http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/double-stud-wall-construction-path-efficiency-budget
Use spray foam in that double wall and even sound is muffled. I was surprised by a simple 6” wall with the stuff deadening street noise such that not even construction noise across the street was heard.
Instead of double stud Walls why wouldn’t you just spray foam?
Six inches of foam is R50. This is more than enough R value in a sidewall of any house in the lower 48. Heat rises, where most houses lose heat is through their ceiling.
Also, spray foam eliminates the need for a vapor barrier. The least expensive way to get at least R fifty in your attic is blowing in bonded cellulose about twelve inches minimum. Your other main losses of heat are through your doors and windows. Lastly, NEVER, EVER, EVER install a skylight in your roof. However, if you do super insulate your house you better install an air to air heat exchanger.
The problem with ninety percent of houses are that they do not have enough insulation. Heat loss causes problems like ice damming, leaks, etc. If you have a super insulated house you could practically heat it with incandescent bulbs.