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The thermal efficiency as a core element of any eco home

You have much to consider when you want a new energy-efficient house, and it can be a challenge. However, recent technological improvements in building elements and construction techniques also allow most modern energy saving ideas to be seamlessly integrated into house designs while improving comfort, health, or aesthetics. And even though some energy-efficient features are expensive, there are others that many home buyers can afford.

As we explained in other post you can get an understanding of your housing footprint by calculating the emissions associated with your fuel use, electricity use, water use and waste disposal.  The estimate for the case of average American’s housing footprint is just over 5 t CO2e per person, based on 2005 data.  Around 3 t or roughly 60% of this results from the use of electricity.  The next major source is natural gas at about 1 t CO2e  per person.  Waste and water account for around 300 kg each with smaller amounts arising from fuel oil and LPG. (see the next image). For this reason in very important to have very energy-efficient homes regardless of whether you live in hot or cold weather.

The energy-efficient homes have some basic elements in common: a well constructed and tightly sealed thermal envelope; controlled ventilation; properly sized, high-efficiency heating and cooling systems; and energy-efficient doors, windows, and appliances.

Thermal Envelope

A thermal envelope is everything about the house that serves to shield the living space from the outdoors. It includes the wall and roof assemblies, insulation, air/vapor retarders, windows, and weatherstripping and caulking.


An energy-efficient house has much higher insulation R-values than required by most local building codes. An R-value is the ability of a material to resist heat transfer, and the lower the value, the faster the heat loss. For example, a typical house in in cold weather might have insulation of R-11 in the exterior walls and R-19 in the ceiling, while the floors and foundation walls may not be insulated. A similar, but well designed and constructed house will have insulation levels that range from R-20 to R-30 in the walls and from R-50 to R-70 in the ceilings. The floors and foundation walls will be insulated. Carefully applied fiberglass batt or rolls, wet-spray cellulose, or foam insulation will fill wall cavities completely.

Foundation walls and slabs should be as well insulated as the living space walls.

Foundation walls and slabs should be as well insulated as the living space walls. Poorly insulated foundations have a negative impact on home energy use and comfort, especially if the family uses the lower parts of the house as a living space. Also, appliances—such as domestic hot water heaters, washers, dryers, and freezers— that supply heat as a by product are often located in the basement. By carefully insulating the foundation walls and floor of the basement, these appliances can assist in heating the house.

While most new houses have good insulation levels, it is often poorly installed. In general, gaps and compaction of insulation reduce its effectiveness.


The typical home loses more than 25 percent of its heat or cold through windows. Even modern windows insulate less than a wall. Therefore, an energy-efficient house in a heating-dominated climate should, in general, have few windows on its northern, eastern, and western sides. Total window area should also not exceed 8 to 9 percent of the floor area for those rooms, unless the designer is experienced in passive solar techniques. If this is the case, then increasing window area on the southern side of the house to about 12 percent of the floor area is recommended. This is often called solar tempering.

A properly designed roof overhang for south-facing windows will help prevent overheating in a hot weather. North, east, and west windows should have low Solar Heat Gain Coefficients (SHGC). South windows with properly sized overhangs should have a high SHGC to allow cold weather sun (and heat) to enter the house. The overhang blocks the high hot weather sun (and heat). If properly sized overhangs are not possible, a low SHGC glass should be selected for the south windows.

At the very least, you should use windows (and doors) with an Energy Star® label, which are twice as energy efficient as those produced 10 years ago, according to regional, climatic guidelines (note: houses with any kind of solar tempering have other guidelines). The best windows are awning and casement styles because these often close tighter than sliding types. In all climates, window glass facing south without overhangs can cause a problem on the cooling side that far exceeds the benefit from the cold weather solar gains.

Heating and Cooling Systems

Specifying the correct sizes for heating and cooling systems in airtight, energy-efficient homes can be tricky. Rule-of-thumb sizing is often inaccurate, resulting in wasteful operation. Conscientious builders and heating, ventilation, and air-conditioning contractors size heating and cooling equipment based on careful consideration of the thermal envelope characteristics.

Generally, energy-efficient homes require relatively small heating systems, typically less than 50,000 Btu/hour even for very cold climates. Some require nothing more than sunshine as the primary source of heat along with auxiliary heat from radiant in-floor heating, a standard gas-fired water heater, a small boiler, a furnace, or electric heat pump. Any common appliance that gives off “waste” heat can also contribute significantly to the heating requirements for such houses.

If an air conditioner is required, it’s often a small unit and sufficient for all but the warmest climates. Sometimes only a large fan and the cooler evening air are needed to make the house comfortable. The house is closed up in the morning and stays cool until the next evening. Smaller-capacity heating and cooling systems are usually less expensive to buy and operate. This helps recover the costs of purchasing more insulation, and other energy-efficient products, such as windows and appliances. Always look for the EnergyGuide label on heating and cooling equipment. The label will rate how efficient it is as compared to others available on the market.

In climates where hot weather cooling requirements dominate, light-colored materials and coatings (paint) on the exterior siding and roof can help reduce cooling requirements by up to 15 percent. Carefully selected and placed vegetation in any climate also contributes to reduced cooling and heating loads.