R-Values.  Exponential going the wrong direction.  After R-15 or so you slip into the long tail.<p>Good article here-  <a href="http:&#x2F;&#x2F;www.energyvanguard.com&#x2F;blog-building-science-HERS-BPI&#x2F;bid&#x2F;76941&#x2F;The-Diminishing-Returns-of-Adding-More-Insulation" rel="nofollow">http:&#x2F;&#x2F;www.energyvanguard.com&#x2F;blog-building-science-HERS-BPI...</a><p>PH is performance based, but the religion is a bit excessive on R.  I do greatly value the air tightness and the quantitative approach.<p>High R-Value walls are tough to build and get expensive quick. Double wall, exterior sheathing, more parts, more fasteners, etc.  We use SIPS walls&#x2F;roof that have very good R, very little thermal bridging and outstanding air tightness while being fast to build.