Introduction
The crawl space material list is short — polyethylene, insulation, tape, vents or a dehumidifier — but each item has options that matter. Poly thickness determines durability, insulation type determines placement, and tape determines whether the vapor barrier stays a barrier. Choosing well here is the difference between a space you finish once and a space you crawl back into in three years.
Quantities for everything below come from the Crawl Space Calculator — floor area for poly and floor insulation, perimeter × height for wall insulation, volume for dehumidifier sizing. This guide covers what to buy; the cost guide covers what it costs.
Polyethylene Vapor Barrier: Thickness
“Mil” is thousandths of an inch — 6-mil is 0.006 in (0.15 mm), 20-mil is 0.020 in (0.51 mm). Thicker isn’t just stronger; reinforced products add a scrim layer that resists punctures and tears dramatically better than plain film.
| Thickness | Type | Typical use | Durability |
|---|---|---|---|
| 6-mil | Plain film | Basic ground cover, vented spaces, rarely entered | Tears on piers/debris; 10–15 yr life |
| 10–12 mil | Reinforced | Heavier ground cover, moderate traffic | Good puncture resistance |
| 20-mil | Reinforced w/ scrim | Full encapsulation, serviced spaces | Walkable; 20+ yr life |
Rule of thumb: if boots will ever walk on it — service access, HVAC maintenance — buy 20-mil. The price step from 6-mil to 20-mil (roughly $0.50–$1.00/ft² vs. $1.50–$2.00+/ft², or $5–$11/m² vs. $16–$22/m²) is small compared to a re-do. Roll sizes and the overlap math that sizes the order are in the formula guide.
Insulation: Batt vs. Foam Board vs. Spray Foam
Fiberglass or mineral wool batts (vented floors)
The standard for insulating between floor joists in vented spaces. Mineral wool costs more but resists moisture and holds its shape better than fiberglass — worth it in a damp-adjacent space. Batts must sit in full contact with the subfloor and be supported (wire stays every 18–24 in / 450–600 mm); sagging batts convect air around themselves and lose much of their rated R-value. Never compress a batt to fit — a squeezed R-19 performs around R-13.
Rigid foam board (sealed walls)
XPS or polyiso, typically 2 in (50 mm) for ~R-10, mechanically fastened or adhered to the interior foundation wall. Continuous — no framing breaks — which is why it outperforms batts per inch in this application. In termite regions, leave the inspection gap at the top of the wall per local requirements (commonly 3 in / 75 mm of visible foundation).
Spray foam
Closed-cell spray foam both air-seals and insulates (about R-6.5 per inch / R-45 per 100 mm) — excellent for rim joists and irregular areas. Usually pro-installed at $3–$6+/ft² ($32–$65/m²). DIY kits exist for small areas but demand careful ventilation and protective equipment.
| Climate zone (US) | Vented floor target | Sealed wall target |
|---|---|---|
| Zones 1–2 (hot) | R-13 | R-5 – R-10 continuous |
| Zones 3–4 (mixed) | R-19 – R-25 | R-10 continuous |
| Zones 5–8 (cold) | R-25 – R-30 | R-10 – R-15 continuous |
These are common planning targets, not code citations — confirm required R-values with your local building authority, as the code compliance guide explains.
Tapes, Fasteners, and Sealants
- Poly seam tape: tape rated for polyethylene sheeting — aggressive acrylic or butyl adhesive. Standard duct tape fails in damp conditions. In full encapsulation, use the liner manufacturer’s matched tape system.
- Mechanical fasteners: concrete nails or powder-actuated pins with large washers for fastening poly and foam board to foundation walls; termination bars at the top edge of wall poly in encapsulation.
- Caulk and foam: silicone or polyurethane caulk for rim-joist joints and small gaps; low-expansion spray foam for larger penetrations. Air sealing the rim joist is the highest-value sealing work in the project — see the pro tips guide.
- Support wire: spring wire stays (“lightning rods”) every 18–24 in (450–600 mm) to hold floor batts in contact with the subfloor.
