When you think about design, there are three aspects of a green home that you should remember.
- Design in terms of site: views, topography, and natural resource inventory.
- Design in terms of passive systems: passive solar, natural ventilation, drainage.
In this chapter, we will look at those in more detail and introduce the third strategy:
- Design in terms of materials efficiency.
We are not saying that design is everything, but it is your single best opportunity for natural resource efficiency and saving money. It’s as easy as 1, 2, 3. Really!
Designing for Your Site
In article Analyzing Your Building Site, we looked at site selection in terms of natural amenities, topography and location. Our site might have other characteristics that influence our design. Desirable views (or objectionable ones) certainly affect site-specific design. Natural site features can be used to improve building performance or, at least, define how we utilize the site to accomplish that with installed components. When we consider the design for our home, we should assess the natural inventory of the site and take advantage of any unique opportunities that it presents.
Site Management
This is really important to think about before you start construction on your land. If you can limit the amount of site that you disturb during construction, you will save money not only on site work, but also on restoration work and installing your landscape. If we truly want to keep housing affordable in the long term, we must consider eliminating all costs that do not add benefit, especially those that increase construction costs and long-term maintenance, increasing our total cost of ownership. There is no need to feel like you have to do something with every inch of your lot. Sometimes less is more.
So this is the time to set your site construction parameters and create a plan for protecting the land outside of the area that you will allow to be disturbed. Consider fencing off that protected area with orange plastic construction fencing, or maybe using straw bales not only to define that space but also to reduce erosion of native soils that are disturbed during construction. This effort is especially important for protecting any existing trees during construction.
Foundation and Roof Design
The type of foundation system you select should primarily depend on what is best suited or required for your building site and climate. For sites with poor soil stability, you should consider deep piers or beams that will connect the structure to a point of ground stability like bedrock or below the frost line. Even if your site does not present those concerns, it’s best to have your home’s foundation designed for your specific site by a qualified engineer. Otherwise, you may overspend on more foundation structure than you need, or end up paying more in the long run for repairs because the foundation was not designed to handle the stress specific to your site.
If you are on solid ground, you may have the option of choosing between a slab foundation, a basement or some type of pier and beam support structure. Pier and beam systems give you long-term access to any systems that run below the floor of the home, including plumbing lines, sewage lines, gas lines, etc. Over the life of your home, you may need to repair or replace one of these service lines, and this is much easier if you can access it than having to jack-hammer out a solid concrete slab foundation. Pier and beam and basement systems also give you the benefit of easily relocating any of those system lines in the event you decide you must reconfigure room arrangement or use.
If you intend to build a basement, it is imperative that issues like bulk-water intrusion, rising groundwater, radon and condensation be addressed in the plans before construction begins. A basement can be a boon or it can become your worst nightmare. The latter outcome is usually due to water, so how this is planned for and handled is the key factor that separates one outcome from the other. We will address the building science of how to build a dry, healthy basement in Chapter 4. For now, let’s just say that it’s no longer true that a basement must be a place with musty odors and occasional standing water.
Sloped sites might require foundation walls that take large quantities of materials to construct. You may need to think about whether or not skirting is really necessary on your project. If it is, to reduce the amount of materials needed, consider stepping the foundation down grade changes or supporting a level floor on a permanent, durable stilt structure. Alternative building components may offer a better value, such as insulated concrete forms (ICFS) or structural insulated panels (SIPs). These systems work very well if you are installing a sealed (unventilated) crawlspace or basement (discussed later in this article).
Your Home’s Umbrella: Roofs are more than just architectural features. The main consideration for roof design is more than just the longevity of the roof itself; it also serves to shade and shield the wall components from sun, rain and other natural weather events, like snow or hail. This means the roof overhangs should provide protection for the home’s exterior walls, doors, and windows. You should assess the area of your site where you intend to build for sun exposure and any protection offered from other nearby features.
This is a big deal when it comes to your home’s durability. Left unprotected over time, door and window frames can leak and walls can suffer water damage and mold. Keeping that water out to begin with is one of the best functions that your roof can provide. Research has shown that the deeper the roof overhangs, the lower the chances of water intrusion on the walls of the home . The hip roof design (roof sloped to all four sides) offers the best overall protection.
Solar Ready: If your lot is not too heavily tree-shaded, designing the roof with the ridge running east to west provides plenty of space for future installation of solar panel components. Whether your plans are to generate onsite energy through solar photovoltaic (PV) systems or provide hot water with a solar thermal system, the roof solar access is critical. To mount the more common type of solar arrays that operate off a common inverter on the roof, you’ll need approximately 100 square feet of roof per kilowatt (1 kilowatt = 1,000 watts) of array. (Micro-inverter panels do not require series installation, so although they are more expensive than series-inverter arrays, their placement is not affected as much by roof penetrations or continuous roof space.)
