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Hong KongInsulation, R-values and vapor retarders
R-value measures resistance to heat flow and U-factor is its reciprocal. A vapor retarder goes on the warm side of the insulation in a heating climate, and putting it on the wrong side causes condensation inside the assembly.
Two measures and one placement rule, and the placement rule is where buildings actually fail.
R-value and U-factor
R-value measures resistance to heat flow. Higher is better insulation.
U-factor is the reciprocal and measures transmission. Lower is better performance.
Windows are usually specified by U-factor, walls and roofs by R-value. A question offering both is checking whether you know which direction each runs.
Assembly R-value
Not just the insulation. Framing conducts heat far better than insulation does, so a wall with R-19 batts performs well below R-19 overall.
That is thermal bridging, and continuous exterior insulation exists to interrupt it.
The vapor retarder
Goes on the warm side of the insulation in a heating climate. That means the interior face in a cold climate.
Warm moist interior air passes into the assembly, reaches a cold surface and condenses. Water accumulates inside the wall where nobody sees it until the sheathing rots. In a cooling-dominated climate the logic reverses, which is why climate zone matters.
Air barriers are different
A vapor retarder controls diffusion. An air barrier controls bulk air movement, and air movement carries far more moisture than diffusion does.
A perfect vapor retarder with a leaky assembly still gets moisture inside it.
Compression
Compressed batt insulation loses R-value. Stuffing R-19 into a cavity sized for R-13 does not give you R-19.
Common questions
What is R-value?
Resistance to heat flow. Higher is better insulation.
What is U-factor?
The reciprocal of R-value, measuring transmission. Lower is better.
Where does a vapor retarder go?
On the warm side of the insulation, which is the interior face in a heating climate.
What happens if it is on the wrong side?
Moist air condenses inside the assembly and water accumulates where nobody sees it.
What is thermal bridging?
Heat traveling through framing, which conducts far better than insulation, reducing the assembly R-value below the batt rating.