Airtightness in buildings refers to one of the critical indicators that ensure the constant temperature within the building's envelope under the influence of wind pressure and thermal pressure, directly related to heat loss caused by cold or hot air infiltration indoors, i.e., indoor energy loss. The higher the airtightness grade, the less the heat loss. In this context, the bold experiment of applying waterproof and breathable membrane, vapor barrier membrane experience from metal roofing, to windows, brought about unexpected airtightness results.
Passive houses require pressure test air change rates, i.e., air tightness n50 ≤ 0.6 h-1. Higher air tightness significantly reduces indoor-outdoor air exchange. During winter heating, high air tightness minimizes heat loss caused by cold winds渗透 into the room, reducing heating energy consumption. Similarly, during summer cooling, it reduces the infiltration of outdoor hot air, lowering air conditioning energy consumption. Of course, air tightness is not the only indicator for reducing energy consumption; it also includes the thermal transmission of components like doors, windows, and walls.
Traditional construction methods cannot address the airtightness issue of windows. Kodebond's self-adhesive waterproof breathable membrane and self-adhesive vapor barrier film can significantly improve the airtightness of windows and reduce heat loss and thermal conductivity through actual application and tracking experiments.
1. Sealing Design Precautions
1.1 Location and Indication of Airtight Layer
The airtight layer is primarily composed of the building's outer protective structures, including doors, windows, walls, roofs, and floors, typically located on the interior side of the exterior walls and continuously wrapping the entire building. In design drawings, it is usually indicated with a red thick line on the interior side of the protective structures. However, some designs omit the airtight layer, causing construction workers to be unable to identify its location, leading to construction oversights that affect the passive house's performance, as shown in Figure 1.
1.2 Commonly Sealed Areas
Passive exterior doors and windows
The Ministry of Housing and Urban-Rural Development issued the "Guidelines for Passive Super Low-Energy Consumption Green Building Technology" in October 2015.
Regulation: "Exterior doors and windows must have good air tightness, water tightness, and wind resistance performance, with an air tightness grade not lower than level 8." Compared to traditional buildings, passive house doors and windows are all installed in an external-hanging manner. At the same time, the connection points between the door and window frames and the walls should use a waterproof vapor barrier (on the inside) and a waterproof breathable membrane (on the outside) to prevent the risk of air or water leakage at the gaps between the frame and the wall, as shown in Figure 2.
Figure 1: Schematic Indication of Airtight Layer Figure 2: Window and Door Design Nodes
Figure 3: Illustration of installation for self-adhesive waterproof and breathable membrane, vapor barrier in airtightness treatment
The "Code for Acceptance of Quality of Construction and Decoration of Building Decoration Engineering" GB50210-2001, issued by the Ministry of Housing and Urban-Rural Development, specifically states in Article 4.2.4: "The plastering work should be carried out in layers. When the total thickness of the plastering exceeds or equals 35mm, reinforcing measures should be taken. Reinforcing measures to prevent cracking should be adopted for the surface plastering at the junctions of different material substrates." The explanation further clarifies: "Due to inconsistent water absorption and shrinkage properties at the junctions of different material substrates, the surface plastering layers are prone to cracking. Reinforcing measures should be taken to ensure the quality of the plastering work." Therefore, in the design of passive houses, both wall construction and plastering must implement airtightness protection measures to prevent cracking due to uneven settlement or shrinkage of the walls or the plastering layers. Particularly in the design of masonry plastering, it is necessary to clearly specify anti-cracking measures and ensure that the airtightness measures are robust and reliable. 1.2.2 Wall Construction and Plastering
1.2.3 Overall Integrity of the Enclosure Structure
To ensure the integrity of the air-tight layer in the building envelope, during design, sealing measures should be implemented for any penetration through the exterior wall, roof, floor pipelines, or pre-installed pipes, as shown in Figure 3.
2. Sealing Construction Precautions
As the saying goes, "Three parts design, seven parts construction." The precision of the construction plays a decisive role in whether a passive house can achieve its energy-saving goals.
