RADONSAFE FOUNDATION, MOISTURE PREVENTION AND AIR EXCHANGE IN A HEALTHY BUILDING

Project leader: Hannu Arvela, Radiation and Nuclear Safety Authority (STUK),  P.O.Box 14,
FIN-00881 Helsinki, Finland, tel. +358-9-75988 470, e-mail: Hannu.Arvela@stuk.fi
 
 

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TIIVISTELMÄ SUOMEKSI

Researchers:
Helsinki Univ. of Technology (HUT), Lab. Of  Structural Engineering and Building Physics
Jukka Bergman, tel. +358-9-451 3762, e-mail: Bergman@rakserver.hut.fi
Reijo Yrjölä, tel. +358-9-451 3714, e-mail: Yrjola@rakserver.hut.fi
Ari-Veikko Kettunen tel. +358-9-754 4090, e-mail: Ari.Kettunen@humi-group.com
Jukka Piironen, tel. +358-9-451 3726, e-mail: Jukka.Piironen@hut.fi
Martti Viljanen, tel. +358-9-451 3713, e-mail: Martti.Viljanen@hut.fi
Helsinki Univ. of Technology (HUT), Lab. Of Heating Ventilating and Air-Conditioning
Jarek Kurnitski, tel. +358-9-451 3609, e-mail: Jarek@cc.hut.fi
Kaj Jokiranta, tel. +358-9-451 3592, e-mail: Kaj.Jokiranta@hut.fi
Miimu Matilainen, tel. +358-9-451 3604, e-mail: Miimu.Matilainen@cc.hut.fi
Radiation and Nuclear Safety Authority - STUK
Anne Voutilainen, tel. +358-9-7598 8474, e-mail: Anne.Voutilainen@stuk.fi
Kaj Vesterbacka, tel. +358-9-7598 8651, e-mail: Kaj.Vesterbacka@stuk.fi
Kari Kuuspalo

Financing SYTTY organisation: Tekes
Funding from SYTTY / Total funding of project (€): 162336 / 238675
Person-months of work funded by SYTTY / Total person-months of work: 30 / 34,2

KEY WORDS: radon, radon prevention, moisture, air exchange, ventilation
 

EXTENDED ABSTRACT

1  Introduction

High radon concentrations of indoor air in low rise residential buildings form an important national health problem in Finland. Decrease in the use of crawl space in house foundations has increased the indoor radon concentration of Finnish housing stock during last decades. The prevailing type of foundation is slab-on-grade. In this type of foundation the flow of  radon-bearing air from soil into living spaces  through gaps between the foundation wall and floor slab should be prevented through special measures. The aim of this project was to develop simple construction practices, which can be utilized for prevention of these leakage flows and which form simultaneously a qualified moisture prevention for the foundation structures /1,2/. Several building companies participated in the project. The project aimed at development of  radon technical practices of the companies. The effect of activated sub-slab radon piping on the drying process of the floor slab was tested in two houses.

The study focuses also on  the applicability of different ventilation strategies, the control of depressure in dwellings  and use of fresh air vents for reduction of indoor radon concentration. One of the objectives of the study was to find improvements to air intake vents in flats with mechanical exhaust ventilation and to develop the radon-safe foundation construction practices of blocks of flats.
 
2  Materials and methods

The construction of  foundation of 20 houses was followed up. In the  first phase  10 houses were studied. In 8 of these the house owner was participated the building work actively. In the second phase the studies were made in cooperation with building companies and suppliers of element based single family houses. The following companies participated:  Skanska Oy , NCC Oy, VVO Rakennuttaja Oy, Pyhännän Rakennustuote Oy ja Kastelli Oy. Katepal Oy brought an important expertise on bitumen felts to the project.  All measures related to the air-tightness of the foundation as well as difficulties and drawbacks were documented. When the houses were ready and in normal use indoor radon concentration, air exchange rate and depressure were measured. The effect of activated sub-slab radon piping on the drying process of the floor slab was tested in two houses.

As a sub-project the indoor radon concentrations of 80 new houses built in the Building Fair 2000 area were measured. These results give a representative view on state-of-art of radon-safe building.

The effect of the activated sub-slab piping on the drying process of the floor slab was tested in two houses. Humidity probes were installed both into the floor slab and sub-floor gravel layer. The moisture content of the slab and sub-slab gravel was measured during 3-20 months.

The influx of radon-bearing soil air was studied in 7 recently constructed flats on the lowest floor and with floor slab in ground contact. In some of the houses sub-floor radon piping had been installed as a preparatory method in order to be able to reduce the radon levels. Through depressurisation of the piping, using a fan, the leakage of radon-bearing soil can be prevented.  Testing the indoor radon concentration with both non-activated and activated fan provided a method to estimate the effect of air flows from soil to indoor radon concentration.

