What vapor retarder means
A vapor retarder is a material that slows water vapor moving through a wall, ceiling or floor. Plastic sheeting, kraft facing and some paints are vapor retarders of different strengths.
Also called vapor barrier.
Where it matters on a insulation job
The wrong vapor retarder in the wrong place traps moisture in the wall. Which class, if any, belongs on the inside depends on the climate zone, so check the local code table before you hang plastic.
What the model code says
- Vapor retarders are classed by perm rating: Class I at 0.1 perm or less, Class II above 0.1 to 1.0 perm, Class III above 1.0 to 10 perm. (IRC 2021, Chapter 2)
Check which code edition your building department enforces for vapor retarder before you quote.
What it costs
Koira's attic insulation cost guide puts a typical job at $2,500, with most between $1,700 to $3,250. The guide shows its sources and what moves the price. The insulation cost calculator prices a job line by line.
Work it out
- Free calculator: Insulation cost calculator
- Cost guide: Attic insulation cost
- Tools, templates and guides: Free insulation resources
Sources: 2021 International Residential Code, Chapter 2, Definitions (ICC). Checked October 9, 2026.