After you have a quality building envelope you can switch your focus to energy efficient systems inside your home, which will now give you your biggest bang for your buck so to speak. Two options instead of forced air regarding how your home is heated is a geothermal heat pump system or radiant heat, also known as hydronic heat. Geothermal heat pumps use the earth's natural energy to heat or cool a home. According to the Environmental Protection Agency (EPA), these systems are the most energy efficient, environmentally clean, and cost-effective space conditioning systems available. Geothermal heat pumps work through a system of underground pipes called a loop. Pumps transfer heat from the warmer earth to the home in the winter and take the heat from home in the summer and discharge it into the cooler ground. Therefore, these systems don't create heat; they move it from one area to another. Once the heat transfers from the pipes it passes through a heat exchanger where the warmth is concentrated and then distributed through the home via a duct system, or even a radiant floor heating system. There are various methods to consider when wanting to install a geothermal heat pump and once again it is going to depend on what option best suits your needs. I would consult with an experienced builder.
Another way to consider heating your home is through radiant floor heating. Most homes use a forced air system with a furnace and metal ducts. Forced air systems can be noisy, spread dust, pollen and germs, and are inefficient in the fact that the heat quickly rises to the ceiling. Radiant floor heating uses plastic tubes which are built into the floor. The tubes fill with hot water to heat the floor, anything sitting on it which warms the house in a quiet, clean, comfortable manner. Energy costs can also be reduced because the thermostat can be set up to 4 degrees lower than a standard forced air system. Radiant floor systems actually become more cost effective the larger the home, and are most efficient in cold climates where heating the home is the larger expense over cooling.
As you start to consider different eco-friendly options for your home keep in mind most of these systems will cost you more upfront, but will pay for themselves multiple times over in the long run. You want to invest in systems that will pay for themselves in a maximum of five to ten years.
The last aspect to discuss in our quality building envelope is the roof. This is an important aspect because that is where most of the heat escapes because of air infiltration and lack of insulation. Two good options for increasing the efficiency of your home is using energy efficient trusses and the second, more expensive but also ingenuous is a green roof.
An easy way to improve the efficiency of a home is to build the attic correctly. Attics need to be built so there is enough airflow for venting heat and moisture. There also needs to be enough room for correct levels of insulation to be installed. Energy efficient trusses allow you to achieve both of these. The difference between an energy efficient truss and a typical truss is the heel height. Standard trusses have approximately a 3 1/2" heel, while an energy efficient truss has up to an 18" heel. The extra height in the energy efficient truss allows for proper insulation to be filled all the way to the corner of the wall and it is relatively inexpensive.
Another thing to consider in being energy efficient is your roof finish. Dark roofing absorbs heat which heats the building up. By choosing a light colored roof it decreases the amount of heat you are absorbing into your home. The most eco-friendly, but more expensive option is a flat roof top garden using native plants. It is built using a standard metal frame, with corrugated metal roof top, weather proofing membrane, a layer of foam, another layer of weather proofing membrane and soil (4” is most economical). It is less likely to leak than any other roof because it is not exposed to elements, therefore it is less likely to crack and weather. The benefits are that it reduce heat island effect, insulates the building, is a wild life habitat, reduces and filters rain water, improve air quality, increase energy efficiency and lengthens life span of roof. The heat island effect is a phenomenon that occurs in developed areas where the replacement of natural land cover with paving, buildings, roads, parking lots, and other structures result in an increase in outdoor temperatures. In the United States it is estimated to cause an increase in temperature of between two and degrees. Do take into consideration that it is a newer building practice and not all county building codes allow them.
U-factors and solar heat gain coefficients can be confusing when it comes to windows, especially depending on the climate you live in. Take into consideration one, two and three pane systems, three panes having the greatest resistance to heat transfer. Also you'll want to consider low-E (low emissivity windows) which reflect heat, not light, keeping spaces warmer in winter and cooler in summer and whether or not tinted windows would be of value. The following is a general guide by region through maze of U-factors and solar heat gain coefficients.
The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. The insulating value is indicated by the R-value which is the inverse of the U-value. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.
Northern Climate: Select windows with a U-factor of 0.35 or less. If air conditioning loads are minimal, windows with U-factors as high as 0.40 are also energy-efficient if the Solar Heat Gain Coefficient is 0.50 or higher. Some double-glazed low-e products have U-factors below 0.30. Some three-layer products have U-factors as low as 0.15.
North/Central Climate: Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.
South/Central Climate: Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.
Southern Climate: A low U-factor is useful during cold days when heating is needed. A low U-factor is also helpful during hot days when it is important to keep the heat out, but it is less important than SHGC in warm climates. Select windows with a U-factor lower than 0.75 and preferably lower than 0.60.
Solar Heat Gain Coefficient (SHGC)
The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.
