Foundation services
Crawl space repair and encapsulation
Vented or sealed, what the code allows, what the research found, and the structural work that has to come first.
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Written by HyreFoundation Research Desk Primary-source research and fact checking
The short answer
Crawl space encapsulation seals the ground and walls with a vapor retarder and conditions the air, instead of venting it.
In humid climates, venting can wet a crawl space, because outdoor air often has a dew point above the framing’s temperature. The code permits both designs. The Department of Energy says sealed, conditioned crawl spaces generally perform better. Encapsulation does not repair sagging floors or rotted framing.
If this is happening now
Standing water, a strong moldy smell, or visibly rotted beams under a floor you walk on needs action now. So does any suspected sewage leak in a crawl space. That is a health hazard, not a do-it-yourself inspection. You can also reach HyreFoundation on (866) 582-8523.
Key takeaways
of floor, dropping to 1 per 1,500 with a Class I vapor retarder on the ground.
with a sealed vapor retarder plus exhaust or conditioned air at 1 cfm per 50 sq ft.
in most climate zones, and do not cost more to build.
where groundwater can rise within 6 inches of the crawl floor, the inside grade must match outside grade unless drainage is provided.
Sagging floors, rotted girders and failed piers are separate work.
What are the signs a crawl space needs repair?
Standing water, or mud that never dries.
Especially on the ground floor, because crawl space air rises into the house.
On ducts, pipes or the underside of the floor.
Batts sagging, falling, or dark and damp underneath.
Mold or efflorescence on framing, piers or walls.
Bouncy, sloping or soft spots underfoot.
At joist ends where they sit on the sill, or at the base of posts.
Rusted pier hardware, cracked masonry piers, or posts sitting on bare soil.
Rodents or termites, both of which follow moisture.
Form boards left from construction.
What does the research say about crawl space encapsulation?
Crawl spaces are one of the few foundation topics with independent, measured building science behind them. The findings surprise most people. Our crawl moisture check walks through the same questions for your house.
DOE: in humid climates "the warm humid air entering the crawlspace is more likely to condense on crawlspace framing than to help dry out the crawlspace".
DOE: "In most climate zones, conditioned crawlspaces perform better than vented crawlspaces in terms of safety, health, comfort, durability and energy consumption", and they "do not cost more to construct".
DOE reports builders sealing and insulating crawl spaces saw up to 20% heating and cooling savings, "while substantially reducing the risk of moisture problems".
Building Science Corporation monitored four conditioned crawl spaces for twelve months and found "crawlspace temperatures, dew points, and relative humidities mirror those of the house interior."
Batt insulation between the joists of a vented crawl space collects condensation on its underside and rots the joists. A vented crawl space can work. That version reliably does not.
DOE’s recipe: a sealed ground cover run up the walls, wall insulation, perimeter drainage if the floor sits below outside grade, and conditioned air.
DOE: "A soil gas venting system should be installed as part of a complete radon-resistant construction system." Sealing changes how soil gas moves, so test.
What does a proper crawl space encapsulation include?
Gutters, grading and paving. Sealing a crawl space fed by a downspout traps the moisture under the house. See drainage and grading.
An interior drain and sump where water enters or groundwater rises. Check the code rule on raising the inside grade where groundwater can reach within six inches of the floor.
Sister or replace joists, repair girders, add or re-shim piers, lift post bases off the soil, and treat any active insects. This is the structural part, and it happens before anything gets covered.
Remove vegetation, organic material, debris and buried form boards. The code requires this, and it is often skipped.
Continuous Class I material over the ground, seams lapped at least 6 inches and sealed, edges run at least 6 inches up the wall and sealed. That wall detail separates encapsulation from plastic on dirt.
Wall insulation, then exhaust or conditioned supply air at about 1 cfm per 50 sq ft, with a path to the house. A sealed crawl space with no air management is a sealed damp box.
Sealing changes how soil gas moves. The EPA action level is 4 pCi/L, a test is cheap, and this is the right time to do it.
Should a crawl space be vented or sealed?
Either can be legal. They are different strategies. Mixing them, with a vented crawl space and insulation stuffed between the floor joists, is the setup that fails most reliably.
| Code section | What it requires |
|---|---|
| Vented (R408.1, R408.2) | Vents of at least 1 sq ft per 150 sq ft of under-floor area, one within 3 feet of each corner. Drops to 1 per 1,500 where an approved Class I vapor retarder covers the ground and vents cross-ventilate. Vent screens have openings no wider than 1/4 inch. |
| Unvented (R408.3) | No vents needed where the ground has a continuous Class I vapor retarder, seams lapped 6 inches and sealed, edges run 6 inches up the wall and sealed. Plus one of: exhaust at 1 cfm per 50 sq ft with an air path to the house, conditioned supply air at that rate, or use as a plenum. |
| Access (R408.4) | A floor hatch at least 18 by 24 inches. Through the perimeter wall, at least 16 by 24 inches. Not under a door into the house. |
| Debris (R408.5) | All vegetation and organic material cleared, and all wood concrete forms removed before occupancy. Buried form boards are a common source of long-running termite and rot problems. |
| Groundwater (R408.6) | Rarely quoted: where groundwater can rise within 6 inches of the finished floor at the perimeter, or surface water does not drain well, the inside grade must be as high as the outside grade unless approved drainage is provided. |
We read this code text through code mirrors and a state agency reprint, because the publisher’s server blocks automated retrieval. Editions and state amendments vary, so confirm your adopted local text.
