Depending upon how cold it gets where you are, the main problem with batteries is that when they are low or discharged, the electrolyte is closer to pure water and it can freeze, bursting the case. The electrolyte in charged batteries freezes at a lower temperature relatively. If the batteries are being used daily, an insulated battery box is likely all that is needed in most locations. If you put them in your attic, remember that a large battery bank can be of significant total weight, and the structure will have to be able to support that weight for the long haul.
I have put a small battery supported in the rafters of a garage attached to a 20 watt panel and a small charge controller for a light. It has been there for a number of years now with no heat, and no problems. I would worry a bit about electrolyte spilling and damaging anything (wood or otherwise) below and nearby, espesially in a worst case scenario of a ruptured case. If you put the batteries in sturdy plastic tubs, or an old cooler that might help mitigate that risk.
Lastly, the batteries during heavy use or overcharging conditions can give off a bit of hydrogen gas do to electrolysis of the electrolyte. Hydrogen gas mixed with air in sufficient quantities is explosive when ignited. Now in a large attic with a single battery and vents this is probably not an issue. If the space is confined, smaller, and not ventilated, then ventilation would likely be in order. Hydrogen gas rises relative to the air. A vent either at the top of the attic (most are already ventilated to prevent moisture build up and remove heat in the summer) would likely be sufficient. Alternatively, if the batteries are in a container, and a vent tube or something similar to collect the gas from the inside top of the container and leading upward, away and outside could be utilized to vent the gas outside.
You can search this site and see some examples of battery boxes that others have built.
Make your project work for you, and have fun, Rich Hagen
