Design Tools

“Before the glazed façade was designed by an architect in collaboration with the trade firms that would execute the work. Today, experts in load-bearing structures and materials, electrical specialists, security and fire-protection consultants, and professionals in thermal currents, com-puter simulation, and wind-tunnel testing are all likely to be involved in the development of the new façade systems. “ — Eike Becker in the book: Headquarters Building of Verbundnetz Gas AG, Leipzig. 1999. Munich: Prestel Verlag.

“Green architecture requires close collaboration between architects and engineers. And a building’s environmental components are not bolted-on attachments; they are designed for particular climate condi-tions and client needs.“ — NYTimes , April 16, 2000, p.37, section 2, by Herbert Muschamp and Architecture League website http://www.archleague.org

Design tools enable architects and engineers to predict the performance of new building systems prior to construction and to improve the performance of existing building systems after occupancy. Performance includes a broad array of parameters: energy use, lighting quality and quantity, building operations, acoustics, condensation resistance, structural, etc. We limit this discussion to tools needed to predict the thermal and daylighting performance of façade systems as related to building energy use and occupant comfort.

One would expect that with the increasing number of buildings that feature advanced façades, a suite of design tools would be available that enables designers (architects and engineers) to determine the impact of advanced façades on building performance. Such tools do exist for many systems, but these tools are generally developed in-house and are proprietary and/or require significant engineering expertise and time that is disproportional to the resources of a conventional project. Many of these tools are used simply to provide performance estimates under the worst-case design conditions. Year-round performance is typically not modeled unless called for by the energy codes or requested by the exceptionally diligent client. Yet, the architectural literature claims increased energy efficiency, improved comfort, improved indoor air quality, etc. Since no post-occupancy field evaluations have been done, how are these claims substantiated?

Pragmatically, A/E firms are tasked to solve complex, multi-dimensional problems within short order to meet the demands of the client and the budget. In this section, we describe the basic concepts or algorithms that are used to predict the thermal and daylighting impacts of façades on building performance (i.e. energy use, comfort, HVAC design, etc.). We explain how models for high performance façades differ from basic algorithms, and describe the research in progress or needed to properly model advanced façades.

In simple terms, we define a façade system’s thermal performance by the total transmitted short-wave solar radiation and heat transfer (conduction, convection, and long-wave radiation) through the façade. This total heat gain places a load on the mechanical system and/or results in a rise in air or skin temperature (for example, if sun shines on an occupant). Mechanical engineers use such information to determine how large to size the capacity of the building’s space-conditioning system. Thermal performance indices are also used in energy codes, which often prescribe minimum or maximum values or allow a whole energy budget to be met if the building is simulated using state-approved calculation procedures.

The underlying indices for daylighting are based on the same fundamentals as thermal indices, but are not as critical with respect to energy codes and standards. Daylighting indices determine the visual performance and impact on occupants. In business-as-usual practices, daylighting indices are typically considered in a qualitative experiential manner (e.g., Building X used a glass with a visible transmittance (Tv) of 0.40 and it seemed fine, therefore it should be applicable to Building Y).

To arrive at such thermal and daylighting performance indices, one needs to look behind the scenes to understand why commercially-available design tools do not have the capabilities to routinely and easily model almost all of the systems described on this website.

The two main thermal performance indices, U-value and solar heat gain coefficient (SHGC), and daylighting indices, visible transmittance (Tv), are derived from measured data and computational models. These indices are based on the optical and thermal properties of individual glass, gap and shade layers. Optical properties include transmittance, reflectance, and absorptance properties. Thermal properties include long-wave emissivity, air film convective conductance, layer conductance and gas fill conductance. Solar radiation transmitted by a system of glass and shading layers depend on the solar transmittance and reflectance properties of the individual layers. Solar radiation absorbed by the façade system enters the glazing heat balance equation that determines the inside surface temperature of the glass and thus the heat gain from the glazing. Transmitted solar radiation is absorbed by interior room surfaces and therefore contributes to the room heat balance (Winkelmann 2001).

For homogeneous transparent glass such as clear or tinted, uncoated or coated glass, simulation modeling is straightforward and accurate. Free software is available that provides optical data for all glass manufacturers’ commercially available products and allows users to build up layer by layer any arbitrary assembly of glass layers, gas fills, and conventional spacers and frames (see WINDOW5, THERM and OPTICS reference below). Energy codes accept calculations made for these standard systems with these approved rating tools (for example, the ASHRAE 90.1-1999 accepts only National Fenestration Rating Council (NFRC) methods for determining U-value and SHGC). European and International Standards, such as EN410, EN673, ISO 9050, and ISO 10292, consider only non-diffusing materials and allow calculation of solar energy properties, color appearance, ultraviolet transmittance, and thermal emittance through spectral measurements performed at (near) normal incidence. For translucent materials, these standards recommend measurements made using an integrating sphere. Measurements for angular-dependent optical properties, view through, glare, redirection, diffusion and scattering have all been defined but not necessarily standardized.