What moves the price
Encapsulation is usually priced by ground area. Structural repair is priced per member. They are often quoted together, so ask for them as separate lines. See what drives foundation costs.
| Factor | Why it changes the number |
|---|---|
| Area and height | Square footage, and whether a person can work in there. A tight crawl space is slow work. |
| Access | Whether materials and debris can get in and out, and whether the opening meets the minimum size. |
| Water management | Interior drain, sump, backup power and discharge route. Often the largest line. |
| Structural repair | Joists, girders, piers, post bases and insect damage. Priced per member, and not fully known until exposed. |
| Material and detail | Vapor retarder thickness, how far it runs up the walls, and how seams and pipes are sealed. Detail explains most of the gap between two similar quotes. |
| Insulation and air | Wall insulation, plus an exhaust fan or a conditioned supply and return path, which may involve your HVAC contractor. |
| Debris removal | Old insulation, old vapor barrier, vegetation, form boards and anything a past owner stored there. |
| Dehumidifier | If one is specified, including whether it drains by gravity or needs a pump. |
HyreFoundation publishes no national average price for this work. Depth to bearing, soil, access and method vary enough between two houses on the same street that a single figure would mislead.
Does encapsulation fix structural problems?
No. A beautifully sealed crawl space can still hide a sagging girder, a failed pier, rotted joist ends or a settling foundation.
Those are separate repairs with separate costs, and they carry the structural risk.
If a proposal covers the ground and walls but says nothing about the framing, ask what shape the framing is in.
What to ask before you sign
Which design, and which code section are we meeting?
If sealed, what moves the air: exhaust, conditioned supply, or nothing?
How far up the wall does the vapor retarder go, and how is it sealed?
What condition is it in, and which structural repairs are in this price?
Where is the water coming from, and what are we doing outside?
Will buried form boards and organic material be removed?
Is one specified, and how does it drain?
Are we testing before and after?
What does it cover, what voids it, and does it transfer?
The full checklist is on choosing a foundation contractor.
What to have ready
The crawl space, if you can take them safely, including framing and ground.
When you notice the smell or damp, and whether it is seasonal.
Whether there is any, and after what weather.
What is there now, and its condition.
Whether they feel soft, bouncy or sloped, and where.
Past plumbing leaks, pest treatment or crawl space work.
Where this goes wrong
The classic failure. Humid air condenses on the batts and the joists above rot. If you have this and it is damp, adding more makes it worse.
Sealing a wet crawl space traps the water under plastic beneath your house. Water management comes first.
The code allows an unvented crawl space only with exhaust, conditioned supply or plenum use. A sealed space without one is not the permitted design and does not behave like it.
Encapsulation is defined by its detail: lapped, sealed seams run up and sealed to the wall. Loose sheeting is a different product, sometimes sold as if it were the same.
Once insulation and liner are up, rot and insect damage are harder to see and to dispute. Repair first.
The code requires wood forms removed before occupancy. Left in place, they feed termites and hold rot.
Sealing changes soil gas movement. A test costs very little next to the rest of the project.
Get connected
Tell us about the work
Three short steps: the job, your home, and how to reach you. Nothing goes to a foundation company unless you agree.
A straight answer first. Matching is still being built, so this is not an instant quote. If a wall is moving or a door frame has racked out of square this week, call a structural engineer first.
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Questions
Should crawl space vents be open or closed?
Is crawl space encapsulation worth it?
Does encapsulation fix sagging floors?
Do I need a dehumidifier in a sealed crawl space?
Will sealing my crawl space cause a radon problem?
My crawl space floods. Should I encapsulate or drain it first?
Written and audited by
HyreFoundation Research Desk
Primary-source research and fact checking
We read the model building code, the federal soil and moisture guidance and the insurer position ourselves, and we publish each figure with the document it came from and the date we retrieved it.
Where a number cannot be traced to a primary source, we say so instead of printing it. HyreFoundation does not inspect, engineer or repair foundations. Authorship is organizational: this desk, not a named persona.
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How this desk works
- Primary sources only. Code statements come from the International Residential Code, Chapter 4, as the model code. Soil statements come from USDA and USGS. Insurance statements come from the policy language and the Insurance Information Institute. We open the source rather than cite a blog that cites it.
- The code is a model, and adoption is local. Your jurisdiction adopts an edition and amends it, and foundation chapters are amended more than most because frost depth and soil are local facts. Every code figure says so.
- We hold no job data. We have no dataset of bids, pier counts or invoices, so no page claims an average from one. Calculators show the arithmetic and the inputs they assumed, and label the result as an estimate.
- We do not diagnose your house. A crack guide can tell you which pattern to measure and when to call a structural engineer. It cannot tell you whether your foundation is failing. An engineer on site does that work.
- We do not repair foundations, and we take no payment for placement, ranking or a favorable mention. Nobody buys a position on this site.
Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
Information on this page is general and varies with soil, climate, foundation type, building code, insurance policy and circumstance.
It is not engineering, insurance or legal advice, and no page can tell you whether your house has a structural problem: that needs a licensed professional on site.
HyreFoundation is an independent foundation resource, not a repair contractor or an engineering firm, and does not perform, supervise or warrant foundation work.