Step 1: Load Minimization - Architectural Elements & Building Envelope

3. Thermal Performance of the Building Envelope: A highly insulated building envelope is a key design element to achieve a low energy building design in heating dominated regions and at the same time reduce the size of the central heating plant and distribution system (perimeter heating equipment).

As described in the “Building Energy Use Statistics” page, heating is the largest end-use, accounting for approximately 32% of the total building energy use in the U.S. and 50% in Canada and also reported to be the largest end-use in the six largest world markets surveyed by the World Business Council for Sustainable Development (WBCSD). [6] As a result, good thermal performance of the building envelope is one of the most important foundations of low energy buildings.

Energy standards for new construction such as those published by ASHRAE continue to increase the envelope thermal performance requirements, driven by the goals towards low energy buildings and net zero buildings. This trend is illustrated below in graphical and tabular form for the ASHRAE 90.1 Standard Series, Standard 189.1 and the Advanced Energy Design Guides (AEDGs) in ASHRAE defined climate Zone 5 and Zone 6. [7] [8] [9] The tables show an approximate doubling in the thermal performance requirements between the 90.1-2004 Standard and the AEDG50 Standard, which is a stringent standard designed to achieve 50% savings over ASHRAE 90.1-2004. For Zone 5, which includes cities such as Chicago and Denver, the Southern Ontario City of Hamilton and the European cities of Berlin and Prague, the AEDG50 calls for performance levels of R30 (RSI 5.3) for the roof, an R13 (RSI 2.3) for mass walls or an R28 (RSI 4.9) for steel stud walls and finally, an overall glazing Uvalue of 0.39 Btu/hr.ft².ºF (2.2 W/m².ºC). Interestingly, the difference in thermal requirements between Zone 5 and Zone 6 are very small and so colder cities such as Minneapolis-St. Paul or the Canadian cities of Toronto and Montreal require essentially the same thermal performance targets.

Some industry practitioners are calling for even higher levels such as R40 walls and R60 roofs or even higher levels that would have been viewed as excessive a few years ago. As an example, Robert Bean, a high performance building consultant in Alberta, Canada stated in a recent article on high performance construction that: [10]

“If you want to set an example of high performance construction and beat ASHRAE 90.1 by 50% or more, we will need

insulation and lots of it placed on the outside of the mass by subscribing to the 10-20-40-60 rule of insulation, meaning

R10 for under slabs, R20 below grade walls, R40 above grade walls and R60 roofs.”

Given the industry objectives towards transforming the market to consistently design net zero energy buildings within the next 10 to 20 years [11] [12], very high levels of insulation are an implicit requirement for energy efficient new construction and yet there is significant debate on the amounts needed to achieve large reductions in energy use. An illustration that provides one answer is presented below in two tables that include the thermal performance of the walls, roof and glazing, the peak heating load and the predicted natural gas use and finally, the energy intensities. The predicted natural gas use and whole building energy intensities are based on energy simulations performed with eQUEST for the same hypothetical 5-storey, 36,000 ft² (3,345 m²) commercial office building used in the previous section in a Toronto location. Three different levels of thermal performance are shown including a 90.1-2004 reference, an intermediate efficient design and a low energy building (LEB) design that displays thermal performance levels beyond the most stringent standards mentioned above.

As shown in the outputs, the significantly better thermal performance of the LEB design achieves a 47% reduction in peak heating load and a 46% reduction in natural gas energy use relative to the 90.1-2004 Reference. The reported natural gas energy use is equivalent to a low space heating energy intensity of 7.7 ekWh/ft².yr (298 MJ/m².yr) compared to the 90.1-2004 Reference space heating intensity of 14.4 ekWh/ft².yr (558 MJ/m².yr). Overall, the whole building energy intensity is reduced by 27%.

The second table shown below includes the required plant size to meet the design heat loss assuming two boilers, each sized to meet 65% of the peak load. The peak hot water flow is also included based on the installed plant capacity. The size of the heating plant, peak hot water flow and circulating pump for the LEB design is reduced by 50% and shown to be approximately proportional to the reduction in peak heating load.

