Summary and conclusions

Erfaringsopsamling – Passiv ventilation under huse

In connection with investigations on contaminated sites, problems with unacceptable vapour intrusion to buildings are often identified. To reduce contaminant impact on the indoor air, a number of mitigation measures based on “passive ventilation under buildings” have been established during the last 10 years. The objective is to reduce vapour intrusion by passive ventilation, whereby the air exchange between atmosphere and the soil gas under the building (subslab soil vapours) is increased thereby causing a dilution of pollutant concentrations in the soil gas. Furthermore, the pressure gradient over the floor construction (slab) is reduced, whereby reducing the advective transport into the building.

The purpose of this report is to provide an overview of experience gained applying passive ventilation under buildings as a remediation technique to reduce vapour intrusion during the last 10 years.

In connection with the preparation of this report, information on existing systems for passive ventilation under buildings has been collected by contacting the Danish Regions, the Danish Petroleum Industry’s Association for Remediation of Retail Sites (OM), and the larger consulting engineering companies working in the field of soil and groundwater pollution. The empiric data collection was then sorted into quantitative and qualitative data. Quantitative data for a total of 125 remediation systems was collected, while more detailed qualitative data on for example the construction of the systems, monitoring, and remedial effect was collected for 38 systems.

The 125 remediation systems for passive ventilation under buildings are located equally divided among the 5 Danish regions, however with a slight predominance in Region South Denmark. In most cases, the passive ventilation is established in the capillary break layer (subslab fill) under the house. For some of systems, ventilation of the crawl space or an equivalent structure is established in combination with ventilation of the capillary break layer.

Approximately 20% of the passive ventilation systems are established in connection with the construction of new buildings, while the rest are established in existing buildings. In half of the systems in existing buildings, a new ventilated floor construction has been established, while in the other half, the systems was established under the existing floor.

According to the quantitative information received, more than 60% of the passive ventilation systems provide the intended protective effect, but that 20% of the systems are ineffective or provide no protection. In other words, the effectiveness of the remediation system has only been evaluated for 80% of the passive ventilation systems. For approx. 60% of the systems, it was stated that there were a good data basis. This implies that the assessment of the effect for a number of the systems is based on an inadequate and uncertain data basis.

38 cases were selected for a more detailed examination of the qualitative data. These cases were selected, so that they represent different types of passive ventilation systems in existing as well as in new buildings. Furthermore, the selection was made with a view to assess variation in the performance of the passive ventilation systems.

Examination of the qualitative data for the 38 selected systems for passive under floor-ventilation identified 4 typical constructions systems for passive ventilation:

Double drain system with the ventilation drain in the capillary break layer connected to a wind driven ventilation rotor on the roof and air inlet in the capillary break layer through a gooseneck inlet pipe or similar structure (under the existing or new floor).
Ventilation drain in the capillary break layer connected to a wind driven ventilation rotor on the roof, but without air inlet (under existing or new floor).
Double drain system connected to gooseneck inlet pipe or similar structure. The driving force for the ventilation is based on a method whereby pressure differences between 2 sides of the building produce a weak air flow (primarily established under new floors).
Ventilation of crawl space (existing floors).

The examination of the selected cases has shown that pilot investigations to enable design and dimensioning prior to the establishment of the passive ventilation were only carried out in few cases. Often, the basis for establishment of the passive ventilation system is solely the risk assessment carried out in connection with preliminary site investigation. For many of the passive ventilation systems is it therefore impossible to assess the design basis and which considerations have been taken into consideration for the dimensioning and design of the individual system. To ensure the performance of the passive ventilation systems, it is of great importance that a detailed design basis be produced prior to the design and establishment of the systems.

In approx. 25% of the 38 selected cases, a R.A.C. membrane has been laid out in connection with the establishment of a new ventilated floor construction. After the laying of the membrane, generally no investigations of the airtight seals and integrity of the membrane have been carried out. In a few cases, however, such investigations have been carried out, and they have then shown leaks at corners, along walls, and along connections.

The costs for the establishment of a passive ventilation system under a building vary greatly and depend partly on the size of the building, and partly on the scale of the remedial measures required. The costs for establishment of a passive ventilation system under an existing floor are typically much less than establishment of ventilation under a new floor construction.

For 23 of the 38 selected cases, information was available concerning the economy for the establishment of the system and for operation and monitoring. The costs to establish a passive ventilation system lies between 100,000 – 500,000 DKK for 16 of 23 systems for which there is information on the economy. For 3 of the systems, the establishment costs exceed 1,000,000 DKK, while the ventilation on 4 sites is established for less than 100,000 DKK. All amounts are in DKK, excl. VAT.

