The Evolution of Indoor Climate, Part II
The Evolution of Indoor Climate, Part II
Last week in this series we covered the evolution of heating and cooling technology in the US. This week we’ll wrap up by looking at ventilation, and the concept of the building envelope.
Ventilation
In the 19th century, one of the primary concerns about the indoor environment was getting sufficient ventilation. Scientific theory at the time held that without proper ventilation, carbon dioxide and other pollutants would build up in indoor air and cause it to become ‘vitiated’, in turn causing a variety of negative health effects. In 1836, the British doctor Richard Baron Howard wrote “The vitiation of the atmosphere by the emanations arising from the bodies of even perfectly healthy individuals, when great numbers are crowded together in a small confined space, and ventilation is neglected, constitutes . . . I believe a very frequent source from whence fever originates.“
Foul, vitiated air was thought to be responsible for up to 40-50% of all deaths, and responsible for such conditions as tuberculosis, diphtheria, rheumatism, gout, and headaches. And of course, while this theory was wrong in the specifics, it was directionally correct. Indoor fireplaces and coal-burning stoves, the most common heating methods of the time, were undoubtedly responsible for severe indoor air pollution and are responsible for a variety of negative health effects.
As early as 1836, engineers were attempting to use the rate of carbon dioxide production to calculate the minimum amount of fresh airflow a building needed. That year, an engineer named Tredgold calculated a recommended airflow requirement of 4 cubic feet per person per minute, based on estimations of a typical person's respiration rate. Over time, more thorough calculations were done, which tended to revise this value upward - over 10 cubic feet per minute in 1845, and a minimum of 30 cubic feet per minute in the 1890s (though 50-60 cfm was preferable.)
However, this rate of airflow was in direct conflict with the desire to heat (and later cool) a building efficiently - a steady flow of fresh air means a large volume of air you have to keep heating or warming. The value of 50 cfm was impossible to achieve while still heating a building economically, and the American Society of Heating and Ventilation Engineers (the organization that would become ASHRAE) settled on 30 cfm as a compromise in their 1914 model code.
Early on, there was limited technology available for achieving ventilation cost-effectively. Large public buildings could sometimes afford steam-driven fans or other complex ventilation systems [0], and some luxury residences were sometimes equipped with “heat extraction systems,” but beyond this, ventilation was mostly limited to vernacular means - keeping doors and windows open, adjusting the layout and shape of the building itself to encourage airflow, etc. The lack of airtightness of buildings at this stage in building evolution was often considered a feature, as it ensured buildings would receive sufficient airflow.
Perhaps ironically, a technology considered important for achieving indoor ventilation was the fireplace itself. People realized that nearly all the heat escaped up the chimney, drawing in outside air behind it - but if your concern is achieving significant ventilation, this becomes a feature not a bug. Achieving ventilation with an open fire was a popular method of ventilation in England well into the 20th century, and as late as the 1880s publications like Popular Science recommended ventilation by way of an open fire in a fireplace.
As mechanical heating and cooling systems became more popular and affordable, it became possible to achieve ventilation by mechanical means. In some arenas, such as the design of schools, this resulted in a conflict between those who favored ‘natural ventilation’ and those who recommended mechanical means - in the years leading up to the second WWII there was a popular movement towards open-air schools, where children were taught in buildings with open windows, or even outside, to receive the maximum benefit of fresh air.
Experiments in the early 20th century eventually conclusively demonstrated that the CO2 hypothesis of air quality was incorrect - even in an extremely crowded, stuffy room, oxygen and carbon dioxide levels changed little. Air “freshness” was instead determined by factors such as temperature, humidity, body odor, etc. But eliminating the rationale behind the ASH&VE recommended ventilation values didn’t stop its adoption - by 1925 22 states had adopted the ASH&VE recommendations of 30 cfm for public buildings.
But these values would continue to fluctuate, based on additional research and other relevant factors (for instance, the requirements were briefly dropped in the 1980s in response to the energy crisis, before ratcheting back up.) Today, ASHRAE has a variety of ventilation requirements depending on the exact sort of building you’re in:
Though the importance of ventilation was long recognized, codified ventilation requirements mostly applied to commercial and public buildings (ASHRAE didn’t introduce a standard for residential ventilation until 2003.) For homes, it was generally considered that natural airflow through them, in combination with people opening doors and windows, provided sufficient ventilation for the number of occupants. Buildings, after all, were always a little bit leaky. But that would soon start to change.
The energy crisis and the building envelope
The trajectory of building and home design was one of increasing reliance on mechanical systems, rather than vernacular methods, to provide a comfortable indoor climate. These systems, rather than working passively like vernacular methods often did, required energy to function, and the post-WWII era was one of steadily rising residential energy consumption.
Though climate control had always been closely tied with efficiency (new technology only spread once it became affordable), the low cost of energy meant that relatively little effort was devoted to residential energy efficiency. Postwar builders often omitted insulation in the name of (upfront) cost savings, and building codes had no minimum insulation requirements.
But the energy crises of the 1970s changed this calculus. Overnight, the price of gas went up by 4x, driving up the price of natural gas as well. All of a sudden, the cost of heating and cooling homes became a major concern around the world.
In the US, the crisis triggered a variety of initiatives to improve building energy efficiency. The Energy Policy and Conservation Act was passed in 1975, which introduced efficiency standards for a variety of consumer products and appliances, including air conditioners, heaters, and furnaces. The Department of Energy was created, with the goal (among others) of promoting energy conservation. The first model energy code for buildings (developed by ASHRAE and published by CABO) was published in 1977. Building codes for the first time began to recommend minimum amounts of insulation, starting with the 1978 BOCA code. Residential energy consumption, which had been steadily rising, would now flatten (and has remained flat.)
