Radon in the Living Environment, 135 RADON MITIGATION IN BLOCKS OF FLATS H. Arvela Radiation and Nuclear Safety Authority STUK P.O. Box 14, FIN-00881 HELSINKI, Finland Fax +358 9 759 88 556, email: hannu.arvela@stuk.fi The average indoor radon concentration in Finnish flats is 80 Bq/m 3. Typically walls have been made using concrete elements. Building materials are the dominant source of indoor radon. However, in the flats of the lowest floor, with a floor slab in direct ground contact, the main source of radon is often the inflow of radon bearing soil air. The number of these ground contact flats is less than 10 % of the total number of flats. The average indoor radon concentration of these ground contact flats is 150 Bq/m 3. Consequently in more than 10 % of these dwellings the concentration limit of 400 Bq/m 3 is exceeded. The data presented is based on a nation-wide radon survey in 900 flats. Highest concentrations observed in flats have exceeded 10.000 Bq/m 3. Best mitigation results were achieved using radon wells and sub-slab-depressurisation, the typical radon concentration reductions being 50-80 %. High depressures in flats decrease the efficiency of both radon wells and sub-slab-suction installations. In comparison, the radon concentration reductions for radon wells, obtained in low rise residential houses where depressures are lower, were typically remarkably higher, 80-95 %. The exhaust ventilation, in combination with the air tightness of the concrete elements and windows, causes remarkable underpressures over the floor slab, increasing the inflow of radon-bearing air from soil. In older flats no fresh air vents have been required. Installation of fresh air vents has been an efficient mitigation method in flats. In the cases studied radon reduction rates exceeding 50 % have been achieved. Sealing entry routes has typically resulted in varying reduction rates of 30-60 %. In order to achieve good results a rather complete sealing of the gap between the wall and floor slab should be performed. However, in many cases a complete sealing work is tedious and expensive. On the basis of the results achieved, it is recommended that a case-specific analysis of the installation of fresh-air vents and entry route sealing should be first considered for radon remedies of flats. If these methods are not adequate, sub-slab-suction and radon well should be considered. Keywords: Radon, radon mitigation, indoor air INTRODUCTION High indoor radon concentrations are a common problem in Finnish low rise residential houses. Similar problems has been observed in blocks of flats in the dwellings of the lowest floor where the floor slab is in direct ground contact. According to the resolution of the Ministry of Social Affairs and Health, in already existing houses the indoor radon concentration should not exceed 400 Bq m -3. Future buildings should be planned and constructed in such a way that the radon concentration does not exceed 200 Bq m -3. According to the nation-wide survey of STUK, 400 Bq/m 3 is exceeded in about 5 % of the Finnish low rise residential houses (Arvela et al. 1993). In flats with floor slab in ground contact, the percentage of dwellings exceeding 400 Bq/m 3 is even higher. STUK has also studied the mitigation measures undertaken, the main emphasis having been in the low rise residential houses (Arvela and Hoving 1993). The results show that active methods sub-slab-depressurisation (SSD) and radon well have given best results. Concerning mitigation in blocks of flats special problems have been met. First the building structures set special requirements. Second the exhaust 1113
135 Radon in the Living Environment, ventilation commonly used in combination with the very tight structures of the flats result in high underpressures. This affects the choice and the efficiency of mitigation methods in flats. In this paper indoor radon concentrations in flats and results from mitigation measures have been considered. MATERIALS AND METHODS Indoor radon concentrations in flats and the effect of substructures was studied using the material of a previous study carried out in 1990-1991 (Arvela et al. 1993). This study measured the indoor radon concentration in the dwellings of 3074 persons, selected randomly from the central population register of Finland. Alpha detectors and two consecutive half year measuring periods were used. Altogether 2171 low rise residential houses and 903 flats were measured. The data concerning the substructures was obtained through a detailed questionnaire sent to the residents. The data regarding the remedial measures taken was obtained from several case-studies (Arvela et al 1999) focusing on the effect of sealing work, sub-slab-suction, radon well and