Passive House and it’s influence on the Cross Shelburne House
Architect: Carol Stenberg, AIA, Certified Passive House Consultant
The Passive House building standard should not be confused with passive solar design.
Passive House is a building standard that was developed in the 1990’s by Dr. Wolfgang Feist in Darmstadt Germany which brings all of the collected knowledge on building science together under one standard. It is also a standard that incorporates new developments in building science as it occurs.
A house has three major heat losses.
- Transmission – where heat moves through the material of a wall, floor, roof, window, door or other building element.
- Ventilation – heat that is lost when fresh cold air is brought into the house by a mechanical ventilation system.
- Infiltration – this is the heat that is lost when cold air is blown through cracks in the walls, between walls and windows or doors and the gaps that occur where the roof meets the walls, the walls meet the foundation or where walls meet walls. This is also called uncontrolled ventilation. The cold air that comes into the house by infiltration must be heated by your furnace or boiler.
Modern building techniques from the 1900’s confronted these losses by putting a really big furnace or boiler into the house. This worked because heating fuel was cheap. This is no longer the case. We need to develop ways to make houses loose less heat and therefore use less fuel and be more affordable to live in.
The name Passive House refers to the passive nature of the approaches to make a house very energy efficient.
A Passive House approach deals with these three losses by applying five basic principles:
Reducing a home’s energy usage through transmission losses means adding more insulation. A standard house built to the Vermont Residential Energy Code has:
– R 20 Walls
– R 40 Roof
– 2” of foam insulation under the slab
A Passive House has:
– R 60 Walls
– R 100 Roof
– 10” to 12” of insulation under the slab
This increase in insulation reduces the use of the fuel by approximately 40%
Passive Houses use a relatively new technology called a Heat Recovery Ventilation system. This is a ventilation system which takes the warmth from the stale indoor air and transfers it to the cold fresh air that is being brought into the house.
The newer Heat Recovery Ventilation (HRV) systems can do this at an efficiency of around 93%. This means that when you bring in air that is 30 °F and your indoor air is 68 °F the fresh air is warmed to 65 degrees Farenheit.
This means that your furnace or boiler needs to warm the air from 65 °F to 68 °F instead of from 30 °F to 68 °F, which uses much less energy.
These HRV’s use very little energy themselves and the indoor air quality they provide is excellent as shown by data collected in Passive Houses in Vermont.
This form of uncontrolled heat loss is the last bit of the puzzle. The Vermont Residential Building Energy Standard (Code) states that a house must not have more that five (5) Air Changes Per Hour (ACH) at 50 Pascals. This means that when a 22 mph wind is blowing, the air in the house must not be replaced more that five times in an hour!
If the air in your house is being replaced by -10 °F five times an hour then your furnace has to heat that air from -10 °F to 68 °F five times in an hour!
A Passive House requires that a house have no more than 0.6 Air Changes Per Hour at 50 Pascals. This is very air tight and reduces the amount of heat that the furnace must produce to keep the house warm by an alarming amount.
If you put these three Passive House principles as well as a few more ideas into a house you get a house that uses approximately 90% less energy to heat and cool.
What other ideas?
Triple glazed windows with thermally broken frames. This means that the frame of the window has a layer of insulative material in it. This takes the R-value from around a 3 to 7 or more.
The effect of these windows is quite remarkable. When you are in a regular house in the Winter, as you move from the middle of the house towards the window, it gets colder. That is why we put radiators under windows, to counter that effect. In a Passive House the inside temperature of the window is the same as the inside wall. All of the surfaces in your room are the same temperature. This leads to a level of comfort that a regular code house just can’t achieve.
Thermal Bridge Free Construction
On your lawn mower engine there are metal fins. These fins are used to dissapate the heat that the engine produces. These are thermal bridges to the outside air allowing the motor to cool of more quickly.
The same principle happens on houses. When you have a balcony where the floor of the balcony is made by extending the floor members of the house. These floor members (joists) sick out into the cold outside world and act like the fins on that lawn mower engine, only this time they are creating a way for the heat of the house to escape around your insulation into the cold outside.
I started by saying that a Passive House is not a passive solar house. That doesn’t mean that we don’t use the sun to help heat the house. Once the house is very well insulated and the drafts are eliminated the sun can add as much as 40% of the needed heat for the house, even in the middle of Winter.
A Passive House orients the house with the larger wall to the South. It puts most of the windows on the South side of the house as well. There are windows to the West, North and East, there are just fewer of them and they are smaller.
Once you apply all of these techniques it leads not only to a house that uses 90% less energy to heat and cool, but one with excellent indoor air quality and thermal comfort.
Since the Passive House uses so much less energy you don’t need a really big furnace or boiler any more. A standard 2,000 square foot house has a 100,000 Btu/hour furnace, with a system of radiators or vents that deliver this all through the house.
