Category Archives: Water-Managed Site And Foundation

Crawlspace Drainage System

In the middle of the crawlspace floor area on top of the building pad we place a perforated drain pipe with gravel and cover it with filter fabric.

The perorated drain pipe is installed with a T-fitting to connect a surface drain for the finished concrete floor of the crawlspace.

The 3″ drain pipe from the floor drain to the outside of the foundation wall is non-perforated and sloped so that any surface water from the crawlspace floor from a severe plumbing leak or flood …or good hose out …will drain to the exterior where it terminates in a dry sump fifteen feet away from the house.

A 3-4 inch layer of drain rock is then backfilled into the entire crawlspace.

Any bulk water intrusion whether flood or otherwise will quickly drain out.

The crawlspace is now ready for a sturdy vapor barrier.

Moisture Management of the Crawlspace – Gravel Base

As designers and builders of high-performance homes, we believe in buildings that will remain durable over a long time (like 100 years or more at a minimum). Persistent moisture on wood will eventually lead to rot and failure of joists, sills and posts. Moisture also creates an ideal environment for mold, mildew and fungus which causes poor indoor air quality. Keeping everything dry is the single most important element of durable, healthy buildings.

Our starting point is the installation of gravel below the insulated crawlspace floor so there is a drainage plane underneath the building. Later we install a perimeter drainage system around the exterior foundation walls, but here we are first concerned with the interior area. We installed about 22 cubic yards of 3/4″ drain rock, or a depth of 3 to 4″ throughout the crawlspace.

Our bobcat delivers the gravel from the street to our crawlspace floor.

Freddy spreads the gravel evenly thoughout the space.

Any moisture that might accumulate under the foundation will channel to the drain. See another story about the crawlspace drain.

The first layer of our moisture-managed foundation is completed. On to Step 2: The Vapor Barrier.

Moisture Management of the Crawlspace: The Vapor Barrier

To protect the house from water vapor entering the crawlspace by evaporation of water contained in moist soil, we cover the gravel base with a high performance vapor barrier. The materials are made using virgin high density polyethylene . This bright yellow product is calledStegoWrap. It’s a tough 10 mils thick and very puncture resistant.

Most crawl spaces are not constructed with vapor barriers and consequently are subject to several gallons a day of water evaporating from the damp soil below. This is the reason that most older crawlspaces are vented by building code so that all that water vapor will be removed from under the house by natural air flow through the crawlspace vents. The trouble is that often there are too few vents, they are blocked or poorly sized and positioned such that the area stays damp…not good.

By installing a vapor barrier none of that water vapor ever enters the crawlspace in the first place.

To be effective, a vapor barrier must be continuous so we overlap the seams by a foot or two and use pressure sensitive tape for airtight sealing of all seams and the perimeter attachment to the foundation wall.

We also drill weep holes in the floor drain riser so that if any moisture does happen to collect on top of the vapor barrier it will weep out the drain.

We are now air, water and water vapor tight with a gravel drainage plane below and floor drain above. This meets the EPA’s tough Indoor Air Plus specification for water-managed foundations and crawlspaces. We are ready to insulate the floor.

Moisture Management of the Foundation: Liquid Rubber and Stucco Coatings

The crew is applying a special blend of adhesive, high strength, elastomeric liquid rubber to the exterior ICF walls which will create a water-proof membrane around the foundation. This coating is waterbased and contains no petroleum. It is non-toxic and VOC-free.

On the protected, interior side of the crawlspace we spray on a thin hard-coat stucco product and back trowel it over the relatively soft EPS foam walls of the ICF’s. No water proof membrane is required on the interior side. It’s easy to apply the stucco with an air compressor, hose and sprayer and provides a uniform protective barrier.

On the exterior side we spray on the same hard coat stucco over the water-proof membrane and then embed a fiber glass mesh material into the base coat for added strength. We finish with a second coat over the fiber glass mesh and trowel the finish.

It’s time to install the perimeter drains and backfill.

Moisture Management of Foundations: Perimeter Sub-Surface Drainage

The finished soil grade will be sloped away from the building so that surface water will naturally drain away from the structure. As an additional best practice for managing site and foundation drainage, we have installed a sub-surface “French drain” system at the base of the foundation walls to insure that sub-surface water can never accumulate and seep into the crawlspace. Storm of the century or flood?? …bring it on. This house will stay high and dry.

The building pad is graded so that sub-surface water will naturally flow to the sides and then the rear of the lot. Gravity put to good use.

A French drain or sub-surface perimeter drain is a trench filled with gravel encapsulating a perforated pipe at the bottom. The pipe and gravel are wrapped in a protective “geotextile” fabric to prevent fine soil from entering and clogging the pipe holes. Moisture accumulating in the trench percolates down and enters the pipe which transports the moisture to a point of discharge away from the building.

Here is where it all comes together at the rear (lower grade) of the foundation. The drain pipe coming from under the crawlspace floor from the 4″ gravel drainage plane joins the exterior perimeter drain pipes and runs down and away 20 feet to a gravel pit. There is a backflow preventer as well so that neither air nor water can enter the building through the drain pipe.

The perforated pipe used for French drains is typically manufactured with two parallel rows of perforations (round holes) which are positioned on the under side of the pipe. This guy must be praying to the gravel gods or something?

The perforated drain pipe has been fully encapsulated with at least 6 inches of 3/4″ drain rock on all sides.

The landscape fabric is carefully folded over the top of the gravel to form a barrier against soil intrusion into the drain trench. This foundation drain system is complete and ready for backfill.

The French drain system properly designed and installed below grade at the base of the foundation walls relieves hydrostatic pressure and is a very valuable component of our site and foundation moisture management. This is a best practice required by the EPA Indoor Air Plus certification we are following on this project.