Passive House Design: A Home That’s Built to Last
There are many misconceptions surrounding passive home design. When most people hear the term, images of eccentric architecture and energy-saving techniques that are as much an eyesore as they are impractical spring to mind. Giant black barrels filled with water for thermal mass, outer walls made entirely of old tires—the stuff only a survivalist would dream of.
Thankfully, that kind of passive house construction is long gone.
Efficient, high-quality, and economical, passive home design is not about fringe building practices for the sake of saving a few trees, but a rethinking of all the inefficiencies that go into outdated construction methods. It’s about making use of passive energy sources already around you and minimizing wasteful heat loss. It’s about maintaining a cleaner, fresher air quality, and saving money as you do. Fundamentally, passive house design is a way to build a better home.
What makes a passive house?
The main goal of passive house construction is to build a home that requires minimal energy generation to heat and cool—if any at all. The average passive house requires 80–90% less electricity to cover all of its heating and cooling needs.
The Passive House Institute of the United States, also known as PHIUS, is the certifying standard for passive house design. This not-for-profit organization establishes the metrics that determine what constitutes a passive home, and the requirements are surprisingly straightforward, albeit ambitious. They are:
The space heating and cooling requirements of the house must not exceed 4.75 kBtu/square foot/year. By comparison, the average home furnace consumes roughly 25-45 kBtu/square foot/year, depending on their location.
Minimal power consumption
The total source power demand of the entire house must not exceed 38 kBtu/square foot/year.
Airtightness – When pressurized to 50 Pa and exposed to a blower door test, the home must experience no more than 0.6 air changes per hour. The average home experiences 5-7 air changes under the same conditions. (Note: The airtightness test is used to measure draftiness within the home and does not denote poor air circulation, as passive houses are notorious for their superior air quality.)
Thermal comfort – The temperature of a passive house may exceed 77°F (25 C) for no more than 10% of the year.
These standards may vary from region to region, as more strenuous requirements may be placed upon lower temperature limits in colder climates, while more stringent upper-temperature limits may be found in warmer areas.
How do you build a passive house?
Although simple in philosophy, passive house construction does require some expertise to execute properly. There are five main components of passive house design that will likely be incorporated into any build.
The reason so little energy is required to heat and cool a passive house is that much of it is derived from the already available warmth of the sun. This form of solar energy differs from solar PV because no electricity is generated. Instead, the house is designed and located in a position where it may passively absorb the maximum amount of the sun’s rays. This means that the home will be situated such that it receives maximum sunlight in the winter months to heat it, and less sun in the summer to keep it cool.
Optimal sunlight absorption entails the use of ample window space, carrying with it the added benefit of plenty of natural daylighting. This cuts down further on the need for electrical lights and reduces power consumption all the more.
Windows are critical to passive house design, not just because they allow warmth and light from the sun to enter the home, but because they are often a key source of heat loss in most houses. To minimize these losses, only the highest quality windows are used in passive house construction. Typically, triple-paned windows often filled with argon gas are installed throughout the home and are sealed to deliver the best insulation and heat gain values possible.
In part because of the exceptional sealing of all windows and doors in a passive home, draftiness is kept to a minimum. As a result, all heated or cooled air within the homestays in the home, and extreme outdoor temperatures are kept outside. This is what is conveyed by the blower door test, whereby a passive house experiences nearly 90% less air turnover than the average home—part of what makes passive houses nearly self-sustaining.
Put more qualitatively, all of the leaks and gaps in the average home through which air escapes amount to a total surface area approximately equal to that of a garbage can lid. In a passive home, the total gap surface area amounts to less than that of a baseball.
Proper sealing plays an important role in minimizing heat losses, but other measures must be taken to ensure that the home keeps all of its warmth if it’s to attain passive house standards. The home must be exceptionally insulated with the highest quality materials, and several design considerations must be taken, too. As an example, one primary source of heat loss in most homes is the presence of cold beams that support the ceiling. These beams may be exposed to or touching adjacent walls which are in turn exposed to the outside air, thus cooling the beam and subsequently the home. This phenomenon is called thermal bridging and is strongly avoided in passive house design.
From ensuring that all beams are kept away from exterior walls to keeping walls that are exposed to the elements from cooling the rest of the home, no detail in passive house design goes overlooked.
Heat Recovery Ventilation (HRV)
Maximum sealing and optimum insulation could have the potential to result in a stuffy house if careful measures were not taken to ensure the greatest indoor air quality. This misconception that passive houses are stuffy is often held by those unfamiliar with them, but they often possess superior air circulation than conventional homes—they just don’t have the heat loss that usually comes with it.
To achieve such superior air quality without thermal waste, designers often implement a heat recovery ventilation system or HRV. Essentially functioning as a dual air purifier and heat exchanger, HRVs bring fresh air in from the outside, filter it, and pass it by the outgoing warmer air. The result is a continuous supply of purified fresh air that has been pre-warmed to the proper temperature.
What a passive house isn’t
Passive homes are often confused with similar construction methods, which results in a misunderstanding of their nature. Here are a few terms that are often conflated with passive house construction.
Although the initial builder of the passive house took much inspiration from passive solar designs, the two are not the same. Passive solar requires huge windows that take up much of a wall and are only in one or a few places and uses thermal mass like heated water barrels to continue providing heat to the home. Although spacious window areas sometimes are part of passive house design, these are more flexibly placed and more functional than in passive solar homes of the past.
All forms of off-grid power supply – solar PV, wind turbines, natural gas, etc.—are actively generated. A passive house need not use any of these energy sources, as passive house design is about minimizing heating and cooling needs so that a home may be almost completely self-sustaining. While the reduction in power demand certainly facilitates the use of such off-grid power generation, it is not necessarily part of a passive house design.
Net-zero building – As structures that will save as much energy over the course of their operation as it took to build them, net-zero buildings are a subset of passive building design. Unlike passive homes, they do require some form of renewable energy generation, and many of the same principles involved in passive house design are used in net-zero building construction.
Are passive houses expensive?
The upfront added expense of passive house design is approximately 3-5% higher than that of conventional construction methods in the United States. This number is dropping as market availability increases—and in Europe, the cost is the same as conventional homebuilding, giving cause to believe that the two will eventually become equal in the US as passive houses increase in popularity.
Even with the extra initial cost, passive house construction presents a clear opportunity for long-term savings. If a 2,000 square foot home is built for $200,000, for instance, an additional 5% cost amounts to $10,000. Assuming the average electrical use of such a home as well as current electricity prices (and neglecting inflation), the annual savings obtained from passive house construction amounts to $717.16/year, meaning the added investment will pay for itself in just under 14 years—less than half the life of the typical 30-year mortgage. Any time spent in the home after this and your passive house is making money for you.
Summing it up
Passive house construction is an innovative overhaul of outdated building practices. The heat sinks and inefficiencies that waste valuable energy and drive up the cost of overpriced utility bills are designed out through comprehensive, thoughtful, sustainable building practices in this common-sense approach to homebuilding techniques, delivering a self-sustaining home that will last long enough to leave a legacy. Passive home designs are versatile and beautiful enough to accommodate any taste, and taken together with their architectural mindfulness and robust durability, it may truly be said that these are the homes of the future—and that the future starts now.
Alquimia In has a team of experts from architects, engineers, and craftsmen who have the knowledge to design and build a passive house design in Houston.