First of all, let's talk about energy use and sustainability. One of the best ways to be "green" is to have a super tight "building envelope" (the line between inside and outside) and therefore minimize the resources needed to heat and cool your house. Insulation -or R-value- is not the whole story; air sealing is actually even more important.
|Spray Foam @ Crawlspace Box Beam|
The energy code allows designers a few different methods for achieving the minimum requirements. Because this is a small house, (Sizing a house appropriately/modestly is the single-most important way to build green) I could use the prescriptive method. I followed Energy Star Standards, meaning the walls have that continuous rigid insulation (CI) everywhere, in addition to the cavity insulation between studs. And because we have the high efficiency CI on the walls, I used plain, old (inexpensive) fiberglass batts in the wall cavities. Then where it really counts, I specified spray foam (a more expensive insulating material, because it is also an excellent air-sealer) for the roof and at each of the floor lines (called the "box beam"). And the roof trusses we designed/installed to allow for full depth insulation over top of the walls- read more about this at the post about the trusses: http://cwb-architect.blogspot.com/2014/01/roofing-and-snow.html
|Spray Foam @ Underside of Roof|
The last item to mention is the location of the building envelope. Many existing houses have insulation in the ceiling of the top floor and between the floor joists of the bottom floor. This was common practice, at least here in the NE... But building science has taught us that rather than having an un-insulated, un-conditioned attic and basement, it is more efficient to condition those spaces and have the exterior surfaces (roof plane, foundation wall) create the building envelope. Different... If you want to learn more, check out: http://www.buildingscience.com/index_html. They have an amazing amount of information.