Solar building materials. Sturdy aluminum bars, which can be
cut to size with a circular saw, are assembled like the pieces of a picture
frame to hold a ribbed acrylic panel. The transparent, double-skinned panel
transmits light as well as glass does, but the acrylic is far tougher, making
it an excellent glazing material for solar collectors. A flexible silicone-coated
gasket fitted into the frame allows the panel to expand and contract without
leaking when the temperature changes.
The slim, black solar collector panels fast becoming a familiar sight in communities across the country are not a novelty born of present- day fuel shortages. Americans began economizing with solar collecting devices before the turn of the century. A simple solar water heater similar to the batch heater was patented in 1891. It featured four galvanized-iron water tanks, painted black and mounted inside an insulated pine box with a glass cover. By 1900, the Climax, as it was called, crowned thousands of American roofs. But its popularity did not last: The advent of cheap natural gas made solar water heaters passé by the 1930s.
The designers of the Climax were not the first to learn that a space enclosed with transparent glass would trap the heat of the sun. Centuries earlier, the comfort-seeking Romans who basked in steamy, glass-walled bathhouses were utilizing the main principle of solar collection: the greenhouse effect.
Sunlight is made up of very short wavelengths of electromagnetic energy, a characteristic that permits it to pass easily through transparent materials such as glass. If the light then strikes a dense, opaque substance, it’s absorbed and turned to heat. But the heat waves, being longer than light waves, cannot readily pass back through the transparent material. Thus, a transparent enclosure becomes a heat trap. The modern, flat plate solar collector works exactly that way: In effect, it’s a small greenhouse.
Solar collectors are at the heart of each of the solar heating devices. Active solar systems employ mechanical means such as a fan or pump to move the collected heat to where it’s needed. Others, called passive systems, rely on the natural principles of heat transference to do the job.
Heat always attempts to flow from a warm area to a colder one. The three methods by which it moves are radiation, conduction and convection. Radiation is the movement of heat waves across space; anything warmer than its surroundings radiates heat to the objects around it. Conduction is the movement of heat through a substance by molecular vibration. The solid masonry Trombe wall shown, warmed on the outside by the sun, conducts heat through itself to the cooler house interior and radiates it into the room.
Convection is the natural circulation of heated air. When the air is warmed it expands, grows lighter and rises. As it loses heat to its surroundings, it grows denser and sinks. The air heated in a wall mounted thermosiphoning air panel travels by convection. It rises and escapes through an upper vent into the house, where it gives up its heat, falls toward the floor, and exits the house through a lower vent, completing a circular current of air.
The Trombe Wall —A Solar Heater Made of Masonry
The Trombe wall—named after its inventor, French architect Felix Trombe—converts a solid masonry wall into a passive solar collector that can provide about half of the required heat for the living space it adjoins. Mounted on a wood frame 3 inches out from the house wall, transparent panels of glass, fiberglass or plastic trap solar radiation in a narrow hot-air sandwich. The sun’s rays heat the masonry; the sun-baked brick, concrete or stone then warms the room inside. When vents are cut in the house wall, convective airflow augments the radiant heat by circulating drafts of heated air through the vents directly into the house.
The principle is simple, but not every wall can become a Trombe wall: Certain conditions must be met before you consider this type of solar retrofit. The wall must be unshaded during the winter and shaded—either by trees or by an awning—in summer, and it must face within 20 degrees of true south. It must be solid masonry—brick, concrete, stone or adobe—at least 8 inches thick, with no insulation behind it and no internal air pockets. Such pockets, often left during construction to provide thermal insulation or to prevent moisture build-up within the wall, prevent the masonry from storing and radiating heat efficiently. Check your building blueprints for evidence of air pockets or, if the blueprints are not avail able, tap the wall over its entire surface for telltale sounds of hollowness.
Finally, check the condition of the wall’s outer surface. The masonry must withstand temperatures that can range from 140 to 180° F., so be sure to replace broken bricks or stones and repair crumbling mortar joints.
Before building a Trombe wall, you should also check with the local building inspector and with any zoning commission or architectural review board that may exist in your community. The masonry surface of a Trombe wall is often’ painted black for maximum absorption and retention of heat. The wall works well enough without the paint, however—which is fortunate because in some communities the black façade, coupled with its cover of transparent panels, may be considered unsightly and, for this reason, undesirable or even prohibited.
