Passive Solar

Passive House, or passivhaus, is sometimes confused with passive solar, and although the latter is an important component in Passive House design, the terms are not interchangeable.

Passive solar refers to the strategy of using the building itself – the windows, walls, floors –  without added equipment, to collect, store, and distribute solar energy as heat. A part of passive solar design is also the control of unwanted solar energy in the summer, through the use of overhangs etc. The idea of passive solar contrasts with active solar, which uses equipment (e.g. photo-voltaic panels, or solar hot water collectors) to do the same.

Passive solar requires thoughtful consideration of the local climate, solar access, building siting and orientation, landscaping etc.

There are several types of Passive Solar. The first, and most basic, is Direct Gain, where the interior space is heated directly through south-facing windows (of course this assumes the building is located in the northern hemisphere).

 

 

 

 

 

 

 

In Indirect Gain, a thermal mass, for example a “trombe wall”, is located between the south-facing windows and the space to be heated. The advantage in this method is that the transfer of heat to the interior is delayed, so a thermal mass heated during the day may release its heat to the interior at night.

 

 

 

 

 

 

 

The third type is Isolated Gain, using a separate Sunspace, or Greenhouse, to borrow heat from as needed.

Some Passive Solar Fundamentals:

  • Orientation – if possible, orient the long axis of the building in the east-west direction, to maximize southern exposure. Ideally there will be unobstructed access to the sun during most of the day, and the principle use spaces of the building will be located on the south side, with service spaces (e.g. example bathrooms, mechanical, storage) on the north side.
  • Windows (free solar heat generators) – in general, optimize the amount of windows on the south side of the building, and minimize the amount of windows on the other three sides.
  • Control – use the architecture itself (eaves, awnings, exterior shades, sliding screens etc.), to block summer sun, but allow winter sun to penetrate interior. The latitude determines the ratio of depth of overhang to height of glazing. You can also use the landscaping for control. Deciduous trees on south side can block unwanted summer sun, but allow the winter sun to pass through. Evergreen trees on the east and west sides can block unwanted solar gain.
  • Thermal mass – Thermal mass refers to a material that can absorb the solar heat that enters a building – it can be an exposed concrete floor, ceramic tile, even gypsum wallboard.
  • Distribution – Thermal mass distributes the heat by radiation; In indirect or isolated passive solar, distribution can be by radiation, convection, or assisted by mechanical means.

Some Passive Solar Challenges:

  • Passive solar design guidelines often assume a large, flat, unobstructed site with no trees. In urban areas, lots oriented east-west typically have (sometimes tall) neighbors tight to the south, while lots oriented north-south will have a short face on the south side, neither of which is ideal. Sites on north facing slopes are not ideal – sometimes the site itself can block the sun (esp. when the sun is low in winter, when you need the solar gain the most). Conversely, sites on south facing slopes are preferred.
  • Seattle homes are sometimes designed as “View Machines”, and often that view is to the west – maximizing windows for view can be at odds with passive solar ideals.
  • Shading or screening of south-facing windows, to minimize summer heat-gain, can make rooms darker in our already gray winter months.
  • Remodels – passive solar design guides often assume you’re building a new house from the ground up, and so have more opportunity for optimal siting, orientation etc. A remodel or addition project has more constraints, e.g. existing architecture to relate to, structural issues that may make large areas of glazing difficult, etc.

That being said, an existing house can be remodeled to incorporate passive solar strategies, e.g. adding more windows on the south side, adding awnings over south facing windows, or adding thermal mass on the interior.

Without going into detail, I’ll list a few innovative ideas relating to passive solar design:

  • Annualized geo-solar – this refers to capturing warm season solar heat and storing it for several months, until it’s needed in the cold season. A variation on the Thermal Flywheel idea;
  • Phase change materials – usually eutectic salts, materials that store solar energy as latent heat. The sun heats and melts the material during the day – at night the material reverts to a solid state, and the stored heat is released. Phase change materials can be incredibly efficient in storing heat – as much as 80 times as effective as water;
  • Living Walls, depending on the plant type, can allow winter sun through, but will block the sun when it’s filled out in the warmer months;
  • Planning for future active solar – I like to think of this as another passive solar fundamental. Configure the roof to maximize solar orientation and access for potential future PV and solar hot water systems. In projects not installing a solar system, pre-pipe for future installation.

