Passive House Definition

The view of Dr Wolfgang Feist
sponsored by the Passivhaus Institut.

The Passive House is not an energy performance standard, but a concept to achieve highest thermal comfort conditions on low total costs – this is the correct definition:

“A Passive House is a building, for which thermal comfort (ISO 7730) can be achieved solely by post-heating or post-cooling of the fresh air mass, which is required to fulfill sufficient indoor air quality conditions (DIN 1946) – without a need for recirculated air.”

This is a purely functional definition. It does not need any numerical value and it is independent of climate. From this definition it is clear, that the Passive House is not an arbitrary standard enacted by somebody, but a fundamential concept. Passive Houses have not been “invented”, but the conditions to use the passive principle has been discovered. One could argue about, whether the noun “Passive House” is adequat to denote this concept. Well – there is no better one. Thermal comfort is delivered in a Passive House by passive measures as far as reasonable (insulation, heat recovery in the temperature gradient, passive utilized solar energy and internal heat loads). To use only passive measures might be possible in some climates – but it will not be reasonable in most of them.

An even better understanding we get from the following practical considerations:

1) In airtight houses one always needs a ventilation system (ask the Swedish). All really energy efficient houses have to be airtight. That means, that with the Passive House concept the technical component “ventilation system”, which one needs anyhow, will be sufficient to heat (and to cool) the building without additional ducts, larger duct diameters, additional ventilators,…

Remark for readers from America: You are used to have air based heating and cooling systems (thats why you call it “air conditioning”). But the systems used in America are almost all just recirculating indoor air at a quite high rate (> 10 ach, but the air is not “changed”, it is just recirculated). The system discussed here is something very different: It replaces the indoor air with a very low rate (0.3 to 0.6 ach) with external air to maintain a good indoor air quality. There is no recirculated air. The airflows are much lower, there is almost no noise and no draft at all. Well, the use of such a system might be very similar to the ones you are used to – but quite more comfortable.

2) This concept makes it possible, to construct buildings with a very efficient heat recovery and to do that cost-effectively. This is difficult in other cases, because heat recovery systems ask for a quite expensive additional investment to the heating system with a too long pay-back time to make them affordable. Therefore it is a good idea to reduce costs of at least one of the two systems: The ventilation or the heating system. If one reduces costs for the ventilation systems by choosing e.g. just an exhaust fan ventilation, then the ventilation heat losses will be quite high and the building will need a conventional heating system – in this case the result could be a low energy house. Or the heating part is simplified in a way that it can be integrated into the ventilation system and, in that case, the building will be a passive house.

The extraordinary low consumptions of passive houses are just a direct consequence of the concept described above. To deliver all the space heating just by heating with fresh air can only work, if the overall heat losses are very low. Therefore the insulation of the building envelope has to be very good at least in cold climates. But the same holds true for hot climates, if the fresh air supply has to be sufficient for air conditioning.

The drawing illustrates the basic principle of a Passive House: Ventilation has to deliver at least the fresh air required for an acceptable indoor air quality. Isn´t it possible to use just this amount of air to heat (and cool) the house? – Yes, in principle this is possible, but the maximum heat load which can be dealt with by this concept is very low.

This is the calculation to derive the “condition for Passive Houses”:

From experience (and DIN 1946) we know, that 30 m³/h is a minimum air rate per person to maintain a reasonable indoor air quality (Yes, in airoplanes you often get only 12 or 15 m³/h. But – is this a reasonable good indoor air quality?). Air has a specific heat capacity of 0,33 Wh/(m³K) (at 21°C). It is allowed to increase the fresh air temperature by 30 K, not more, to avoid pyrolysis of dust. You get

30 m³/h/Pers · 0,33 Wh/(m³K) · 30 K = 300 W/Pers

That shows: 300 Watt per person can be delivered by a fresh air heating system. If you have e.g. 30 m² living space per person, you get 10 W per m² living space. This value is independent of the climate. So far all values are peak load values, that is the maximum heat capacity needed at design conditions. In dependence of the external climate Passive Houses will have to be insulated to a different level: More insulation in Stockholm, less in Roma.

It is important to distinguish heat load values (power in W (Watts)) from annual consumption values (heat or energy in kWh). In a Central European climate from experience and simulation we now, that typical heating energy consumptions of Passive Houses are some 15 kWh/(m²a) – but these are only raw figures. In Stockholm it could be up to 20, in Roma more like 10 kWh/(m²a).

Passive Solar Home Design

A View from Energy Saver, a US Government Agency

Passive solar design takes advantage of a building’s site, climate, and materials to minimize energy use. A well-designed passive solar home first reduces heating and cooling loads through energy-efficiency strategies and then meets those reduced loads in whole or part with solar energy. Because of the small heating loads of modern homes it is very important to avoid oversizing  south-facing  glass and ensure that south-facing glass is properly shaded to prevent overheating and increased cooling loads in the spring and fall.

ENERGY EFFICIENCY FIRST

Before you add solar features to your new home design or existing house, remember that energy efficiency is the most cost-effective strategy for reducing heating and cooling bills. Choose building professionals experienced in energy-efficient house design and construction and work with them to optimize your home’s energy efficiency. If you’re remodeling an existing home, the first step is to have a home energy audit to prioritize the most cost-effective energy efficiency improvements.

