Thanks Jeremy, but I think my question is different than how you may have interpreted it. I'm wondering why does the article
recommend changing both the U-EFFECTIVE and the ficticious material.

The article says in one place that the heat transfer, Q = [U-EFFECTIVE]*A(Tg - Ti). Then, it later says, as you quoted, "The
fictitious insulating layer is required to give the correct effective resistance for the construction..."

So which is it??? Is there some sort of heat balance that needs to happen with the two? Does the ficticious layer somehow
influence the weighting factors even though it has no mass???

-Eric