Passive House Principles: Passive Elements

When people think of houses being passive they think about "passive solar".  Passive solar is a different kind of home and although it integrates some passive elements, a passive house is still more energy efficient and can be tailored to a wider degree of climatic conditions.  Passive House aims to meet some of the energy demand of the building using passive elements.  The effect on the building is less energy usage.

What is a passive element?  A passive element is a part of the building that offsets the energy demand of the building passively.  It is some element that is used to gain a desired effect without having to add energy actively.  When a building envelope is super-insulated, air tight, and thermal-bridge free, the total energy inside a building stays constant for longer periods since the envelope resists the flow of heat inwards or outwards.

There are several ways to gain heat passively.  South facing windows can be a huge contributor to heat.   However they can also be like a huge hole in the envelope where heat leaks out of the building.  Lets put this in perspective.  Take a wall 8 ft high and 20 ft long.  Without a window with R20 batt insulation the effective R-value is about 17.  Insert a large 4ft x 4ft window in the wall which has an R-value of 2.  It can be shown that the total R-value of the whole wall assembly becomes R11!!!  So the effect of having a R2 window is detrimental to the performance.  Even though the window only makes up 10% of the total wall area it effectively halves the R-value and makes a 5% difference to the heat flow out of the building.  Now add a couple more windows....and you're living in a green house:  Its cold when it's cloudy and sweltering hot when the sun is shining!.  Triple glazings should be used for all windows regardless of their orientation on the building.  Glass with expensive coatings aren't really required for areas of the home where the sun doesn't shine!  However on the south side of the home, glazing with high solar heat gain coefficients are necessary (SHGC).  They will ensure maximal transmission of solar energy through the glass.  When the energy requirements of a building are as low as a Passive House, the solar gains can make up to 1/3 of your total energy for heating.  The key is balancing the size and number of windows so the amount of energy gained is at least equal to or more than the energy lost.  This is part of the Passive House design process.  Adding thermal mass, such as concrete floor act to buffer over heating of the air inside the home.  The concrete will soak up some of that heat which will radiate into the space as the sun goes down.  That same thermal mass will help keep the house cool in the summer time.

People, appliances, showers, lighting can all make up a large component of the internal gains of a building.    People are walking 60 W radiators.  All appliances expel heat in some manner.  They are a huge contributor passive heating.  Cooking a couple of times a day using a stove top or oven can contribute up to 1/3 of the necessary heat to keep the building comfortable!    These elements are purely passive.  Their intent is not to heat the inside of the house; but the final effect is a significant contribution to the heat added to the building.  This being said, since heat is being added passively, there is less necessity for active heating.  Have a look at this energy balance diagram below.  This is from a simulation for a passive house.  the bar on the right shows the thermal gains for the building.  Solar gains make up almost 1/3 of the energy!  Internal gains from all of the elements mentioned above make up almost another 1/3!

Ventilation can be passive.  In the summer, a home will warm during the day but at night when it cools, the windows can be opened on either end of the house to encourage cross ventilation.  Orienting a house so prevailing winds act to sweep through the home can be effective for night time flushing.  When the morning comes, the windows are closed for the day to slow the flow of heat from the outside-in.  Simple trellis shading over windows can passively decrease solar gains through windows so homes gain less heat and require less cooling.

All of these passive elements are designed into the home.  The home is planned for performance from the beginning.  Don't quote me on this but I would say that at least 50% of the energy savings of a building are from air tightness and insulation.  We see this effect with R2000.  The rest of the savings is achieved through good design: A Passive House design.

Here is a link to the Passipedia site which has a nice video called "Passive House Explained":

Some more links to consider,


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