A curtainwall differs from a window or window-wall in one fundamental way:
Whereas windows fit within an opening, curtainwalls fit outside the structure. Curtainwall mullions and glass run vertically past the slabs. Curtainwall mullions are anchored back to slab edges.
There are categories of curtainwalls:
Stick built curtainwall – The fabricated parts are delivered to the site like a Tinkertoy(tm) set and are assembled and glazed on site. This is in contrast to…
Unitized curtainwall – Glazed panels are assembled and glazed at the factory and “hung” on the building. To continue the toy metaphor, this would be more like the Kenner Girder and Panel Set(tm) .
I-Beam – The primary structural mullion is an unfinished I-beam shape. Once anchored, other parts such as horizontals or trim are attached to it, hiding the structural mullion from view.
Captured glazing – All four edges of the glass are mechanically captured within the framing, as opposed to…
Butt or Structural Silicone Glazing – Here, some or all glass edges are visible, usually from the exterior, separated only by weatherseal sealant. On the interior, the glass is “glued” to the mullions using structural silicone. Yes, Matilda, there are buildings where the only thing holding the glass on the building is silicone caulk! But don’t worry, the glass hardly ever falls out.
Pressure glazed – The glass is set onto the face of the frame, then another piece – the pressure plate – is screwed down, clamping the glass between frame and pressure plate. Matilda, you will feel much safer walking walking under a pressure glazed curtainwall.
Jiggle glazed – to install the glass, remove one stop only (usually at the head) slide the glass over the lower stop into a deep pocket at one jamb, close the glass like a door, drop it into the sill pocket, then slide it into the shallow jamb pocket.
CURTAINWALL WATER CONTROL SYSTEMS
Curtainwalls have been around since the 50’s (that’s 40 years before the internet) so there has been time for innovation and experimentation. But with all of this experimentation, there are only a few types of water control systems in use.
End dammed / compartmentalized – In this design, the area immediately around the glass edge is segregated from adjacent lites through the use of sealant or baloney sandwiches, usually sealant. Actually I wasn’t kidding about the baloney sandwich. One time, after a failed water test, we disassembled the frame and found a partially eaten baloney sandwich inside. It did not have sufficient water resistive properties to form a credible end dam, so the test failed. In compartmentalized walls, there will be weep holes for each lite because water is not allowed to flow from lite to lite.
Water diverter – The glass in almost all walls sits on a shelf at the bottom of the lite. Due to the way the frame joinery works, this shelf – sometimes called a stem, does not run all the way to the corners. In a compartmentalized wall, this space is filled with a plug which prevents water from traveling. In the case of a water diverter wall, the stem is continued almost all the way to the end. It diverts water laterally (sideways) so that it does not drip onto the top of the glass but rather runs down an internal passageway within the mullion and drains somewhere below.
Now that you are a curtainwall expert and will be able to captivate cocktail party audiences with your knowledge of curtainwalls and drainage paths, here is what you will need to know about how curtainwalls can leak. I am presenting these in the order of frequency found in my own experience.
- Corner sealant and plugs/diverters not installed properly.
- Water gets on top of glass, enters the building between glass and head gasket. I once witnessed a test where the curtainwall passed a 15 minute test, but as soon as the pressure was turned off, water poured down between top of glass and interior gasket. The pressure of the test compressed the glass against the gasket, sealing the area against the water. When the pressure was turned off, the glass was no longer compressing the gasket, and water sitting on top of the glass easily flowed past the loosely fitting gasket, which brings me to…
- Insufficient gasket pressure. A well compressed gasket will repel air and water quite well. If the compresion is too loose, air and water performance will suffer.
- Poorly sealed splices. Since even extruded aluminum has a finite length, eventually you will have places where one piece ends and the next begins. These splice areas must be sealed in such a way as to preserve what I call the “sealant plane” of the system. There can be no discontinuities between the seal of the interior gaskets, the end dam seals and the splice seals, or leaks can occur between these elements.
Since each manufacturer has their own recommended details, it is wise to study the installation manual and sealant details provided by the manufacturer for the exact system you are using. To know your curtainwall, know the intended water path. This will guide your thinking in determining why the water is going down an unintended path. Happy curtainwalling!
Have curtainwall leaks?