Home › Crawl space

20-mil barrier · Rim joist · Dehumidifier · Sistering

Crawl space encapsulation and joist repair calculator

Encapsulating an average crawl space — a heavy vapor barrier up the walls, sealed vents, and a dehumidifier — commonly costs $3,000 to $15,000. A liner alone is less. Sistering rotten floor joists is a separate structural line, often $8 to $25 per foot of joist, and it is not included in a liner quote.

Reviewed 25 October 2026 · Written by Mantu Kumar

See the photo · See the cost diagram · Open the calculator

Written and maintained byMantu Kumar

Evidence review17 October 2026

Calculation rulesRead the methodology Quality standardEditorial policy

Key facts at a glance

• Liner only: about $2–$5 per sq ft of crawl floor for a heavier barrier, seams and a wall turn-up.
• Full encapsulation: often $5–$9 per sq ft, including the liner, sealing and a dehumidifier around $1,200–$2,500.
• 20-mil reinforced liner is the thickness people should ask for. A thin 6-mil sheet is a different product.
• Rim-joist foam is part of drying the perimeter, not an attic product. Attic insulation is a separate calculator.
• Joist sistering for rot or termite damage is often $8–$25 per linear foot, more if the sill or a beam is gone.

2026 planning bands for residential crawl encapsulation and carpentry sistering. Structural repairs need someone who can see the wood.

2026 planning ranges

1. 1Encapsulation · $3–$10 / sq ft
2. 2Liner up the walls & fastened
3. 3Rim-joist foam
4. 4Dehumidifier · $1,000–$3,000 installed
5. 5Drainage & sump · $800–$3,000
6. 6Joist sistering · separate structural line
7. 7Vents sealed, access door
8. 8Liner mils + taped seams
9. 9Mold on framing

Total, most homes: $3,000–$15,000; a 1,000 sq ft crawl often $3,000–$10,000.

• Drainage first if water stands on the soil. A liner over a pond fails.
• Vents are sealed in a true encapsulation. Leaving them open fights the dehumidifier.
• Joists that are gone at the ends need more than a sister.

Crawl space estimator

No email · nothing sent

Work out the cost & payout

Diagram, part by part

Crawl space encapsulation and joist repair: every numbered part explained

The crawl space encapsulation and joist repair drawing above has 9 numbered parts, from Encapsulation to Mold treatment on joists. Each card below frames one of them, and after you run the calculator it shows what that part adds to your estimate.

1. 1 Encapsulation

   $3–$10 / sq ft

   Encapsulation covers the dirt floor with heavy plastic so ground moisture cannot evaporate into the crawl space and rise into the house. Price follows the floor area and the liner thickness, with thicker reinforced liners costing more but surviving foot traffic and storage.

   What moves it: Floor area, liner thickness and how many piers the liner must wrap.

   On the quote: Ask for the liner thickness in mils and whether the seams and piers are taped.

2. 2 Liner up the walls & fastened

   Included with encapsulation

   The liner continues up the foundation walls and is mechanically fastened and sealed near the top, so the whole crawl space becomes a closed, dry box rather than a floor covering.

   What moves it: Wall height and the condition of the foundation walls.

   On the quote: Ask how high the liner runs up the walls and how it is fastened and sealed.

3. 3 Rim-joist foam

   The rim joist, where the floor framing sits on the foundation, is one of the biggest air leaks in a house. Closed-cell spray foam or cut rigid foam seals and insulates it.

   What moves it: The perimeter length and the foam type.

   On the quote: Ask whether foam is spray-applied or cut board, and its thickness.

4. 4 Dehumidifier

   $1,000–$3,000 installed

   Once the space is sealed, a crawl space dehumidifier keeps relative humidity low, usually under 60%. It needs a condensate drain to a sump or to the outside and a dedicated outlet.

   What moves it: Crawl space volume, local humidity and drain routing.

   On the quote: Ask for the dehumidifier model and capacity, and where the condensate drains.

5. 5 Drainage & sump

   $800–$3,000

   If water enters the crawl space during storms, a perimeter drain and sump pump must come first. Sealing a wet crawl space only traps the water against the liner and framing.

   What moves it: How much water gets in and how far the drain must run.

   On the quote: Ask the contractor to fix water entry first and price drainage separately from encapsulation.

6. 6 Joist sistering

   separate structural line

   Damp crawl spaces often have sagging or rotted joists. Sistering new lumber alongside them or replacing them is structural work, priced separately from encapsulation.

   What moves it: Linear feet of damaged joists and access height.

   On the quote: Ask how many joists will be sistered and whether rotted sill plates are included.

7. 7 Vents sealed and doors closed

   Included with the liner

   Old foundation vents are closed and sealed so humid summer air cannot enter, and the access door is weatherstripped and insulated. Without this step the dehumidifier fights outside air all summer.

   What moves it: The number of vents and the access door size.

   On the quote: Ask how many vents are being sealed and whether a new insulated access door is included.

8. 8 Liner thickness and taped seams

   12 to 20 mil, seams taped

   Thicker liners, often 12 to 20 mil, resist punctures from foot traffic and stored items. Overlapped seams are taped and the liner is sealed around piers and pipes so moisture cannot slip through the gaps.

   What moves it: Liner thickness, reinforcement and the number of piers.

   On the quote: Ask for the liner thickness, whether it is reinforced and how seams and piers are sealed.

9. 9 Mold treatment on joists

   Priced on the mold tool

   Joists and subfloor in a damp crawl space often carry surface mold. It should be treated and cleaned before the space is sealed so it does not keep growing inside the closed box.

   What moves it: How much framing is affected.

