A: R-value is resistance to heat flow.  The higher the R-value, the more insulating the material is.  These R-values assume that there is no air moving through the materials.  These are average values, and may vary depending on brand, thickness installed, installation quality, etc.

Gosh darn yes!   Insulation that is in batts or sheets can have voids, gaps, cracks and other spots where the insulation is not completely covering the area.  Even small voids can have dramatic impact on its R-value.  The new "Field Guide- Residential New Construction" for New Hampshire has an illustrative graphic showing effective R-values of R-38 fiberglass batt insulation in an attic.

In other words, nominal R-38 insulation in attic with a 3% void area provides an effective R-20 insulation for the entire area.  A 6% void area would result in an effective R-15, a whopping 60% reduction in insulating value!  And we at Building Energy Technologies, LLC have seen a lot of attics with 5+% voids, particularly after electrical work.  (Hello electricians are you hearing us?!)

Why is this?  Humans tend to think we can average out R-values, but physics tells us differently.  The trick is to first convert the R-values of each component to U-values (inverse of each other), do the math for the proportional areas, and finally convert the resulting number back to an R-value.

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