3  Results and discussion

3.1  Radon-tight foundation
The radon-tight construction practices published in the guidance of the Ministry of Environment  was studied in the project /3/. In addition a new more easy-to-do construction was studied. The construction given in the guidance requires a high level carefulness. Along the results of this study the sealing of the joint between the foundation wall and floor slab using elastic sealants fail often in real building site conditions.

A new construction for a radon-tight joint between the foundation wall and floor slab was developed for houses with slab on ground or houses with basement. The results were very promising. In the new construction a wide (0.5-1.0 m) strip of bitumen felt will be installed above the foundation wall. The other edge of the strip will be installed beneath of the concrete slab in direct contact with the casting. The key advantage is the exclusion of the elastic sealants, which was often failed in the earlier construction. The tests made in laboratory ascertained that the adhesion of the felt to concrete was good.

In the sealing of the feedthroughs the methods presented in the guidance of the Ministry of Environment showed to be working. The practical experiences showed that special attention should be paid to the development of the guidance of sealing the water pipes and the electric wiring which are installed in protective piping beneath the floor slab. Above-slab installation would be preferable.

The results showed that all foundation constructions should be constructed airtight. Omitting the sealing work for example in storerooms or in rooms for house technology may lead to increase in radon concentrations in living spaces. In these auxiliary spaces radon concentration was in some cases 5000-10000 Bq/m3, when the sealing was omitted.

The results obtained from the Building Fair 2000 area showed that air-tightness of the foundation was best in the terraced houses and weakest in houses with walls backing soil. In terraced houses and in houses with crawl space indoor radon concentration exceeded the target level of 200 Bq/m3 only in a few cases. In houses with walls backing soil in none of the houses radon concentration was below 200 Bq/m3. Lightweight concrete blocks used in the wall increase the flow of radon-bearing soil air into houses.

3.2  Air exchange and underpressure
Low rise residential houses
The measurements in the 40 low-rise residential houses resulted in air exchange rates of 0.3-0.7 1/h, when the adjustment was in the normal position.  However, the possibilities for control of ventilation were in some cases limited. The maximum ventilation rate was in some houses 0.5 1/h or the minimum was 0.7 1/h. The underpressure in houses with mechanical exhaust ventilation was typically 7-10 Pa, which was remarkably higher than in houses with supply and exhaust ventilation, 1-5 Pa.  This difference affects also the indoor radon concentrations. Mechanical supply/exhaust ventilation seemed to be the only safe ventilation solution in order to control the indoor radon concentration in radon prone areas. In one of the houses the depressure remarkably distorted the operation of the sub-slab-depressurisation system used for decreasing the indoor radon concentration.
 

Apartments
Underpressures of 10-40 Pa were measured in apartments /4/. This increases significantly flow of radon-bearing air from soil into indoor spaces. The tests showed that the under-pressure and indoor radon concentration in the apartments could be reduced by 50 % by installing air intakes. Due to the pressure drop in the air-intake, greater reductions in radon concentration can not normally be achievable.  A new type of air-intake was developed in the project. According to the results of the simulation, only limited pressure drop can be achieved by installing the second air intake into rooms. Higher reductions in radon concentration can be achieved through installation of a new supply and exhaust ventilation system or through other remedial measures. The normal sealing procedures used by the builders provided in none of the cases measured, a complete barrier against the leakage of radon-bearing soil air into the apartments.

3.3   Effect of sub-floor ventilation on floor slab drying
Radon piping to be installed below the slab can be utilised in improving of the drying process of the floor slab /5/. The study focused on the improved drying process of the floor slab through the use of sub-slab ventilation. A significant difference of several weeks in the drying time was observed as a result of the ventilation. Simultaneously, significant amounts of water were transported out from the sub-floor soil through the piping.   In certain cases radon piping can be utilised also in connection of water damages

4  Conclusions and outcomes

- A new construction for an airtight joint between the foundation wall and floor slab was developed for houses with slab on ground or houses with basement. The construction prevents the flow of radon-bearing air from soil into the house.
- The results will be utilised when improving the present guidance material.
- The bitumen felt used in the construction works also as a qualified moisture insulator.
- A new guide to be published in the Finnish RT-Building Information File is under preparation.
- The practice will be recommended for a wide use in the whole country.
- The key detail in the construction is the right installation of bitumen felt to the joint between foundation wall and floor slab.
- The companies that participated in the project have renewed their radon-technical practices.
- Depressor caused by the mechanical air exchange should be taken into account in the planning of both low-rise residential buildings and flats. Mechanical exhaust ventilation may increase significantly both depressure and indoor radon concentration.
- In flats with no air intakes, significant reductions in radon concentration are achievable through installation of new air intakes.
- Mechanical supply/exhaust ventilation seemed to be the only safe ventilation solution in order to control the indoor radon concentration in radon prone areas.
- The possibilities to control the air exchange rate in low-rise residential buildings are sometimes limited.
- Radon piping to be installed below the slab can be utilised in reduction of the indoor radon concentration and also in reduction of the moisture in floor slab and sub-slab gravel.
- In certain cases radon piping can be utilised also in connection of water damages
 

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