By Marina Golden, EIT, LEED AP
Today many building envelope elements, especially in high-rise construction, are pre-assembled at the factory — a trend that has led to the widespread use of curtain wall, window-wall, metal panels, and framing modules that can slide and snap together easily. Shipped to the site and ready for construction, pre-fab components provide cost and time savings for builders on a tight budget and schedule. But, internal to these modules are aluminum and steel components that are conductive, and in cold climates can cause thermal bridging.
Thermal bridging, occurs when metal elements inside a building’s walls are not properly insulated and results in the increased transfer of energy between materials. This decrease in thermal performance is most significant when differences between the interior and exterior temperatures are highest. The greater the temperature difference, the greater the heat transfer. For example, in the winter when it is 75°F inside and only 20°F outside, energy will travel out of the building faster than in a more moderate climate or during the summer months when the temperature differential is less.
Why not just add more insulation? Conventional wisdom has led us to believe that adding more insulation to the empty cavities in opaque envelope assemblies will increase thermal performance proportionally to the added insulation R-value. However, unless care is taken to ensure potential thermal bridges have been “broken,” wherever metal building components are present, the rate of heat conduction will still be high. It’s time for a total shift in the way building envelopes are constructed.
Hidden Underperformers Revealed
One dimensional thermal performance analysis is widely used today by architects and engineers to specify building envelopes. However, the 1D method does not accurately incorporate the effect of thermal bridging. The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) recognized a need for greater accuracy and commissioned engineering firm Morrison Hershfield to study the thermal performance of typical opaque envelope assemblies using 3D modeling techniques. The result of this study is ASHRAE Research Project 1365-RP and a catalog that lists the actual 3D R-value of common envelope types, which found that 1D calculations grossly overestimate the thermal performance of opaque envelope assemblies.
In the fall of 2014, my firm, Environmental Systems Design, Inc. (ESD), studied how envelope thermal performance calculation methods affects the modeled energy consumption of 16 newly constructed buildings, primarily in and around Chicago. For each modeled building, the team determined opaque assembly thermal performance using conventional 1D R-value calculations for comparison against the more accurate 3D R-values found from look-up tables in the Building Envelope Thermal Analysis (BETA) Guide developed by Morrison Hershfield. ESD also factored in the building’s shape and window locations in order to more accurately analyze each building’s modeled energy penalty gap.
When comparing estimated 1D calculations versus actual 3D thermal performance, the data set revealed an average 8% increase in heating demand and a total of $272,000 increase in annual utility bills for the 16 buildings, plus the following (see Figure A):
- Actual, 3D energy and operational costs increased on average between 2% to 14% over 1D design.
- An actual peak heating demand increase between 1% and 29%.
- An actual peak cooling demand increase between 1% and 8%.
Buildings that featured unitized curtainwall spandrel and conventional curtainwalls did not fare well in the study (see Figure B), regardless of insulation type (spray foam or insulated slab bypass). Even when conventional mineral wool or back pan insulation was doubled (from R-8.4 to R-16.6), assembly R-values increased only slightly and did not translate to an improvement in performance.
Some buildings in the analysis saw more drastic penalties, like the 100,000 sq. ft. LEED Platinum office in Figure C. Because of its thinner profile and lower window to wall ratio (32%), this building’s energy consumption penalty was 8%, with a peak cooling increase of 7% and peak heating increase of 30%.
Full-height window-wall systems were the only opaque glass wall assembly that reported high R-values of 18 or 19. The window-wall systems had fewer aluminum parts, no slab edge cover and featured double-glazing with R-12 interior spray foam insulation (Figure C, dark blue window assembly and line on graph).
Impacting the Building Envelope
Better understanding of how building envelopes actually perform will be the key to designing more efficient buildings in the future. The design engineers, architects, owners, and operators of today’s buildings must work together to build efficient facilities inside and out. A few best practices include:
- Engage a competent envelope façade consultant that will advise on the best options for the building’s aesthetic.
- Utilize energy modeling early on in the concept phase to assess the impact of different wall types, the shape, orientation and window-to-wall ratio of the building on future utility bills. The design team, together with the façade consultant, should analyze the results and make the best life cycle costs decisions for the building.
- Reduce framing area as much as structurally possible by choosing larger floor-to-floor, window-wall spandrel units with built-in extensions over the slab edge and other envelope design strategies (see table below).
When these design phase strategies are employed, a truly efficient envelope can emerge.
Out with the old, In with the new
Ensuring that tomorrow’s envelopes perform as efficiently as they are designed will require a new set of materials and design principals. Here are just a few below…
Golden, EIT, LEED AP, is a mechanical engineer in the Environmental Systems Design, Inc. (ESD) Energy & Eco Services Group. Her expertise includes MEP design, building physics, and analysis of energy saving measures in a wide array of building typologies. She also has experience working on various phases of commissioning, retrocomissioning and LEED certification within financial, healthcare, and governmental facilities.