Later in this article, we will discuss energy modeling, which is useful in determining the size of solar array you’ll need to manage your electrical loads. For now, just remember that you will need that amount of roof area with the proper orientation and roof pitch in order to maximize your solar PV production. This area needs to have a clear southern exposure (no shading) with minimal or no penetrations (roof/plumbing vents). Roof vents are usually required above any combustion appliances (furnace, gas water heater, cook top), as well as over bathrooms, kitchens and laundry rooms (plumbing and ventilation pipes). With direction and planning, contractors can find alternate locations for roof vents, on adjacent roof areas or even exterior walls. Make sure that you communicate your intent to keep roof space dedicated for future solar access.
Even if you do not plan on installing any solar devices in your construction project immediately, think about the life of the home. Most homes built today in America will still be standing long after fossil fuel energy sources are completely depleted. You may realize in as little as five, ten, or fifteen years, that having the roof design such that it will allow you to add onsite active systems is a great hedge against rising costs. And certainly over the course of a hundred years, other owners and occupants may value that the home’s roof design allows that option, which will help in resale value.
Avoid chopped up roof designs. The most basic, cost-effective roof design is a straight ridge running east to west (a gable roof). Although a complicated roof design might add architectural interest, every turn and valley increases the potential for water leaks, which adds cost in flashings and other materials to keep water out and to ventilate the assembly. These costs are incurred not only during the initial construction, but every time the roof has to be replaced. Also, the more complex the roof design, the greater the number of areas for water to eventually penetrate, thus increasing the frequency of costly roof repairs and/or replacement.
Vented versus Sealed Attics and Crawlspaces: Building science research has made many recent advances regarding ventilation strategies for attics1 and crawlspaces.2 These studies indicate that when the HVAC or ducts are located in the attic, unventilated assemblies outperform their ventilated counterparts in all climates, which saves you money over the life of the home. And, yes, these types of assemblies are approved by the international building codes. We now know that in hot, humid or mixed-humid climates, ventilated assemblies can cause a number of issues in the home. In a humid climate, any vent openings in your attic and/or crawlspace can provide a path for outdoor humidity to enter the cool crawlspace and cause condensation; in the case of your attic, the extreme heat forces your cooling system and ductwork to work in a very hostile environment. If your attic and/or crawlspace are ventilated with this humid summer air, it allows that humidity much greater access to your home via all of the many penetrations. Vented crawlspaces should only be built where they can be ventilated with dry air for most of the year.
The alternative to ventilated attics is to seal up these spaces, just as you seal up other parts of your building assembly, and install insulation at the exterior of that sealed space. This means insulating with spray foam at the roof deck and rafters, just as you would at all other exterior walls of the structure.
The unvented attic is a clear winner if you intend to have any mechanical equipment or ductwork in the attic, as is often done in warm and mixed-climate zones. By sealing up the attic, applying spray foam insulation to the underside of the roof deck and rafters, you bring that system within the air boundary and thermal envelope. In other words, the system components are inside the insulated air-sealed space so they are protected from heat in the summer and cold in the winter. In a traditional vented attic, temperatures can exceed 130 degrees in the summer, so placing our air conditioner equipment and ductwork there is like putting them in an oven and still expecting them to keep our home cool. The same is true for our furnace (and its ductwork) if located in a vented attic, having to overcome winter’s cold while trying to heat our home.
Many times locating the equipment and ductwork within the insulated space means that we can downsize the size of the system. That saves you money initially on the installation of the system. You will also save money on utility costs over the life of the home since a smaller system will typically use less energy to operate. This operational savings will be even greater because of the decreased heat or cold loads on the equipment and ductwork that results from locating them inside our insulated house, further increasing its efficiency.
In cold and mixed climates, the foam insulation effectively prevents ice dams by keeping the roof deck from becoming warm enough to melt the underside of the roof snow pack. We now know that the melting of the underside of a snow pack caused by rising heat from the house is what begins the process of ice dam formation. This meltwater runs down to the cold exposed eave and re-freezes to start the process of ice dam formation. A sealed attic assembly eliminates the need for heaters at the eaves or working on dangerous ladders to control the ice buildup at the overhangs, since the snow pack never melts and the ice dam never gets a chance to start. Installing foam insulation also prevents hoar frost because there are no surfaces in the attic cold enough to be a condensing surface, much less have frost form on them. Building science gives us the power to address home problems at their source, rather than trying to control the symptoms after they have begun.
