Fabric “skins” for building exteriors
The challenge is finding the right materials in architectural applications.
Fabric Architecture | August 2014
By Bruce N. Wright, AIA
Is the building cladding going to be massive and highly insulated, or is it going to be layer upon layer of thin insulating materials? Will the south and west sides be completely glazed or mostly opaque?
The most important challenge in designing and constructing a building is a waterproof, energy-efficient exterior cladding, and that is harder than you may think. Why else do building material manufacturers continue to invent new materials and new ways of “skinning” the building enclosure?
Many new textiles for architecture on the market today are refinements or improvements of new materials introduced three to seven years ago, and they tend to fall into two or three categories:
- Materials to improve solar control (reduce energy gain),
- Coatings on fabric to improve performance (mostly energy performance), or
- New high performance materials (for precision energy control).
The solar challenge
Perhaps more than any area of building design research, solar control has the aura of the “Holy Grail” about it. With roots in the 1960s, solar design concepts have been on a continual refinement and improvement track and easily have the attention of public and architect alike. The greatest technical advances and market growth for solar have come in the tech sector of photovoltaics (PV), and flexible PV is one of the fastest developing areas.
A recent student–manufacturer collaboration serves to highlight how popular this technology has become, with three universities spanning two continents joining together to research, develop and prototype the integration of flexible PV for their entry in the biennial European solar house challenge.
Architecture schools around the world have enthusiastically embraced the U.S. Department of Energy (DOE)-funded program of research and design called the Solar Decathlon, which challenges collegiate teams to design, build and operate solar-powered houses that are cost-effective, energy-efficient and attractive.
This year’s USA design team includes architecture students and faculty from Rhode Island School of Design (RISD), engineering students and faculty from Brown University, and heating/cooling systems engineering students and faculty from the University of Applied Sciences Erfurt, Germany.
Key manufacturing partners for the project (dubbed “TechStyle Haus” in a nod to the house’s main cladding material and the German-based design movement called Passivehaus), include Saint-Gobain Performance Plastics (fabric cladding), Pvilion (flexible PV and tracking software), and Birdair Inc. (fabrication).
Throughout the more than a year-long process of developing TechStyle Haus, Pvilion met with students, faculty and project partners to help with design and technical issues, all the while developing its next generation PV technology flexible membranes. The modules of flexible PV used on the solar house are monocrystalline silicon cells in a flexible encapsulation (patent pending). The modules are around 18 percent efficiency, at the top end for silicon modules.
Pvilion co-founder and vice president Robert Lerner, AIA, says the project allowed the company to try out its new technology on a small, measurable case study. “We devised a way to attach our PV membranes to the glass fiber skin without adversely affecting the membrane,” says Lerner. “That actually solves a lot of problems such that the PV skin can be installed separately from the main skin, it can be removed and replaced, if necessary, without having to remove the permanent glass membrane,” a method he says also is being patented.
Coatings improve performance
One of the more exciting and widely adopted coatings for textiles over the past 10 years is TiO2 (titanium dioxide), discovered and developed more than 20 years ago in Japan. The process associated with this coating—ultraviolet photocatalytic oxidation (UV-PCO)—involves the interaction of light striking a mineral in the coating that triggers a chemical reaction that breaks up or decomposes organic matter. The TiO2 coating has been most notably used in medical environments to produce self-sanitizing surfaces and as an agent for breaking down smog and odors in exterior environments when used on structural fabrics, such as with tensile architecture roofs and façades. Today there are a number of manufacturers producing TiO2 coatings for tensile structures.
Much of the activity in high performance coatings occurs at the nanoparticle level and many research labs and universities have made this the focus of their research. Brigham Young University mechanical engineering professors Julie Crockett and Dan Maynes have recently published their study of superhydrophobic surface treatments that have the potential to improve the performance of any surface that could benefit from extreme waterproofing, such as solar panels or exterior building cladding. This self-cleaning capability is created with micro-structured surfaces using a process similar to photo film etching onto CD wafers, with the addition of water-resistant coatings.
Another area of active development is in the integrated use of phase-change materials (PCM) especially for improving the thermal management of buildings with membrane enclosures. According to Dr. Barbara Pause, president of Textile Testing & Innovation LLC, Longmont, Colo., who specializes in research and product development with phase-change materials, “PCM changes its physical state within certain temperature ranges. When the melting temperature is reached in a heating process, the phase changes from solid to liquid.”
There are many advantages to using this phenomenon when it comes to building materials, especially with insulation used to regulate heat gain/loss. Dr. Pause recommends using PCM material treatment for building roofs and walls as an improvement over more traditional construction methods.
“For roof systems, the PCM controls the heat flux into and out of the building through the roof components by either absorption or release of latent heat as soon as the PCM’s temperature increases above a certain value or decreases below a certain value. By controlling the heat flux through the roof, the PCM adapts the roof’s thermal insulation to the need.” In other words, a PCM application reduces and smoothes out the peak energy demands during the heat/cool cycle.
Pause estimates that using a PCM application in a roof or building envelope can reduce the annual heating demand of a residential building by 25 percent and energy savings of AC use during summer months could improve by 40 percent.
Coatings also are being developed to improve performance of existing products, such as shade fabrics and meshes. Last year SEFAR Architecture introduced a new exterior sunscreen PTFE fabric with an improved coating that enhances light transmission. EL-30-T1-UV has a UV-absorbing additive in its coating that reduces the amount of UVA and UVB rays transmitted through the fabric. According to SEFAR, “this results in a sun protection factor (SPF) value of 80, yet still provides 30 percent light transmission with minimal color shift.” The fabric is colorfast, dirt and water repellent, has low weight/square meter and high tensile strength.
Serge Ferrari recently introduced Flexlight FX 701, a glass-based architectural fabric with a fluopolymer coating on both top and bottom surfaces that improves natural light and see-through visibility (rated Tv direct 46 percent) and has a high UV transmission rate of 57 percent that allows plants to grow beneath roofs of this material. FX 701 meets the ASTM 136 noncombustible standard and is recommended for tensile and façade applications.
Material science is continually innovating, and the same can be said for new textile-related developments for architectural application. Two developments to watch for in the near future are a thermoelectric fabric that converts heat into electrical current and a flexible, energy-storing fabric battery—both with exciting possibilities for integrating into architectural projects when the technologies are more robust and fully realized.
Developed by researchers at the Center for Nanotechnology and Molecular Materials at Wake Forest University, Winston-Salem, N.C., the power fabric device is comprised of carbon nanotubes encased in flexible plastic fibers that use temperature differences to create an electrical charge. A recently published paper in the Journal of Applied Physics reports that the material can produce 14.7 nanowatts of power per thermocouple, more than 40 times the output of a previous iteration of the device.
Researchers at Wake Forest predict many possible applications for the technology, including insulation for pipes, collecting heat under roof tiles to lower energy use or providing low-level electrical power that can run control systems.
A fabric battery made of a nickel-coated polyester yarn that collects current is being developed by researchers at the Korea Advanced Institute of Science and Technology, Daejeon, South Korea. Research scientists have tested the fabric with flexible polymer solar cells made of PCDTBT, for an efficiency rate of 5.49 percent.
With more improvements in the pipeline of material development, we can be sure architects and fabric architecture manufacturers will benefit from what research presents to the world. Saving the planet, through reduced energy use with these products, will be part of that legacy.
Editor’s note: The initial development for TiO2 coatings for architectural fabric use was done by the collaborative team of Taiyo Kogyo Corp. and Nippon Soda Co. Ltd in 1998. For a good background article on the history of the technology see: "Photocatalyst technology adds value to fabric"