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The skins game: economy of scale in fabric structures

May 1st, 2012 / By: / Feature, Structure Basics

The low and high cost of insulating fabric structures.

It has been 45 years since the German Pavilion was erected for Expo ’67 world’s fair in Montreal, Q.C., Canada. The structural system of this early fabric structure was a steel cable net that supported a pre-stressed textile membrane. The 7,200m2 structure was open on all sides, with wind-deflecting glass walls on the ground floor and an operable skylight on top that would be open for natural ventilation in summer and closed with aprons against the roof membrane for heating in winter.

The purpose for using a fabric structure then was to create a low-cost roof with a translucent material that allowed natural daylight to filter in and would reduce the need for artificial lighting, heating and cooling.

Some things never change—or do they?

Today’s membranes are more durable and translucent than ever and some materials—like ETFE and insulated membranes with components such as aerogel—offer higher R values and additional benefits, but at what cost to the owner? Membranes come from all over the world and can have a life span of more than 30 years, yet the perception is that some materials are becoming so expensive that they are competing in price with traditional materials such as wood, glass and metal. However, the membrane may be the least of your cost.

When it comes to choosing the right “skin” for your building (new or re-cover), consider the structure’s other costs (steel, installation, access) and weigh the initial cost of all components against the operational, service and maintenance costs required on the roof over time. The biggest advantage fabric has over traditional materials is its translucency and its ability to reflect direct UV rays.

The solution to choosing the right skin is to balance the “needs” versus “wants” of your project and do your homework.

  • When is your facility being used? Some facilities are used more often during the day and may not need as much artificial lighting.
  • Where is your structure located? Structures in cooler climates tend to require more energy than structures in warmer climates.
  • Are stainless steel cables necessary? In many cases PVC-jacketed galvanized cables, which are lower in cost, may be acceptable.
  • What paint finish does the project need? Paint finishes vary from painted steel to powder coated steel to stainless. They all come with different initial cost or long-term servicing.

Choosing the membrane may be the most difficult decision to make because most people are unfamiliar with its performance as both building envelope and structure.

To insulate or not to insulate

There are a number of ways to insulate a fabric structure, including applying batt insulation to a liner, installing a single layer with a liner below it and blowing air between the layers or using state-of-the-art ETFE or insulated membranes.

“The primary issue with insulation of tensile membrane roofs is whether and how much light transmission is desired,” says David Campbell, PE, president of Geiger Engineers. “If daylighting through the assembly is not a design objective, tensile membrane roofs can be insulated with any conventional insulation in a ceiling assembly or liner. For example, the student center building at the University of LaVerne, California, is insulated with conventional fiberglass bat insulation. The assembly is opaque.

“Blowing air between an outer fabric layer and a liner does not constitute ‘insulation,’” says Campbell. “What it does is keep dry air at the underside of the outer fabric to reduce condensation.” One should also study the operational cost involved in blowing air between membranes. The blower does not need to be on all the time. However, it will increase net heat loss and therefore is less energy efficient than not blowing air at all.

Batt insulation is primarily used in frame supported membrane structures and not tensile structures and reduces, if not eliminates light transmission. This insulation also requires an additional step in the installation process.

ETFE, which is a film not a fabric, comes in translucency as high as 98%. As a cushion or foil, ETFE can come in two and three layers and offer R-values in the 2 to 4 range, but it requires a blower running 24/7 to be structurally stable. According to Campbell, “ETFE cushions are all well and good but air movement from convection and the makeup air in the cushion cells results in heat loss. The ETFE cushions have leakage that requires replacement no matter what the thermal goals might be.”

Tensotherm™ is a proprietary material created by Geiger Engineers, Birdair Inc. and Cabot Corp. and manufactured by combining Teflon®-coated fiberglass and aerogel. Aerogel is a synthetic, porous material derived from a gel in which the liquid component of the gel has been replaced with a gas. It provides thermal insulation with light transmission in a compact composite membrane panel installed in a single operation. The aerogel insulation medium is hydrophobic, maintains its thermal insulation value when compressed and does not deteriorate with age.

To insulate or not to insulate

Another perspective in the skins games comes from Richard Nelson, inventor of the Liquid Foam Insulation (LFI) technology, which uses bubbles in between layers of highly translucent fabric for a system he calls SolaRoof. According to Nelson, “The SolaRoof building technology provides full environmental control in all climates, especially temperate and northern locations, at a low cost.”

The SolaRoof is ideal for cold climates, efficiently collecting solar heat gain during the day and conserving this low temperature energy using the LFI overnight. In warm climates, the SolaRoof can prevent overheating by using the LFI to reduce heat gain and a water cooling process for temperature and humidity control within the SolaRoof building.

Nelson’s goal is not so much the use of insulated membranes for long-span structures but for sustainable and more humanitarian purposes. In collaboration with Life Synthesis and Phoenix Planning Design, Nelson has designed the AgriPOD, a greenhouse project suitable for both urban and rural use. It is particularly suitable for areas considered unusable for growing due to lack of water and good soil. The AgriPOD can be used on rooftops, car parks but also on ground or rocks. By using hydroponic and other revolutionary growing techniques, crops can be produced in places previously believed unsuitable.

“What makes this project different is that it aims to develop a revolutionary new solar bubble greenhouse that reduces the amount of water required and extends the growing season, thus increasing food production,” says Nelson. “Essentially, the benefits are that the system creates a totally controlled environment and allows food to be produced year round extending the growing season. This means having two, three or four crops a year depending on the situation and location. There are also added benefits such as reduction in the amount of water required and, if set up properly, purifying saltwater to enable it to be used in the food production process.”

Nelson is working with Norway’s Hydro Group to develop the AgriPOD project for the Rio + 20, the United Nations Conference on Sustainable Developments to be held in Rio de Janeiro, Brazil, in June. Prototypes have been made (6m by 9m) but the concept shows promise in sizes larger than 900m2. The goal now is to make the system low in cost. Clear glass scrim/HDPE film laminate with an aluminum frame is currently used, and the cost and size of the bubble mechanical system is based on the size of the structure. The structure has recently gained interest from the equestrian market.

So how do you like your fabric structure?

A fabric structure is primarily made of three components: steel, cables and fabric. The structural system used can vary depending on collaboration between the owner and the design team (designer, engineer and fabricator). Some clients prefer a structure with more steel than tensile fabric. The steel and structural components of a tension fabric structure can be more than 50% of the overall cost. It is up to the designer to find the most efficient design based on the structure’s criteria and purpose.

The same could be said of the cables or edge treatment. Cables are associated with tension fabric structures and come with their own set of costs. End fittings and installation play a big role. By replacing catenary edges with clamping and straight edges, the price increases.

Finally, the owner or client’s representative needs to do homework on the membrane. Fabric structures can come in single layers, multilayers and insulated membranes. They all come with a cost beyond the cost of rolled goods. Although these types of membrane systems work for enclosed roofs, many fabric structures are “open air” structures and these systems present another issue: sound absorption and sound reflection.

Whatever materials are used, the addition of light-transmitting thermal insulation results in roughly 50% more cost than a structure with a single layer PTFE and a liner.

Samuel J. Armijos, AIA, is vice president of FabriTec Structures, a brand of USA Shade and Fabric Structures. He is author of Fabric Architecture: Creative Resources for Shade, Signage and Shelter.

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