Hello, 

    First off all I have little to non experience in construction. I do live in Europe and we have full seasons so temperature is from +30c to below -20c.
    I’m thinking to renovate old house and start with floors. I want heated flooring, but there is small crawl space from 50cm to 20cm under current floor.
    So plans are to rip out old floors and fill crawl space with sand and put hot water heated floor at foundations level.
    My question is should I or how to insulate house foundation? Foundation is made of concrete with some rubble inside. As there is already crawl space inside would be simple to insulate from inside.

    Attached a simplistic drawing for reference of current situation.

Thank you for your time.