People began experimenting with homes that minimized energy use by way of large amounts of insulation. The Lo-cal House, which used large amounts of insulation to reduce energy use by 60%, was built by the University of Illinois in 1976. The Saskatchewan Conservation House, a similar Canadian project, was built in 1977, the same year Gene Leger built his superinsulated house in Massachusetts [1].
This era also saw the beginnings of what would become modern building science - understanding how moisture, air, and energy flow through a building. Princeton’s Center for Energy and Environmental studies received a grant to study residential energy consumption in 1974. This work would eventually lead to the discovery of the thermal bypass in 1977 - that even in a highly insulated building, small leaks around gaps and edges were responsible for significant amounts of heat (and energy) loss. Princeton researchers also developed the blower door, a machine used (and still used) to test home air leakage by pressurizing the inside of the home. Along with providing sufficient insulation, building energy efficiency became increasingly focused on sealing the exterior of the house completely.
Completely sealing the exterior of a building was difficult, if not impossible, to do with traditional methods of construction. Buildings were made from thousands of individual pieces, which were seldom uniform, resulting in thousands of tiny gaps that air could work its way through. And construction was done on-site, with imprecise human actions. Even dense masonry walls tended to leak around the doors and windows. As late as 1970, ASHRAE scientists claimed that “practical limitations on construction techniques preclude elimination of all leaks to outdoor air.”
But the changing landscape of construction and building materials increasingly made minimizing air leaks achievable. Industrial production of large sheet materials (drywall, plywood, etc.) meant newer buildings had fewer joints that air could work its way through, and the joints that did exist could be tighter and more uniform. Newly developed polymers such as Tyvek could be produced in thin sheets and used to cover building surfaces, and polymer foams could be used to fill in any gaps that remained.
Designing a building to work this way required the creation of new fields of expertise. People began to consider the exterior skin of the building, the “building envelope”, as a system with its own performance criteria and design requirements. Today there’s a variety of experts (building scientists, building envelope engineers, enclosures consultants) who study and design the exterior skin of the building.
Initial efforts at producing tightly sealed buildings were often unsuccessful. Air leaks in a home allow energy to escape, but they can also provide a way to dry interior moisture. Sealing the building without accounting for this (such as by carefully choosing a climate-appropriate wall assembly, and ensuring the sealed building doesn’t trap moisture) was allegedly the cause of several building failures in the 1970s due to rot, mold growth, etc. The new best practices didn’t always play nicely with the old ways of building - in traditional masonry buildings for instance, masonry’s tendency to absorb water means that it’s often reliant on interior drying to function properly, and adding interior insulation and vapor barriers can induce decay. Often the easiest way to get a high-performing old building is to build a new envelope around it.
Today, building scientists and performance experts pay extremely close attention to minimizing uncontrolled airflow through a building. For instance, of the top 5 priorities for creating a comfortable, efficient home in “The Home Comfort Book”, the first 3 are “Air seal the house”, “Air seal the house more”, and “Keep air sealing the house.” And blower door testing has become mandatory in new homes as of the 2015 International Energy Code. And beyond this, there’s also a growing emphasis on considering climate management as a whole system - that, rather than working against the natural environment, mechanical systems and vernacular methods can be combined to work with it. By siting a building correctly, arranging windows to minimize heat gain, and including eaves and overhangs, the size and energy requirements of the HVAC system can be minimized.
These interventions are also increasingly being couched as ways to make the home more comfortable, and as a way to lower a building's carbon footprint, rather than simply a way of lowering utility bills. Even with higher energy prices, building a home to extremely high energy efficiency standards has at best an extremely long payback period, and can easily cost far more than it will ever save in energy.
Partly this is due to the equipment a highly sealed house requires to introduce effects that you get for ‘free’ with a leaky house. As mentioned previously, ventilation standards had mostly not applied to residential construction, as it was thought (and studies showed) that the natural leakiness of homes provided sufficient ventilation. But this stops being true in a highly sealed house - getting sufficient ventilation while minimizing energy use can require systems such as heat recovery ventilators or energy recovery ventilators.
This hasn’t been a smooth, steady process. Many groups opposed increased energy efficiency standards, and states varied in when they adopted energy efficiency provisions. And because buildings tend to stick around for a long time, the building stock takes a long time to catch up to new efficiency standards - around 1/3rd of the US’s housing stock consists of homes built before 1980, for instance, and these homes use around twice the energy per square foot than post-1980s homes do. This is one reason the DOE has such a large focus on retrofitting old buildings to be energy efficient. And while knowledge of building science best practices is gradually spreading, the majority of new homes aren’t built using them, and instead are built to whatever the local code minimums are.
But those code minimums are gradually creeping up over time, and there’s a steady increase in home energy efficiency - it seems likely that this trajectory will continue. Increasing emphasis is placed on buildings being ‘net zero’ or ‘net zero ready’ - that with the installation of a sufficient (and achievable) number of solar panels, buildings won’t require any outside energy to function, while still maintaining a comfortable indoor climate.
[0] - For a good example of what these systems could look like, see the design for the mechanical systems in the House of Lords.
[1] - There seems to be disagreement on this date - I can find sources that say 1977, 1978, and 1979.
In a sense they were right that “vitiated air” was responsible for tuberculosis, given that we now know that tuberculosis (like flu and COVID) is an airborne disease, and one of the best ways to prevent airborne disease is through constant replacement of the air! They were wrong about the mechanism, but right about the things to do to prevent tuberculosis.
Great articles!
If houses are increasingly sealed envelops, what are the strategies for ventilating them in an energy efficient way?