1、 Analysis of condensation water on steel roof

 When the water vapor in the air reaches its saturation value at a certain temperature, it condenses into liquid water, that is to say, condensation occurs (it is more common in northern winter and southern summer).In the light steel structure building, the general roof system condensation is divided into “surface condensation” and “internal condensation”.”Surface condensation” is mainly shown on the facing layer (veneer) on the external surface of thermal insulation cotton, especially at the place where cold bridge occurs; “internal condensation” is mainly manifested in the side of insulation cotton surface facing the outdoor, and this part of moisture will remain in the insulation cotton, so it is called internal condensation.According to the actual situation of dewing on the site, it can be seen that there are two kinds of condensation in the workshop, especially on the surface of insulation cotton veneer. 

Condensation water phenomenon of steel structure roof

First of all, the phenomenon of “internal condensation” can prove that part of water vapor has penetrated into the interior of insulation cotton, and there are mainly two ways of penetration: one is from the surface of thermal insulation cotton; the other is from the longitudinal lap joint of thermal insulation cotton.The first is the reason why the insulation cotton must use moisture-proof veneer. VR veneer, also known as imported white polypropylene plastic veneer, has a water vapor permeability of about 5.17ng/n.s (i.e. under test conditions, the water vapor permeability of 50000 square feet of surface layer is 5.17 × 490 pounds in a week). It can be said that the veneer is still a good moisture-proof veneer (but worse than W38).The second case can indirectly explain the importance of longitudinal lap construction of thermal insulation cotton. The correct way of overlaying and tightening after 360 crimping is adopted for longitudinal lapping of thermal insulation cotton. This method can play a certain role in air blocking, but if a special insulation cotton repair film is pasted on the longitudinal lap joint, the effect will be better.

 Secondly, because the factory’s “surface condensation” is displayed on the surface of the thermal insulation cotton veneer, and considering that there is no flame-retardant extruded polyvinyl foam block under the roof panel connections, the location of condensed water is not mainly concentrated on the surface of the upper flange of the purlin. Therefore, the actual phenomenon from “surface condensation” can also indicate that the upper part of the ceiling of the workshop is too hot and humid.The main reason is that there is no effective ventilation and dehumidification measures.

 2、 Analysis of measures to solve the condensation water of steel structure roof

 According to the two ways of causing “internal condensation” water vapor penetration, and considering that the thermal insulation cotton veneer itself is the use of moisture-proof veneer, the solution should be mainly in the improvement of longitudinal lap joint of thermal insulation cotton.Therefore, the original part of the insulation cotton can be repaired, and part of the longitudinal lap joint is sealed with the insulation cotton repair film.For some seriously damaged parts, a layer of insulation cotton can be added. The two ends of the insulation cotton are compacted on the abdomen of the roof purlin through galvanized strips, and then locked with self tapping screws.Considering that the original insulation cotton, especially near the roof ridge, is soaked in condensed water to a certain extent, the roof ridge cap can be removed to cool the insulation cotton under this part for a certain period of time. When the edge closing is re installed, some new thermal insulation cotton will be added to the roof ridge. 

Measures to solve condensation water of steel structure roof

According to the dewing temperature of 18.4g/m3 in winter, the condensation temperature of 18.4g can be calculated according to the dew density table, that is to say, the dew temperature of 18.4g should be taken for each dew point.If the design adopts effective measures to reduce the relative humidity of the upper space to 45%, the dew point temperature will drop to 5 ℃. After the connection piece is increased and the corresponding thermal insulation measures are taken, the outdoor calculation temperature that can ensure no condensation can be reduced to about – 10 ~ – 15 ℃, which can avoid condensation in general areas.Secondly, there is a controversy about the way of setting up the fire retardant plastic extruded polyvinyl foam board between the connecting piece and purlin. In the light steel structure system, there has been controversy. Sometimes it is really too much to sacrifice the structure’s tight connection to solve the condensation problem. For this reason, the use of the heightened roofing connection piece, this non polyvinyl foam insulation board is the overwhelming majority.It is effective to prevent the condensation on purlins by several areas (at least according to the standard design conditions. Maybe considering the trampling situation in construction, it seems that it is not very serious to add insulation board).Based on the above analysis, it can be said that the most effective anti condensation measures are to strengthen the ventilation and dehumidification of buildings.

