I'm not a fan of tar paper cover on exterior walls. Problem, it does not allow moisture to escape from between the walls. This is where 'house wrap' is much better. However, that said, 2x4 walls do not allow much insulation. Don't know your climate but would suggest sprayed in foam insulation. Yes, it is more expensive, but does a much better job of sealing air leaks and does not support mold growth if it should get damp.

The sprayed-in type is wet going on. In Houston, where everyone is nuts over mold, there has been a lot of resistance to it. And I understand the R-value is only 1/2R more than batts. Does that sound right?

Thanks.

Look up Icygene or something like that on the web.

It is sprayed on but it expands immediately. It is great stuff. And has much better R-value than fiberglass for same thickness.

Try: http://www.icynene.com/ While it does go in 'wet', it does not support the growth of mold.