From a practical perspective, walls with large amounts of insulation need to be thick, which can be a design challenge. The location of the insulation and the design of the wall structural elements are equally important in order to minimize thermal bridging and ensure that the effective thermal performance of the wall is as close as possible to its nominal thermal performance. The examples used in this section assume that the wall Rvalues used in the energy models and load calculations are effective Rvalues, but the vast majority of wall construction used in commercial buildings exhibits significant thermal bridging and achieve much lower effective Rvalues. Designing a wall with an effective R32 such as what is used by the LEB is not easy and this is especially true for steel stud cavity walls and spandrel glass panels, which exhibit significant thermal bridging to the point of cutting their effective thermal performance by as much as 50% or more. [13] [14] [15] [16]

The best approach to minimize thermal bridging in an exterior wall construction is via placement of the insulation as a continuous layer to the exterior of the wall as opposed to the stud cavity. [17] Doing so will also provide additional benefits including better sound attenuation, increased durability (cracks in back-up and cladding due to thermally induced expansion and contraction are minimized), condensation of moisture will occur in the exterior and finally, the internal mass of the building is able to function as heat storage. [18]

4. Downsizing of Perimeter Heating Systems: A wall that is highly insulated has the added ability to maintain more even interior space temperature throughout the seasons. In winter, the surface temperature of exterior walls will stay warmer with a corresponding increase in comfort and more importantly, the rate of temperature drop during the unoccupied period will be slower, allowing the building to “ride-through” or coast with minimal or no heating. In milder climates where winter design temperatures are above 23 ºF (-5 ºC), it may be possible to reduce the size of perimeter heating. This potential downsizing is something that is not typically considered by designers since steady state calculations of design heat loss explicitly requires that thermal storage effects be ignored. [19] Yet, there is evidence in the form of operational performance data of existing buildings showing that they maintain the interior temperature during the unoccupied period better than predicted, especially in the cases where there are constant internal heat gains such as office equipment and other plug loads. Additionally, there are anecdotal reports of some buildings designed with no perimeter heating systems and research work including one study completed in 2009 that measured the indoor temperature in a building in Montreal, which concluded that the use of a high performance building envelope could potentially eliminate the need for perimeter heating. [20]

Space temperature can be used as an indicator of the required capacity of the perimeter heating system. However, there is almost no technical information on the subject of calculating rates of temperature drop, as it is not an important consideration when sizing heating equipment. In addition, prediction of indoor space temperatures requires dynamic, non-steady state calculations that are based on sets of differential equations or hourly computer programs in order to consider thermal mass and thermal inertia. To illustrate this behavior, two computer-based approaches that take into account the building thermal mass were used to create the indoor space temperature profiles presented below. The profiles represent a perimeter zone of a typical multi-storey, commercial building that goes on temperature setback for 9 hours starting at 10:00 PM and assumes a space that is sandwiched between two 8 inch (200 mm) concrete slabs. One set of results were derived using the Passive Design Assistance (PDA) computer program. [21] [22] [23] This program uses the CIBSE Admittance Method developed in the 1960s to predict the thermal performance and indoor space temperature taking into account dynamic effects of heat storage in a building. [24] The second set of results were derived with DOE 2.1E by requesting hourly reports of the “average perimeter zone temperature”. DOE 2.1E uses a weighting-factor technique to calculate thermal loads and heat flow from the building mass. [25]

The results are presented as three sets of data and graphs that are arranged in two separate columns. The three graphs on the left column display temperature profiles created with the PDA program at different outside air temperatures (OAT) while the graphs on the right column show the temperatures calculated by DOE 2.1E. Each graph displays multiple curves for the different levels of thermal performance used in the previous examples and show that the higher the level of insulation, as depicted by the LEB scenario, the lower the rate of temperature drop. The first graph is for an OAT of 14 ºF (-10 ºC) and shows the final indoor temperature of the 90.1-2004 Reference scenario to be between 48 and 56 ºF (9 to 13 ºC), depending on the analysis tool used. In contrast, the LEB displays a final interior space temperature of approximately 55 to 57 ºF (13 to 14 ºC). At a warmer OAT of 28 ºF (-2ºC), the final interior temperature for the highly insulated walls is estimated to be in the range of 59 to 62 ºF (15 to 17 ºC), meaning that the perimeter is almost able to ride through the unoccupied period with no need for heating. Finally, at an OAT of 36 ºF (2ºC), the final temperature of the space is estimated to be in the range of 60 to 66 ºF (16 to 19 ºC), again indicating minimal need for heating.

