Structure: Exterior Walls
Walls Must Be Durable, Well-Insulated, and Weather-Resistant
Besides strength, consider insulation and materials
Your first concern when building a home's outer walls might be whether they are strong enough, but you should also consider several other questions, including how you can minimize material demands and how the walls will be insulated and air-sealed. There are quite a few ways to build a structural wall — each with pros and cons.
See below for:
Platform framing is an economical way to build a house
Use less wood, get better performance
Whether you call it optimum value engineering (OVE) or advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope., this is a way of reducing the amount of lumber used in a wood-frame building without compromising strength. Lower material and labor costs are two benefits; a third is reduced heating and cooling costs because this technique allows more room within the walls for insulation.
Use more wood, get better performance
Builders looking for the least expensive way to build exterior walls with high R-values usually settle on double-stud walls. By spacing parallel 2x4 walls 5 inches apart, a builder can leave room for a full 12 inches of cellulose insulationThermal insulation made from recycled newspaper or other wastepaper; often treated with borates for fire and insect protection..
Steel studs are better used for interior walls
Light-gauge steel framing can be attractive to builders who are turned off by the unpredictable quality of dimensional lumber. Galvanized steel studs are up to 50% lighter than wood, won’t rot, are uniformly straight, and are impervious to insect damage. Moreover, steel can readily be recycled into new building materials. Unless insulation is carefully installed, however, steel framing can weaken a home's thermal envelope.
SIPs combine insulation and structure in a single component
Most structural insulated panels (SIPs) consist of an inner core of insulating foam sandwiched between outer layers of oriented strand board. SIPs take the place of conventional wood framing and, like ICFs, sharply reduce energy losses from air leaks.
Timber frame is often combined with SIPs
Timber framing is a beautiful structural system, and if the wood is gathered on-site, it can also be a very green one. The beams are often left exposed inside a wrap of SIPs. Such an approach combines responsible resource use with improved energy performance.
Building blocks make fast and tight walls
There are a number of masonry wall options, including conventional concrete blocks (CMUs) or blocks made from autoclaved aerated concrete (AAC). Those looking for a poured-concrete wall will probably gravitate to insulated concrete forms (ICFs). Unlike stick-built walls veneered with brick, stone, or block, these materials replace the wood frame altogether. They can be finished on the outside with a variety of materials, including stucco, fiber cement, or traditional wood siding.
ICFs are layered like a sandwich: Outer forms of insulating material hold an inner filling of concrete. The blocks have an internal network of voids. Steel reinforcing is put in place as the blocks are stacked, and the forms are then filled with concrete. Insulation and structure are intertwined.
ICFs are made by a number of manufacturers in a variety of types. One consists of an inner and outer layer of foam insulation separated by plastic or steel connectors. Another is made with a composite of cement and another material — recycled polystyrene, for example, or wood chips.
Some older forms of concrete walls used in homes include hollow concrete masonry units (CMUs). AAC blocks are durable and provide thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. .
Concrete block, AAC, and ICFs produce walls with a higher mass than standard wood-frame construction. Dense materials with high mass tend to be poor thermal insulators — that is, they have fairly low R-values. But that same high mass also can store a lot of heat, thus flattening out temperature extremes. Manufacturers often argue that the “effective R-value” of these wall systems is higher than conventional tests for thermal resistance would indicate. Using high-mass walls has energy benefits only under limited conditions in some, but not all, climates. Since "effective R-value" has no legal or scientific meaning, its use in marketing materials should set off alarm bells.
Natural materials like straw and adobe perform well
Natural walls includes both old and new materials: straw bale, rammed earth, and adobe. Alternative walls are low-tech and rely on the use of indigenous or low-cost materials. They are well suited to specialty builders or owner-builders who are looking for alternatives to conventional wood framing.
Separating the outside from the inside
The exterior walls of a house have several functions. Not only do they define the shape of a house, they also support the floors, walls, and roof. Equally important is their role in separating the house’s interior from the outdoors, and to do this effectively they have to block the weather with systems that insulate, shed water, and repel moisture and air infiltration.
While it's important to understand the different roles walls play, if we treat them and their functions separately, we miss great opportunities to improve material efficiency, operating efficiency, and overall building performance. Green building integrates them all.
Wood-frame walls have been the predominant choice for houses in the United States for more than three centuries, with masonry walls a distant second. But today’s alternative products and techniques — many which are more energy efficient and have lesser environmental effects — are certainly worth examining.
A lot of time and materials go into building a house’s walls, and with the exception of a timber frame, all that structure is covered up when the project is finished — out of sight and out of mind. Yet decisions about wall construction have consequences that last as long as the building does, including how much maintenance it will need, how energy efficient the envelope will be, and how difficult the structure will be to repair or modify.
ABOUT CHOOSING A TYPE OF WALL
Energy efficiency. Where will the insulation go? Will the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the completed wall be high enough? How will the wall be sealed against air leaks? A focus on energy efficiency pays dividends over time.
Sustainability issues. Can the resources used to build the structure be produced on a sustainable basis?
Local building requirements. Areas prone to hurricanes or earthquakes, for example, may have specific rules to help structures withstand extreme natural events that affect specific areas of the country.
Durability and initial cost. Keep in mind that differences in initial cost may not seem quite as dramatic when weighed against the expected life span of the house.
Combined functions. Wall systems that combine structure with finish have an inherent material efficiency advantage and should be seriously considered.
MORE ABOUT WALLS
Add more foam for a better wall
The performance of almost any wall, in any climate, can be improved by adding a layer of exterior foam. If the wall already has exterior foam, it can be made greener by making the foam thicker. Remember, depending on the type of foam and thickness, foam-sheathed walls may need to dry only to the interior. For walls with more than three inches of any foam or with any thickness of foil-faced polyisocyanurate foam, never include interior polyethylene or other impermeable interior finishes.
Siding is the first line of defense
Walls are a house's "skin," and as such must protect the building from rain, wind, and sun. Siding is the first line of defense, but how siding is applied and the kind of water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB) installed beneath it have a lot to do with how durable the walls will prove to be. The skin also includes doors and windows, important components of a home's thermal envelope but also sources of damaging air and water leaks if not properly installed.
Interior walls define spaces and affect livability
Floors and interior walls don't keep weather out, but they often do more than just define spaces. How you lay out partitions can affect airflow, solar heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss., natural lighting, and even how efficiently pipes, wires and ducts are laid out. Structural demands may dictate where interior walls and floors go, which is why you should plan your mechanical systems and framing at the same time.
Doors can help or hinder
Exterior wood doors just under 2 inches thick don't offer much in the way of insulation, just R-2 or less. When weatherstripping is of poor quality or worn out, the effects are magnified. Doors don't represent a huge amount of wall area, but they can help nullify all the effort of insulating outside walls carefully. Insulated doors will help, along with high quality weather-stripping. Window area in doors, along with sidelights, should be kept on the small side or eliminated altogether.
Storm doors may seem like an antiquated idea, but they can be helpful in reducing energy losses while providing an extra weather barrier. They're especially useful when the primary door is exposed to the elements and not protected by a roof overhang or porch.
- Daniel Morrison/Fine Homebuilding #174
- Chuck Lockhart/Fine Homebuilding #174
- Dan Thornton/Fine Homebuilding #188
- Chuck Bickford/Fine Homebuilding #166
- Brian Pontolilo/Fine Homebuilding #170
- Gary Williamson/Fine Homebuilding #131
Fri, 07/02/2010 - 13:42
Fri, 07/02/2010 - 12:49
Mon, 06/28/2010 - 14:06
Mon, 06/28/2010 - 13:36