Once these preliminary requirements have been met, you are ready to plan the design of the wall. Trombe wall panels come in a standard 4-by-5-foot size and are available from solar-equipment dealers. Plastic and fiberglass panels are generally more popular than glass because they are light and easy to work with. They come with rubber gaskets at the top arid bottom; be sure to remove the bottom gaskets before you begin the installation. Glass panels, though heavy and fragile, are also a possibility; all types of panels have double walls for maximum heat retention.
You can buy all of the necessary framing materials for the panels from the same source; they come in plastic, aluminum or wood, along with the appropriate gaskets, depending on the type of panel. To save money, you can buy pressure- treated lumber and cut the framing pieces yourself, as shown opposite, and substitute a black rubber called neoprene for the preformed gaskets; it comes in strips and rolls from hardware stores.
For optimum efficiency, the area of the wall should equal approximately one third the floor area of the room the wall is expected to heat. But in planning the wall, take the size of the panels into consideration. It’s usually easier to adjust the dimensions of the wall than to cut numerous glass or plastic sheets to odd sizes. Ideally, the only time you should cut panels is when you plan to frame around a window, either to provide a fire exit or to avoid interfering with a view, as in the case of the right-hand window shown opposite.
A Trombe wall can be designed to function with or without vents, depending on the time of day when heat is most desired. An unvented wall, heating by radiant heat alone, begins its heating cycle late in the day and continues to provide heat into the night. A vented wall, heating by convection as well as radiation, begins its heating cycle earlier in the day—as soon as the sun warms the air trapped under the glazing.
The openings for a vented Trombe wall should always be installed in pairs, in vertical alignment, one just above floor level, the other below ceiling level. The size and number of vents is determined by the size of the Trombe wall they will service. The combined area of the vents should be approximately equal to 1.5 per cent of the total Trombe wall area.
To calculate the size of the vents, first calculate 1.5 percent of the total area; then divide this figure into an even number of vents. Each vent should be about three times as wide as it’s high; no vent should exceed 20 inches in width. If the Trombe wall will cover a double-hung window, you can use the upper portion of the window for a top vent, as shown opposite, and adjust the size of the other vents accordingly.
The installation of a Trombe wall is a fairly simple job, but one that requires painstaking measurements. An error of even as much as 1/4 inch in the placement of a framing piece can make it impossible to complete the installation, since glass and plastic panels are quite difficult to trim accurately.
At the same time, the framing must allow for slight expansion and contraction of the panels caused by changes in temperature—requirements that will be specified by the panel manufacturer. The measurements given in the Trombe wall instructions are for acrylic panels set into a wood frame. If you install a different framing system or another type of panel, refer to the installation instructions that accompany these materials for the correct measurements.
For this installation, you will need only a few basic tools—a circular saw for cut ting the wood framing and plastic panels; a hammer drill, which can be rented from a tool-rental store, if you will be cutting vents in the masonry; and a socket wrench for fastening expansion bolts to the masonry wall.
Anatomy of a Trombe wall --- This Trombe wall is made up of six 4-by-8-foot panels of double- walled acrylic (inset, bottom right) mounted on a rectangular frame of doubled 2-by-4s that allows a 3-inch air space between the panels and the masonry house wall. The 2-by-4 framing members are fastened to the masonry wall with expansion bolts (inset, bottom left). The outer edges of the panels at each end of the wall are sandwiched between 1 strips of neoprene gasket, butted against 1½-by-1/4-inch separator strips and covered by 1-by-4-inch battens; this entire assembly—gaskets, separator Strips and battens—is screwed to the framing.
Interior panel edges butt against separator strips mounted on 2-by-4 mullions. The mullions are offset from the house wall by spacers made of 1½-inch lengths of PVC pipe, through which run expansion bolts that fasten the mullions to the masonry. The joints between interior panel edges are covered by 1-by-4 battens; screws driven through the centers of the battens into the separator strips anchor the joints firmly.
Here, the left-hand section of the Trombe wall is vented; three vents are cut into the masonry, and the fourth vent is the top of a double-hung window. The right-hand section is unvented and its window is left uncovered, providing a fire exit. The window is surrounded by a frame constructed of doubled 2-by-4s in the same manner as the perimeter framing, and the plastic panels are cut to fit around the window frame. The two sections of the wall—vented and unvented —are sealed off from each other by a mullion made of doubled 2-by-4s.
