For the past two weeks my addition to green building knowledge has been focused on the Passive House or Passivhaus Standard of energy efficient building developed by the Germans in 1988. It seems that every week or two I latch onto a new obsession in the world of modern and green building, suck as much information as possible out of it, and then move on to the next concept to obsess over.
Passive House Definition
The basic idea of a Passive House is to reduce the energy usage of a home by 90% over traditional code built homes. I grabbed this definition off of a site which will hopefully become clearer later in the post: “A Passive House is a building for which thermal comfort 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 without a need for recirculated air.”
Key Elements of a Passive House
From what I can find, there seem to be four main aspects to the average Passive House although each one will vary slightly. I have included two things we all like below before going into further detail – a numbered list and a lovely diagram:
- Super Insulation that is airtight and minimizes thermal bridging
- Highly Efficient Windows
- Mechanical Ventilation with Heat Recovery
- Innovative & Efficient Heating Technology
There are a few key elements of super insulating a passive house:
- High R-Value or Low Thermal Heat Loss Coefficient – For the climate in central Europe which is a tad colder than the northeast US here, they will achieve R-Values of 38 – 52 on all external walls, slab foundation and roofs. This level of insulation reduces the heat lost during the winter and the heat gained during the summer to extremely low levels. It then becomes very easy to keep the home at a comfortable temperature with very little energy. Surfaces in the home will also remain at a constant temperature and enable the home to be kept within safe humidity levels for occupants, furnishings and electronics.
- Construction Reducing Thermal Bridging – Heat will flow through the path of least resistance such as wood, metal or certain foundation materials. Therefore it is important to not only have high insulation values, but to eliminate thermal bridges from the inside of the home to the exterior that are common in typical construction. Thermal bridging will waste the time and money spent on extra insulation if left unchecked.
- Airtight Construction – Building an airtight thermal envelope is important for energy savings, humidity control and ensuring the longevity of the building structure. Gaps in the building envelope will allow moisture to seep in, raising humidity to unsafe levels in the home and damaging the structure of the home over time.
High Efficiency Windows
Windows in a Passive House must be extremely efficient as well to complement the super insulation. In Europe they seek an R-Value of just over 7 which is no easy feat. They use triple pane windows with low-e coatings and Argon gas to reach this goal. They also seek a low U-value of less than 0.20 where a very good Energy Star window in the US will be closer to the 0.30 mark. Below are the three main requirements of windows for a Passive House according to the Germans:
- Triple glazing with two low-e coatings
- “Warm Edge” spacers between the panes of glass
- Super-insulated frames
Mechanical Ventilation with Heat Recovery
Proper ventilation of a Passive House is critical especially due to the air tightness in the home that does not exchange the stale air with fresh outdoor air very much at all. Opening the windows is not a convenient strategy, nor one that can be performed year round. For these reasons a mechanical ventilation system in the form of an HRV or ERV is used to exchange stale air from the most polluted rooms (kitchen, bath, utility) and fresh air is vented into the living quarters (living room & bedrooms).
A Heat Recovery Ventilator (HRV) or Energy Recover Ventilator (ERV) is used in order to recover 75% to 95% of the heat by passing the warm exhaust air past the incoming cold air in a method that does not mix the two streams in order to make sure only fresh air is being vented into the home and no air is being recirculated.
Innovative & Efficient Heating Technology
The heating requirement is so low in a Passive Home due to all of the other factors that usually the home can be heated by simply heating the fresh air that is being brought into the home via the mechanical ventilation system. Various methods can be used to heat the incoming air inline which eliminates the need for additional ducting in the home. Some of the common methods to heat the air in a Passive House include the following:
- Small heat pump
- Small condensing gas burner
- Small combustion unit for biomass fuel
- Compact unit for all in one heating, ventilation and domestic hot water
Finally we’ll end with one of the charts I found from a case study (linked below) on a passive house built in Europe. It shows the effect on the heat requirements of a typical new home in Germany that each measure implemented has on the home.
- Passive House Institute US
- Passive House Case Study (143 page PDF)
- Passive Houses Directory (German site translated to English)
- E-colab.org (US Energy Consultants that have built 3 Passive Homes in Illinois)