purging using your kegging equipment or by dropping a few cubes of dry ice into the container, letting it change to gas, purging/burping the container/bag and sealing it.
Neat thing about the CO2 purging is it will kill any critters & their eggs already in the grain (Weevels, I hate weevels!), and due to it's slightly heavy than air attributes, it will keep a high concentration in the stored grain when you carefully scoops some out.
Adding bags to your storage containers, whether they're the fancy mylar ones I mentioed earlier or just trash bags, helps alot too becuase it helps eliminate head space and reduce oxidation further.
I think if you make your bulk purchases so you'll never have more than 12-18 months of grain on hand, and develop a good rotation plan, you have nothing to worry about, unless your storage area is really warm and/or humid (though the humid part can be overcome with good sealing).