 In view of the above analysis, it can be said that from the ceiling system of the workshop, it should be a very effective method to make the flat ceiling into a sealed ceiling. The principle is to use steam blocking to reduce the air humidity in the upper space.In fact, the keel system of the suspended ceiling of the workshop is open frame type, while the keel system of the sealed ceiling plate is often of the concealed frame type, which is not easy to be sealed.The surface of the workshop may be gypsum board ceiling (similar in appearance), but it should be calcium silicate board (characterized by better waterproof), and some areas are empty.According to the characteristics of the ceiling, two aspects should be considered for sealing and steam blocking. Firstly, the hollow ceiling board should be treated, and the joint powder of the special ceiling board can be used for treatment. Secondly, the sealing between the ceiling board and the keel should be carried out. Considering the vibration, elastic putty can be used for treatment.

 After sealing and steam blocking treatment for the ceiling of the workshop, it can be said that other treatment for the upper space is relatively unimportant.Considering the actual effect of ceiling sealing, it is better to install two sdf-ii series wing type textile axial-flow fans on both sides of the longitudinal walls. It is recommended to use 8 × 10 fans (the exhaust air volume of the fans will reach more than 20000m3 / h, which is more economical), and the 4 fans can be basically emptied every 2 hours, which can be said to greatly reduce the probability of condensate generation.Of course, the installation of exhaust fan in the upper space has a certain impact on the lower environment, that is, the investment in the lower space for constant temperature and humidity environment is increased (that is, the equipment power is increased, and the energy consumption is increased).

 Adding two rows of steam pipes at the bottom of the two rows of longitudinal purlins on the roof ridge can increase the dehumidification effect and effectively reduce the probability of water vapor reaching its saturation value at a certain temperature.Since there are many ceiling hangers at the bottom of purlins, these suspenders can be used for this purpose. In this way, it can be considered that the steam pipes can be fixed by pipe clamps and suspenders in the range of about 1m below the two rows of longitudinal purlins on the roof ridge.Relatively speaking, this method will also increase the construction difficulty due to the limitation of the upper space.

 3、 Summary of treatment scheme of steel structure roof condensate

 After the detailed analysis of the above improvement measures, the treatment scheme is summarized as follows:

 　　1Firstly, remove the edge closing of the steel structure roof cap, let the insulation cotton under the part cool for a certain time, and then add some new thermal insulation cotton at the ridge to increase the thermal insulation effect;

 　　2At the same time, the special insulation cotton repair film is used to seal part of the longitudinal lap joint. The special insulation cotton repair film is about 4 inches wide and belongs to self-adhesive type; 

Treatment scheme of condensate water on steel structure roof

　　3In some places, the newly added insulation cotton can be directly inserted into the upper flange surface of purlin, which can improve the thermal insulation performance of some positions where the cold bridge may be;

 　　4If the above three items are completed, a layer of insulation cotton can be added to some seriously damaged parts. The two ends of the insulation cotton are compacted on the abdomen of the roof purlin through the “galvanized strip”, and then locked with self tapping screws;

 　　5Take down the empty ceiling board (calcium silicate board), plug it with the joint powder of special ceiling board, and then install it again after leveling (the joint powder is 2000 yuan / ton);

 　　6Considering the vibration, the elastic putty can be used to fill and seal the joint between the ceiling and the keel (the elastic putty is about 10000 yuan / T), and the construction amount is relatively large.If the construction quantity is too large, the adhesive tape with good quality can be simply pasted on the joint of the lower surface of the ceiling board, but the validity period of this method is not long;

 　　7Two sets of sdf-ii series wing type axial-flow fans for textile industry are installed on both sides of the longitudinal wall (brick wall). It is recommended to adopt 8 × 10 fan, the exhaust air volume of the fan will reach more than 20000m3 / h, and the power is only about 3KW, which is relatively economical;

 　　8Two rows of steam pipes are added at the lower part of two rows of longitudinal purlins on the roof ridge. Many ceiling hangers at the bottom of purlins can be used. Therefore, the steam pipes can be fixed by pipe clamps and suspenders in the range of about 1m below the two rows of longitudinal purlins on the roof ridge.Only one method can be selected between this method and the seventh method. The difference is not much in economy, but it is relatively difficult to increase the steam pipe construction;

 　　9After the treatment of the upper space, the lower air-conditioning environment should be improved.After adding exhaust fans to the upper space, it is necessary to appropriately increase the investment in the lower space on the requirements of constant temperature and humidity environment (that is, with the increase of equipment power, the energy consumption will increase).

 The processing time of the above insulation cotton should be carried out before the arrival of summer. Otherwise, the upper space environment will be extremely bad in hot weather. At the same time, the suspender of ceiling board should be used properly.

 It is better to install the wing type flow fan on the civil wall than on the roof. After all, it is very difficult to install the fan by opening holes on the steel structure roof and increasing the skeleton.