Cob sledgehammers and heat retention

[GENTH, APRIL]

Help, Help

        Does anyone know how much damage can be done to a cob wall of
16-18inches with a car going 25mph -the same damage done to a brick walled
house?  I ask because someone said cob has low resistance to impact.  I
wondered how little resistance compared to the modern home's wall (outside
wall).
        As well, do you know if a cob home would be energy efficient in
heating costs in a region where winters are cold with a mean temp. of 22
degrees F in January- where it's not very sunny?
        What kind of floor and ceiling insulation would you use to increase
heat retention?
        What kind of roof system would trap heat best?

        I'm trying to prove, in a paper, that a cob home would save the
owner money in heating costs in temperate cold, cloudy winter regions.  If
there are limitations like low r-values, then what are the low-cost remedies
I can offer?  
sorry for asking so much.

Desperately,
        April