How many rolls of vapor barrier do I need?

Multiply floor area by (1 + overlap) to get the order area, then divide by the roll coverage and round up: rolls = ceil(area × (1 + overlap) ÷ roll coverage). A 1,000 sq ft floor at 10% overlap and 200 sq ft/roll needs 6 rolls. The vapor barrier tool does it.

How much overlap should vapor barrier seams have?

About 6 to 12 inches at each seam, roughly 10% of area, is a common planning band — more on uneven ground. Check your product instructions, since the right overlap depends on the liner and how seams are taped.

What size dehumidifier does a crawl space need?

As a planning tier by area: up to 1,500 sq ft the "50-pint class", 1,500–2,500 sq ft the "70-pint class", and above 2,500 sq ft the "90-pint class". These are planning tiers, not specs — actual sizing also depends on how wet the space is and the climate.

Do I need both a vapor barrier and a dehumidifier?

In a sealed, encapsulated crawl space they work as a pair: the barrier blocks ground moisture and the dehumidifier removes airborne humidity. Sizing the dehumidifier is easier once the space is sealed, because it is not fighting humid outside air.

Vapor barrier material & crawl-space dehumidifier sizing

Q: Which vapor-barrier thickness should I buy?

Thickness is measured in mils. A heavier liner, roughly 12 to 20 mil, is more durable and better if you will move around the crawl space; thinner liner is cheaper but tears more easily. Thickness changes the price per roll you enter, not the roll-count formula, so pick for durability and let the tool handle the quantity.

Two of the everyday numbers in a crawl-space project: how many rolls of vapor barrier to order, and how big a dehumidifier to buy. Both come down to simple area math plus a labeled sizing tier.

By Francesco Zinghinì

Once you have decided to seal and dry a crawl space, two practical questions follow: how much vapor barrier to buy, and how large a dehumidifier to run. Neither needs a price list — the barrier is area-and-overlap arithmetic, and the dehumidifier is an area-based sizing tier — so both estimates stay correct as long as you supply your own prices. The vapor barrier tool and dehumidifier tool handle each.

Counting vapor-barrier rolls

Vapor barrier is bought by the roll, and you always order more than the bare floor area because seams overlap. The formula is:

order area = floor area × (1 + overlap), then rolls = ceil(order area ÷ roll coverage)

Overlap is a labeled planning band — around 6 to 12 inches at each seam, roughly 10% of area — and roll coverage depends on the product (a common figure is about 200 sq ft per roll, but check your label). Worked example: a 1,000 sq ft floor at 10% overlap is an order area of 1,100 sq ft; at 200 sq ft/roll that is 1,100 ÷ 200 = 5.5, which rounds up to 6 rolls. You round up because you cannot buy half a roll. The vapor-barrier coverage table lists mil thicknesses, roll coverage and overlap conventions.

Which mil to buy

Vapor barrier comes in thicknesses measured in mils (thousandths of an inch). Thicker liner (say 12–20 mil) is more durable and better suited to a crawl space you will move around in; thinner liner is cheaper but tears more easily. Thickness affects the price per roll you enter, not the roll-count formula. Because you supply the $/roll, the estimate reflects whichever product you choose.

Sizing a crawl-space dehumidifier

Dehumidifiers are commonly described by their pint-per-day capacity, and for planning you can pick a class by crawl-space area. The tool uses labeled tiers:

≤ 1,500 sq ft → "50-pint class"
1,500–2,500 sq ft → "70-pint class"
> 2,500 sq ft → "90-pint class"

These are planning tiers, not a spec — actual sizing also depends on how wet the space is, the climate, and whether the space is sealed. A 2,000 sq ft crawl space lands in the "70-pint class"; if your unit is $1,100 and install is $200, the cost line is $1,300. Enter your own unit and install prices in the dehumidifier tool.

They work together

The vapor barrier stops ground moisture from entering; the dehumidifier removes what is left in the air. In a properly sealed and encapsulated crawl space the two are a pair, which is why they usually appear together in an encapsulation quote. Sizing the dehumidifier for a sealed space is easier than for a vented one, because you are not fighting a constant inflow of humid outside air.

Running costs, not just the purchase

The dehumidifier tool prices the unit and install, but a dehumidifier also has a running cost worth keeping in mind when you choose one. A crawl-space dehumidifier that runs much of the year uses electricity, and an energy-efficient model (often described by its efficiency rating) can cost more up front and less to run — the classic trade-off between purchase price and operating cost. In a properly encapsulated and insulated crawl space, the unit works less, because it is drying a sealed volume instead of an endless stream of humid outside air, so the sealing work pays back partly in lower dehumidifier running costs.

The same logic applies to the vapor barrier: a thicker, more durable liner costs more per roll but resists punctures from foot traffic and debris, so it does not need patching or replacing as soon. When you price the two together, think of them as a moisture-control package with both a purchase cost and an ongoing cost, rather than two isolated line items. Sizing the dehumidifier correctly — not oversizing "to be safe" — also keeps both the purchase and the running cost sensible, which is why the area-based tier is a starting point you refine to your space, not a mandate to buy the biggest unit.

Moisture, not health

These tools are about quantities and cost: rolls to order, capacity class to buy, dollars to budget. Moisture control is not a substitute for a professional mold assessment, and this site makes no mold, radon or air-quality health claims. If air quality is your concern, have it evaluated by a qualified professional. The figures here are planning estimates from your own prices — confirm quantities against your product labels and quotes before ordering.