Formulas For A Warmer Floor

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Right now, venting seems to be a bone of contention. Though field research suggests that a well-prepared crawlspace doesn't need foundation vents, building codes literally require them: A typical recommendation over a soil base calls for at least four vents with a total net area of 1/15o the floor area, located within 3' from corners and unobstructed by shrubbery or ductwork. Concerns over radon contamination also favor subfloor ventilation, even if it is only passive.

To compound the problem, vented spaces insulated to the limit have shown an alarming ability to collect moisture during the summer. The reason? Cool earth temperatures sustained by the super thermal barrier are often below the dew point of the humid summer air. Any incoming air is cooled beyond saturation, causing condensation.

Regrettably, untreated wood with a moisture content of much more than 20070 is prone to fungal attack and deterioration. To put that figure in perspective, a 2 X 10 in open air averages 12% to 15010 moisture; in a constantly damp environment, the same framing member may well contain over 50070.

There are solutions. The simplest is to close the vents in the warmer months, which may work well if moisture isn't a concern throughout the rest of the year. If it is, consider these additional measures:

Vapor retarder:
In regions with more than 20" annual precipitation (this excludes the desert states, parts of Colorado and a wide swath from eastern Washington to the Dakotas), a 6- or 10-mil polyethylene vapor barrier can be laid over the floor of the crawlspace once it's leveled and cleared of wood scraps, rubble and other sharp debris. Each section should overlap 6" at the seams, and the membrane should be carried to the top of the foundation wall and secured with industrial sealing tape.

Surface drainage: Rainwater mustn't be allowed to collect within 10' of the foundation. The surrounding earth should have low permeability (clay is ideal) and be sloped away from the structure so there's a 6" drop in the 10' distance. There shouldn't be a sidewalk or driveway impeding run-off: If the roof has less than a 3' overhang, consider adding gutters with downspout extensions.

Subsurface drainage: Chronic moisture may indicate a high water table or poorly drained soil. Simply keeping the crawlspace floor above the exterior grade level will help; if this isn't practical, a perimeter drain or, at sloped sites, a curtain drain buried on the upper side-may be needed to carry water from nonporous soil to a lower exit point or a sump pump. A good drain consists of 4" perforated ABS pipe or drain tile set in a bed of coarse gravel within 8" or 12" of the exterior foundation footing. A silt filter covers the stone, and sand or small gravel can be used as backfill to about 6" of finished grade. Once the excavation is done, it's convenient to apply a standard-duty waterproofing material just in case. Polyethylene, PVC sheeting and bituminous coatings are all well suited to the job.

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