Depending upon how big your cave is you might want to go with a spray foam insulation company. If you are doing a cave of any size its the only way to go. We have a 50 foot cave and have had it sprayed and re-sprayed several times. Just make sure that you get the foam that cures with a hard shell verses some of the foam that stays soft. The hard shell will help protect it and makes it feel like a cave. The fist time we sprayed we were only 25 feet long and we spent about $2200 bucks spraying it. If we had used the foampacks we would have been well over that. I do use the foam packs now just to fill in cracks and that type of stuff. Once sprayed and painted it looks cool!