Thinking of a House As a System Can Keep a Home's Carbon in Check

Before you fire up that humidifier, think about the walls and windows.

A white and blue humidifier in front of a window

Stebastian Gorczowski/Getty Images

It's the time of year when there are articles everywhere that tell you to get a humidifier, particularly in these pandemic times. It's recommended you keep the humidity in your home between 40% and 60%. As noted in an earlier, archived post, that's the humidity range that reduces virus transmission.

But in the colder parts of the country, if you try to crank the humidity up to 60%, it can cause real problems because cold air is dry air. With our leaky old houses, the air changes a lot, bringing in a lot of cold air. So we add moisture back into the air with our breathing, our cooking, our showering, and if those don't work, with humidifiers.

If the humidity gets too high, moisture can condense on windows, on walls, and even inside walls. Physicist Alison Bailes says a humidifier can rot your house. He uses an important term to explain why:

"Controlling the indoor conditions for health is absolutely a good idea. But you’ve got to understand the big picture. A house is a system. When you make a change to one part of the system, it has impacts on other parts. That’s the case with trying to crank up your indoor humidity to keep the viruses at bay. You may be creating other problems inadvertently, which is another reminder that a house is a system." 

A house is a system. This is a relatively recent concept: It used to be that one would design a house, and then give the drawings to a consultant or contractor who would throw a plumbing system or an electrical system or a mechanical system as a layer on top. But everything connects and interacts. As Natural Resources Canada writes in their wonderful guide, "Keeping the Heat In":

"A house operates as a system. All the elements of a house, the environment, envelope, mechanical systems and occupant activities affect each other, and the result affects the performance of the house as a whole. The secret to avoiding problems is in understanding these relationships. For example, reducing air leakage provides more comfort to the occupants and protects the envelope from moisture damage, but it also increases humidity levels inside the house since less water vapour can escape. This can mean an increase of condensation on windows. If a house is tightened to this degree, it will now need more ventilation. The lesson here is that a change to one component of the house can have an immediate effect on another component. Many small changes over time can also affect the balance of the system."

In an era when we are thinking about carbon instead of energy, House as a System (HAAS) becomes particularly important. That's because the embodied or upfront carbon emissions are as or more significant than the operating emissions. So if you design a crappy building envelope and need a bigger heat pump (because every new building should have a heat pump) you are increasing the embodied carbon. A lot.

New research published in The CIBSE Journal found that in the United Kingdom, the heating and hot water systems can account for up to 25% of the home's embodied carbon. They use pipes and radiators in the U.K. rather than ducts so the numbers might be different in North America. But, in either case, better walls and windows mean smaller mechanical systems with less upfront carbon emissions.

HAAS includes the occupants, their behavior, and their comfort. That's why we worry about the walls as much as the heating system, and as we recently discussed, the mean radiant temperature. Because, as engineer Robert Bean has noted, everything connects: "No matter what you read in sales literature, you simply cannot buy thermal comfort — you can only buy combinations of buildings and HVAC systems, which if selected and coordinated properly can create the necessary conditions for your body to perceive thermal comfort."

My Awair monitor looks at a few data points.
My Awair monitor looks at a few data points.

Lloyd Alter

The HAAS approach explains why that smart thermostat never worked as advertised. Because it is all so much more complicated than one point in the home, measuring one thing, when there are so many things going on—temperature, humidity, and the carbon dioxide (CO2), carbon monoxide (CO), volatile organic compounds (VOC), particulate matter, decibel levels— and it's hard to keep up with it all.

The HAAS approach is particularly important in renovations and upgrades of homes because changing one thing affects everything else. For instance, we keep saying that sealing the building is the first and most important thing to do but then you might have a ventilation problem or even a humidity problem. If you have a gas furnace and water heater, you might even have dangerous combustion spillage of exhaust gases that come into the house because there is not enough air to let them go up the chimney.

But it is all another reason I pitch Passivhaus: It gives you the building envelope and ventilation you need. Then I recommend that you electrify everything—get rid of that gas because of the carbon dioxide, but also for your health. Don't forget about the upfront carbon of the materials you select. And don't forget that your home is part of a much larger system: the community. Then, you truly have a House as a System.

View Article Sources
  1. "Relative Humidity of 40-60% in Buildings Will Reduce Respiratory Infections and Save Lives." 40 To 60 RH.

  2. "Embodied Energy: The Whole Picture." CIBSE Journal, 2021.

  3. "Combustion Gases in Your Home – Things You Should Know About Combustion Spillage." Government of Canada.