Image Credit: Image #1: Mike Anderson First floor layout for the Flatrock Passive. The second floor layout for Flatrock Passive. The chart shows total energy balances in winter and summer, with the blue column representing energy lost by the building and the red column showing energy gained by the building. The charts are produced by the Passive House Planning Package, the energy modeling software that predicts energy consumption based on the house design and the climate. Heating and cooling loads during winter and summer. Energy used for heating and cooling during the year. Energy modeling suggests the house will use about 75% less energy for heating than one built to code.
Editor’s Note: This is the second installment in a series of blogs by David Goodyear describing the construction of his new home in Flatrock, Newfoundland. The home will be the first in the province to be built to the Passive House standard. The first installment of the GBA blog series was titled An Introduction to the Flatrock Passive House. You can find Goodyear’s complete blog here.
Our designer, Mike Anderson, has been busy! The house now measures 2,176 square feet. The current refinements have been finalized enough that we are ready to submit them to the town for our building permit…. Well, we already have a building permit but it didn’t have an attached garage. The town of Flatrock will only allow you to build a garage once the house has started. To avoid this, we have attached the house so both the house and garage can be built at the same time.
In its current orientation, the long axis of the house is less than 15° from geographical south. This will ensure that our solar coverage will be greater than 95% during the winter months. There will be some energy penalty in the summer, since the setting sun will lead to solar gains later in the day. Some trellis shading over the main level windows should take care of this problem, but only the energy modeling will tell the tale.
The garage facade has been changed around a little. Rather than a double garage door, there is now a single. The garage will be a woodworking shop. Garage doors are a horrible R-18 at most and leak energy like you wouldn’t believe. Minimizing the size of the door will help retain heat as well as give me more wall area for my equipment. The garage will have a huge storage room upstairs that’s about 14 feet by 25 feet. This will accommodate the loss of basement storage. However, getting things upstairs could be a pain and I don’t want a fixed staircase in the garage taking up valuable floor space. We have decided to install an attic stair to access the space from inside. To facilitate lifting large items to the second level, the designer proposed a simple solution: a hayloft door and a pulley. Simple yet effective!
There is no chimney for a wood stove yet since the energy modeling needs to be completed before we determine the heating requirements. Upstairs, things have been changed around a little (for floor plans, see Images #2 and #3 below).
For the most part I like the layout but we have proposed to close in the laundry upstairs with a wall that has two double sliding doors with frosted glass panes. It will allow us to have extra storage for linens, towels, etc., and will enable us to close the doors while still allowing light to spill into the hallway. The ensuite has been resized to accommodate a makeup table for my wife. The master is a little smaller but still about 14 feet square, which is big enough. Besides these changes, the floor plan is likely finished.
Some details yet to come
I am not sure about the vertical board-and-batten siding. Board-and-batten requires a double rainscreen, which takes longer to install, and it doesn’t shed water as well as standard bevel siding. I am proposing we use bevel siding and install board-and-batten under the eaves, with stained cedar shakes to break up the east and west walls.
I am also proposing a multi-colored building, with the flavor of downtown St. Johns and the traditional Newfoundland saltbox seen in outports. It is likely that the home will be one color and the garage will be another, such that the color draws the eye to the house as a focal point on the lot.
At this point, the house is ready for energy modeling. Passive Design Solutions will use the Passive House Planning Package (PHPP) software to see if there are any gross issues with the geometry. The software will determine issues with loss of winter solar gains and summer overheating. Based on those results we can determine the need for larger south-facing windows, minimizing north-facing windows, and window overhangs to prevent overheating. Finally, we will obtain a better idea of the heating and cooling requirements for house.
The results of the energy modeling are encouraging
The first round of energy modeling has been completed. Based on the model and the orientation of the house on the lot, the energy usage looks great. There too many numbers to talk about so I’ll focus on the important numbers.
The PHPP software used to model the home is quite complex. The data entry is exhaustive and it contains information from the transparency of windows to the air change rate related to the wind blowing on the building.
First, let’s look at the total energy balances (see Image #4, below). The blue columns show the energy lost by the building and the red columns show the energy gained by the building (based on our climate). Windows typically drain energy from a building. However, if oriented properly, the windows can provide solar gains in winter which can offset the heat loss through the windows.
In our model, the south windows gain slightly more energy than the total lost through all the windows in the envelope! This is a great start! In the summer, the south-facing windows gain quite a bit more than is lost but ventilation alone takes care of much of the heat. The main windows causing this are on the main level since there is no shade structure. The model shows that added solar screens to the outside of the building cuts this by 60% since they decrease the solar irradiance by about 50%.
Heating and cooling loads
Now lets look at loads (see Image #5, below). Loads are the instantaneous use or gain of energy. Here the loads are shown in Btu/h, but we typically use power in kW to represent this. The winter balance is on the coldest day of the year. The summer balance is on the warmest day of the year.
What does all of this mean? Well, let’s decipher the balances and try to put this in perspective. First, I want to talk about demand or total yearly energy usage for heating. I’ll use kWh to represent this rather then the Btu/h convention used in the diagrams. Look below at Image #6. Summing up the red bars (in Btu/h and converting to kW) in the plot gives a total energy usage of 17 kWh/square meter for heating during the whole year.
How does this compare to a code-built home? A typical code-built home uses about 70 kWh/square meter! So the model estimates a 75% savings in heating cost alone.
How about the load on the coldest day of the year? In the load bar plot, the total heat used by the building is about 13,000 Btu/h or 3.8 kW. This is amazingly small for a house that measures 2,176 square feet. In fact, the heating load per square meter was determined to be about 13 watts. Most code-built homes are about 5 to 10 times this number.
Predicted electricity use
If electricity were used for everything in the building, the total yearly electricity usage would be about 12,400 kWh, including heating, appliances, hot water, etc. Keep in mind, we’re located in Climate Zone 6. At today’s rates, the cost-averaged bill will be about $100/month. Using a heat pump for space heating and a heat-pump hot water heater, this drops to 10,300 kWh or $83 per month. A typical code-built house of the same size would use almost 24,000 kWh or about $195 per month. At our current rate, the savings would be about $1,330 per year, and the total monthly electricity bill would be $1,001 per year. Not too shabby!
One of the principles of passive house design is adding renewable energy to further decrease the total energy usage. Do you think that you could be on the grid and minimize energy usage below this while still living comfortably? You betcha! By combining heat and hot water systems and installing a wood gasification boiler plus a heat pump, we have further minimized our electrical consumption by another 3,000 kWh. (A separate post on that system is coming up.)
Our total estimated energy usage will be around 7,300 kWh. If all goes as planned, our estimated monthly electricity bill will be $86.30, -ncluding tax and the service fee. Compare that to a code-built home, where the bill would be $245.67. That’s a savings of 65%. I’m pleased.
With a total energy savings of 65% and the heating requirement that’s 75% less than a code-built home of the same size, the Flatrock Passive House is on the road to be the most energy-efficient detached home in Newfoundland!