ventilation repairs. Alpha detectors with a measuring period of two months has been used both before and after the remedial measures for indoor radon measurements. RESULTS Factors affecting indoor radon concentration Table 1 shows indoor radon concentration for different categories of flats. All residents did not answer the questions concerning the existence of a cellar. Thus the results do not give a representative view on the prevalence of flats with floor in ground contact in Finnish blocks of flats. In the ground floor flats with ground contact (no cellar) the average radon concentration, 212 Bq m -3, is about 200 % higher than the average concentration in the upper floors, 68 Bq m -3. The difference in the geometric means is about 100 %. In old flats with brick structures the concentrations are only slightly lower than in flats with concrete structures. The mean radon concentration of flats with wooden structures is strongly affected by an old wooden house (3 floors) on an esker and with a very leaky substructure. The results presented in Table 1 show clearly the effect of substructures on indoor radon concentration of the flats on the lowest floor. The arithmetic and geometric means are higher by a factor of 2-3 in the flats with floor slab in ground contact than in ground floor flats with cellar. The percentage of flats with floor slab in ground contact, exceeding the reference concentration of 400 Bq/m 3 is higher than 10 %. Correspondingly the percentage of this type of flats exceeding 200 Bq/m 3 is estimated at higher than 20 %. Sealing work The chance of succeeding in sealing the entry routes in flats is better than in low rise residential houses. This is because the foundation walls and wall elements are normally made of concrete elements, which do not allow the radon bearing air to enter the wall element and further from the wall into living spaces. This the difficulty met, when trying to seal the leakage flow through the gap between the foundation wall and floor slab in low rise houses with wooden bearing structures in walls. In blocks of flats this gap is in most cases attainable and possible to treat with elastic sealants. However in many cases other fixed structures like lockers and separating walls make it difficult to 1114
Radon in the Living Environment, 135 seal the gap totally. However, a complete sealing work may be required in order to achieve high radon reduction efficiencies. The efficiency of sealing work was studied in 7 flats. The work was carried out either by home owners or by building enterprises, which had no special former experience in indoor radon mitigation. The average indoor radon concentration in these flats was 700 Bq/m 3 and 380 Bq/m 3 before and after the remedies. In six cases 400 Bq/m 3 was exceeded, before mitigation and in three flats the concentration was lower than 400 Bq/m 3 after the remedy. Typical reduction rates achieved were 30-60 %, the maximum being 88 %. The ventilation conditions and underpressure in the flats may have been varied, affecting the indoor radon concentrations measured. This problemacy has been considered below. In older blocks of flats built in the 19 th century or in the beginning of the 20 th century the floor constructions of the lowest floor flats may be wooden. STUK tested different remedial methods in two old flats of this kind. Sub-slab-suction did not result in remarkable radon reductions. The subfloor spaces were complicated and inaccessible. Therefore, it was neither possible to create underpressure nor to reduce the sub-floor radon concentration through ventilation. Best result was achieved through sealing the joint between the plastic flooring and wall, the radon concentration being reduced from 1000 Bq/m 3 to 400 Bq/m 3. A better reduction would have required expensive reconstruction of the separating walls which prevented a complete sealing work. Ventilation studies Table 2 illustrates underpressure measurements carried out in a flat in Helsinki. The exhaust ventilation operates at intensified level during some hours in the morning and during dinner time. The air exchange rate was measured from the exhaust vents and the underpressure over the floor slab. The intensified ventilation increases relatively more the underpressure than the air exchange rate. The front door of the flat comprises an outer door and an inner door, which together dampen noise from the stairway. The inner door is tight and affects remarkably the underpressure in the dwelling. The underpressure was measured in the middle of the floor slab, the pressure difference on the borders of the floor slab being higher. There were no fresh air vents in the flat. The indoor radon concentration in this flat was 630 Bq/m 3 during the first winter