Insulation glossary

R-value is thermal resistance. It is quoted per inch for loose materials and as a total for a finished layer. Fiberglass runs about 2.5-3.5 per inch, cellulose about 3.2-3.7, mineral wool about 3.0-3.7, open-cell spray foam about 3.8, and closed-cell about 7, per Fixr. Multiply per-inch value by depth to get the total. U-value (or U-factor) is roughly 1 divided by R and describes the whole assembly, which is why windows are rated in U and insulation in R.

Thermal bridging is heat taking a shortcut through a conductive element - a wood stud, a steel beam, a concrete slab edge - that bypasses the insulation around it. It is why a wall with R-13 batts does not perform like a solid R-13 wall, and why continuous exterior sheathing insulation is valuable.

Air barrier is the continuous layer that stops air moving through the building envelope. It is a different job from insulation, which is why sealing and insulating are two tasks. Vapor retarder slows water vapor diffusing through an assembly. Which side of the wall it belongs on, or whether you should have one at all, depends on your climate - getting this backwards traps moisture inside the wall.

ACH50 is air changes per hour at 50 pascals of pressure, the number a blower-door test produces. It quantifies how leaky a house is. It is the single most useful diagnostic figure in this industry because it turns "the house feels drafty" into something measurable before and after the work.

How to choose

Ask for total R-value, not material R-value

Per-inch figures only become meaningful once you know the installed depth. The contract should state both.

Ask about thermal bridging in wall quotes

Cavity insulation alone leaves the studs as a heat path. A contractor who raises this unprompted is worth listening to.

Confirm the vapor retarder strategy

Climate determines which side it goes on, or whether you need one. This is the detail most likely to cause hidden moisture damage.

Get an ACH50 number before and after

If you are paying for air sealing, a blower-door test at both ends is how you know you got it.