The heat-gain benefits of passive solar design should always be complemented by strategies to minimize heat-loss, such as adding insulation (beyond code), using high-performance windows, making the building super air-tight, using an HRV, using high-efficiency lighting, plumbing fixtures, appliances and systems, etc. This meshes with the goals of Passive House (you knew I was going to circle back to that, didn’t you?) – to equalize, as much as possible, the heat loss through the envelope of the building, with the heat gains, both external (solar) and internal (peoples bodies, appliances, lighting, etc.).

Mean Radiant Temperature – an Important Factor in Comfort

The Mean Radiant Temperature (MRT) indoors is an important factor in determining if a home is comfortable or not. The MRT is essentially a measure of the average temperature of all the objects in a space, including the walls, windows, furniture, people etc. It, along with the ambient dry air temperature, determines how thermally comfortable a space is. A newer, well-insulated building with high-performance windows will have a higher MRT than a conventional older building.

The importance of the MRT can be illustrated by something we’ve all experienced. In an older home in the middle of winter, the ambient air temperature indoors can be 68 to 70 degrees, and yet we feel cold when we’re near a window. That’s because the cold interior pane of glass in that window is literally sucking warmth away from us, or, to put it more accurately, our bodies are emitting heat to that cold surface, which causes us to feel cold. This is the inverse of the experience of feeling warmed by the sun outdoors on a cold winter day. Our skin has high emissivity and absorptivity, meaning we’re very sensitive to radiant heat loss and gain.

You might have noticed that heat registers in older buildings are typically located along the exterior wall, at windows – that’s precisely to counteract this effect. In a high-performance building, such as a Passive House, the MRT will be higher. Their triple-glazed windows will ensure the interior pane of glass is warm, so you’ll be comfortable next to them even on a cold day. This leads to other benefits – for example, because the heating system doesn’t have to counteract the cold glass effect, the conditioned air can be delivered to the living spaces on the inboard side of the rooms, rather than at the exterior walls. In this way the size (both of the heating equipment, and the size and length of ductwork) and complexity of the heating system can be drastically reduced, thus driving down the cost of the mechanical equipment too.

We’re all well aware of the effect of air temperature on comfort, but once you’re cognizant of the importance of Mean Radiant Temperature on comfort you’ll become more aware of it too.

 

Passive House – a Strong Component in Green Building Rating Systems

Passive House is sometimes criticized for limiting itself to Energy and IAQ, and not being a more comprehensive green rating system, such as LEED, or Built Green, or the Living Building Challenge. In fact that’s one of the things I like most about it, that it focuses on one thing, and does it very well. When it comes to creating energy-efficient, healthy, comfortable buildings, it’s clearcut, definitive, and unambiguous.

After doing green building for years, and dealing with the often ambiguous choices that have to be made, when I learned of Passive House it was an epiphany and, well, REFRESHING. There are many aspects of Green Building that can be equivocal, if not  downright fuzzy. For example, I remember when fly-ash in concrete was everyone’s green strategy du jour (a by-product of coal combustion, fly ash can be substituted for cement in concrete, significantly decreasing its embodied energy). But when many began specifying it in projects here, and the fly ash had to be imported from areas of the country that actually had coal-fired power plants, the environmental benefit flew out the window. Insulation has got to be the best example of a material or strategy that no one can agree on. There often seems to be an inverse relationship between effectiveness (i.e. R-value per inch) and Global Warming Potential, or GWP (GWP compares a product’s contribution to global warming to carbon dioxide – some rigid foams had a GWP of 1600 – yep, 1600 times that of CO2!). Nothing is sacred, not even Photovoltaics. Sure, renewable energy is a good thing, but what’s the embodied energy in PV panels – how much energy was used to refine the silicon, to produce the aluminum frames, to transport it to the distributor, etc? I could go on and on…

Back to my main point, I respect that Passive House doesn’t try to be all things to all people, but just focuses on how to create energy-efficient buildings, and does it better than anyone else. That being said, Passive House can form a strong component in all of the Green Building rating systems – LEED, Built Green, the Living Building Challenge – in fact it’s the best method for achieving a good part of their Energy and IAQ goals. Don’t think of Passive House as competing with them, but as complementing them.

