Chris, here is an excerpt from another great article from Allison Bailes III.
"Now, we can't just average the R-values. If we did that here, we'd get R-30, and we'd be wrong. ***Heat will take the path of least resistance, and the less resistance you give it, the more heat will flow***. If you've studied physics, engineering, or building science, you've probably seen the equation for heat flow by conduction: Q = UA ΔT = 1/R ∙ A ΔT"
Read more: http://www.greenbuildingadvisor.com/blogs/dept/building-science/there-downside-lumpy-attic-insulation#ixzz4n3BpLIjx
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I did some research and found this article particularly helpful:
To answer your question again... suppose you had a wall that consisted of a 2" inner layer of [new]XPS foam and an outer layer of 5" of Type II EPS foam. The inner layer would be R-10 and the outer layer would be R20 (assuming its about 65 degrees...). I 'think' this wall would have an average R value of R15.
Things change though when we introduce studs! Because a stud extends from outside to inside, warm indoor air now has a choice how to get outside to the colder air. It can travel through the insulation or it can travel through the stud. The XPS/EPS wall above doesn't encounter this problem because the indoor heat encounters only one layer at a time any where on the interior side of the wall.
This is why you have to convert the R Value to the U factor. R values deal with uniform, singular layers. It was designed to provide ratings for insulation materials to keep insulation companies honest. U factor is more helpful as it deals with whole wall assemblies where thermal bridging and multi-dimensional heat flow may exist. Part of this may be because U factor deals with more than just conductivity like R value. It takes into consideration radiation and convection as well (I think). This is why windows use U-factors rather than R value as they are complex enclosures all within themselves.
You can see why a lot of people want the R value metric to go away altogether as it isn't too helpful in measuring heat flow through an entire assembly.
Check out the article I posted. So much wisdom at Greenbuildingadvisor.com
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