GREEN BUILDING: Thermal Building Envelope
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David K. Horvath, GMB, CAPS, CGP, LEED-AP
General Manager, Certified Builder
Office: (615) 791-5678
Cell: (615) 319-0000
Consisting of a home's foundation, walls, roof, windows, and doors, the building envelope protects the occupants, helps regulate the indoor environment, and controls the flow of energy between the home's interior and exterior. A well-designed envelope responds efficiently to heating, cooling, ventilating, and natural lighting needs to provide an attractive, more comfortable and healthier home environment. A building envelope’s energy efficiency depends on a proper balance between these systems:
Advanced techniques for sealing holes and cracks in the home’s “envelope” and in heating and cooling ducts help reduce drafts, moisture, dust, pollen, pests, and noise. Air sealing efforts will complement your insulation efforts and improves comfort and indoor air quality while lowering utility and maintenance costs.
Properly installed insulation in floors, walls, and attics ensures even temperatures throughout the house, reduced energy use, and increased comfort.
Proper moisture control and ventilation strategies will improve the effectiveness of air sealing and insulation, and vice versa.
In mixed climates where houses require both heating and cooling, energy efficient windows use advanced technologies, such as protective coatings and improved frames, to help keep heat in during winter and out during summer. High efficiency windows offer:
- Energy Savings. Advanced technologies such as invisible glass coatings, vacuum-sealed spaces filled with inert gas between the panes, improved framing materials, better weather stripping, and warm edge spacers reduce undesirable heat gain and loss.
- Improved Comfort. Compared to less efficient windows, high efficiency windows help keep homes warmer in the winter and cooler in the summer. This is because they can block 70 percent or more of the solar heat gain in the summer and reflect radiant heat indoors during winter and reduce air infiltration.
- Protection of Your Home's Interior. Photographs, furniture, flooring, and window treatments can fade or discolor after repeated exposure to direct sunlight. A high efficiency window with special (Low-E) coatings can block damaging ultraviolet sunlight and reduce fading by up to 75 percent.
- Reduced Condensation. If an inefficient window or window frame gets too cold, water can condense (or even freeze) on the interior surface and then pool on the sill. Over time, chronic condensation can damage window sills, cause paint to crack, and encourage the growth of mold. Advanced frames, glass coatings, spacers, and other technologies enable high efficiency windows to keep the inner surface of the glass and frame warmer, reducing the potential for condensation and ensuring a clearer view on winter mornings.
Windows: Independ Testing for Energy Performance: To find energy-efficient windows and skylights, simply look for the ENERGY STAR rated window unit tailored to the mixed climate zone. The energy performance of all ENERGY STAR qualified windows and skylights is independently tested and certified according to procedures established by the National Fenestration Rating Council (NFRC). NFRC is a third party, non-profit organization that sponsors certified rating and labeling programs to help consumers compare the energy and performance features of windows and skylights.
Based on the label, compare window properties such as U-factor and Solar Heat Gain Coefficient to find the most efficient products among those with Energy Stars.
HEATFLOW. Heatflow is indicated by U-factor (U-value). Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes. Some double glazed low-e products have U-factors below0.30.
SOLAR HEAT GAIN. Solar heat gain is indicated by solar heat gain coefficient (SHGC). In mixed climates, select windows with a SHGC of 0.55 or less. If you have significant air condining costs or summer overheating problems, look for SHGC values of 0.40 or less.
INIFILTRATION. Air infiltration is indicated by air leakage rating (AL). The air leakage rating is an important window property in cold climates. Select a window with an AL of 0.30 or below.
DAYLIGHT. Daylight is measured by visible light transmission (VT). A window with VT glass above 0.70 (for the glass only) is desireable to maximize daylight and view. This translates into a VT window above 0.50 for the total window including the frame.
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BEST TECHNOLOGIES FOR EXTERIOR THERMAL ENVELOPE:
Wood, by all nearly all modern quality measures for building materials, lacks the ability to meet the demands of a high performance thermal envelope. Two of the best affordable building systems for external building envelopes are:
Structural insulated panels (SIPs) are high performance building panels used in floors, walls, and roofs for residential and light commercial buildings. The panels are typically made by sandwiching a core of rigid foam plastic insulation between two structural skins of oriented strand board (OSB). Other skin material can be used for specific purposes. SIPs are manufactured under factory controlled conditions and can be custom designed for each home. The result is a building system that is extremely strong, energy efficient and cost effective. Building with SIPs will save you time, money and labor. SIPs with cellulose reinforced cement board skins are now offered by ThermaSave which offer maximum durability at the lowest cost. Insulating Concrete Forms (ICFs) are rigid plastic foam forms that hold concrete in place during curing and remain in place afterwards to serve as thermal insulation for concrete walls. The foam sections are lightweight and result in energy-efficient, durable construction.
ICFs consist of insulating foam, commonly expanded polystyrene (EPS) or extruded polystyrene (XPS). ICFs can be used to form various structural configurations and provide backing for interior and exterior finishes. Insulation values of ICF walls vary depending on the material and its thickness. Typical insulation values range from R-17 to R-26, compared to between R-13 and R-19 for most wood-framed walls. ICF walls are designed as reinforced concrete, having very high wind and seismic resistance.