There is less variation in the operation and monitoring costs as there is little variation in activities carried out in connection with operation and monitoring of the 23 systems. Typically, the annual operation and monitoring costs lie in the interval from 20,000 to 40,000 DKK, excl. VAT.

After the establishment of the system, the aim of monitoring phase is primarily to document that “passive ventilation under buildings” is sufficient to ensure observance of indoor air quality criteria. Thus, the primary documentation method is by measurement of the indoor air quality. In certain cases, where there is a risk that other pollution sources can have an impact on the indoor air quality, soil gas measurements are often made under the floor only. This problem primarily concerns systems established to prevent vapour intrusion due to subslab oil pollution, since other sources of hydrocarbons in the indoor and outdoor environment can affect indoor air quality.

Other central monitoring parameters are measurements of air flow and pollution components in exhaust air. By measuring the air flow, it is possible to estimate the air exchange achieved in the ventilated capillary break layer by establishment of passive ventilation under buildings. Based on the content of pollution components in the exhaust air, the amount of pollution removed over time (emission) can be calculated and assessed. In connection with the measurements of the air flow, point measurements and continuous measurements over periods of several weeks have been carried out. Point measurements achieve a momentary value of the air flow under the actual wind conditions. Continuous measurements provide a description of the average air flow and thus reveal periods with less air flow, corresponding to periods with weak wind conditions.

On basis of the experiences collected in this study, recommendations for further activities with the remediation method “passive ventilation under buildings” have been prepared.

Due to the generally limited dimensioning basis for the established systems for passive ventilation under buildings, it is recommended that an evaluation tool to assist in the preliminary assessment of the potential suitability of the remediation method in relation to the actual concentrations of pollution components in the soil gas under the building be developed.
In this study, it has been difficult to compare the air flow measurements as they are made under different wind conditions and for different types of passive ventilation systems. In order to get a better description of the expected air flow, it is recommended that future activities focus on a detailed investigation of the correlation between the wind action and the air flow generated by passive ventilation under buildings.
If passive ventilation under buildings proves to be a suitable method of remediation, it is recommended that a conceptual model be prepared to describe and improve understanding of the possible spreading routes to the indoor climate in the building. The conceptual model can be supported by information from a technical examination of the building. The conceptual model is important in order to target the design of the overall remediation solution before implementation.
In the establishment phase, in the case of laying of a diffusion limiting R.A.C. membrane in a new ventilated floor construction, more focus should placed on the documentation of the air tightness of the membrane. Furthermore, the function of the ventilation system by for example ventilation and/or a tracer test should be documented as well as can be achieved.
For documentation/monitoring of the effect of the system, it is recommended that measurements of the indoor air quality in combination with monitoring of soil gas in the ventilated layer be carried out. Especially, in connection with indoor air measurements in connect with remediation systems to reduce oil pollution, it is important to be aware of other contributions. Empirically, it is difficult in the case of oil pollution to distinguish contributions from vapour intrusion under a building from other contributions within the building (household cleaning agents, wood burning stoves etc.). In such cases, it may be more appropriate to only document the effect based on monitoring of the soil gas in the ventilated layer in order to avoid sources of error.
For documentation of the ventilation in the ventilated layer, it is recommended that measurements of the air flow are carried out. It is further recommended that continuous measurements are made in order to include periods with a low air flow under weak wind conditions. For comparison, it is recommended in the same period to measure the wind speed locally or alternatively collect data from the nearest measuring station. This will help define expectations concerning the average level of ventilation achievable in the ventilated layer.
At the same time, it is recommended that monitoring of the content of pollution components in the exhaust air be carried out. Compared with the air flow measured, the amount of pollution removed over time (emission) can be estimated and assessed.
Furthermore, it is recommended that a method for carrying out differential pressure measurements between the ventilated layer and the indoor air be prepared in order to be able to assess the absence of a driving force producing advective gas transport from the ventilated layer into the house.
Monitoring results achieved for the individual sites during the monitoring period demonstrate large variations, and therefore, in order to document the effect of the system, it is recommended that monitoring should be carried out several times under different conditions after the establishment of passive ventilation under floor before the monitoring is completely stopped.
To support future application of the remediation method, it is recommended that a method catalogue be prepared. This catalogue can include guidelines and descriptions of practical tools/methods for dimensioning, design, establishment, and monitoring of systems for passive ventilation under buildings.