The rules on vapor retarders differ by climate zone — and getting it wrong can cause moisture damage. Here's what you actually need.
A vapor barrier (more precisely, a vapor retarder) slows moisture migration through the building assembly. It doesn't stop bulk water, air movement, or condensation — those require air barriers and waterproofing. Understanding the difference prevents the most common and expensive insulation mistakes.
| Class | Permeance | Examples | When to Use |
|---|---|---|---|
| Class I (vapor barrier) | < 0.1 perms | Polyethylene sheeting, foil | Cold climates, below-grade concrete, under slab |
| Class II (vapor retarder) | 0.1–1.0 perms | Kraft-faced batts, vapor-retarder paint | Exterior walls in zones 5–8 |
| Class III (vapor permeable) | 1–10 perms | Latex paint, housewrap | Mixed climates, hot-humid zones (inward drying) |
The rule: vapor retarder goes on the warm-in-winter side of the insulation. In cold climates, that's the interior. In hot-humid climates, vapor drive can reverse seasonally — a Class I barrier on the interior can trap moisture. This is why "always use a vapor barrier" is wrong advice.
| Climate | Zones | Recommended Approach |
|---|---|---|
| Cold | 5–8 | Class II vapor retarder (kraft-faced batts) on interior — warm side |
| Mixed (Marine/Continental) | 3C, 4 | Class III (latex paint) is typically sufficient |
| Hot-Dry | 2B, 3B | No interior vapor retarder — allow inward drying |
| Hot-Humid | 1A, 2A, 3A | No interior vapor retarder — unfaced batts preferred; exterior vapor control instead |
Kraft-faced fiberglass batts have a Class II vapor retarder (the brown paper) integral to the batt — no separate sheet needed. They're the standard for exterior walls in zones 4–8. Unfaced batts are appropriate for interior partitions, hot-humid zone exterior walls, and anywhere the code doesn't require a vapor retarder on the insulation itself. All the fiberglass products we stock are available in both faced and unfaced versions — specify when you call.
Concrete foundation walls and slabs are inherently vapor-open — ground moisture migrates through concrete constantly. Below-grade applications require either:
In basements, never place kraft-faced batts against a concrete wall with the kraft touching the concrete — the paper can trap moisture and grow mold.
Call or text — same-day response, next-day delivery on stocked products.
Leave your number and what you need — we text or call you back with a quote the same day.
Or reach us directly: (929) 466-1426 · Text us