Pioneers of High-Performance Buildings

I’m inspired by pioneers in any field, but particularly those who were doing low-energy buildings long before most of us even heard of “Green Building”.

A little background may be in order. I remember in the early seventies when the oil embargo hit, and my parents complained about the price of gas jumping (the price of oil quadrupled from $3 to $12 a barrel!). A concern for energy efficiency and energy conservation grew out of this “crisis”. Energy codes were born or broadened, requiring us to insulate our buildings, and to make them less leaky. There was a lot of trial and error – two steps forward, one step back. Insulated buildings saved energy, and so did tighter envelopes. But the latter also led to the unhealthy build-up of toxins and stale air indoors (can anyone say “Sick Building Syndrome”?)*. In response to this, fresh-air ventilation was introduced. This led to more energy loss as the warm indoor air was exhausted, to be replaced by cold air from outside. In response to this, heat-recovery ventilation was introduced, which has become more efficient over time.

*There were other unforeseen impacts of these “improvements” – for example, tighter buildings no longer allowed the easy passage of water vapor through walls. More about this in a later blog!

The abbreviated history above describes the slow response (really still occurring) within the mainstream building industry, to the energy crisis. But there were those who took a more holistic view of building and energy, and from the start integrated many effective strategies together. One of these innovators (a hero of mine) was William Shurcliff, a well-known physicist with a background in nuclear physics. He issued a press release in 1979 (!), listing the necessary components of super-insulated houses. It reads today, 33 years later, like a checklist of strategies to achieve the Passive House Standard:

1. Truly superb insulation. Not just thick, but clever and thorough. Excellent insulation is provided even at the most difficult places: sills, headers, foundation walls, windows, electric outlet boxes, etc.
“2. Envelope of house is practically airtight. Even on the windiest days the rate of air change is very low.
“3. No provision of extra-large thermal mass. (Down with Trombe walls! Down with water-filled drums and thick concrete floors!)
“4. No provision of extra-large south windows. Use normal number and size of south windows — say 100 square feet.
“5. No conventional furnace. Merely steal a little heat, when and if needed, from the domestic hot water system. Or use a minuscule amount of electrical heating.
“6. No conventional distribution system for such auxiliary heat. Inject the heat at one spot and let it diffuse throughout the house.
“7. No weird shape of house, no weird architecture.
“8. No big added expense. The costs of the extra insulation and extra care in construction are largely offset by the savings realized from not having huge areas of expensive Thermopane [windows], not having huge well-sealed insulating shutters for huge south windows, and not having a furnace or a big heat distribution system.
“9. The passive solar heating is very modest — almost incidental.
“10. Room humidity remains near 50 percent all winter. No need for humidifiers.
“11. In summer the house stays cool automatically. There is no tendency for the south side to become too hot — because the south window area is small and the windows are shaded by eaves.

The developers of the Passivhaus Standard acknowledge their indebtedness to William Shurcliff, and other pioneers experimenting in super-insulated houses (many of whom were from the U.S. and Canada). Shurcliff wrote many books on this and related subjects, including “Solar Heated Buildings of North America”, “Thermal Shutters and Shades”, “Super-Insulated Houses and Double-Envelope Houses”, and “Air-to-Air Heat Exchangers for Houses”. More about Shurcliff, and others including Harold Orr and Eugene Leger, can be found at: http://www.greenbuildingadvisor.com/blogs/dept/musings/forgotten-pioneers-energy-efficiency

The 2000 Watt Society – a Good Metaphor for Passive House

The 2000 Watt Society is a vision for the future, formulated in 1998 by the Swiss Federal Institute of Technology, in which the energy use of every First World citizen is limited 2000 Watts. The current world average per capita is 17,520 kWh, which translates to roughly 2000 Watts of continuous energy use – in other words, the goal is to limit everyone’s energy use to the current world average.

