Vapor Barrier Calculator
Estimate poly sheeting, rolls, and seam tape
Area to be covered by the vapor barrier
Seam overlap between sheets
Width of one poly roll
Length of one poly roll
Rolls Needed
—
rolls
Tape
—
ft
Poly Sheets
—
sqft
Effective Roll Coverage
—
sqft
Disclaimer: Results are estimates based on standard formulas. Not a substitute for professional advice from a licensed contractor, engineer, or architect. Verify all calculations before making purchasing or construction decisions.
How to Use This Calculator
Vapor barriers need overlap at the seams and a waste allowance to cover the full floor. This tool sizes your poly rolls and seam tape in one pass.
- Measure the floor or crawlspace area in square feet
- Enter the seam overlap (6 inches is standard)
- Set the roll width and length from the product label
- Seal every seam and penetration with vapor-barrier tape
Expert Insights
Smart analysis for your Insulation project
Overlap Efficiency
A 6% overlap loss is typical for standard 6-inch seams. AI deducts overlap from usable roll coverage so you order the right number of rolls.
Waste Allowance
Adding 10% to the floor area covers cuts, seams, and wall wrap-up at the perimeter without leaving you short.
Seam Tape Sizing
Plan roughly 1 linear foot of seam tape per 10 sq ft of barrier. AI sizes tape for every seam, penetration, and edge seal.
Calculation Formula
Effective Roll = Roll Width x Roll Length x (1 - 0.06)
Needed = Floor Area x 1.1
Rolls = ceil(Needed / Effective Roll)
Tape = ceil(Needed / 10)
Poly Sheets = ceil(Needed)
Vapor Barrier Material Comparison
| Material Type | Thickness (mil) | Permeability | Cost per Roll | Best Use |
|---|---|---|---|---|
| Polyethylene (Standard) | 6 | 0.06 perm | $40-$70 | Crawlspaces, floors |
| Reinforced Poly | 10-15 | 0.04 perm | $80-$150 | High-moisture areas |
| Cross-Laminated | 10-20 | 0.02 perm | $100-$200 | Below-grade walls |
| Faced Insulation | N/A (kraft) | 1.0 perm | Included | Wall cavities |
Authority References
Our calculations are based on guidelines and standards from these authoritative sources:
- American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) — Moisture Control & Vapor Retarder Handbook. Learn more
- International Residential Code (IRC) — Chapter 7 — Vapor Barriers. Learn more
- U.S. Department of Energy (DOE) — Vapor Barriers & Moisture Control. Learn more
Frequently Asked Questions
What mil thickness should a vapor barrier be?
Use 6 mil polyethylene as a minimum for crawlspaces and floors. For high-moisture areas or long-term durability, 10 to 15 mil reinforced poly is recommended.
How much should vapor barrier sheets overlap?
Overlap seams by at least 6 inches and seal them with vapor-barrier tape. At penetrations and edges, wrap the poly up the wall 6-12 inches and seal to the foundation.
Where should a vapor barrier be installed?
In cold climates, install vapor barriers on the warm (interior) side of walls and ceilings. In hot, humid climates, exterior vapor retarders are often preferred to avoid trapping moisture.