The other option is to go old school and ventilate your attic. The new twist is that we now know that if we do this we must create an airtight seal with foam insulation or other rigid materials (wood or drywall) at the ceiling of your home. If you choose this option, you must take special precautions to ensure that the entire ceiling plane with all of the framing irregularities like utility chases, dropped ceilings, furrdowns, can lights and trey ceilings is fully air-sealed. The key is to cover and air-seal all these gaps and irregularities. If you employ this option, you can’t efficiently use the attic for your ducts or mechanical equipment, so you should consider placing your ducts in dropped ceilings below the attic insulation and air barrier. This will allow you to still reap the benefits of your mechanicals within the thermal and air barrier of your home.
The best venting system to use includes both continuous ridge vents that run along the top of the highest ridge points of the roof and continuous soffit vents that are placed in the eave underside of the roof overhangs or otherwise at the base of the roof assembly. This combination uses the free physics of a thermal chimney effect to draw the hot air up and out of the attic and replace it with the cooler air from the shade of the eaves. Vented crawlspaces should only be built where they can be ventilated with dry air most of the year.
Roof Truss Design: If you are designing a home with a vented roof assembly, it is critical that the design include details that will allow the home to be constructed for the best thermal performance. Remember, all homes should provide the best possible shelter from extreme weather conditions, but a green home should strive to achieve this by using good applied building science, better building methods and sound, passive design strategies. These efforts result in a durable, high-performance home that will continue to provide operational cost savings throughout its lifetime.
Building better requires us to look at how effectively traditional building practices have performed and whether we can improve upon those practices. Roof truss design is a good example of this. In traditional wood truss design, the roof rafters sit right on the ceiling joist, so there is not room to take full-depth insulation all the way out to the edge. This means that we end up with a fairly large gap between the end of the ceiling insulation and the top of the wall insulation, along the soffit line. In a traditional wood-framed home, this is one of the most commonly missed areas of insulation, and since hot air rises, it provides a good escape route for heat to move outward from the conditioned space to the outdoors in the winter. In cold climates, the ceiling gets cold in this area, causing water condensation that leads to mold growth.
A better way to design trusses is with energy trusses or raised heel trusses. Energy trusses or trusses designed to cantilever out over the top plate provide an opportunity to get the insulation out all the way over the top plate of the exterior wall assembly. If your goal is net zero energy, the improved design and thermal performance work together to reduce loads on mechanical systems, i.e., building energy use. The International Energy Conservation Code also recognizes the inherent energy performance benefits of energy and cantilevered truss designs by allowing you to reduce the overall attic insulation level if they are used.
Fenestration (Windows)
Glazing: Where you put windows, commonly called the “fenestration” or “glazing area” by building designers, should have much more to do with their function than with the aesthetics of the design. Unfortunately, in traditional home design, this is seldom the case. Try to think of windows in terms of their usefulness when placing them in your design. Once you have determined how the windows can best serve your needs, then you can incorporate them into your architectural detail.
It’s important to recognize that even the best windows available on the market today still only perform about a third as efficiently as the average code-required wall assembly in terms of thermal performance. For this reason, in relation to the overall thermal efficiency of your building envelope, think of windows as large holes in your walls. While having fewer windows is better, those that you do have should serve one or more of the purposes described below. When planning your windows, plan to accomplish as many of these benefits as you can with each window.
Since the sun’s path is east-west, you will want to design so that most of the windows are on the sides of the home that support or reduce passive heating or cooling, as appropriate for your climate. If you live in a hot climate, you will want to reduce the amount of window glazing area on the east and west sides of the house. This will help you save money on your summertime cooling costs. If you are trying to maximize heat gain in a cold climate, then you would want most of the windows on these sides, as well as the critical south side.
Also remember that windows are available in a variety of shapes and sizes. The only reason to install windows in any location that you expect to keep covered all of the time with privacy treatments is to provide an emergency exit in case of fire or other interior threat. Don’t get caught up in thinking that all your windows must be three feet by six feet or four feet by five feet (in construction jargon, 3060 or 4050). Instead, think about how you can use the light. The same amount of glass can be installed in one large window or several smaller ones. Placing a few two-foot-by-two-foot windows at standing height eye level would give you nice views, while also providing daylighting, and yet would not interfere with furniture placement or add privacy concerns. Adding a lot of windows where they serve no purpose is just wasting your construction budget and will also increase your utility, maintenance and repair costs over the life of the home.
Views: Windows can perform the important function of connecting indoor space to outdoor space and capturing any views that will help you stay in touch with the natural environment and events outside. This is important not only for your health but also security, by letting you see what is going on outside the home. However, that works both ways-windows added as architectural features may serve no other purpose than providing unwanted intrusions into your privacy.