2.1 Window and Door Installation Project
Passive house exterior doors and windows are installed in an overhung manner, with wooden blocks and angle brackets used for fixation on the outer side. The inner and outer sides are sealed with moisture-proof and vapor-proof membranes, respectively. The intricate process can lead to potential air tightness issues. Be mindful of the following concerns:
(1) During the application of waterproofing membranes on both interior and exterior sides, it's challenging to adhere them at the corners of doors and windows, as well as at angle brackets and wooden blocks. Therefore, reinforcement layers are required at these areas. The exterior waterproofing membrane should be applied from bottom to top, ensuring that the overlapping edges face downwards to minimize water seepage risks.
(2) When applying the waterproof film, ensure the base is completely dry to prevent the film from not adhering properly due to damp walls.
(3) During the rainy season, it is advisable to use waterproof mortar to level the exterior walls, especially around door and window openings, to prevent prolonged rain from washing away the walls and seeping into the waterproofing membrane, affecting the adhesive quality.
(4) For non-adhesive vapor barrier and waterproof breathable membranes, the construction method can be simply summarized as "stick, paint, scrape, and press." This means first adhering the waterproof membrane to the window frame, then applying the adhesive in an "S" shape on the wall, followed by evenly scraping the adhesive flat and attaching the membrane, and finally pressing it firmly. It is worth noting that when scraping the adhesive, ensure the adhesive remains continuous without breaks.
2.2 Concrete Engineering
Ensure the concrete is compactly tamped during pouring to prevent defects like honeycomb or rough surfaces. After demolding, seal the air tightness of the tie bolt sleeves with an airtight plug. Due to the numerous tie bolts, it's easy to overlook some, so make sure to seal all sleeves to avoid losing the big picture over small details.
2.3 Masonry and Plastering Work
During masonry construction, factors such as shrinkage cracks and cracking in the masonry should be fully considered, and strict control should be exercised over the selection of masonry units, age, mortar density, and masonry height. For example, with autoclaved bricks and concrete bricks, due to their significant early shrinkage values, they should not be used until 28 days after curing. If the masonry units have defects like damage or cracks, it can adversely affect the strength of the masonry, leading to cracking. During construction,严格控制 the thickness and density of the masonry mortar, as well as the daily masonry height and the time for diagonal laying at the gap between the top of the filled wall and the main structure.
The plaster layer can form an airtight layer and also compensate for defects in wall masonry. Therefore, crack prevention measures should be taken during plastering, especially at junctions of two materials, where measures like mesh hanging are necessary for crack prevention.
Due to the significant impact wall cracking can have on the overall airtightness of a building, it is crucial to fully consider wall cracking factors during the construction and plastering processes. This ensures the reduction of wall cracking risks, thereby guaranteeing the airtightness of the maintenance structure.
2.4 Installation Engineering
Air tightness in passive houses should be addressed during both the design and construction phases, yet the issue often gets overlooked in the operational stage. Taking the floor drain and lavatory sink as examples, air leakage may occur at these areas during the air tightness completion test after the renovation of a passive house project. Article 4.5.9 and 4.5.10 of the national standard GB50015 "Code for Design of Building Water Supply and Drainage" stipulate that the water seal depth of floor drains with water seals should not be less than 50mm and that water seals with anti-drying function should be preferably used. However, the author believes that due to the water seal's tendency to dry out over time and the reduced resilience of anti-drying floor drains after long-term use, odors from the pipes may still enter the indoor space. Therefore, permanent and externally force-resistant air tightness measures are necessary.
3. Airtightness Testing and Evaluation
We employ the "baffled door method" to assess a passive house or a passive house unit, along with reverse pressure testing. A fan is installed in a door or window opening of the exterior protective structure, and all other non-permanent openings indoors are sealed. We sequentially establish a 50Pa micro-positive and micro-negative pressure and measure the air volume flow rate of the fan's suction under this wind pressure. It's noteworthy that larger buildings are more likely to meet the requirement of n50≤0.6h-1. In reality, large buildings with n50 reaching 0.6 may still have significant air leakage issues. Therefore, for large passive buildings where Vn50≥4000m³, it is necessary to test both the air change rate per hour (n50) and the building's permeability (q50), with q50 being less than or equal to 0.6m/(h.m).
