Walls
Different framing materials and wall products are available to suit design needs, environmental factors, and climate zone variation. Some of these choices are described below.
Wood Framing
Builders can choose from a variety of wood sheathing products that range in cost, strength, insulation value, and ease of installation. Of the options available, plywood and oriented strand board (OSB) are the strongest and most durable. Wood sheathing panels add shear and racking strength. Important characteristics that are engineered to help a structure withstand the forces of high winds and earthquakes. Wood-sheathed walls are also easy to build and easy to insulate for high R values.
Structural insulated panels (SIPs)—which commonly consist of rigid foam board sandwiched between structural wood sheathing (plywood and OSB)—can be used in place of stud-framed construction for both walls and ceilings. Manufactured wood products often structurally perform as well or better than lumber. Engineered wood products made with exterior-type glues (phenolic resins) and urethane (polyurea) adhesives give off some of the lowest emissions.
Steel Framing
Steel is commonly used in the construction of commercial buildings. Builders have only recently started using steel more in residential construction.
Steel buildings use nearly the same framing techniques employed in wood-framed buildings. Construction costs also run about the same. Unlike wood, however, steel is impervious to termites. It provides added resistance to fire and earthquake. Steel ceiling joists can span greater distances than wooden ones, allowing new design possibilities for architects and builders.
But steel conducts heat more than 300 times faster than wood. Steel studs can create "thermal bridges" to the outside of the house. Even the fasteners become a heat loss issue. Screws attached to steel studs can reduce the insulating value of the foam sheathing by 39 percent. Therefore, when selecting this framing method, you need to make sure you insulate properly. In cold climates, the additional insulation required to prevent heat transfer might reduce the cost-effectiveness of steel framing.
Concrete
As a whole, concrete framing provides structural integrity, termite protection, and thermal insulation, and it helps reduce air infiltration in buildings. It also readily absorbs heat, making it ideal as thermal mass in passive solar building design. There are many concrete framing options and products:
- Masonry
- Autoclaved aerated concrete
- Insulating concrete forms
- Cast-in-place
- Precast
Masonry
Masonry is concrete blocks assembled with mortar. See the Building Envelope: Foundation section for more information on masonry.
A Trombe wall is a special type of masonry wall used for thermal storage in passive solar building design. A typical Trombe wall consists of an 8- to 16-inch-thick masonry wall coated with a dark, heat-absorbing material and faced with a single or double layer of glass. The glass is placed from 3/4 inch to 6 inches from the masonry wall to create a small airspace. Heat from sunlight passing through the glass is absorbed by the dark surface, stored in the wall, and conducted slowly inward through the masonry.
Applying a selective surface to a Trombe wall improves its performance by reducing the amount of infrared energy radiated back through the glass. The selective surface consists of a sheet of metal foil glued to the outside surface of the wall. It absorbs almost all the radiation in the visible portion of the solar spectrum and emits very little in the infrared range. High absorbency turns the light into heat at the wall's surface, and low emittance prevents the heat from radiating back towards the glass. Although not as effective as a selective surface, painting the wall with black, absorptive paint will also help the wall to absorb the sun's heat.
For an 8-inch-thick Trombe wall, heat will take about 8 to 10 hours to reach the interior of the building (heat travels through a concrete wall at rate of about one inch per hour). This means that rooms remain comfortable through the day and receive slow, even heating for many hours after the sun sets, greatly reducing the need for conventional heating. Rooms heated by a Trombe wall often feel more comfortable than those heated by forced-air furnaces because of the radiantly warm surface of the wall, even at lower air temperatures.
Architects can use Trombe walls in conjunction with windows, eaves, and other building design elements to evenly balance solar heat delivery. Strategically placed windows allow the sun's heat and light to enter a building during the day to help heat the building with direct solar gains. At the same time, the Trombe wall absorbs and stores heat for evening use. Properly sized overhangs shade the Trombe wall during the summer when the sun is high in the sky. Shading the Trombe wall prevents the wall from getting hot during the time of the year when heating is not needed.
Autoclaved Aerated Concrete
Autoclaved aerated concrete comes in plank or block form. These units are much lighter than traditional concrete blocks because they use a special mixture of sand, limestone, cement, and an expanding agent.
Insulating Concrete Forms
To construct an insulating concrete form (ICF), builders pour concrete into a foam form. The form then stays in place to provide insulation. Builders construct walls by stacking ICFs, cutting them where needed to fit windows and corners, etc. They also place steel rebar horizontally and vertically within the form to provide strength. Although all ICFs are identical in principle, the various brands differ widely in the details of their shapes, cavities, and component parts. See Foundation and Insulation for more information on ICFs.
Cast-in-Place Concrete
Unlike ICFs, cast-in-place framing construction involves setting up removable or temporary forms for the pouring of concrete walls. Rigid foam board insulation is usually placed between the removable form. Steel rebar is also generally used to add strength to the wall. Such concrete buildings are typically constructed in one of three ways:
- Only the exterior walls are cast-in-place concrete.
- Both exterior and interior walls are cast-in-place concrete.
- The floor/ceiling, exterior walls and interior walls are all cast-in place concrete.
Precast Concrete
Manufacturers construct precast concrete walls or panels off-site. Most of them are pre-insulated as well with rigid foam board. But additional insulation of your choice usually can be added inside the wall cavity to achieve a high R-value. The panels typically come in lengths of up to 16 feet and in standard heights of 4, 8, and 10 feet. Once constructed, they're transported to the building site. A crane is needed to lift them in to place. Precast concrete walls have been shown to be very effective in passive solar design.
Structural Insulated Panels
Structural insulated panels (SIPs) provide both structure and insulation. They consist of carefully engineered laminate with a foam core 4 to 8 inches thick, with a structural facing on each side. The most common types of facings are drywall and/or structural wood sheathing such as plywood and oriented strand board (OSB).
R-values for SIPs range from about R-4 to R-6 per inch of thickness, depending on the type of foam core used. Manufacturers construct most SIP foam cores from expanded polystyrene (EPS), also known as beadboard. But some manufacturers choose to use polyurethane and isocyanurate as the insulating material.
Although SIPs cost more than traditional building materials, they require less labor to install. Therefore, the total cost is roughly the same.