Crawl space moisture in a high desert valley

Where the moisture comes from

Not from the air. Outside air here is dry for most of the year. The moisture in a Grand Valley crawl space comes up out of the ground, and the ground is wetter than the climate suggests because the valley has been irrigated for more than a century.

US Geological Survey work on the Grand Valley found that irrigation recharge occurs through leakage of unlined canals and ditches and through flood irrigation, and that the shallow water-table system in the unconsolidated material here is largely sustained by irrigation rather than by natural recharge. Soil survey data records wet-season water tables under parts of Grand Junction as shallow as roughly 14 inches in the poorly drained Cojam series, with several other series classified as aquic or oxyaquic, meaning periodically saturated.

A crawl space with bare earth on the floor sits directly on that. Soil evaporates moisture continuously into the space above it, and the framing overhead absorbs it.

What that does to a house

• Framing moisture content rises. Wood that stays above roughly 20 percent moisture content becomes hospitable to decay fungi. Joist ends bearing on a damp stem wall are the usual first casualty.
• Posts rot at the base. A wood post standing on a masonry pier or directly on soil wicks moisture upward. The rot is at the bottom, hidden, and it progresses until the post loses bearing.
• Insulation falls down. Fibreglass batts absorb moisture, get heavy, sag out of the joist bays, and stop insulating.
• Ductwork sweats. Cooled ducts in a humid crawl space condense, which wets the framing further and can drip onto the ground.
• Odour reaches the living space. Air moves upward through a house, so crawl space air ends up indoors whether anyone intended it to or not.

The structural half of the problem

Moisture is one story. Support is the other, and they interact. A crawl space carries the floor of the house on a grid: perimeter stem walls, interior piers, posts sitting on those piers, and girders spanning between the posts. Any one of those can fail.

The symptom is almost always the same and it is not a crack. It is a floor that bounces when you walk across it, or that has developed a dish toward the middle of the house. Homeowners often assume that means nothing because there is no cracking upstairs. In fact it means the support below has moved, which on an older Grand Valley house usually traces to one of three things:

1. A masonry pier that has settled. Frequently set on a small pad or on nothing at all, in soil that changes volume.
2. A post rotted at the base. The moisture story reaching the structural story.
3. A girder spanning too far. Support spacing that was optimistic when the house was built, plus decades of load, produces a permanent sag.

What encapsulation does

A crawl space vapor barrier is a reinforced sheet laid across the ground, run up the stem walls, and sealed at seams and penetrations. It does one thing very well: it stops soil moisture evaporating into the space.

What it does not do is stop liquid water. If groundwater or surface water is entering the crawl space, a barrier laid over it traps water underneath and solves nothing. Drainage comes first: exterior grade and downspouts corrected, and where necessary interior drainage and a sump inside the crawl space itself. Then the barrier.

Encapsulation runs roughly $3.00 to $7.00 per square foot in the Grand Valley, so a 900 square foot crawl space lands somewhere between about $2,700 and $6,300 depending on preparation, debris removal and how much sealing detail is involved.

Vents: the question everybody asks

Whether to close crawl space vents is a building science question that depends on how the space is built, whether it is insulated at the floor or at the walls, and how the house is conditioned. It is a decision for whoever specifies the work rather than a universal rule, and anyone who answers it confidently without seeing the space is guessing.

What is consistent is that ventilation alone does not solve a wet crawl space here, because the water is coming from the ground rather than from the air. In a dry climate, ventilating a crawl space with outside air does help with humidity for much of the year, which is exactly why the underlying ground-source problem gets overlooked for so long.

Order of work

1. Get the water out. Exterior grade and downspouts first, then interior drainage if liquid water is present.
2. Fix the structure. New footings and adjustable steel columns at failed support points, sistered or replaced framing where decay has taken it, and additional supports where a girder span is too long.
3. Replace what decayed. Rotted joist ends, damaged sill plates, degraded insulation.
4. Then seal. Ground barrier up the walls, sealed at seams and penetrations, with access maintained.

Doing these in the wrong order means sealing a barrier over a wet floor, or laying a new barrier and then cutting it up to install piers.

What it costs

Structural re-support points run roughly $700 to $1,600 each, and a typical Grand Valley crawl space job needs between three and eight of them, so $3,500 to $12,000 covers most work. Encapsulation is priced by area as above. Framing repair is priced by the piece. Where the perimeter stem wall itself has settled rather than an interior support, helical piers under the crawl space perimeter come in at normal pier pricing, roughly $1,400 to $2,600 each.

Older housing, and where these jobs cluster

About 18 percent of Grand Junction housing was built in 1959 or earlier, with a median year built of 1992 across the city as a whole. The pre-war and immediately post-war stock is where crawl spaces are most common, and it clusters in the older parts of the city and in the historic cores of Fruita and Palisade, both towns incorporated well over a century ago. Those are also the areas with the longest history of ditch irrigation on the surrounding ground.

Which is the whole point: the oldest housing sits on the ground that has been wettest for the longest, on the least engineered foundations. It is not a coincidence that crawl space re-support is one of the most common scopes in this valley.

Scope detail on the crawl space page, and the water background in the irrigation guide.