Northern Climate: To reduce heating, select the highest SHGC you can find (usually 0.30-0.60 for the U-factor ranges required in colder climates) so that winter solar gains can offset a portion of the heating energy need. If cooling is a significant concern, select windows with a SHGC less than 0.55. Use a computer program such as RESFEN to understand heating and cooling trade-offs.
North/Central Climate: If you have significant air conditioning costs or summer overheating problems, look for SHGC values of 0.40 or less. If you have moderate air conditioning requirements, select windows with a SHGC of 0.55 or less. While windows with lower SHGC values reduce summer cooling and overheating, they also reduce free winter solar heat gain. Use a computer program such as RESFEN to understand heating and cooling trade-offs.
South/Central Climate: A low SHGC is the most important window property in warm climates. Select windows with a SHGC less than 0.40.
Southern Climate: A low SHGC is the most important window property in warm climates. Select windows with a SHGC less than 0.40.
To properly figure the size of HVAC system the installer should take into consideration the R-value of your walls, roof and foundation along with the SHGC and U-factor of your windows. Depending on the quality of your building envelope it will have a tremendous effect on the size of HVAC system you need to heat and cool your home, regardless of how large it is.
Now there is a plethora of options to consider when it comes to building components for the walls of your home. Instead of going into the pros and cons of each one I would rather discuss the general building principles to keep in mind and maybe introduce you to some new materials that you might not be familiar with. Because one again, there is no one size fits all answer to what is best, it will depend on the variables of your needs and priorities. The overall main concern is air infiltration and the R-value (resistance to heat flow).
Air infiltration happens all over the house with the wall cavities only contributing a small amount (14%). To solve air infiltration you need to address air flow throughout the structure, with the most important places being gypsum board, gaps, and penetrations, not the cavity. To get a continuous air barrier you need to seal under sill plates, fill band joists, and seal around penetrations such as recessed lights and electrical boxes. Also you need to seal all edges of your gypsum board and exterior structural sheathing. All of which are typically not done in the residential construction industry.
The optimum wall insulation value is an R30+, while industry standard and typical building codes only call for an R19. There is a myriad of options when it comes to insulation, such as: spray foam, clothing insulation, rigid insulation, extruded polystrene (EPS), expanded polystrene (XEPS), Cellulose, even straw bale and the most commonly used fiberglass. I learned as a child not to play in the fiberglass "pink" stuff because you will be very itchy afterward. Each material has various R-values, pros and cons. Which material you choose to use will depend on your needs, wants, concerns, budget and also the construction of your wall. Once again beyond just the typical stick framed home there are many other options such as metal framing, structurally insulated panels (SIPs), insulated concrete forms (ICF), straw bales and even rammed earth. One system I would like to discuss more in depth is the straw bale home. It is currently being touted as the greenest, most energy efficient cost effective way to build.
Straw bales 2’ thick stacked on top of each other in a running block pattern like bricks are plastered inside and out. It eliminates the need for insulation and is entirely non toxic. The California Energy Commision officially regards a plastered straw bale wall to have an R-value of 30, while other studies have shown straw bales to have as high as a R-53 value. It matters little whether the final truth about the R-value of straw bales walls is R-33 or R-43 or even R-53. Above R-30, the differences are minor and will usually be overshadowed by windows, floors, doors and ceiling/roof details. Whatever the value, it is at least three times better than the average “R-19” wood studwall system. Rice, barley wheat or flax straw bales can be used. Rice is an optimal choice because it has silica which is resistant to fire and because of the sand in it it is less prone to rot.
As odd as it sounds to build a home out of straw bales Dr. Burritt in 1938 in Hunstville, AL used straw bales as insulation in between a post and beam structure when building his mansion. The idea came to him while making rounds he stopped in a barn to rest. He noticed how cool and comfortable it was and decided to build his house this way. His home is a historical landmark and can still be toured today. Straw bales are less prone to rot or burn than a typical stud. It is less likely to burn because it is very dense, there isn’t space inside wall for fire to travel. Regular walls are 90% air and fire races through it. It also creates a healthy indoor are quality. The wall is breathable so water vapor can move in and out of wall and water doesn’t condense and create a breading ground for mold. It is roughly the same price of installation alone. Also you can get this product directly from the farmer which gives it an extremely low embodied energy. It is also an fantastically affordable product, approximately $2,600 for 4,000 sq. ft. home. Take into consideration you will need to have a post and beam structure instead of a typical stick framed home, but on average your post and beam structure with straw bales will cost you half as much a lumber package and go up twice as fast.
Whatever construction products you choose to build your walls out of keep in mind the goal of minimum air infiltration and maximum r-value while keeping a healthy environment for your family. Who knows, maybe the three little pigs were onto something when they built their house out of straw. And yes, all the homes featured on this post were built out of straw bales.
