A south pitched roof is employed for solar collection directly onto a metal roofing with chocolate brown color (without any glazing cover). The roofing is structural and is suspended over plywood decking so as to create an air space which receives input from the coolest and lowest basement air of the house interior. Air heated beneath the metal roofing is returned to a basement storage wall. Full length plenum cavities are formed into the ordinary rafter truss framing - at the knee wall and collar tie spaces. Preliminary testing of BTU gain at known air flows is acquired with a microprocessor system continuously collecting input and output temperatures at the roof collector into disk data files.
Ambler House is a three level house with a twin gable (east/west ridge) roof. In a manner similar to a "solar attic", the south facing roof is employed as an opaque collector surface with no glazing cover. The rafter truss framing is covered with full plywood decking followed by bi-directional purlinswhich suspend a structural steel roofing which is colored dark brown. The space between plywood and roof metal communicates through full length slot openings into lower and upper plenums formed respectively in the knee wall and collar tie spaces.
The south roof is vented to a pair of gable vents which may be closed in winter, while the suspended north roof is permanently vented to a full length ridge vent. A view of one gable end of the house shows both the ridge vent and gable vent required. With the two roof surfaces vented separately, the south roof is joined to the basement space by ducts which provide intake from the basement and outflow to a storage plenum at an insulated basement retainer wall. The configuration was tested in 1996 with a computer log from sensors of input and output temperatures at the knee wall and collar tie plenums.
In recent years, more residences have been built with industrial style metal roofing which can span purlins at 4 to 5 foot spacing on very shallow pitches typical of warehouse buildings. These commercial systems are usually formed in 26 guage steel with the chosen enamel color applied over a "galvalume" rust protection. The metal is formed into channels at 1 foot intervals in the panels which are usually 3 feet wide by any length limited only by logistics of transport and installation. The channels have the cross-section shape of a truncated pyramid about 1 1/4" tall, 3/4" at the top and 3 1/2" at the base. This commercial material is stronger than traditional "barn tin" which has 5 "vees" and requires frequent purlins. Standing seam roofing which is attractive (and expensive) often requires a full support by a decking surface and is unsuitable to creating an air space directly beneath metal for air circulation. Traditional asphalt shingles and felt laid on plywood are not only unusable in creating an air space, the transfer of heat from the solar (outer) surface is impeded by the poor conductivity of the layered fibrous and wood materials. Metal roofing offers a weight advantage compared to asphalt shingle; this is important because no heat can be transfered until the entire mass is brought to an operating temperature exceeding the house interior. A 26 guage metal has only 100 lbs per 100 sf. to be heated while good asphalt shingle and felt laid on plywood will average 350 lbs of mass to be heated to baseline.
Metal panels and suspended purlins have some general advantages disregarding solar heating use. The metal panels can be safely handled on steep pitches by using the suspended purlins as a vertible climbing gridwork. The full plywood sheathing beneath purlins is coated with aluminum paint before applying the panels, so that all wood has temporary weather protection during construction.
Although installed for its heat potential,the air space used on both north and south roofs is passively vented in summer to reduce heat gain upon all wood components of the house and reduce metal expansion during hottest months. Finally a metal roof system sheds leaf litter and does not form shingle sludge.
In the decision path to choose a metal roof for reduced maintenance it is only natural to seek a solar collection method that is also low maintenance with quick payback. By avoiding the use of glazing over the metal absorbing surface, the house acquires little solar benefit on extremely windy or overcast,diminished sunshine days, but this is balanced by the benefit of having a roof based solar apparatus that does not complex glazing to be maintained.
Metal roofing does its roofing duty 24 hours per day while a simple system of air handling ducts is the only added cost to acquire heat which is available on some winter days.
Fig.1 and Fig.2 are provided on a combined drawing to emphasize that the roof and basement (or lowest floor) of a house should not be designed as disparate structures. The lowest floor is the logical site to receive heat which will subsequently rise in the house and it is the floor most likely to contain some masonary surfaces for storage especially if and when solar heat is abundant. Ambler house has a full basement with its concrete retainer wall insulated from earth by polystyrene foam panels.
As shown in Fig.1 a basement plenum applies positive pressure from the squirrel cage fan (SF) to the hollow space behind a drywall applied on furring strips. The drywall ends with a 2" floor slot (FS) along the entire basement slab floor. It is intended that the wall receive and temper excesses of heat available on sunny days of late autumn and early winter. During midwinter heat loads, any bonus of solar heat which is obtained will be needed directly in the living space, in which case the storage wall is bypassed by opening a bypass door (BD) depicted at the plenum. All ducts between roof and basement are formed out of sheet rock built out from the initial drywall interior skin of the house (to form 4" by 24" duct chambers).
Fig.2 shows the metal roof (MR) with a corrugation thickness of 1 1/4", held on suspended horizontal purlins (SP). The space (S) which is beneath the metal does not show the vertical spacer purlins. The south roof is pitched 50 degrees from horizontal while the north roof is at 38 degrees . The collar tie space runs the length of the house and is in negative pressure as a plenum; during solar operation gable vents at each end are sealed. Heated air from the roof is received at the slot below the blocking strip (BS) so that air is pulled under the metal. Air is delivered to the roof space from the knee wall plenum through the lower slot (LS). At both ends the knee wall plenum receives paired input ducts extending from the basement floor at the southeast/southwest corners of the house. In summer the collar tie space is passively vented through open gable vents in which case the main floor of the house can be vented through optional ceiling height vents (CV) to the knee wall plenum and roof.
