The Necessity for Improvement of Building Air Tightness, Water Tightness and Permeability

The building‘s air tightness, water tightness and permeability are closely connected with its functions. Now they are respectively described as follows:

 

1. Improving the air tightness and water tightness of building

It has been demonstrated through practices that the tightness of envelope structure can hardly be ensured no matter what form of vertical plane or what form of roof pavement is adopted. Taking the exterior wall as an example, the render cracking is unavoidable, and the board can hardly be tightly overlapped, while the wall simply made of masonry bricks and stones are not “a wall that no wind can pass through”, which are particularly obvious in the female and male corners of building, on the lapping joint and around windows and doors. Through the porosity which is not air tight, the air flow and vapor can intrude the envelope structure, developing serious impact upon the energy consumption, durability and comfort ability of the building.

（1）energy consumption of building

——The rain water and vapor intruded into the envelope structure via the airflow shall reduce the effective thermal resistance value of the insulation layer, resulting in more energy consumption. Of course, the vapor has different impact on different insulation material.

——The inbreaking airflow forms a convection circulation within the envelope structure of the building, also reducing the effective thermal resistance value and developing more energy consumption. In the same manner, the airflow has different impact upon different insulation material.

——With the airflow breaking into the indoors, people has alternative but to compensate the heat /energy loss using huge heating and ventilation facilities, increasing the energy consumption of building.

（2）Endurability of building

The vapor invaded into the envelope structure shall condensate in the wall cavity and speed up the aging of building material, such as corrosion of steel and rotting of wood, seriously affecting the endurability of building

（3）Comfortability of building

——the invasion of airflow shall have bad effect upon the indoor thermal comfortability

——the contaminants deposited on different building components and radioactive emanation from building material or outdoors shall be brought in , affecting the indoor air quality indoors.

——The vapor entered envelope structure of building shall create conditions for the vapor’s compensating and development of mold, bringing affection upon the indoor air quality and the health of occupiers. 

It can be seen that the improvement of air tightness and water tightness of building is of vital importance to the energy conservation and environment protection.

 

2. Permeability of building

Moisture condensation within the envelope structure is unavoidable due to the existence of temperature difference across the house. But as a matter of fact, the damping problem which reversely affects the energy consumption, endurability and conformability of the building (as mentioned above) shall happen only when the envelope structure has been in a watery condition for a long time. Therefore, it is particularly important to avoid such problems, ensure the permeability of the building and realize a natural drying out by diffusing the vapor toward outdoors.

To sum it up, the energy consumption, endurability and conformability of the building are closely related to the air tightness and water tightness as well as permeability of its envelope structure, while the weather barrier technique is just intended for archiving the purpose of energy saving, protection of residence environment and improvement of endurability of building by controlling those factors. Moreover, research shows that such technique can effectively ensure the drying out of envelope structure (exterior wall and roof) and provide an obvious heat insulation result in combination with the usage of air layer.