Building a New Climate-Friendly Home
Congratulations! You are about to undertake an exciting adventure.
Architect and Builder
The most important decision you may make is the selection of your architect and general contractor. A good place to start is to find those who have been certified by Passive House Institute US [phius.org, then search under “find a professional.” Even if you aren’t interested in certification, you can be assured that they will understand the techniques. Nothing is more frustrating than having to deal with an architect or builder who balks at every deviation from standard building practices.
Don’t be discouraged over the length of the discussion below. If you have a qualified architect, he/she will understand this process very well. If you have a tight budget, the architect should understand where you can get the most “bang for the buck” in efficiency measures.
Designing a net-zero passive house follows a logical sequence. Depending on your goals, not all these steps may be required, of course.
Certain design choices will determine how easy or difficult it will be to meet your energy goals, among them:
- Size, shape, placement, and orientation of the house
- Number of floors
- Roof shape and slope
- Floor type
- Room arrangement
- Window and door size and placement.
For a typical two-to-four-bedroom, single family house, the most efficient shape is generally a two-story, perfectly square structure. It would include one roof facet large enough for your solar system that is facing due south and tilted at the ideal angle for your local conditions. It would have larger windows facing south than north and few windows facing east or west. It would be oriented on the lot to minimize shading of the solar panels.
Of course, your house is unlikely to fit this mold perfectly, but the closer it is, the less materials it will take to build it and the less energy it will consume, everything else being equal.
Insulation and Floor, Wall, and Ceiling Structure
Heat will seep out of (or into) your house through any structure than has conditioned air on one side and outside air on the other – generally that means at least the floor, walls, windows, doors, and ceiling. If air ducts are run under the floor or through an unheated attic, they also lose heat.
Floor. If you elect to use a slab-on-grade floor, you can take advantage of the natural insulating properties of the underlying soil and will need to add less insulation that if you have a raised floor over an uninsulated crawl space. Either way, a good rule for the effective insulation of the floor structure in our area is R30 to R40 – more if you are using a heated slab.
Walls. Unless you are building at higher elevations, typical energy-efficient homes in the Pacific Northwest use wall structures rated at about R40. There are many ways of constructing a well-insulated wall. Regardless of construction, however, the walls will probably be about one foot thick.
Ceiling. In our climate, a typical efficient home will specify around R60 for the ceiling, regardless of whether there is an unheated attic above it or the house has vaulted ceilings.
It is important that the places where ceiling meets wall and wall meets floor be thought out so that the insulation layers are continuous from one plane to the other to avoid thermal “bridging” – places where heat can flow unimpeded.
It is also important to realize that the R values your architect puts on the plans (especially the walls) only consider the insulation material itself, whereas the effective insulation of the structure will be lower due to the presence of framing members. The architect should also calculate the effective insulation since those will be required in your energy calculations.
Windows and Doors. One of the most important decisions you will make is windows and doors. Even the best windows have far less heat flow resistance than the walls in which they are mounted – typical double-pane windows have less than 1/10 the resistance of an R40 wall for instance.
Other. If your heating system will use ducts and they are run outside your insulated space, they will need to be heavily insulated to avoid excess heat loss. Better, if possible, is to avoid ducts altogether or, if used, to run them in duct chases that are within the conditioned area.
Similarly, water pipes should be run completely within the conditioned space – you’ll get hotter water to the far ends of the house and avoid freezing the cold water pipes in winter. External faucets should be one of the freeze-protected types.
The other way heat flows into or out of a house is when inside air is exchanged for outside air that then must be heated (or cooled) to room temperature. In traditional building, exhaust fans are provided in kitchen, bathrooms, and sometimes laundry and utility rooms. When these are activated, replacement air leaks into the house through any place where the house in imperfectly air sealed.
Two problems: first, that outside air has to then be heated (or cooled) to bring it to room temperature and, second, that those leaks are there all the time, and so you typically get far more air leaking in than is required for health and comfort.
Heat Recovery Ventilation Systems. For these reasons, efficient houses are almost always equipped with a central air flow management system known as a heat recovery ventilator (HRV). These systems draw air from all those locations where you would otherwise put in an exhaust fan and send it outside. They also draw fresh outside air in and distribute it throughout the house – usually to living rooms and bedrooms.