Of course that average is not evenly distributed throughout the world – people in the more developed countries, such as the U.S., use significantly more than those in the less developed parts of the world. The current US average is just over 12,000 Watts – six times what would be allowed within this vision. In contrast, the current average in Bangladesh is less than 300 Watts, and only Canada, Scandinavia, and some oil-producing countries in the Middle East use more per person than we do.

Because this vision looks to achieve its goal without lowering our standard of living, it requires that we dramatically improve the energy conservation in our buildings, and the energy efficiency in all aspects of our lives (the 2000 Watt metric is not based only on household use per person, but everything, including our automobile use). Over time, hopefully, this energy will be increasingly produced by renewable sources.

In this way the 2000 Watt Society is very much like the Passive House Standard – it looks to set an energy limit on those who currently use more than they should, a limit that may seem radical to some, but is reasonable and achievable. Clearly, building our buildings to the Passive House Standard would be an effective component in achieving the 2000 Watt Society’s goals. And as with the Passive House Standard, the 2000 Watt Society’s metric is not a requirement everyone has to exactly meet, but is only an upper limit. You can certainly use less energy than 2000 Watts, and more power to you (no pun intended) if you do!

For more information on Passive House, contact Seattle architect Jim Burton at www.blipdesign.com.

Building to the Passive House Standard Equals Money Saved

Building to the Passive House standard not only ensures that your house is comfortable, healthy, and energy efficient, but also benefits you economically in the long-term, through drastically reduced energy bills.

How is this done? Through strategies that take advantage of free heat (passive solar of course, but also the waste heat of appliances, lighting, and even our bodies), and keep that heat in the building through added insulation, triple-glazed windows, airtight construction, and elimination of thermal bridges. This means the house doesn’t need an extensive heating system, but a relatively small one. (It’s sometime mistakenly claimed that a Passive House doesn’t need a heating system – it does. But that heat source won’t be a conventional furnace – it can be as small as a hair dryer!). A balanced ventilation system with heat recovery then ensures superior air quality and comfort, provides fresh air from the outside, and at the same time recovers most of the heat in the exhaust air before it leaves the building. Passive Houses will only use 25% or less of the energy of conventional new houses.

While it’s true a Passive House will cost more than a conventional built-to-code new house (around 5-10%), some of the extra investment in thicker walls, better windows etc. is offset by not needing a large, complicated heating system. In the first few years, the additional mortgage payments will be offset by reduced utility bills, and when the investment is paid off the energy savings continue for the life of the house. It’s really a win-win scenario – you get a healthier, more comfortable, longer lasting house, that’s easy on the environment, and in the end it even saves you money!

Seattle Green Design and Passive House Consultants

Jim Burton Architects, aka BLIP Design, is dedicated to the integration of sustainability and modern design.

Buildings should touch the land lightly (like a “blip” on the landscape).  Using this dogma, BLIP Design and Jim Burton incorporate energy-efficiency, materials and water efficiency, healthy home strategies, and environmental stewardship into each project.

Passive House

Passive House is a rigorous, proven methodology for creating ultra low-energy buildings. It uses a variety of strategies to ensure that the heat losses through the envelope are roughly balanced by the heat gains from passive sources, and thus eliminates the need for a conventional heating system. It is the most cost-effective path to Net-Zero Energy. Jim Burton is a Certified Passive House Consultant, and can help your project meet the Passive House Standard.

Backyard Cottages

With the recent passage of Seattle’s Backyard Cottage Ordinance, Detached Accessory Dwelling Units are now allowed throughout the City. If you’re looking for more space for a studio, for aging parents or boomerang kids, or extra income from a rental property, a Backyard Cottage may be just what you need.