A Passive house energy analysis

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

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.

 
 

understanding THE BUILDING ENVELOPE

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 results

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.

 
 

evaluating cost

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

At the end of the next section, you will see how these numbers translate into real-world cost savings.

 

Part II: Case Study

THE DOWNTOWN CARRIAGE HOUSE

This Gehrung + Graham project is an accessory unit that we designed as an addition to an existing family home. It features a first floor garage and a second floor studio apartment with a cathedral ceiling, large windows, and a simple yet graceful floor plan designed to bring a sense of openness and lightness to the space. The owners were interested in exploring energy efficient measures primarily for economic benefit, but were not trying to achieve a particular energy standard. The quoted building included the following assemblies:

  • Wall is 1.5” of mineral wool insulation over 2x6 wood framing with dense pack cellulose insulation, (R28)

  • Windows are double pane

  • Roof has 1.5” of mineral wool insulation over 2x10 wood framing with dense pack cellulose insulation, (R37)

  • Floor over Garage is 11-⅞ TJI wood framing with dense pack cellulose, (R48)

  • Slab-on-Grade has no insulation, (R0)

  • Airtightness will come in close to 0.6 ACH

This assembly most closely resembles that modeled in Test Case 4. We will evaluate this in detail below.

 

comparison with test cases

Case 4

The differences between the quoted assembly above and Case 4 are as follows:

  1. The quoted wall’s total R-Value (R28) is R4 less than the model’s versus (R33), which reflects 0.5” less exterior insulation and slightly less cellulose

  2. the quoted roof has an R-Value that is R3 less than the model’s (R40 versus R37), which reflects 0.5” less exterior insulation and slightly less cellulose

  3. The model’s slab is highly insulated (R30) whereas the quoted slab is uninsulated, however the model does not account for the insulated garage ceiling (R48) of the quoted building

Case 5

Case 5 removed the mineral wool from Case 4, but upgraded to triple pane windows. It also added slab insulation. The differences between the quoted building and Case 5 are:

  1. The quoted wall’s total R-Value is R9 greater than the case’s (R28 versus R19), which reflects an additional 1.5” exterior insulation and slightly more cellulose

  2. The quoted windows are double pane, whereas the case’s windows are triple pane

  3. The quoted roof has an R-Value that is R1 less than the case’s (R37 versus R38), which reflects the removal of the exterior insulation and an almost equivalent addition of cellulose

  4. The model’s slab is insulated (R10) whereas the quoted slab is uninsulated, however the model does not account for the insulated garage ceiling (R48)

Case 4 vs. Case 5

The essential difference in these two cases are the upgraded wall assembly versus the downgraded window performance. The slab insulation also differs, but will be ignored for now. The primary considerations were exterior insulation, which totaled $1350 including installation for the 1.5” thickness we specified. and windows. These are a high-ticket item with a great impact on overall performance, and therefore require a careful analysis of cost vs. benefit. The average price of the double pane windows was approximately $10k with a minimum at $8k and a maximum at $12k; the average price of triple pane quotes was $18k with a minimum at $14k and a maximum at $21k. The ranges do not quite overlap, but are somewhat consistent with our experience that upper mid-range double pane window costs approach low-range triple pane window costs. Triple pane UPVC frame windows came in between $14-15k, the triple pane metal frame window was quoted at $16k, and the triple pane aluminum clad wood windows ranged from $18.7-21k. Triple pane quotes notably include overseas freight, avoiding additional hidden costs. The initial cost difference for upgrading to triple paned windows started at $6k.

Case 6, AKA Passive House

Whereas Case 4 and 5 separated mineral wool and triple pane windows as assets, Case 6 included both.

 

evaluating energy savings

The EIA study we referenced previously does not specify the way the average home’s area was measured, and whether it includes the gross, net, conditioned, and/or non-conditioned spaces is unclear. Our Carriage House case study, therefore, accounts for for the boundary conditions, the gross conditioned area (635 SF) and the gross total area (945 SF). The following shows the minimum and maximum total annual energy savings per year for each case:

  • Case 0 = $0/year

  • Case 1 = $17-25/year

  • Case 2 = $63-93/year

  • Case 3 = $123-183/year

  • Case 4 = $154-229/year

  • Case 5 = $188-280/year

  • Case 6 = $212-316/year

Applying these numbers to Case 4, the near Passive House envelope with double pane windows, we estimate that an investment in exterior insulation of approximately $1350 would be paid off in around 7 years. In Case 5, assuming the the least expensive UPVC window in the Code+ envelope, an investment of $6k for triple pane windows would be paid off in approximately 25 years.

 

cASE STUDY CONCLUSION

After performing the comprehensive energy analysis outlined above, we decided that the addition of mineral wool exterior insulation seen in Case 4 would be a much more economically effective way to improve the Carriage House’s energy efficiency and durability over time. In addition to it’s insulative properties, the mineral wool layer acts as an additional drainage plane, a fireproofing layer, and a thermal bridge decoupler. Durability, comfort, and annual energy costs will be improved significantly without the upfront price tag associated with triple pane windows. In addition, retrofitting a house with windows is far less complicated than retrofitting its entire envelope. Note that this decision was greatly informed by the client’s goals with regards to overall energy efficiency. Each project has unique goals, and we evaluate and weight the data accordingly.

 

looking beyond the model—additional passive house benefits

A Passive House envelope wIth its airtight and well-insulated envelope, triple pane windows, and thermal-bridge free construction offers a number of additional benefits beyond the energy model’s numbers. These benefits include thermal comfort, durability, air quality, and resiliency. An airtight building without thermal bridges substantially lowers the opportunities for the growth of mold on interior surfaces and rot within the envelope itself as cold spots are negated and moist air is prevented from becoming trapped in cavities. This improves the durability of the structure and the air quality of the occupied spaces. Without the cold spots found at thermal bridges and low-insulated expanses (like double pane windows), a passive house offers thermal comfort throughout. With its well-insulated and airtight envelope, the extreme effects on the interior temperature of a passive house due to power outages are softened, resulting in a more resilient, healthy, and environmentally sound building.