To reduce the energy required to bring that outside air to room temperature the HRV incorporates a heat exchanger that transfers energy from the outgoing air to the incoming air, thereby heating or cooling most of the way to room temperature. HRVs are made by several manufacturers. The best of these will pre-condition the outside air about 85% of the way to inside temperature, whereas others may only be rated at 70% or so. Typically, HRVs run continuously.
When comparing HRVs, look carefully at how much power they consume, how efficient they are, and how much noise they generate.
Limiting Uncontrolled Air Leakage. The HRV will get rid of all the stale air and replace it with an adequate supply of fresh air. Having done that, the design and construction of the house should eliminate any other uncontrollable air leakage points. This does not mean that the house is sealed up, however; you want to be able to open up the house on mild days to let the breezes through. What you want to avoid is cold drafts in the middle of the winter.
Limiting uncontrolled leakage does not add a lot to the material cost, but careful planning and good construction workmanship is essential. It requires designing a complete air barrier surrounding the conditioned space, including floor, walls, ceiling, windows, and doors. There should be no gaps where any of these elements meet or where there are penetrations such as exterior outlets and faucets, roof vents, or pipes coming up through a slab floor.
To test how well houses are sealed, current building code requires testing for uncontrolled air leakage. As covered in the Performance topic under Explore the House, new houses cannot exceed 3 ACH50 leakage levels, while the Passive House standard allows only 0.6 ACH50.
Peak Heat Flow Calculation
Once the insulation and air flow parameters are known, the next step is to calculate the maximum rates of winter heat loss for the house on a cold winter day and heat gain on a hot summer day. Manual J – Residential Load Calculation [See Resources] gives the procedure, though other tools are available.
The total peak heat loss and gain can be compared with the suggested maximum in the Passive House standard (or some other goal, if you have one) to guide whether or not to adjust the design.
Comparing the relative loss through the floor, walls, ceiling, windows, doors, and due to air flow will give you an idea of where to most productively make improvements if you are not meeting your goal or, alternately, where you have overdesigned some structure and could consider saving some money.
Heating/Cooling System Selection
The peak rates of winter heat loss and summer heat gain are then used to size the heating and cooling system. Some form of heat pump is almost always the best choice for an energy-efficient house. Resist the temptation to oversize it – heat pumps work most efficiently at near maximum capacity.
The least expensive system is often a ductless mini-split air-to-air heat pump and many utility companies encourage their use with rebate programs. Other options include:
- Mini-split heat pumps that use air ducts to distribute conditioned air throughout the house.
- Multi-head mini-split heat pumps which pair multiple indoor units with one shared outdoor unit for better control flexibility and air distribution than a single ductless unit.
- Air-to-water heat pumps which distribute conditioned water through the floor structure rather than blowing conditioned air into a room. These require a higher level of floor insulation because the floor will be warmer than the room, but have a comfort advantage.
- Ground-source heat pumps which replace the outdoor fan unit with a pump that circulates water through buried pipes. These have an efficiency advantage over fan units when the underground temperature is warmer than the surface air temperature in the winter and cooler than the surface air temperature in the summer.
Calculating Annual Heating and Cooling Energy Requirements
The Manual J calculations are used to calculate the peak heating and cooling loads on the heat pump. They can also be used to determine the equivalent heating and cooling thermal resistance R for the entire structure. Two other factors, heating degree days (HDD) and cooling degree days (CDD) can then be used to estimate the yearly heating and cooling requirements in thousands of BTUs per year.
Using the heating (HSPF) and cooling (SEER) efficiency ratings of the heat pump those can then be converted to annual energy requirements for heating and cooling and added to get the total amount of electrical energy required.
Calculating Annual Electrical Requirements for Water Heating
In conventional construction, water heating is the second largest electrical load after space heating and cooling. For an energy efficient house, there are three dominant choices:
- Using an air-to-water heat pump water heater.
- Using a solar thermal water heater.
- Using a combined space and hot water heat pump.
Conventional Heat Pump Water Heaters. Heat pump water heaters are the simplest solution and local power companies often offer rebates to encourage their use. They work by moving heat from the surrounding air to the water in a tank and require only about one-third as much energy as a conventional electric water heater.
