[gtojohn]

With manual j there's no heat gain from underfloor crawl space. Heat loss for winter is there but still minimal compared to other losses. I've been experimenting with sealing up my crawlspace. This winter I blocked all the underfloor crawl vents with rigid insulation board. It seemed to help, my gas usage was lowest ever, but we also had a ridiculously mild winter. I'm keeping the crawlspace blocked off thinking it couldn't hurt unless it gets too moist under there. My house was built in '58, its taken some doing but not impossible to make efficient.