Energy Efficiency In Theory And Practice, Pt. I



As a firm that focuses on both architectural design and energy consultation, we provide clients with a range of services depending on their unique project, vision and needs. Most come to us with an established interest in sustainable design, but we see a wide range in prior knowledge of the methods, materials, and technologies that are used to achieve the desired energy standard. As such, our role is often to educate clients and sometimes even contractors about the wall, roof, and slab assemblies we design. Encouragingly, the Passive House standard, with its focus on creating a tight envelope that greatly reduces the burden of heating and cooling energy, is becoming more well-known in the US as general interest in creating a more sustainable built environment rises. Below is a detailed explanation of how we perform an energy analysis on a building envelope to explore a range of energy-positive options, followed by a case study from one of our projects.

Part I: Test Case Modeling



Using Passive House techniques, we can reduce heat transfer through the exterior surfaces of the house known as the building envelope. These measures include increasing effective insulation, increasing airtightness, and eliminating opportunities for thermal bridging. R-value measures resistance to heat flow: the higher, the better. ACH (air changes per hour at a given pressure) measures airtightness: the lower, the better. Thermal bridges bring unwanted conditions into the envelope, creating conditions for water vapor to condense and mold to form.

Windows play a vital role in the envelope’s performance. If not treated properly, they act as thermal holes in an otherwise continuous envelope. Typically, u-values (the inverse of r-values) are provided per window type and measure the rate of heat transfer through the assembly expressed in BTUs per hour per square foot per degree Fahrenheit. Lower u-values indicate less transfer, thus better performing windows. Depending on the manufacturer’s location, the u-values are expressed in different ways. In the US, the NFRC (National Fenestration Rating Council) relates an average u-value for an average size window, which takes into account performance values for both frame and glazing in a particular type of window. For European windows, these sub-values are given their own independent ratings. As you may guess, the former rating system makes evaluation somewhat opaque. In general, upgrading to triple pane windows and thermally-broken frames improves the overall performance. Our studies use values from double and triple pane windows from both US and European brands, interpolating values where necessary to compare across these different evaluating systems.

Airtightness can be measured by a blower door test, which pressurizes a building and measures the amount of air that escapes through the envelope. Given the pressure exerted and the volume of the building, the air changes per hour (ACH) can be found. As the ACH is decreased, exfiltration/infiltration decreases, and the internal environment can be controlled more easily. Instead of allowing air to leak through the envelope haphazardly, air movement is centralized. As airtightness improves, however, fresh air ventilation becomes a requirement. In order to control air movement with minimal energy loss, heat exchanging ventilators such as a heat recovery ventilator (HRV), or enthalpy recovery ventilator (ERV) are employed. The resulting airtight envelope with controlled air exchange can markedly decrease the magnitude of heat loss, which in turn decreases the magnitude of the design load on the HVAC equipment, and the required size of the equipment itself. 

The factors above—building envelope, windows, and airtightness—combine to influence the energy performance of the building. The peak heat load and the estimated site energy consumption dictate the size of your HVAC equipment and your monthly conditioning bill. The test case below will show that a well insulated, airtight building with triple pane windows can result in as much as a 33% improvement in energy efficiency.

test case METHOD

Using energy modeling software, our team ran several tests on a hypothetical 30 foot x 30 foot, two-story building, situated in Charlottesville, Virginia. Each test varied the effectiveness of the envelope, the windows and the airtightness, ranging from the least stringent standards (that of the Building Code), improving to what we call “Code +”, and finally reflecting a Passive House envelope.


The summaries below compare the energy savings for heating and cooling with the envelope improvements we tested. These are given case numbers 1-6, and compared to control case based on standard building practices in the US, which is numbered 0 and referred to as the base case.

  • From the base Case 0’s insulated 2x4 walls (R15 by code), to Case 1’s 2x6 walls (R19), there is a 3% reduction in annual load

  • From base to Case 2’s 2x6 walls plus an insulated slab, we see a 10% reduction

  • From base to Case 3’s 2x6 walls, insulated slab, and airtight assembly, there is a 19% reduction and a considerable decrease in the heat pump capacity required (from 2.50 to 1.25)

  • From base to Case 4’s Passive House envelope (assuming exterior insulation but still with double pane windows), there is a 24% reduction and a considerable decrease in the heat pump capacity required (from 2.50 to 1.00)

  • From base to Case 5’s 2x6 walls, insulated slab, airtight assembly, and triple pane windows, there is a 30% reduction and a considerable decrease in the heat pump capacity required (from 2.50 to 1.00)

  • From base to Case 6’s full Passive House assembly including triple pane windows, there is a 33% reduction in energy costs and a considerable decrease in the heat pump capacity required (from 2.50 to 0.75)

The differences between Cases 4 and 5 are particularly interesting to us, as they compare a building with a high performance Passive House envelope with double pane windows to a less insulated envelope (no external insulation) with triple pane windows. This will be particularly relevant to our project case study below. It is also important to note that the full benefits of these envelope enhancements can not be fully described by the energy performance increases. The increased durability, comfort, and improved air quality that such envelope improvements bring are partially discussed in the example below.


According to the Energy Information Administration (EIA), the average Virginian household spends an average of $2200/year on energy. Given an average home size of 2,227SF, this expenditure can be understood as roughly $1.00/SF/year, reflected in Case 0 below. Assuming the average Virginia home’s energy efficiency does not exceed today’s code expectations, we can use this number as an upper value for the cost of energy per square foot per year. Based on this baseline, we estimate the energy costs associated with our test cases as follows:

  • Case 0 = $1/SF/year

  • Case 1 = $0.97/SF/year

  • Case 2 = $0.90/SF/year

  • Case 3 = $0.81/SF/year

  • Case 4 = $0.76/SF/year

  • Case 5 = $0.70/SF/year

  • Case 6 = $0.67/SF/year

In Part II, we will present a case study and discuss how these numbers translate into real-world cost savings. Stay tuned!

EnergyJaime JusticeComment