If conventional heat pump water heaters are located within the house, the downsides are that they do generate some noise when running and also cool the room in which they are placed. The cooling effect can be eliminated by running ducts to outside the house, so that the cool air they generate is blown outside the house, but of course that increases the cost of the installation. Also, the heat pump isn’t designed for a wide range of incoming air temperature and so the unit will revert to standard electric heat mode when the outside temperatures are too cool, thereby reducing the average efficiency in the winter.
An alternative is to use a Sanden® Heat Pump water heater. In this product the heat pump is separated from the tank and is placed outside where neither the noise nor cooling the air is a problem. It is designed for a much wider incoming air temperature range than conventional heat pump water heaters and so does not even include an electric backup mode. Finally, it uses a refrigerant that has a much lower climate-warming potential. As of this writing we are not aware of an equivalent product from another manufacturer.
Solar Thermal Water Heaters. Water can be directly heated by the sun using solar thermal panels designed for that purpose. These generally consist of a black metallic sheet enclosed in an insulated box with glass on one side. Tubes attached to the sheet carry water that is then heated by the sun-warmed metal. The water is cycled between the collector and a storage tank. Generally the system is backed up by a conventional electric water heater (which may use a second tank fed from the first) in the event of several cloudy days.
Solar thermal water heaters work best in environments with reliable sunshine year around, but have fallen from favor in most locations as the price of solar electric systems have decreased.
Combined Hot Water and Hydroponic Heating Systems. Where slab floors are heated by pumping warm water through imbedded tubes, a single heat pump can be set up to provide both domestic hot water and less-hot water for slab heating. Sanden offers such an option, as do other companies. Although there is probably no efficiency advantage over separate heat and water systems, the overall installation may be more compact. A disadvantage is that space cooling, if required, must be separately provided for.
Calculating energy requirements for a conventional heat pump water heater is simple as the manufacturers provide a tag on the unit that includes estimated yearly energy use. It’s a lot harder for the other options.
Calculating Annual Energy Requirements for Ventilation
The HRV runs continuously, but consumes very little energy and at a constant rate (not counting occasional boosts to a higher level). The specifications for the unit should provide the required information. In practice we have found our HRV’s energy usage to be essentially invariant over three years of operation.
Other Equipment in House
In an efficient home, energy for heating, cooling, ventilation, and water heating are greatly reduced compared with a conventional home so that the remainder of the in-home electrical equipment (cooking, laundry, dishwasher, lighting, entertainment, etc.) may together require most of the energy.
Lacking power consumption and usage data on each individual device, Manual J provides heating loads for various appliance scenarios, which are part of the calculations for peak and yearly heating load.
That same data can be used to estimate the electrical requirements by converting the heat load information to the electricity required to generate that heat in a year.
At best, this is a rough approximation, but we haven’t found a better tool so far.
Total Yearly Energy Requirements for House
Adding the yearly totals for heating, cooling, hot water, ventilation and an estimate of other equipment gives you a yearly total.
Total Yearly Energy Requirements for Property
If your property includes other energy-using equipment, you can separately add in these items if you also intend to power them from a solar system or other on-site energy source. Other items might include, for instance:
- Electric or plug-in hybrid vehicles
- Well pump
- External lighting
Sizing the Solar System
Now that you know your total yearly energy requirements, it is time to figure how much solar system capacity it would take to generate that much, assuming you plan on using one.
Solar systems are sized and sold by “DC” capacity, which is a measure of how much power it is capable of generating under ideal conditions on a bright sunny day at a certain temperature, not including system inefficiencies, weather variation, and yearly sun-angle variation. Getting from there to how much usable energy an array of panels will generate over the course of a year is a complex process.
Fortunately, the National Renewable Energy Lab has an online calculator that does all the calculations and estimates for you. Simply go to pvwatts.nrel.gov/pvwatts.php and it will bring up a form where you enter your address, the tilt and orientation of your panels, and the DC capacity of your proposed array and it will give you the estimated average usable power the array will generate in kWh on a typical year. By varying the proposed array size you can easily determine how big an array will be required to offset any percentage of your energy requirements. If your plan is to generate all the required energy using the array, you should plan to make it slightly over-sized to cover:
- Aging of the array (its capacity will decline slowly over time)
- The many estimates involved in calculating yearly energy requirements
- Year-to-year variations in average cloud cover and average temperature.