season measurement. The gap between floor slab and the wall was sealed, resulting in an radon concentration of 260 Bq/m 3. No fresh air vents were installed. Figure 1 shows the effect of underpressure on indoor radon concentration in a flat in Vantaa town. Before the remedial measures the radon concentration was 600 Bq/m 3. Before decisionon mitigation method STUK recommended to test the effect of installation of fresh air vents. The test was carried out by keeping the ventilation window slightly open during a period of four days, simultaneously the indoor radon concentration was monitored. During this period radon concentration decreased to below 100 Bq/m 3. Later fresh air vents were installed to window frames, resulting in a radon concentration of 250 Bq/m3. 1115
135 Radon in the Living Environment, Sub-slab-depressurisation In blocks of flats the foundation constructions often complicate the introduction of sub-slabdepressurisation (SSD) installations. Load bearing walls with deep foundation walls divide the floor area into many parts which require each a separate suction pit. The underpressure resulting from the exhaust ventilation and tight concrete wall structures decrease the effect sub-slab underpressure. The underpressures created by the exhaust ventilation are typically 5-20 Pa. However, the low power fans (50-100 W) normally used for SSD in single family houses create typically sub-slab underpressures of 1-5 Pa, on the borders of the floor slab. The efficiency of SSD installation can be improved either by increasing the fan power or preferably by decreasing the underpressure in the flat. Radon well Radon wells have been utilised with success in areas where the building soil is coarse gravel. In the town of Lahti, in the southern Finland, there is an suburb with 25 blocks of flats built on esker. In the lowest floor flats of these houses high indoor radon concentration were measured, the average indoor radon concentration in 24 flats exceeding 400 Bq/m 3 was 4200 Bq/m 3. Altogether 23 radon wells were installed and the average radon concentration decreased to 400 Bq/m 3. Only in four flats the radon concentration remained above 400 Bq/m 3. After chancing the position of two radon wells and increasing the power of a few radon wells none of the flats exceeded 400 Bq/m 3. The underpressures in the flats were not considered when building the radon wells. Most probably installation of fresh air vents would decrease the radon levels further. CONLUSIONS On the basis of the results achieved, attention should be paid on decreasing the underpressure in flats, when considering radon remedies in flats. This a special problem of older flats, because fresh air vents are required today in new blocks of flats. Also in the case of new building the pressure drop in intake air vents should be considered. It is recommended that a case-specific analysis of the installation of fresh-air vents and entry route sealing should be first considered for radon remedies of flats. If these methods are not adequate, sub-slab-suction and radon well should be considered. REFERENCES [1] Arvela H. Mäkeläinen I. and Castrén O. Residential radon survey in Finland. Radiation and Nuclear Safety Authority, STUK-A108, pp. 28, Helsinki 1993, (abstract in English). [2] Arvela H. and Hoving P. Finnish experiences in indoor radon mitigation. Proceedings of the 6th International Conference on Indoor Air Quality and Climate, July 4-8, 1993, Vol 4:563-568, Helsinki 1993. [3] Arvela H. Rissanen K. Kettunen A-V and Viljanen M. Radon mitigation in blocks of flats. Radiation and Nuclear Safety Authority, STUK-A162, Helsinki 1999, (in press, abstract in English). 1116
Radon in the Living Environment, 135 Table 1: Annual average radon concentration in different categories of Finnish flats, results from the nation-wide study of 1990-91. Type of flat N Radon Conc. Mean Bq m -3 Radon Conc. Gmean Bq m -3 Blocks of flats, concrete - ground floor, no cellar 31 212 122 - ground floor, with cellar 69 78 68 - ground floor all 99 121 82 - upper floors 563 68 61 All concrete flats 674 76 63 Brick structures 70 62 56 Wooden structures 17 468 81 All flats 903 82 63 Table 2: Air exchange rate and depressure over the floor slab in a lowest floor flat in Helsinki. Normal air exchange rate Intensified air exchange rate Air exchange rate 0,35 h -1 0,62 h -1 Underpressure, inner front door open 2,6 Pa 8,3 Pa Underpressure, inner front door closed 4,1 Pa 12,5 Pa 1117
135 Radon in the Living Environment, Figure 1: Reduction in indoor radon concentration in a lowest floor flat as a results of opening a ventilation window slightly for a period of 2.5 days. Later installation of fresh air vents decreased the radon concentration to 250 Bq/m 3. 1118