SITE SELECTION

If you’re planning a new passive solar home, a portion of the south side of your house must have an unobstructed “view” of the sun. Consider possible future uses of the land to the south of your site—small trees become tall trees, and a future multi-story building can block your home’s access to the sun. In some areas, zoning or other land use regulations protect landowners’ solar access. If solar access isn’t protected in your region, look for a lot that is deep from north to south and place the house on the north end of the lot.

HOW A PASSIVE SOLAR HOME DESIGN WORKS

In simple terms, a passive solar home collects heat as the sun shines through south-facing windows and retains it in materials that store heat, known as thermal mass. The share of the home’s heating load that the passive solar design can meet is called the passive solar fraction, and depends on the area of glazing and the amount of thermal mass. The ideal ratio of thermal mass to glazing varies by climate. Well-designed passive solar homes also provide daylight all year and comfort during the cooling season through the use of nighttime ventilation.

To be successful, a passive solar home design must include some basic elements that work together:

  • Properly oriented windows. Typically, windows or other devices that collect solar energy should face within 30 degrees of true south and should not be shaded during the heating season by other buildings or trees from 9 a.m. to 3 p.m. each day. During the spring, fall, and cooling season, the windows should be shaded to avoid overheating.
  • Thermal mass. Thermal mass in a passive solar home — commonly concrete, brick, stone, and tile — absorbs heat from sunlight during the heating season and absorbs heat from warm air in the house during the cooling season. Other thermal mass materials such as water and phase change products are more efficient at storing heat, but masonry has the advantage of doing double duty as a structural and/or finish material. In well-insulated homes in moderate climates, the thermal mass inherent in home furnishings and drywall may be sufficient, eliminating the need for additional thermal storage materials.
  • Distribution mechanisms. Solar heat is transferred from where it is collected and stored to different areas of the house by conduction, convection, and radiation. In some homes, small fans and blowers help distribute heat. Conduction occurs when heat moves between two objects that are in direct contact with each other, such as when a sun-heated floor warms your bare feet. Convection is heat transfer through a fluid such as air or water, and passive solar homes often use convection to move air from warmer areas — a sunspace, for example — into the rest of the house. Radiation is what you feel when you stand next to a wood stove or a sunny window and feel its warmth on your skin. Darker colors absorb more heat than lighter colors, and are a better choice for thermal mass in passive solar homes.
  • Control strategies. Properly sized roof overhangs can provide shade to vertical south windows during summer months. Other control approaches include electronic sensing devices, such as a differential thermostat that signals a fan to turn on; operable vents and dampers that allow or restrict heat flow; low-emissivity blinds; operable insulating shutters; and awnings.

REFINING THE DESIGN

Although conceptually simple, a successful passive solar home requires that a number of details and variables come into balance. An experienced designer can use a computer model to simulate the details of a passive solar home in different configurations until the design fits the site as well as the owner’s budget, aesthetic preferences, and performance requirements.

Some of the elements the designer will consider include:

The designer will apply these elements using passive solar design techniques that include direct gain, indirect gain, and isolated gain.

DIRECT GAIN

In a direct gain design, sunlight enters the house through south-facing windows and strikes masonry floors and/or walls, which absorb and store the solar heat. As the room cools during the night, the thermal mass releases heat into the house.

Some builders and homeowners use water-filled containers located inside the living space to absorb and store solar heat. Although water stores twice as much heat as masonry materials per cubic foot of volume, water thermal storage requires carefully designed structural support. An advantage of water thermal storage is that it can be installed in an existing home if the structure can support the weight.

INDIRECT GAIN (TROMBE WALL)

An indirect-gain passive solar home has its thermal storage between the south-facing windows and the living spaces. The most common indirect-gain approach is a Trombe wall.

The wall consists of an 8-inch to 16-inch thick masonry wall on the south side of a house. A single or double layer of glass mounted about one inch or less in front of the dark-colored wall absorbs solar heat, which is stored in the wall’s mass. The heat migrates through the wall and radiates into the living space. Heat travels through a masonry wall at an average rate of one inch per hour, so the heat absorbed on the outside of an 8-inch thick concrete wall at noon will enter the interior living space around 8 p.m.

ISOLATED GAIN (SUNSPACES)

The most common isolated-gain passive solar home design is a sunspace that can be closed off from the house with doors, windows, and other operable openings. Also known as a sunroom, solar room, or solarium, a sunspace can be included in a new home design or added to an existing home.

Sunspaces should not be confused with greenhouses, which are designed to grow plants. Sunspaces serve three main functions — they provide auxiliary heat, a sunny space to grow plants, and a pleasant living area. The design considerations for these three functions are very different, and accommodating all three functions requires compromises.

PASSIVE SOLAR HOME DESIGN FOR SUMMER COMFORT

Experienced passive solar home designers plan for summer comfort as well as winter heating.

In most climates, an overhang or other devices, such as awnings, shutters, and trellises will be necessary to block summer solar heat gain. Landscaping can also help keep your passive solar home comfortable during the cooling season.

PS. The featured image is a North Carolina home that gets most of its space heating from the passive solar design, but the solar thermal system (top of roof) supplies both domestic hot water and a secondary radiant floor heating system. | Photo courtesy of Jim Schmid Photography.