Basement and Crawl Space Strategy – AVID (Air, Vapor, Insulation, and Drainage)

The AVID System was developed over the course of nearly thirty years by Henri Fennell to address the need to safely install a complete thermal envelop in basements and crawl spaces with un-reinforced stone foundations. This approach evolved from work in hundreds of residences and commercial buildings with stone foundations and poor or unknown drainage characteristics that could be at risk for structural damage from frost if retrofitted with insulation. It also addresses the need to maintain healthy indoor air quality and reduce materials costs for irregular, un-pointed (large open spaces) stonework. This system is designed to provide the following:

1. Excellent insulation at the sill and top of the foundation wall where it is exposed to the outside temperature extremes

2. Continuous air and vapor barriers

3. A drainage path to manage water that typically migrates through this type of construction, at least on a seasonal basis, which is isolated from the conditioned space

Before: Classic dry-laid stone foundation with signs of seasonal water infiltration and frost damage

After: Completed AVID crawl space installation

While this system does not maximize the potential for thermal performance, warmer below-grade temperatures and the overriding needs for conserving energy, controlling cost, and protecting the structure make this approach the best possible compromise. Testing at a 90-unit multi-family complex showed that insulating to a shallow depth, while providing complete air and vapor barriers, still yielded superior performance in this cold New England climate.

This test demonstrates that the humidity of the air inside the crawlspace of the modified unit is very close to that of the living space upstairs. This reduces the likelihood of condensation and related mold, mildew, and insect infestations.

This test demonstrates that the inside temperature of the air in the crawlspace of the modified unit is very close to that of the living space upstairs. This reduces the likelihood of condensation and reduces the likelihood of pipe freeze-ups, frost damage, and comfort issues in the rest of the unit. Even though the duct work had been air sealed and the crawl spaces were not actively conditioned, they were able to maintain “indoor” conditions.

This system includes spray-applied polyurethane foam (SPF) insulation from the bottom of the first floor deck sheathing to a level below grade that provides optimal insulation value against heat loss due to outdoor temperatures, usually about one or two feet. Limiting the insulation depth allows some heat to penetrate to the soils outside the lower foundation, while saving energy by insulating where the temperature difference is the highest at the top of the foundation.

The top of the wall is insulated; then, the A/V barrier is suspended from the SPF just above the high-water level to complete the envelope and provide a drainage outlet for seasonal water infiltration.

Leaving the bottom of the foundation uninsulated and exposed to warmer interior temperatures reduces the risk of frost pressure caused by freezing of the water present in the soils immediately on the outside of the lower portion of the foundation wall. The SPF controls air leakage and vapor diffusion at the top of the foundation. In order to complete the envelope and maintain drainage, the air and vapor (A/V) control is extended down to the bottom of the wall using an airtight, watertight, vapor retarder membrane. This material also provides a “drainage plane” or water-proof barrier that separates ground or surface water passing through the stone wall from the conditioned space. This separation does not prevent the flow of water down to the bottom of the foundation wall, where it is conducted to a drain or sump.

Early project sketch of the AVID System in section showing the configuration at, and beyond, a bulkhead window location.

Diagram showing how winter snow accumulations that creates ponding and directs water into the basement even though the finished grade is pitched away.

The top of this drainage plane membrane is anchored into position by the SPF at a level determined to be above the highest elevation of seasonal and ground water ingress, thus guaranteeing an outlet for any hydraulic pressure. Ideally, there is a gutter or other means around the perimeter of the foundation, outside of and under the floor membrane. The bottom of the membrane connects to the vapor retarder incorporated into the floor of the basement or crawlspace, or is sealed to the edge of the slab to make it airtight. The slab should be sealed with a vapor retarder paint or sealer if there is no sub-slab vapor retarder.

This method forms a continuous vapor retarder in the crawl space, preventing water or rising damp from the soils, concrete slab, or foundation walls from evaporating into the indoor space, and completes the air barrier system. Ballasting is sometimes required in crawlspaces to stabilize the A/V barrier in residences with significant mechanical depressurization or stack effect from a poorly-performing air barrier at the upper-level of the residence.

The membrane on the lower portion of the wall has no appreciable R-value and could allow moisture entering from the ground side (outside) of the membrane to condense on the outside of this relatively cool surface; however, mold growth in this location poses no risk to the indoor environment as the membrane also forms a barrier which isolates the moisture and potential mold growth location from the conditioned space.

Detail of membrane (blue line) trapped at bottom of the SPF insulation providing drainage pathway and A/V Barrier.

Condensation on the conditioned or inside of the cooler membrane is limited to minimal amounts as the moisture content of the air in the conditioned space is small and not being continually replenished by venting. Because the AVID system creates conditions similar to those inside a normal above-grade living space, the limited amount of moisture and food content on the inside surface of the membrane is unlikely to support mold growth or rot on the interior side of the membrane.

The AVID system has been acknowledged by Dr. Nathan Yost of Building Science Corporation as a viable system for addressing air leakage, vapor control, insulation, and drainage while reducing the risk of mold development and frost action in foundations with poorly drained soils and/or ground water infiltration.

Finally, the air barrier should always be tested to confirm continuity. Air leakage quality assurance protocols ensure that the air barrier is complete. This not only provides protection against energy loss, it reduces the likelihood of attracting vermin and indoor air quality problems caused by below-grade soil gasses and mold/mildew. Quality assurance testing is low cost, non-destructive, and prevents localized failures and callbacks.

In summary, the AVID System offers a practical solution for improving energy performance without risking frost damage at a reasonable cost. It has been proven in hundreds of applications including commercial buildings, historic landmark buildings, and many residences. It protects against frozen pipes, vermin, and moisture-related indoor air quality and rot problems.

Additional Notes:

1. The drainage system can be designed to also allow for the creation of a sub-slab, low-pressure zone for soil gas (radon, etc.) remediation. Radon testing is recommended after air sealing the foundation and sills.

2. Basements and crawl spaces typically should not be ventilated (from the outdoors), but this is especially important when the outside humidity is higher than the inside.

3. Indoor air quality problems may occur if the vapor retarder is not extended down to and across the floor of the space. It is possible to determine if the slab (or similarly, a foundation wall) is a source of moisture migration with a simple do-it-yourself test.

4. The AVID System does not eliminate moisture in basements and crawl spaces with internal moisture sources. For example, dryers should be vented outside and washing machines should be plumbed directly to a drain, not run across the floor to an open floor drain.

1- Dry slab surface-not apparently a source of moisture

2- Lay a bag on the floor overnight (or hang one against a suspected foundation wall)

3. Remove the bag and see if the slab is wet. This indicates that the water vapor is migrating into the interior space from the ground through the floor or wall.