Crawl Space Encapsulation Before and After: Humidity, Wood Moisture, Energy and What Stays the Same

The short answer: before encapsulation, a vented crawl space in the humid Southeast tracks the outdoor weather; after a properly sealed install with a drying system, it follows the house. In the best-documented field study, vented crawl spaces sat above 70 percent relative humidity 98 percent of the time in a wet North Carolina summer, against 5 percent for the closed ones. The closed spaces never reached condensation conditions that summer, had about 30 percent lower wood moisture, and used 15 to 18 percent less heating and cooling energy. Three things do not change on their own: water coming in from outside, rot already in the framing, and radon.

Where the Numbers Come From

Most of the figures here come from the most detailed public study we found of vented and closed crawl spaces in occupied homes. Advanced Energy, a Raleigh nonprofit, ran it from 2001 to 2005 with Department of Energy funding and published it as A Field Study Comparison of the Energy and Moisture Performance Characteristics of Ventilated Versus Sealed Crawl Spaces in the South. The setup tells you how far the results transfer to your house:

• Twelve identical houses on one cul-de-sac in Princeville, eastern North Carolina, each 1,040 square feet with a crawl space about 28 inches high, all-electric heat pumps and ductwork under the floor.
• Three groups of four. The controls kept their vents, R-19 floor batts and a 6 mil ground cover over all the soil. The two closed groups had vents sealed and a sealed liner on the ground and walls, one with floor insulation and one with foam board on the walls.
• A drying mechanism. From 2003 the closed crawl spaces got a small stream of conditioned air from the heat pump, about 1 cubic foot per minute per 30 square feet of floor. Retrofits today more often use a crawl space dehumidifier.

Most of the study compares houses side by side, but it contains one true before-and-after: in mid-2004 the researchers closed three of the vented control crawl spaces and tracked what happened. The report also notes the control houses were better built than typical homes, so an older house may start from a worse "before."

Before and After at a Glance

This summary draws on the final report and the team's 2004 conference paper.

Relative Humidity: The Biggest Change

Relative humidity shows the largest gap, and it held in very different summers: 2002 was a record drought year in central and eastern North Carolina and 2003 a record wet one. This is the share of summer days (June to August) each type spent above each level:

On humid days the vented crawl spaces were cooler than the dew point of the outdoor air, so air coming through the vents left water on joists, ducts and pipes. The ground cover was not the weak point: the controls had a careful 6 mil cover over all the soil and still ran wet, which led the researchers to conclude that dry crawl spaces need the air controlled too. That is the gap our encapsulation vs vapor barrier comparison describes.

A partial job showed up in the data as well. The three crawl spaces closed in 2004 got sealed vents and the drying air but kept an unsealed, lapped ground cover with no liner on the walls. They held daily averages below 70 percent, not 60. The EPA advises keeping indoor humidity below 60 percent, and our ideal crawl space humidity guide explains why 55 percent is a good setpoint.

Wood Moisture Content

Wood in the closed crawl spaces read about 30 percent lower in moisture content than in the vented ones, across ten marked spots per crawl space: sills and band joists by the access and at the worst location, floor joists above and below the insulation, a center beam and the subfloor. Three results matter if you plan to measure your own:

• Closed spaces dried fast. After dehumidifiers ran in the newly closed crawl spaces in July 2001, average wood moisture in both closed groups was still under 10 percent a month after the units were switched off.
• Cheap fixes counted. In the vented houses, completing the ground cover, fixing plumbing leaks and draining water away from the access door coincided with a drop of about 2 percentage points within a month.
• The vented controls did not rot. Their wood moisture rose in summer without reaching critical levels, yet they still had condensation and surface mold.

For thresholds, use the USDA Forest Products Laboratory's Wood Handbook chapter on biodeterioration. It says air-dried wood at 20 percent moisture content or less has a reasonable margin of safety against fungal damage, that serious decay needs moisture above the fiber saturation point (about 30 percent), and that humid air alone will not support significant decay but will let some molds grow.

Readings in the bottom two rows are a reason to read about rotted crawl space framing before pricing an encapsulation.

Condensation, Mold and the Musty Smell

Condensation is the change you can see. The vented crawl spaces reached condensation conditions repeatedly in summer 2003 and the closed ones never did. In one 48-hour stretch that August, with less than 0.2 inches of rain, the outdoor dew point sat above the vented crawl space temperature most of the time. Water beading on ducts, wet streaks on block and damp insulation are classic "before" signs that should be gone afterward.

The mold data is more mixed. In July 2001 air samples, taken after heavy rain, viable spores ranged from almost none to 400 per cubic meter in the closed crawl spaces against 400 to 3,500 in the vented ones; by September 2002 both types read mostly 100 to 200. With only two rounds of sampling, the researchers concluded that vented crawl spaces appeared more vulnerable to fungal growth. The report notes that most molds release alcohols and sulfur compounds, the source of that musty odor. Drying stops new growth, but moldy insulation and debris keep smelling until removed, so a smell that lingers weeks after the job means contaminated material or an active moisture source. See our guides to the musty crawl space smell and mold removal cost.

Temperatures in the Crawl Space and at the Floor

Closing a crawl space moves it from outside the house to inside it. The vented controls mixed freely with outdoor air through more than 7 square feet of vent openings, and the report calls them tempered outdoor air zones whose temperatures approach outdoors. Closed, they became semi-conditioned zones, and heat and air leaking from the ductwork stayed in the crawl space instead of going outside. Where the insulation sits decides what changes at the floor:

• Floor insulation kept: the floor stays the thermal boundary, but the space below is no longer open to outdoor air. This version did best in winter.
• Insulation on the walls: the crawl space and floor sit inside the insulated envelope. This version did best in summer but used 2 percent more heating energy than the vented houses over one winter.

