Pam. Lots of questions. To rip up the sub floor would be a lot of work, but maybe the only way to do it. Have you looked at the crawlspace to determine how high it is? If it's too low for someone to get under, you may have issues with termites because the wood is too close to the earth. If there's not good access from the outside, sometimes after a section of subfloor is removed, someone can get under an install the insulation. just remeber the vapor barrier goes against the heated side of the flooring. Good luck! I would go with vented gas logs if you have a choice, but that's just my opinion. A chimney sweep can evaluate your chimney and give you an idea what would work the best.