The insulation pattern is depicted schematically and it should be noted that the north roof insulation extends to the peak so that there is an insulation barrier (IB) which separates the collar tie space from the north roofat there is an insulation barrier (IB) which separates the collar tie space from the north roof and its full length ridge vent (RCV). All insulation batting within plenum cavities is covered with Tyvek and a furnace filter is provided at the output vent (OV) in the collar tie space. During fan operation at 300 CFM there is no perceptable air movement within the collar tie and knee wall plenums due to their relatively large corss-sectional areas in comparison to typical ducts.
It is necessary to have back flow dampers or closable vents at the intake vents (IV) to prevent cool air descent from the roof and knee wall plenum at night. The output vent (OV) in the collar tie space must also be closed nightly. The installed duct system as described has an alternative non-solar use for delamination of interior house air. There is an optional recirculation vent (RV) which may be opened to move warmer upper level air to the base ment (the output vent OV must be closed to use the system for recirculating interior air rather than solar heated air.
A microprocessor circuit with solid state temperature sensors was installed in late Feb/Mar 1996 to record minute by minute temperature samples to digital disk file. During this winter the house was awaiting sale and unoccupied with no other heat sources (gas heat shut off) and no heat conservation methods (such as window curtains).
As Table of daily data demonstrates there is a window of opportunity for a pulse of solar heat on sunny winter days in an amount equivalent to a small gas wall heater. February is typically a cloudy month with only an occas ional clusters of sunny days in Eastern climate.
Ambler House is a demonstration of unglazed metal roofing as a solar collector using a minimalist approach with simple ducts integrated into the drywall interior of the house and distribution plenums built into existing framing structures (knee wall and collar tie plenums). The choice of metal roofing stands on many reasons other than solar collection, especially regarding maintenace and durability which justify the slightly greater cost of metal over typical asphalt shingle. Some residential designers mistakenly grasp for the small savings in structural metal roofing by empoying bare purlins which allows for the elimination of plywood sheathing over rafters. This is a mistake because the suspended purlin method described here offers more than the benefit of segregating the south roof for solar collection. Both northand south roof can be better ventilated during summer heat due to the well formed air space and the reflective film applied over the plywood skin.
The plywood skin beneath the purlins can be minimal (structural strength is provided by structural rate roofing and purlins alone); the minimal along with the grid of purlins to suspend the metal roof provides safety and ease of metal assembly on steep pitches. With the entire gridwork and plywood painted with aluminum reflective paint, there is even weather protection during construction (a Z formed polyethylene must be placed over the full length plywood slots before applying purlin spacers which are nailed through to the rafters). It is not necessary to have a storage wall destination for solar heat and the massive masonary wall provided in Ambler House may well be too large for the amount of heat flow. But even if the wall cannot be charged to high temperatures, the wall will serve as a tempering surface particularly during early winter when the wall can serve as some of the "heat flywheel" of the house entering into full winter conditions. In a typical house with stud and sheet rock walls and (no masonary) the mass of sheet rock walls alone may be enough for tempering the heat provided from a south roof metal collector. In such a case a secondary drywall is furred out over an existing expanse of drywall and a delivery plenum is provided along the length of the wall (analogous to the basement storage plenum depicted in the system diagram.
It is not unreasonable to consider this method of solar collection in a full house retrofit where an older house needs a new roof covering and the house interior is receiving a full facelift. The heated air should be routed to a storage wall that has a minimum of windows. The interior ducts are simply furred out on existing walls and the existing north and south roofs must restructured to their segregated ventilation systems. Houses with either a ridge vent or gable vents require both vent systems to serve segregated roofs. In a rehab operation the benefit of the reflective paint is again realized in the form of temporary weather protection during construction with as phalt shingles stripped.
An unglazed metal roof collector is like the man who occasionally picks apples on his walks near an orchard... but doesn't like the expense of carrying a bucket. If it isn't rainy he can take off his coat and use it to gather apples. However if it starts to rain he is without a coat he can't expect to wear a bucket ... he must have a coat.
A "roof" is more than a "finishing touch" on a house...it's necessity even if it isn't needed on sunny days. On the other hand, a dedicated solar collector is a great way to collect the rays, but a poor way to have to protect a house from the elements. Continuing the pun ,if a bucket is not something to "wear", a glazed solar collector certainly does "wear"....it must be "maintained" much more than typical continuous roof surface.
The value of an unglazed roof surface for collecting solar heat is that the roof material and most of the construction is not counted as a cost of the solar system... and the metal roof has merits over shingle even without considering its solar value. The unglazed approach avoids roof top glazed solar collectors which are inconvenient and dangerous to inspect and maintain.
For monitoring equipment and software special thanks to: Brian Boothroyd,Charlotte NC and Barry Wilson and Ross Hile at PGC Inc. Black Mountain, NC.