The impact on the size of the heating and cooling loads from varying the WWR and SHGC is shown in the next set of tables. In a cooling dominated climate, decreasing the WWR from 55% to 25% decreases the cooling load and the size of the air handling equipment by 20% to 25%, which is significantly more than the potential energy use reduction of 7%. In milder and heating dominated climates, the heating and cooling load decrease from the lower WWR is even larger, with savings of 25 to 35%. Readers will notice that the heating plant reduction is constant across all geographical locations due to the same thermal performance applied to all regions.

The series of results presented suggest that optimum selections of WWR and SHGC can in general, achieve a reduction in building energy use of 7% to 20%, depending on geographical location with a corresponding 20 to 35% reduction in the size of the heating and cooling loads.

6. Shading Strategies: The use of shading elements such as a window recess or other external shading can have a small impact on building energy use that can be positive or negative depending on the climate, the type and amount of glazing, its SHGC and the design of the shading device.

In heating dominated climates, shading devices can reduce the summer solar heat gain and the cooling load, but will also reduce the winter solar heat gain with a consequent increase in overall building energy use. This behaviour is illustrated below in two separate sets of tables and graphs for a 4” (100 mm) window recess, which would act as a horizontal shading device. The results were derived with eQUEST for the same hypothetical 5-storey building used throughout this page. [35]

As shown in the first set of results, adding a window recess while keeping a constant window SHGC (0.4 in heating dominated climates and 0.25 in Atlanta and Houston) will increase total building energy use by less than 0.5% (negative values shown in red font). However, the peak electrical demand and the space peak cooling load will be reduced by an average of 1% and 2%, respectively.

In a cooling dominated climate such as Singapore, the window recess will reduce the building energy use, as well as, the peak electrical demand and space peak cooling load by 0.3%, 0.4% and 1.5% respectively.

Adding the window recess and simultaneously increasing the SHGC will help reduce the building energy use in heating dominated climates by as much as 2% as shown in the second set of results, but at the expense of increasing the peak electrical demand and space peak cooling load by 3% and 7%, respectively.

Different combinations of window SHGC values applied to each building orientation in combination with shading elements will produce results that would be in-between the two sets of outputs presented above. Depending on the project objectives, designers can focus on minimizing energy use while incurring a penalty on equipment size and peak electrical demand or employ the opposite design approach. However, as a general rule, optimum design practices for heating dominated climates should typically consist of shading elements coupled with the use of windows with higher SHGC values selected for southern exposures and windows with lower SHGC values selected for the east and west exposures to minimize the afternoon solar heat gain during the summer period. In cooling dominated climates the design practice should consist of shading elements and windows with low SHGC selected for all orientations.

7. Putting it All Together: The graphic shown below provides a summary that compares the design specifications of a 90.1-2004 design with a WWR of 45% against a low energy building design that incorporates the architectural elements covered in the previous sections. These elements include a highly insulated envelope, high performance glazing, a close to ideal WWR of 30% based on punched windows and finally, recessed windows to control the summer heat gain.

By combining all the architectural design elements illustrated in this page, the low energy building design achieves energy savings of 30 to 35% in heating dominated climates and 25% in a milder climate such as San Francisco compared to the ASHRAE 90.1-2004 Reference Building. Additionally, the highly insulated envelope contributes to cutting the design heat loss by half while the reduced WWR and use of recessed windows cuts the space peak cooling load and required air flow by approximately one-third and peak electrical demand by approximately 10%. These results pointedly illustrate the concept of “bundling” of design features in order to magnify the energy savings and achieve very large reductions in equipment size discussed in the “IBD Process” page.

In cooling dominated climates such as Singapore, the energy savings and space peak cooling load reduction are significantly smaller and in the order of 6% and 13%, respectively since the savings are only from the reduced WWR and recessed windows.