Under Michigan's Energy Code, what is the minimum R-value requirement for wall insulation in Climate Zone 6A?
Correct Answer
C) R-20
Michigan Energy Code requires R-20 wall insulation for Climate Zone 6A, which covers most of Michigan.
Why This Is the Correct Answer
R-20 is the minimum R-value for wall insulation in Climate Zone 6A under the Michigan Energy Code (IECC). This is typically achieved with R-20 full-cavity insulation or the equivalent R-13+5ci (continuous insulation) combination. Michigan adopted these values to meet energy efficiency standards for its cold northern climate.
Why the Other Options Are Wrong
Option A: R-13
R-13 is the minimum wall insulation for Climate Zone 3 and portions of Zone 4. In Michigan's Zone 6A, R-13 alone is insufficient and would not satisfy the energy code. It is a common distractor because R-13 is a well-known standard for 2×4 stud walls.
Option B: R-21
R-21 is associated with fully-packed 2×6 fiberglass batt insulation and meets or exceeds R-20, but it is not the stated code minimum. The code specifies R-20 as the threshold; R-21 would comply, but the question asks for the minimum required value.
Option D: R-15
R-15 is the minimum for Climate Zone 5 (a slight upgrade from R-13) but falls short of the R-20 requirement for Zone 6A. Michigan's colder climate demands a higher thermal barrier than Zone 5 states.
Memory Technique
For Zone 6A Michigan: 'Ceiling 49, Wall 20.' Think of it as a temperature scale — the ceiling is coldest (highest R), walls are in the middle (R-20), floors over unconditioned space are lower (R-30). Pair '6A → 20 → 49' as a sequence.
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