For systems that use any of the following types of materials and assemblies, tools do not exist or can be significantly inaccurate while energy codes require special procedures to arrive at approved ratings or indices:

• Materials that transmit or reflect light in a non-linear manner, such as light-redirecting daylighting materials, prisms, metallic sun shades, angular selective coatings, louver or blind systems, woven metal fabrics, fritted, patterned or etched glass, etc.
• Systems that have between-pane air gaps with an aspect ratio greater than 1:40, between-pane shading layers, or that have forced or natural ventila-tion within the gap.

The first item is categorized as optically complex: for transparent or non-transparent materials with two-dimensional or three-dimensional complex geometries which scatter solar radiation in an unpredictable manner, determining thermal performance indices is not straightforward.

At this time, samples of such systems are measured by approved testing agencies (see NFRC reference below). Imagine, for example, a Venetian blind with a semi-glossy painted surface tilted at a particular angle. Some solar radiation passes directly through the open portions of the blind. The remaining solar radiation strikes the blind and is either reflected toward the room interior, or to an adjacent slat, or towards the outdoors, or is absorbed by the blind surface. The direction of scattering is determined by the surface properties and geometry of the slat. If the slat is blue, it scatters incident flux differently from a white slat (both spectrally and with outgoing angle). If the slat has a “satin” finish versus a “glossy” finish, or if the blind angle changes, the reflected pattern of flux also changes. This function of transmittance and reflectance as a function of incoming and outgoing angle is known as the bi-directional transmittance and reflectance function (BDTRF) property of a material or fenestration system (since transmittance, absorptance, and reflectance properties add up to 1.0, absorptance is determined by deduction).

Measuring the optical properties of complex systems is expensive, time-consuming, and requires significant expertise. Every combination of material, color, geometry, angle, or mode of operation must be measured separately. Data files can be enormous. Test procedures are being discussed within the NFRC Solar Heat Gain Subcommittee. For example, the NFRC 201 is an interim test procedure to determine the SHGC of non-homogeneous glazing systems (glass blocks, other diffusing glazing systems, and projects with shading systems) which currently cannot be simulated. ASTM E-06.51.08, ASTM C-16.30.04, and ASTM C1199 SHG and Thermography task group are also working on test procedures to quantify U-value.

There are no comprehensive databases of BDTRFs for such systems and there are few simulation programs that routinely incorporate such data in their  calculation of thermal and daylighting performance. Coming up with simple, inexpensive methods to characterize complex systems has been a problem facing researchers for years. An elegant alternative currently being explored by researchers is to determine optical properties computationally. This involves measuring the optical properties of individual materials. Forward ray-tracing programs are used to determine transmission and reflection coefficients based on the unique geometry and the measured optical properties of each layer in a façade system. The forward ray-tracing assumes a position of the source then traces that ray through the system as it is scattered, reflected, interreflected, or transmitted through the system. The end result is a percentage of total flux reflected or transmitted or reflected through the system as a function of angle of incidence and, if needed, spectral wavelength.

This method would allow any system or combination of systems to be mod-eled, if one simply knows the basic optical properties of the material. This method is as yet unproven. A proof-of-concept is currently underway at LBNL in collaboration with the École Polytechnique Fédérale de Lausanne (EPFL). Optical measurements of complex systems, done with a new digital scanning method in Switzerland (Andersen et al. 2001), are being compared to a ray-tracing model of the same system. Results are promising.

The coefficients will be generated in a unique program that contains optical libraries of basic materials or layers. A user would assemble their unique complex system properties to create a fenestration system. If this method proves to be valid and accurate, transmission and reflection coefficients could be available for any complex system. These coefficients could then be used to arrive at standardized U-value and SHGC values and also used in new building simulation software programs to predict the daylighting and solar heat gain performance of complex systems in buildings.

For systems that involve ventilation within or through the envelope, the problem is equally challenging. Again, standard models have not been developed and implemented in a user-friendly manner. Computational fluid dynamic (CFD) software packages can be used to solve the problem, however users often obtain multiple solutions – any of which can be wrong unless one is an expert. Computational time for one design day condition can take a full day for a relatively coarse three-dimensional grid. Often, basic parameters are difficult to define: effective areas of ventilation, discharge coefficients (Cd), pressure coefficients (Cp), direction of flow, etc. Weather data and wind tunnel measurements are often needed to obtain these basic parameters. Algorithms for dampers and penetrations in the wall form some basis for this work (e.g., one assumes simply a large-area ventilation damper), however heat transfer through the window’s transparent portions add to the complexity of the problem. Coupling between the thermal zone of the window and the whole building must also occur. Building energy simulation programs either do not implement such coupling or use of such features requires significant expertise to model (see description of EnergyPlus capabilities in this regard below).

In lieu of such developments, engineers are forced to approximate thermal indices or accept default or worst case ratings in order to pass energy codes. For many of these advanced building systems, engineers must work with code officials to arrive at acceptable computational methods for special cases.

Question/Information: eslee@lbl.gov