Designing curtain walls to water penetration plays critical role in keeping the safety, thermal performance and comfort of the structure and residents.
Wind driven rains and condensation are considerably problematic and difficult to deal with while curtain wall is designed in addition to gravity, kinetic energy, capillary action and surface tension which increase the possibility of water ingression.
The design of curtain wall against the ingression of water and condensation are discussed.
Fig.1: Waterproofed Curtain Wall
Design of Curtain wall for Water Penetration and Condensation
- Wind driven rain and the likelihood of water ingression
- Design of curtain wall to control water penetration
- Design of curtain walls to control condensation
Wind Driven Rain and Likelihood of Water Ingression
There are five different types of forces such as gravity, kinetic energy, capillary action, surface tension and air pressure difference that contribute either partially or as a whole to the ingression of water.
It is claimed that, the ability of curtain wall to withstand forces, which increase water intrusion possibility is based on glazing details, drainage details, frame construction, weather stripping and frame gaskets, perimeter flashings and sealings and interior sealants.
Wind loads generate pressure differentials which may lead to windblown rain that surpass gravity force and consequently force water to move upward.
Surface tension properties and capillary effect of curtain wall elements are considerably influenced by thermal expansion of different building materials. For instance, expansion or contraction of materials due to temperatures may tight expansion joints excessively and eventually increase capillary action between various components of the curtain wall.
Curtain wall surface tension properties may vary due to contraction and expansion and lead to undesired results. Therefore, it is required to design movable joints, seals and gaskets to make rooms for differential movements between different members.
Design of Curtain Walls to Control Water Penetration
There are different curtain wall systems that employed different techniques to prevent water penetration. It is important for the designer to be familiar with manufacturers details to prevent undesired situations. That is why the detail of curtain wall systems to prevent water ingression is explained in the following sections:
Structural Silicone Glazed System
This system whether it is two sided or four sided have the most severe performance requirements for sealant adhesion and water penetration at both exterior face and frame-glass interface.
In structural silicone system, moisture ingression is controlled through the provision of cavities that force water to weep holes at the stack joint.
Unitized Curtain Wall System
Unitized curtain wall system is placed using pressure equalizing principle that disrupts forces which push water across barrier.
The air pressure differential is a major force that drive rainwater into the curtain wall assembly. This force is blocked by pressure equalized concept that composed of airtight self-contained compartments and air vents permits air pressure to vent and equalize and finally discharge any moisture at the sills which might penetrate the system.
Wet glazing and pocket sills which accumulate and disposed penetrated water out of the system, need to be sloped properly to have a satisfactory performance.
Stick Built Curtain Wall System
The water proofing performance of stick built curtain wall system is dependent on the completeness and integrity placement process of seals at shear blocks and glazing gaskets at glass interior side in addition to pressure plates that need to be placed appropriately.
The diversion of water from vertical to horizontal pressure plate is conducted at zone plugs which are specifically designed for this purpose.
Design and Detailing of Flashing and Weep Holes
The provision of perimeter flashing contributes in the waterproofing of curtain walls and their links to the adjacent wall elements. An all situations, the drainage system need to be designed to control rain and condensation.
Curtain wall drainage may be provided by large weep holes placed at small intervals, frames sloped to the exterior and drainage at each horizontal mullion.
To prevent the blockage of drainage path, both designers and producers are required to communicate properly and coordinate the installation of setting blocks with the weep holes.
Design of Curtain Walls to Control Condensation
In most cases, curtain walls are designed to withstand surface condensation by introducing certain measures such as frame thermal breaks and insulations. These measures along with other considerations are discussed in the following sections:
Aluminum is employed in the curtain wall frame construction has a considerably high thermal conductivity. So, materials with low thermal conductivity such as neoprene, PVC, polyurethane and polyester reinforced nylon is provided to enhance condensation performance of the curtain walls.
Such material isolates interior aluminum material from exterior aluminum material and consequently breaks thermal conductivity from inside to outside and in opposite direction.
Thermal breaks in forms of protective gaskets are provided in the pressure glazed system. This protective gasket, which works as a thermal breaker, cannot sustain waterproofed for long time, so it is required to dispose water and vapor which have penetrated the system at the corner of the gasket employing proper means for instance weep holes.
Insulations of Curtain Wall
Thermal performance of curtain wall parts, which are not translucent, is based on not only insulation but also vapor barriers. Condensation problem and the loss of energy cannot be tackled unless insulations are placed adequately at the perimeter of curtain wall.
An example of insulation is mineral wool insulation installed at the perimeter of buildings. Not only does it offer thermal insulation but also fire protection as well.
Finite Element Analysis
If condensation play significant role in certain structure for instance high interior humidity buildings, then it is required to specifically conduct finite element analysis for the project.
Finite element analysis permits the estimation of energy efficiency of the product and local temperature patterns. This information may be associated with issues of condensation, moisture damage and structural integrity.
There are software that may be applied for finite element analysis, such as Therm which is capable of modeling two dimensional heat transfer effects like wall and door in which thermal bridge are of great importance.