Crawlspace Vapor Barrier Calculator
Floor area plus laps to poly rolls and mil advice.
What it does
Damp soil breathes moisture into the house all year. A sealed ground sheet breaks that path, holding crawlspace humidity down and joists drier. Area math plus overlap sets the order.
The tool above multiplies floor area by an overlap share, then rounds up to 1000 sq ft or 100 m2 rolls. It also flags whether the chosen mil rating meets the usual 6-mil minimum. Pick thickness for traffic, not price.
DOE weatherization and moisture guidance pairs ground covers with drainage and drying, and EPA moisture-control pages press the same sequence: bulk water first, then vapor. Six-mil poly lapped a foot and sealed is the common ground-cover floor both point to.
Order the poly, then detail it. Run sheets up the walls, tape every lap, seal piers and penetrations, and keep standing water off the top with outside grading and, where needed, a sump and dehumidifier.
How to use
- Pick imperial (ft, sq ft) or metric (m, m2).
- Measure floor length and width.
- Set overlap percent, 12 % for lapped seams.
- Pick poly thickness in mil.
- Read material, roll count, and thickness note.
- Lap 12 in, seal seams, and run up walls.
Formula
- L
- Crawlspace floor length in ft or m.
- W
- Floor width in ft or m.
- overlap %
- Extra share for laps and waste.
- mil
- Poly thickness: 6, 10, or 20 mil.
Rolls assume 1000 sq ft (imperial) or 100 m2 (metric). Overlap covers 6 to 12 in laps plus cutting waste around piers. Thickness guidance follows the common 6-mil minimum; traffic and storage argue for 10 to 20 mil.
Worked example
Example: 30 by 40 ft floor
30 by 40 ft crawlspace, 12 % overlap, 6 mil poly.
- Floor is 30 x 40 ft = 1,200 sq ft.
- Overlap 12 %: 1200 x 1.12 = 1,344 sq ft.
- Rolls: 1344 / 1000 = 1.34, round up to 2.
- Mil: 6 meets the usual minimum.
- Lap seams 12 in, seal, and run up the walls.
Reference table
| Mil | Use | Note |
|---|---|---|
| 6 mil | Minimum ground cover | Lap and seal carefully |
| 10 mil | Traffic and storage | Puncture resistant |
| 20 mil | Heavy duty | Rocky or commercial |
| Woven corded | High traffic | Staked and taped |
| Pipe boots | Penetrations | Seal every pier |
Limits
- Ground cover only. Walls, vents, and drainage need their own plan.
- Closed crawlspaces need conditioning and code review.
- Punctures and open laps void the math.
- Bulk water beats poly; fix grading first.
Common mistakes
- Butting seams with no lap.
- Skipping tape on laps and piers.
- Buying exact area with zero waste.
- Laying thin film over sharp rock.
- Sealing vents without conditioning the space.
Try it yourself
Quick checks. Answers sit tucked under each question.
Poly for 20 by 25 ft at 10 % with 10 mil?
Material is 500 x 1.10 = 550 sq ft. That is 1 roll. 10 mil exceeds the minimum.
Poly for 6 by 8 m at 10 % with 10 mil?
Material is 48 x 1.10 = 52.8 m2. That is 1 roll. 10 mil exceeds the minimum.
Questions
What does the ground sheet actually do?
A ground sheet that slows vapor rising into the house and joists. Poly keeps the soil's moisture out of the air, which cuts mold risk, musty smell, and cupped floors above.
How thick should the poly be?
At least 6 mil polyethylene for ground cover in most guidance, lapped and sealed. Thicker 10 to 20 mil suits traffic, storage use, and rocky soil that punctures thin film.
How much do seams overlap?
About 6 to 12 inches of overlap per seam, taped or mastic-sealed, plus extra up the foundation walls. The overlap percent here buys that lap plus cutting waste.
Does a wet crawlspace need more than poly?
Yes, where the water table is high or walls weep. Pair the sheet with drainage, a sump, and a dehumidifier sized to the space. Poly alone cannot pump.
Should I seal the vents too?
Only where codes and moisture reports support it. Closed crawlspaces need sealed vents, conditioned air or dehumidification, and pest and combustion-air review. Get local sign-off first.
How do I know the barrier still works?
Tears at piers, open laps, daylight at the band, and standing water on top of the sheet. Walk it yearly with tape and check humidity through summer.