Evening all. Planning on doing some over night camping soon with friends but my only experience of winter camping is field exercises in bivvi bags. Im wondering how sleeping in tents effect temperature ratings of sleeping bags (while on inflatable roll mats), can they increase the ratings? Its was -18 recently in the highlands so planning on that as an extreme even with the incoming "warmer" weather, so not sure if i could push it using my Softie 9 or best go with the bulky issue bag. Thanks and merry Christmas.