How a vented Trombe wall works. The diagram at near right illustrates the daytime pattern of natural circulation that keeps warm air flowing from a vented Trombe wall into the adjoining living area. The air in the space between the glazed panels and the masonry wall is heated by the sun-warmed masonry. The heated air rises, creates a current strong enough to push open a flexible damper flap that covers the up per vent, and flows into the house. At the same time, cool air from the room is drawn through the lower vent, pushing open a similar damper flap, and is heated to continue the cycle. This convective circulation continues until two or three hours after sunset, when the masonry wall be comes too cool to produce an updraft of warm air.
At night, the air in the wall space cools, tending to drop and flow into the room through the lower vent. This change in the direction of airflow, which would draw warm air out of the room through the upper vent, is checked by the flexible damper flaps, which are drawn closed against a screen or a register by the reversed circulation pattern. But the masonry wall still radiates heat it has absorbed during the day.
Planning a Trombe wall frame. The dimensions of a Trombe wall frame are planned to accommodate the parts it will hold—the glazed panels and the separator strips against which the edges of the panels are butted; these strips may vary in width with the size of glazing used. In addition, the expansion allowance needed for the panels must be taken into account. Usually this allowance is 1/4 inch per panel—½ inch on each side—but manufacturers may specify more.
To determine the length of the top and bottom frame sections, add together the width of all the panels, the width of the separator strips at all the mullions, and the width of the separator strips along each side of the frame; then multiply the number of glazed panels by the expansion allowance, usually ¼ inch, and add this to the result. To calculate the overall height of the mullions and side sections of the frame, add the height of one panel to the width of two perimeter separator strips, plus the expansion allowance. For the side sections, subtract the combined width of the top and bottom frame sections—usually 7 inches, since most 2-by-4s are 3½ inches wide—from the overall height.
To determine the position of a mullion on the plan, locate the separator Strip for the mullion, using the panel width and expansion allowance as a guide. Then locate the strip’s center line, and mark that center line on the top and bottom frame sections. On each side of this mark, measure out half the width of the mullion.
1. Breaking through the brick. Prepare the in door space for dust and debris: Pull back furniture from the wall, cover the floor and furniture with drop-cloths, and shut room doors. Mark the vent locations on the outside wall; usually they should be centered between mullions. Then, wearing protective goggles and gloves, use a 3-pound mallet and a cold chisel to cut into the mortar joints along a vent outline. Try to remove whole bricks within the outline in one piece, but break them if you have to. To split cleanly through bricks that are crossed by the outline, score them along the outline with a brickset (inset), then cut along the scored line with a brickset or cold chisel. Re peat for every vent.
After the outer course of bricks has been re moved, drill a hole through the center of each vent with a carbide-tipped bit. Working from inside the house, use the holes as reference points to mark the vent outlines, and re move the inside bricks in the same manner.
2 Installing the sleeve. For each vent, construct a box of ¾-inch wood, the same depth as the vent and dimensioned to fit snugly inside it. Fit the boxes into the vents and, wearing gloves, stuff fiberglass insulation into the gaps around the edges. Cover the fiberglass, on both inside and outside walls, with a ½- inch-wide line of mortar. Block each vent temporarily, while you finish the Trombe wall, with a piece of rigid foam insulation, wrapped in poly ethylene to simplify its removal.
Constructing the Vents
Installing a Trombe Wall Frame
1. Establishing string guides. At one of the up per corners for the Trombe wall location, drill a 1-inch-deep hole into a mortar joint and tap in a 4½-inch nail; make sure that the nail is perpendicular to the wall and projects from it approximately 3½ inches. Mark the planned height of the frame on the string of a plumb bob, and suspend the plumb bob from the nail; using this for reference, make a mark for the lower corner of the Trombe wall. Then measure the planned length of the wall along the mortar joint holding the nail, and mark the other up per corner of the Trombe wall. Hold the plumb bob against that corner, and mark the corner be low it. Check to make certain that the four corners form a perfect rectangle by measuring the diagonals between them, adjusting the marks as necessary until the diagonals are exactly equal.
Drill pilot holes and drive nails into the three marked corners. Tie a string near the head of one nail and run the string taut around the others, fastening it to the first nail. At 1-foot intervals, measure the distance between the string and the wall, and slide the string along the nails until the minimum distance between string and wall is exactly 3 inches. Paint the area within the string with flat exterior latex paint.
2. Framing the perimeter. Assemble doubled 2- by-4s for the perimeter frame, leaving 3-inch gaps in the outer 2-by-4s of the top and bottom sections for the mullion joints. Hold the lower edge of the bottom frame section to the guide string and, at one point where the inner 2-by-4 touches the brick, drill through the frame and into the brick. Apply construction adhesive to the wall side of the inner 2-by-4, and bolt the frame section to the wall with an expansion bolt. Add other bolts at 3-foot intervals; remember that mullion ends will also need bolts. Use shims, if necessary, to hold the face of the outer 2-by-4 flush with the string guide.
