What is the difference between conductive and convective heat loss and does this play a role in the effectiveness of R-value?
R-value is the measure of resistance to heat flow. The higher the R-value, the better the resistance to heat flow. This measure is considered conductive heat transfer. However, this is only one part of the whole story that is insulation effectiveness.
More heat is lost out of a building from air leakage than due to heat transfer through a material. This is called convective heat loss and it accounts for as much as 2/3 of the total heat loss in a structure. Examples of contributors to a leaky building envelope are poorly sealed drafts on doors and windows, air leakage around door and window rough openings, poorly sealed vapor and or air barriers, and insulation that does little to stop the flow of air.
R-value does not take air leakage into consideration. It is true that a high R-value means better resistance to heat flow, but it does not mean better resistance to air flow. A true air barrier coupled with high R-value s the key to an excellent thermal insulation.
Spray Foam insulation comes in many species. There are Closed Cell foams, Open Cell foams, and a variety of densities in between for different applications and uses. The R-value of foam can range from 3.5 - 7 per inch thickness and higher still for industrial grade foams. While these numbers are typically higher than fiberglass, rockwool or cellulose insulation, the main benefit is that foam is a seamless air barrier.
By sealing a building with foam, it essentially stops the leaking heat. By reducing convective heat loss, the true R-value of the spray applied foam insulation can work to cut energy costs by as much as 50 percent over traditional insulation.
A great resource for all things energy efficiency, including tips and tricks for DIYers is http://www.greenbuildingadvisor.com/
Good luck on your projects and stay warm friends!