Vents and Dehumidifiers
- Foundation vents: sized by net free area from the manufacturer’s rating — typically 40–60% of the nominal vent size. Buy to the vent-area number from the calculator (8 ft² / 0.74 m² for a 1,200 ft² space under the 1-per-150 rule), not to a vent count someone guessed.
- Dehumidifiers: 70–90 pint/day class for a typical 1,200 ft² (111.5 m²) space; look for a built-in humidistat, auto-defrost (crawl spaces run cool), and a condensate pump or gravity drain to daylight or the sump. Ductable units can serve adjacent sealed areas.
- Hygrometer: a $20–$40 unit left in the space — the cheapest verification that the whole system works.
Worked Example: Material List for 30 × 40 ft
For the 30 ft × 40 ft × 3 ft (9.144 m × 12.192 m × 0.914 m) crawl space — 1,200 ft² (111.5 m²) floor, 140 linear ft (42.7 m) perimeter, 420 ft² (39 m²) of wall — planned as a sealed encapsulation:
| Item | Quantity | Spec |
|---|---|---|
| Vapor barrier | 1,320 ft² (122.6 m²) → 1 roll 20×100 ft | 20-mil reinforced poly |
| Seam tape | 4–6 rolls | Poly-rated, 4 in wide |
| Wall foam board | 420 ft² (39 m²) → ~14 sheets 4×8 ft | 2 in XPS, ~R-10 |
| Rim-joist sealant | 6–10 tubes + 2 cans foam | Polyurethane caulk, low-expansion foam |
| Fasteners | ~200 | Concrete pins + washers / termination bar |
| Dehumidifier | 1 unit | 70–90 pint/day, condensate to drain |
| Hygrometer | 1 | Logging type preferred |
Cross-check in metric — 122.6 m² of poly; 39 m² of foam ≈ 14 sheets of 1.2×2.4 m (2.88 m² each = 40.3 m² ✓). Pricing this list is the cost guide‘s job; the measuring behind it is the estimating guide‘s.
Related Guides
- Cost Guide — price ranges for every item here.
- Pro Tips — installation techniques for these materials.
- Estimating Guide — turning dimensions into order quantities.
- DIY vs. Pro — which materials suit DIY installation.
- Pre-Close-Up Checklist — verify materials installed correctly.
Frequently Asked Questions
What mil poly is best for a crawl space?
6-mil for basic ground cover in rarely entered vented spaces; 20-mil reinforced for encapsulation or any space with foot traffic. The upgrade costs more once; replacing torn thin poly costs more twice.
Is mineral wool better than fiberglass in a crawl space?
Generally yes where moisture is a concern — mineral wool resists water, holds its shape, and doesn’t lose R-value when damp the way fiberglass does. It costs more per batt, which the drier performance usually justifies.
Can I use duct tape on vapor barrier seams?
No. Standard duct tape adhesive fails in damp crawl space conditions. Use tape rated for polyethylene, ideally the liner manufacturer’s matched system for encapsulation.
XPS or polyiso for foundation walls?
Both work. XPS tolerates ground-contact moisture well; polyiso offers slightly higher R-value per inch but can lose performance if it stays wet. Either meets typical R-10–R-15 wall targets at 2–3 in (50–75 mm).
What size dehumidifier do I need?
Size by volume: a 3,600 ft³ (102 m³) space — e.g., 1,200 ft² at 3 ft clearance — typically needs 70–90 pints/day. Choose a unit with a humidistat, auto-defrost, and condensate drainage, not a portable room unit.
How many rolls of poly do I need?
Divide the coverage (floor area × 1.10+) by the roll’s square footage and round up. For 1,320 ft² (122.6 m²): one 20×100 ft (2,000 ft²) roll, or two 10×100 ft rolls — the wider roll means fewer seams.
Get Your Number
Materials chosen — now size the order. Run your footprint through the Crawl Space Calculator, build the list against the table above, and price it with the cost guide before buying. Verify R-value and vapor-barrier requirements with your local building authority, since code minimums vary.
Disclaimer: Product descriptions are general planning guidance, not endorsements. Verify specifications with manufacturers and your supplier, and consult a licensed professional or your local building authority for code-related material requirements.