With the Passive House of the same size the furnace can be 10,000 Btu/hour (90% reduction in use) which can be achieved with a small much smaller heating system.
Use of materials that have low Embodied Energy
Embodied energy is the amount of energy that is required to produce a material. Foam insulations all have rather high embodied energies. Extruded Polystyrene (XPS) uses the most energy of all. This is your standard blue board (sometimes pink as well).
Cellulose is produce from used newspapers and other paper and cardboard products. It has one of the lowest embodied energies of all of the insulations. Rockwool also has a relatively low embodied energy.
In this project we chose to use materials that have low embodied energy. The foam we used for the foundation is EPS (Expanded Polystyrene) which has the lowest embodied energy of all of the foam products.
How were these principles applied to the Cross Shelburne House?
We applied all of the concepts of a Passive House when we were designing the Cross Shelburne House.
Highly insulated walls:
ñ R-60 Walls
Non structural Truss Joist I-shaped (TJI) on end filled with cellulose
Super insulated slab (no basement with this house)
ñ R-50 insulation under the slab which is 12 inches of EPS
Super insulated roof: R- 104 roof
Yes an R-104 roof which is 30” of cellulose blown into the cavity above the ceiling. The ceiling was made with 5/8 Oriented Strand Board (OSB) which can handle the load of so much insulation.
To quote a builder friend of mine, Chris Corson, from Eco Cor in Maine,”Cellulose is cheap!”
Thermal Bridge Free Construction
When you have an outside corner of a building, or where the wall meets the foundation or a wall meets the roof, these are places where thermal bridging can occur. These were modelled in this building to check that the critical joints of the building weren’t thermal bridges. The Passive House standard defines a thermal bridge as having a thermal conductivity of 0.006 Btu/ft hr F.
Here is the analysis of the footing to wall detail.
The purple color is cold the weather outside (14 F in this case) and the white is inside (68 F). The minimum temperature on the inside corner shows 63.4 F. The dew point at 68 F and 70% Relative Humidity is 57.3 F. This model shows very low risk of moisture at the coldest point thus showing safe construction regarding as far as mold is concerned.
Achieving high levels of air tightness great care must be used in the building. Quality is of the utmost import if you are trying to get 0.6 Air Changes Per Hour at 50 Pascals!
In our construction as in any super air tight building you need to take care an clearly define your air barrier. This begins in the design phase and is carried through all phases of construction.
In this house the main air barrier is the OSB layer on the outside of the 2×4 frame. Each of the OSB panels were caulked and then the seams were primed an taped over.
The walls are not solid. There are holes in them. Windows, doors and the entrance and exit of various pipes and ducts.
Each of these have been painstainkingly detailed by professional air sealers. The windows are all either fixed or casement windows. No double hung windows. The installation of these windows will include not only water sealing but air sealing as well.
All duct and pipe penetrations will be carefully air sealed.
Once the windows, door and mechanicals are in we will do a blower door test to see just how tight the house is. Let’s hope we make our 0.6 ACH50!
The windows being used on this project are Thermotech made in Canada. The client was very specific about using only products that came from the North American continent. These windows are triple glazed and have a thermally broken frame.
These windows have been used in other Passive House and near Passive House projects in Vermont with great success.
The project is using a high efficiency ventilation system from Ultimate Air that keeps the warmth from the ventilation air in the house and maintains the balance of moisture. This type of system is called an ERV (Enthalpy Recovery Ventilation).
The house has a wood stove as well as solar hot water panels and resistance electrical hot water for heating.
The heating load of this house is 6,248 Btu/hr! This is the same heat as is given off by a 1,500 Watt hair drier!
If the client desires she can heat her house by having a party with 15 friends because each person gives off approximately 100 Watts.
Other principles used for the mechanicals:
ñ Short hot water runs
ñ Insulated hot water runs
ñ Insulated ventilation duct runs
ñ Ventilation ducts tested for air tightness (increases efficiency)
ñ Ground sourced pre heater for ventilation (keeps core from freezing)
ñ Solar Domestic Hot Water heater with tank
ñ PV Panels
ñ Any penetrations through the air barrier must be noted and air sealed.
All of these elements combined with high quality construction on site ensures that this house will perform as we have predicted.
The owner will be keeping track of the temperatures in the house, of the intake and output air of the ERV (Ventilation system) as well as the performance of the solar hot water, PV and ground sourced pre-heating loop. This way we can see how well the home is performing and compare that with our computer models.
You too can have a home that is as efficient as this one will be. A Passive House costs approximately 15% more than a standard Code house to build new.
Tom Moore and his crew build to the level of quality that Passive House requires. I recommend him highly.
Certified Passive House Consultant
President, Passive House Alliance US, Vermont Chapter
Chair, HBRA Norther Vt Green Council
Eco Houses of Vermont, LLC