Any time a hydro-carbon fuel is burned, a main byproduct is steam. You really do need to exhaust it outside your garage. As for covers, please do not use them unless they breath. As for helping with moisture they really won't help. To do any good there you must remove the water from the air inside your garage. One good think about running a dehumidifier and especially the special "Low Temp" models is that they help heat the air some. Try putting a thermometer by the discharge of one of them and let it run about 15 minutes and then read it. As for really running up your electric bill, unless you have a super powerful one or several running non stop, it really shouldn't cost all that much. Any method used to heat your garage or such will cost you some $ for the needed energy. The lowest cost, but also the most dirty and smokey would be to use scrap wood and/or paper trash as your fuel source. That's providing you are allowed to burn such as a heating fuel in your area. Please note that moisture is doing harm to the internals of your machinery. You might do yourself a big favor to look over the web sites for CRC and LPS chemicals.