Complementary instructions

Complementary instructions are additional instructions related to the application of the RT reference card RT 81-11099, LVI 37-10513, KH 27-00510.

The instructions are based on STUK’s own studies and experiences of the authorities, companies and developers.

1. Exhaust pipe must be installed open all the way out

The exhaust pipe is installed above the roof during construction. This eliminates alterations which are necessary in retrofitting. The end of the pipe must be left open and equipped with a suitable rain cap. Passive radon piping reduces radon concentration even without a suction machine. However, the effect is much less efficient than what is achieved with suction.

2. Looped or branched piping

Branched piping is the best solution for semi-detached houses and row houses. As extremely coarse backfill materials that are highly permeable to air are nowadays used under the slab, the efficiency of looped piping even in a semidetached house may be sufficient only in the part of the house in which the exhaust duct is installed. On the other side of the house, air flow may be insufficient.

3. Sub-slab suction may be inefficient

The capacity of sub-slab suction to reduce the radon concentration is limited. For houses that have coarse backfill material and, possibly, additional coarse material underneath and outside the foundation wall, sub-slab suction may be sufficiently efficient only at very strong currents of air. An airflow that lowers the radon level enough may exceed the rated airflow multifold. Exceeding the rated airflow may lead to problems with cold or frost.  Therefore, sealing must be installed with care, and radon prevention must not rely on the radon piping only.

4. The need for sealing

The sealing of the joint between the ground slab and footing with bitumen felt and sealing of penetrations are essential radon prevention measures. Sub-slab suction may not function properly in demanding conditions, which is why radon prevention should not solely rely on it. Sealing can also promote the functioning of sub-slab suction. The guidelines are based on the assumption that both the sealing and the installation of the radon piping are carried out.

5. Sealing of the bitumen felt joints

Unsealed bitumen felt joints will leak. Bitumen felt joints in straight sections and corners are made to overlap by at least 50 mm. Joints in weldable products are sealed by heating. If glueable (not weldable) products are used, the joints are glued using suitable adhesives (such as sealing adhesive Katepal K-36 or hot bitumen). The seams of molten corner joints are secured with sealing adhesive when necessary. Practical instructions can be found in manufacturers’ RT reference cards, such as Perustusten kosteuden ja radonin eristys, RT L-37529, RT/KH 382-37529. Katepal Oy.

6. Avoid penetrations in base floor

Penetrations that break the bitumen felt reduce tightness and cause demanding additional work. Try to avoid electric conduits in the floor slab. Joints remaining at the bottom surface of the slab can be leakage routes. Using seamless conduits reduces the probability of leakage. Concentrating penetrations to the engineering and utility services room, for example, facilitates the sealing work.

7. Sealing of penetrations

In unfavourable conditions, a single unsealed penetration can increase the radon concentration above the action level. Penetrations should be sealed as early as possible, before they are encased inside partition walls. When necessary, additional casting and suitable elastic sealing compounds are used for sealing in accordance with the RT reference card.

8. Sealing is required also in the engineering and utility services room

Unsealed penetrations and floor joints in the engineering and utility services room can increase the radon concentration of the room to several thousand Bq/m3. Air usually moves from the engineering and utility services room to living areas, increasing their radon concentration.

9. Ground-supported walls and stepped structures

Lightweight concrete block walls that are in contact with soil are significant routes for leakage. The entire outer surface of such a wall that is underground must be sealed with bitumen felt in accordance with the manufacturer’s instructions. A similar sealing requirement applies to stepped structures. In stepped structures, the joint between the vertical wall and the slab is sealed with bitumen felt, applying the instructions provided by the RT reference card and manufacturers.

10. Crawl space construction and monolithic slab are safe solutions

With regard to radon control, crawl space foundation and monolithic slab are technically easier and more reliable foundation solutions than the normal solution with a ground-supported floor slab and separate foundation wall. In houses with these foundation solutions, the maximum value is exceeded considerably less often compared with ground-supported floor slab solutions. In houses with crawl space, the base floor and its joints and penetrations must be carefully sealed, and the crawl space must be properly ventilated. Monolithic slabs do not have leaking seams or joints, but it is still important to seal penetrations.

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