An Active Approach to Passive House Design

by Shaun May, EIT

This article is part of Wood Harbinger’s newsletter series.

Back in October 2016, the AIA Seattle’s Committee on the Environment (COTE) gathered for a “Sustainability Slam,” which I attended. The evening featured many informative presentations. Two of them, by NK Architects, caught my attention. They both focused on Passive House building design. Passive House is not a new concept but it seems that it has not yet “been discovered” in our local area. As a commissioning provider, I have been exposed to myriad building designs and facility types, yet I have not come across a single Passive House project. I think if more building owners and designers knew about it, we’d see these methods integrated into every building design!

What is Passive House?

I first learned about Passive House a couple years ago while researching my “Sustainable Europe” article, which featured the many ways that European countries are pushing progressive sustainability design. Passive House’s methodology is sound and data has proven its performance over the past three decades. There are numerous excellent resources for Passive House design, guides, references, etc. from the Passivhaus Institut (origin Germany) and others all over the Web[1].

In a nutshell, there are three key Passive House characteristics:

  • Eliminate thermal bridging
  • Assure airtight construction
  • Utilize heat recovery ventilation

There are real monetary advantages to this building strategy:

  • Reduced energy demand and lower operations costs
  • Reduced maintenance costs
  • Higher rent potential for housing facilities
  • Insignificant cost premium

The “More Expensive” Myth

That last bullet item is one of the most impressive elements of Passive House design and was the detail from NK Architects’ presentations that surprised and impressed me the most. Passive House design doesn’t really cost more! According to NK’s presentation, Passive House building design typically yields approximately 80-percent energy savings, higher indoor air quality, higher thermal comfort, a superior building design—all at a measured 3-percent cost premium.

Pushing the Envelope

The greatest energy demand in a typical commercial building is for heating and cooling. For a fully efficient system, the energy expended to heat or cool the interior should remain within the building. Building envelopes already work passively, as they don’t require input energy. However, the envelope can either support or hinder energy retention in the building. Energy seeping out of the building envelope via air leaks and thermal bridging (imagine a hot rod extending out of an oven) represents the greatest energy demand for the building system. Passive House design promotes better building envelope performance, which greatly reduces building load by retaining energy. With better performance, the building energy equation is reduced from the outset. Less energy in and less energy out results in a stable, high thermal-comfort level in the environment, which is maintained with minimal input energy.

Design that yields energy savings is most often the domain of mechanical engineers and architects, but it’s relevant for all disciplines. My educational background is in electrical engineering, so here’s an analogy for the electrical engineers in the room. Imagine a building is an electrical circuit. The resistors (the “heat sinks,” the loads) in the circuit are the fresh ventilation air, which is always induced into the building plus all of the exhausted air and thermal energy expelled from the building. Now, via Passive House design, we can almost eliminate some of those resistors (the leakage resistors, the exhaust air, and energy expelled) by an order of magnitude. Disconnect that heat sink from the circuit! There are huge real power savings to constructing buildings airtight and thermal bridge-free.

Our first step in sustainability is to eliminate unnecessary demand—that is, energy consumption that provides no benefit, like heating and cooling energy that escapes through an inefficient envelope. The second step is to efficiently supply that demand. Let’s look at an example. If we have a building envelope that requires 20% of traditional baseline energy to maintain thermal comfort, and we supply the energy with equipment at high (~90%) efficiency, we reduce consumption. Even with an efficient envelope and inefficient equipment, we’re still better off. But, if we start with a building that requires five times as much input energy to maintain thermal comfort (because 50% leaks out/is wasted), it will not matter how efficient our engineered energy systems (equipment) are, we still will not be able to meet the low demand of the more efficient building envelope. Here’s a case in point:

Passive House with low efficiency HVAC equipment [Efficient envelope/inefficient equipment]

  • 20% demand x 200% equipment energy (50% efficiency) = 40% energy consumption

Traditional envelope with high efficiency HVAC equipment [Inefficient envelope/efficient equipment]

  • 100% demand x 110% equipment energy (90% efficiency) = 110% energy consumption

Realizing Potential

With a significantly reduced building energy demand, renewable energy production onsite becomes a viable means to power significant percentages of facilities. The threshold at which a building generates as much energy as it consumes is called Net-Zero Energy; this is building for the future. Expanding out to the city infrastructure (utility) scale, energy generation and transmission demands decrease as each facility consumes less energy. In this sense, Passive House design pairs well with distributed renewable generation (smart grid) quite beautifully.

Passive House can also improve health and wellness through design. It incorporates HEPA air filtration for higher indoor air quality; the increased building insulation and airtight construction also helps keep out distracting outdoor noise and prevents water intrusion, reducing the risk of structural rust and mold. These concerns decay more than just building materials!

Passive systems can offer not only function, but aesthetic opportunity too. For example, daylighting systems help create open, bright environments with healthy, full-spectrum lighting. Wind chimneys transform water features into cooling systems.

Despite the name, the tenets of Passive House apply to commercial buildings as well, such as schools, office buildings, even hospitals. Commercial buildings shape our collective experience while making the biggest impact on our environment. By actively applying Passive House design to our community infrastructure today, we can carry the weight of a thriving, sustainable society for decades to come.

[1] Here are just a few!

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