Our crawl space insulation guide compares the two approaches.

Energy Use

The closed crawl space houses used 15 to 18 percent less heating and cooling energy than the vented houses on the heat pump sub-meters. The 2004 paper breaks down June 2003 to April 2004:

Keep the study's caveats in mind. These were small all-electric homes with ducts in the crawl space, where duct losses end up inside a closed crawl space instead of outdoors. The researchers had expected no summer savings, and they warned that adjusting for differences in household electricity use could shrink the result. Our energy savings guide shows how to estimate your own savings and payback, and the benefits overview covers the wider case.

Pests

The study did not count pests, so this before-and-after rests on what pests need. The Wood Handbook says subterranean termite hazard is greatest over crawl spaces that are poorly drained and lack a moisture barrier, that carpenter ants need high humidity while their young develop, and that powder-post beetles generally need wood moisture around 15 percent or more. A dry, lined crawl space removes much of that, but it does not replace termite treatment or rodent exclusion. The study liners stopped 3 inches below the top of the foundation wall to leave a termite inspection gap. Our page on crawl space pests covers entry points.

What Encapsulation Does Not Change

Encapsulation controls water vapor. It does not stop liquid water, repair damage or treat soil gas, and the study was explicit that closed crawl spaces still need ground and surface water kept out.

• Water from outside. Downspouts that empty at the foundation, grading that slopes toward the house and a high water table put water on or under the liner. That needs gutters, grading, drainage or a sump; see our standing water guide.
• Plumbing leaks. The report says closing the crawl spaces nearly eliminated the risk of rot not caused by plumbing leaks. A dripping drain line still rots the subfloor above a perfect liner.
• Rot already in the wood. The Wood Handbook says existing decay will not progress in wood kept air dry, but drying does not restore strength. Soft framing needs repair before the liner goes in; see crawl space repair cost.
• Radon. Crawl space radon was higher in the closed houses in both monitoring periods: 2.9 pCi/L against 0.8 in the first, 1.1 against 0.5 in the second. Living spaces averaged 1.9 against 0.5 in the first period and showed no difference in the second. All of it stayed under the EPA action level of 4 pCi/L at a low-radon site. A liner is not a radon system, so test before and after; our radon and encapsulation guide covers mitigation.
• The need for drying. The study concluded that closed crawl spaces need a supplemental drying mechanism, because some vapor still reaches a sealed space and builds up. Our guide on whether an encapsulated crawl space needs a dehumidifier covers the options.

What a Good "After" Looks Like

A fully encapsulated crawl space looks like a sealed, lined room rather than a floor with plastic on it. In words, a good after shows:

• Liner over the whole floor and up the foundation walls, held at the top by a termination bar, stopping a few inches below the sill where an inspection strip is required
• Seams overlapped and taped along their full length
• Each pier wrapped and taped to the floor sheet, with no soil showing
• Vents sealed from the inside and a gasketed access door
• A dehumidifier hung or on a stand, with a condensate line to a drain, pump or the outside, and its own power
• No debris or old wet insulation left behind, and any sump pit under a sealed lid

Our encapsulation process guide shows how a crew gets there, and what is crawl space encapsulation explains each component.

How to Judge a Contractor's Before-and-After Photos

Most pages about before and after crawl space encapsulation are contractor galleries showing their best jobs under work lights, where a white liner looks good in almost any photo. Look past the color:

Readings tell you more than photos. Ask for dated humidity and wood moisture readings from the inspection and a written promise of a return reading once the space has dried. Our contractor questions list covers the rest of the quote review.

Measure Your Own Before and After

You can document your own results with a $20-$40 hygrometer and a pin-type moisture meter in the same price range; a wireless hygrometer that logs to your phone is worth the small premium. This is a simple plan we recommend, modeled on how the field study took its readings:

1. Log humidity first. Set the hygrometer mid-space on a joist or block, away from vents and the access door, for at least a week, ideally in summer. Note outdoor humidity on the same days.
2. Mark six to ten wood spots with a permanent marker: band joist and floor joist by the access, the same at the wettest corner, a joist near plumbing, the center beam. Skip OSB subfloor and pressure-treated sills; the researchers left those out of their averages because they read misleadingly.
3. Test radon in the lowest lived-in level, and save twelve months of utility bills.
4. Repeat after the work in the same spots. The three crawl spaces closed in 2004 reached their new humidity level within about four weeks; re-read the wood after one to two months, and retest radon.

If the readings do not move, look for an unsealed opening, liquid water still getting in, or a drying system that is undersized or not draining. Our crawl space inspection checklist helps narrow it down.

Questions About Encapsulation Results

When you collect quotes, ask each contractor to record humidity and wood moisture readings at the inspection and to come back for a reading after the space has dried. You can find a crawl space contractor in your area, and our cost by size guide shows what the work typically runs for your square footage.

Sources

1. A Field Study Comparison of the Energy and Moisture Performance Characteristics of Ventilated Versus Sealed Crawl Spaces in the South (Final Report, 2005), Advanced Energy for the US Department of Energy (OSTI)
2. Moisture Solution Becomes Efficiency Bonanza in Southeastern United States, Advanced Energy (Davis and Dastur), ACEEE 2004 Summer Study on Energy Efficiency in Buildings
3. Wood Handbook, Chapter 14: Biodeterioration of Wood (FPL-GTR-282, 2021), USDA Forest Service, Forest Products Laboratory
4. A Brief Guide to Mold, Moisture and Your Home, US Environmental Protection Agency
5. Radon, US Environmental Protection Agency