Vapor Barrier Calculator

Estimate vapor barrier plastic sheeting area, rolls, overlap waste, and cost for slabs, crawl spaces, basements, and walls. Supports feet, inches, and meters.

Units & basic settings

in

Typical: 6–12 in (0.15–0.3 m)

%

For cuts, obstacles, offcuts.

Rectangular area dimensions

ft
ft
ft

Optional: how far the plastic turns up the wall.

ft

If empty, perimeter = 2 × (length + width).

Roll size & cost

ft
ft
$

Set to 0 if you only need area, not cost.

$

Per ft². Used if roll price is unknown.

How this vapor barrier calculator works

This tool helps you estimate how much plastic sheeting (poly vapor barrier) you need for slabs-on-grade, crawl spaces, basements, and wall assemblies. It also estimates the number of rolls and total material cost.

1. Floor area

The first step is to compute the floor area you want to cover:

Rectangular area

\( A_\text{floor} = L \times W \)

where:

  • \(L\) = length
  • \(W\) = width

L-shaped area is modeled as two rectangles:

\( A_\text{floor} = L_A \times W_A + L_B \times W_B \)

2. Wall / foundation upturn

Many details require the vapor barrier to turn up the wall or foundation by 6–12 inches (0.15–0.3 m). The calculator adds this perimeter area:

\( A_\text{upturn} = P \times h \)

where:

  • \(P\) = perimeter length
  • \(h\) = upturn height

3. Overlap and waste

Seams must overlap and you will always have some waste from cuts and obstacles. The calculator applies a combined factor:

\( A_\text{subtotal} = A_\text{floor} + A_\text{upturn} \)

\( f = 1 + f_\text{overlap} + f_\text{waste} \)

\( A_\text{total} = A_\text{subtotal} \times f \)

Internally, the tool approximates the overlap factor from your chosen seam overlap and a typical seam spacing, then adds your explicit waste percentage.

4. Roll coverage and number of rolls

Coverage of one roll is simply:

\( A_\text{roll} = W_\text{roll} \times L_\text{roll} \)

\( N_\text{rolls} = \left\lceil \dfrac{A_\text{total}}{A_\text{roll}} \right\rceil \)

5. Cost estimation

You can estimate cost in two ways:

  • By roll price: \( \text{Cost} = N_\text{rolls} \times \text{price per roll} \)
  • By unit area price: \( \text{Cost} = A_\text{total} \times \text{price per ft² or m²} \)

Typical vapor barrier thicknesses

  • Under slab: 10–15 mil Class A vapor retarder (ASTM E1745)
  • Crawl space: 6–20 mil depending on durability and traffic
  • Interior walls: 4–6 mil where required by code

Thicker membranes resist punctures better but cost more. Always follow local building codes and manufacturer recommendations.

Best practices for installing vapor barriers

  • Overlap seams by at least 6–12 inches and tape them with compatible seam tape.
  • Seal penetrations (pipes, columns, posts) with tape or boots.
  • Extend the membrane up walls or piers as required and seal to the structure.
  • Avoid unnecessary punctures; repair any holes with patches and tape.
  • Protect the membrane from damage during concrete placement or crawl space work.

FAQ

How much extra vapor barrier should I order?

For simple rectangular slabs, 10% extra is often enough. For complex crawl spaces with many piers, steps, or obstacles, 15–20% is safer. This calculator lets you set the waste percentage explicitly.

Can I mix metric and imperial units?

No. Choose either feet/inches or meters for all dimensions. The calculator automatically updates labels and area units when you switch the unit system.

Does this calculator replace a professional takeoff?

No. It is a planning and budgeting tool. For large commercial projects or where code compliance is critical, a detailed takeoff and specification review by a design professional is recommended.