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Fresh Air

by: Austin Trautman, Nov 30, 2024

We have all heard the cliche you are what you eat. We are also what we breathe. In the simplest sense, this is all we can be. Our bodies continuously break down and rebuild from what we put in them over time.

A crucial part of this process is our lungs, one of the world’s impressive gas exchange systems. With more surface area than the average human occupies in their home, our lungs efficiently absorb the gases and tiny particles in our air. Without this efficiency, we would cease to exist. With it comes a great responsibility to be mindful of what we breathe in.

Living in a modern city brings new risks to our health, not previously impacting humans a few generations ago. Core to a healthy, vibrant existence is breathing clean air.

Of high concern are chemicals (VOCs), particles (PM2.5s), humidity and CO2 (which we breathe out). We have two main strategies to address these four items: dilution and filtration.

Dilution

Dilution means mixing air high in a certain concentration with air with a lower concentration of that same thing. Think about an airplane. Many people are breathing CO2 into the same small space. That plane must bring in outside air that is low in CO2 concentration for dilution. Without this, everyone on the plane would get a headache, lose all higher-level thinking, become grumpy, and eventually pass out.

Filtration

With particles (PM2.5s), we have the added trick of filtration to remove them from the air. We tend to think of HEPA as the gold standard, and while this is partially true, we need to consider a balance of the amount of air moved, energy used, the particles to remove, and the size and cost of filters. This leads to a MERV13 filter with a large surface area being a nice balance.

Put Them Together

Let’s put dilution and filtration together in the context of a house. Dilution means bringing in outside air. Filtration means adding a filter to the outside air supply (typical in passive houses) or recirculating the inside air through a filter (the only option in most houses).

Old Homes

50-plus-year-old houses have little control over the exchange of inside and outside air. Their single trick available is maximum filtration. In this case, we recommend a Corsi Rosenthal box concept. Maximum airflow, maximum filter surface area, minimum noise. This will outperform fancy air filters in a leaky home. We have tested and like the kits available from Clean Air Kits

2020’s Code-compliant Homes

Then, we jump to code-compliant homes of the last few decades with one-way air exchange. This is commonly exhaust only through bath fans. You turn on the bath fan, depressurizing the house and sucking in equal outside air through random cracks. The insulation in your walls (plus whatever else is in there) becomes your effective air filter. You get rough dilution with no filtration.

High-performance Homes

Now, we get to what we believe is essential to a healthy home: balanced ventilation. These systems remove air from bathrooms, kitchens, laundry, or storage rooms and supply an equal amount of fresh, outside air to bedrooms and living areas. The systems used in a Nopal home also balance heat and humidity and filter that air. We currently use ERVs, a MERV 13 filter with another box to optionally add MERV15 and carbon (commonly used during wildfires).

Finally, the essential support system is extreme air tightness. All Nopal homes are built to exceed Passive House airtightness, the world’s strictest standard. While extreme insulation adds risks and isn’t always the best approach, the data shows that extreme airtightness is always a good move. The caveat is that your airtightness layers must allow moisture to move as nature intended.

We now have complete control of the air we breathe. Nothing is passing through random cracks. Our fresh air system is equipped with adjustable speeds and sufficient filtration. A house without people could then be set to a minimum design flow, and indoor air would remain in near-perfect balance regardless of what happens outside.

System Control

Now we add in humans. What are those humans doing today? They are breathing, which means CO2. Can the base flow handle that CO2? Let’s start measuring and add a boost speed when CO2 rises. Are these humans cooking food and showering? Let’s add boost control for those actions.

Now, we have a dynamic system capable of diluting and filtering our four major concerns: VOC, humidity, PM2.5, and CO2. The system adapts as people live in the house or host friends and family without calling attention. Near-perfect indoor air is maintained regardless of what happens inside or outside the home.

The end system is simple, with a couple of fans as the only moving parts, a filter or two, and a few sensors for dynamic function.

PS - How much fresh air do you need? This is a fun one. Without getting into too many specifics, it’s a hotly debated topic. Luckily, we have real-world measurements to support our current conclusion (which has evolved).

You want a system that can ramp down to 40-50 cfm on an away mode and scale up to at least 25 cfm/person, ideally 40-50 cfm/person. It is essential that this system can supply that 25 cfm/person with low noise, low power draw and high recovery efficiency (in the case of an HRV or ERV). Bonus points if the system performs similarly at those max speeds. This is one of the many reasons we have leaned into Zehnder for our fresh air system in Nopal complete builds.

PPS - How many plants would it take to offset human CO2? Too many. Way too many.

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