Bolt the side and top frame sections to the wall. If you are using glass panels, add a brace to the wall and bottom frame piece. Trim the shims, stuff fiberglass into any gaps between the frame and the wall, and seal the perimeter with silicone caulk. To build around a window, frame it with doubled 2-by-4s, working in the same manner as for the larger frame.
3. Affixing the mullions. Drill mounting holes in the mullions as for the perimeter framing. Position a mullion in its gaps in the top and bottom frame sections, and bolt it to the wall. In stall the remaining mullions in the same way. If you are constructing vented and non-vented areas within a single Trombe wall, separate them with a doubled 2-by-4 mullion to prevent air from flowing between the two sections. To construct the doubled mullion, add a 2-by-4 the length of a side piece to a 2-by-4 the length of a regular mullion.
4. Attaching the spacers. At 3-foot intervals along each mullion, hold a piece of 3 diameter PVC plumbing pipe against the house wall; mark where the pipe touches the inside face of the mullion. Cut the pipe at this mark. Then drill a bolt-hole through the center of the mullion and a corresponding hole in the wall. Insert an expansion bolt through the mullion, through the PVC spacer and into the wall. Tighten the bolt.
Adding Panels and Flashing
1. Assembling the separator strips. Drill ‘, diameter weep holes through the separator strips that will be fastened to the bottom frame section; usually 1 to 3 weep holes per panel is sufficient, but check the panel manufacturer’s instructions for recommended intervals. Sandwich a gasket between each bottom separator strip and the bottom frame and, holding the bottom edge of the strip flush with the bottom edge of the frame, nail the strip to the frame with 1½-inch nails every 6 inches. Continue in this fashion, attaching separator strips and gaskets to the sides and top of the frame, but don’t drill weep holes into these strips. In the same manner, at tach gaskets and separator strips along the centers of the mullions. As each raised frame of separator strips is completed, check its interior dimensions to be sure a panel will fit into it.
2. Securing the panels. With a helper, hoist an end panel into place within its frame of separator strips. While your helper stabilizes the panel, hold another gasket against the edge of the panel, and top the gasket with a batten. Be sure that the gasket lines up with the outer edge of the bat ten and that the outer edge of the batten lines up with the outer edge of the frame. Screw through the batten into the separator strip every 2 feet, using 3-inch wood screws. Continue in stalling panels in this fashion until all the panels and battens are in place.
When panels must be cut, as for fitting around a window, use the tool recommended by the manufacturer; for most plastic panels, this will be a circular saw fitted with a plywood-cutting blade. Apply masking tape to panel areas along which the saw will travel, to prevent the saw base from scratching the panel. Use gaskets to cap the bottom edges of any cutout in order to help keep out moisture.
3. Installing the flashing. At the mortar joint immediately above the top of the Trombe wall, use a circular saw with a masonry blade to rout a ¾-inch-deep groove along the entire length of the wall. Fashion—or have a fabricator fashion—a strip or strips of aluminum flashing wide enough to fit into the groove, project over the Trombe wall and bend over the wall’s front edge about 1 inch. Fill the groove with mortar and insert the flashing edge; hold the flashing there until the mortar stiffens.
Finishing the Vents
Adding the draft flaps. Working inside the house, remove the rigid foam insulation from the vents. Cut a piece of the wire mesh called hardware cloth to fit the exterior dimensions of an upper vent, and staple the mesh to all four edges of the vent. Cut a 2-mil polyethylene flap to fit the interior dimensions of the vent—but add % inch to the length and width of the flap. Bend duct tape over the top ½ inch of the polyethylene for reinforcement, and staple the flap to the top edge of the vent (left, top). Add a flap and screen to each upper vent in the same fashion.
For a lower vent, first staple a polyethylene flap, cut 3/4 inch less wide than the draft flaps for the upper vents, to the edges of the vent; then add the mesh. Frame the vents with molding of your choice; here, clamshell molding is used. Fasten the outer portion of the molding to the wall surface with a construction adhesive, and nail the inner portion to the edges of the vent with 1½-inch finishing nails.
As an alternative to this arrangement of screening and draft flaps, construct the vent cover shown, or slip store-bought air-duct registers into the vents and screw the registers to the wall. Omit the molding—the register flanges cover the sleeve sides, Open and close the registers night and morning to control the direction of the airflow.