   On the quote: Ask whether mold treatment is included and get it priced separately if it is not.

Under the floor

The liner, the rim and the joists are three lines

A finished encapsulation looks like a clean room under the house: barrier up the wall, wood visible overhead, and a machine keeping the air dry. The photo is the end state. The bid should still list the liner thickness, the dehumidifier and any wood repair separately.

Read the quote against the crawl you actually have. These close-ups are crops of the same AI-generated illustration.

01Ground liner

Water on the ground

Standing water, a failed gutter, or a foundation leak has to be handled before a liner. Otherwise you wrap the problem.

02Dehumidifier

Liner thickness and the dehumidifier

Ask for 20-mil, taped seams, and a dehumidifier sized to the volume. A 6-mil sheet and no machine is a vapor barrier, not an encapsulation.

03Floor joists

The wood

Probe joists and the sill. Sistering is carpentry. Powder-post or termite damage may need treatment before the new wood goes in.

Before you sign a quote or file a claim, verify these three details

Square feet

Of crawl floor, and the wall height the liner climbs.

Machine

Dehumidifier model, drain, and who services it.

Joists

Linear feet, both sides, and whether a beam or sill is included.

Water is coming through the wall, not the soil? Open the foundation calculator →

Crawl space cost (2026)

Why encapsulation, vapor barrier and joist repair are priced together

Contractors sell “encapsulation,” “vapor barrier,” “crawl space insulation” and “joist repair” as separate visits. Under the house they are one condition. Ground moisture loads the joists. A liner without a drainage path floats. A dehumidifier in a vented crawl runs forever. Foam on the rim joist does nothing about a sill that is already soft.

Price the liner by the thickness and by whether the seams are taped and the vents are closed. Price the machine on its own line even if it is bundled, so you can see it. Price sistering by the foot of bad joist, on both faces if both faces are specified, and do not let it hide inside a per-square-foot liner rate.

Attic insulation is the lid of the house, not the floor. Use the attic calculator for R-value overhead. Use the foundation and sump tools if the water is hydrostatic against a wall rather than humidity off the soil. Mold on the joists gets the mold tool only after the water source is named.

Worked example: 1,200 sq ft crawl, full system, 40 ft of sistering

Insurance and a damp crawl

• Encapsulation is an improvement. Insurers do not buy vapor barriers because a crawl feels clammy.
• Sudden water from a burst pipe or a covered flood can pay to dry and to replace damaged joists. It does not automatically pay to encapsulate the whole footprint.
• Rot, insects and long-term groundwater are the usual exclusions. A sistered joist after years of moisture is often yours.
• If a covered loss did break a joist, age and ACV apply to the wood. Leave the encapsulation lines in the quote so you can see which dollars are the claim and which are the upgrade.

Crawl space encapsulation cost by region

The calculator's default crawl space encapsulation job, repriced for each of its six regions. At the national average it runs $5,537 – $11,673; South / Lower Midwest is the lowest at $4,817 – $10,156, and West Coast metro is the highest at $7,198 – $15,175, a gap of about $3,700 at the midpoint.

What “Moisture scope” does to the price

Every other input stays at its default and only “Moisture scope” changes. The cheapest choice, Sealed 20-mil liner only, comes to $3,137 – $7,973; the most expensive, Liner, seals and dehumidifier, comes to $5,537 – $11,673.

What “Rim-joist foam” does to the price

Every other input stays at its default and only “Rim-joist foam” changes. The cheapest choice, Not in this quote, comes to $5,120 – $10,700; the most expensive, Foam the rim, comes to $5,537 – $11,673.

How age changes the crawl space encapsulation claim payout

Using the default job, a $1,000 deductible and a 40-year expected life. At 10 years old, about $2,151 is held back as depreciation; a replacement cost policy releases it after the work is finished, while an actual cash value policy keeps it.

Quote checklist: 7 items to verify

Encapsulation quotes vary widely, so compare the same scope line by line.

1. Water entry fix. Drainage and sump work, if water gets in, priced separately.
2. Liner. Thickness in mils, reinforcement and how far it runs up the walls.
3. Seams and piers. Taped seams and sealed piers and pipes.
4. Vents and door. Vents sealed and an insulated access door.
5. Rim joist. Foam type and thickness.
6. Dehumidifier. Model, capacity, drain route and outlet.
7. Structural and mold work. Joist repair and mold treatment as separate lines.

How long the job takes

Ask for a humidity reading at handover and again a month later, so you can see the system working.

Common mistakes that raise the cost

• Sealing a wet crawl space. Water must be stopped before the liner goes down.
• Leaving vents open. Outside air defeats the dehumidifier.
• Thin liners. Thin plastic tears with foot traffic.
• No condensate drain. A dehumidifier needs somewhere to send its water.
• Ignoring the rim joist. Air leaks at the rim keep the space damp and cold.

Ways to keep the cost down

Fix the cause first and size the job correctly.

• Fix gutters and grading. Less water entering.
• Compare liner thickness. Pay for what the space needs.
• Combine with joist repair. One access setup.
• Choose an efficient dehumidifier. Lower power bills.
• Ask about utility rebates. Sometimes available.
• Keep the access clear. Less labor time.

Signs your crawl space needs work

Moisture problems show up in the house too.

• Musty smells upstairs. Air rises from below.
• Standing water. Drainage needed.
• Sagging floors. Joist damage.
• Mold on joists. High humidity.
• Cold floors in winter. Missing insulation.

Sources and further reading

• NAIC — Actual cash value vs replacement cost coverage
• ReplaceCost foundation calculator
• HomeGuide — Crawl space encapsulation cost

Frequently asked questions