A Breakdown of Thermal Efficiency Construction
THERMAL EFFICIENCY CONSTRUCTION (TEC) is a technological breakthrough in building envelope design developed and perfected over the past 40 years. TEC enables the construction of low cost, wood frame homes that drastically reduce heat loss through the framing system, and provides ample space for low cost insulation materials. Depending on location, size, window area, and occupancy TEC homes may be designed with a heating load that can be largely met by interior heat sources—the energy expended in household appliances and lighting even if they are energy efficient. Architects and designers will find advantages with TEC that extend beyond the system's ability to defeat heat loss.
A typical residential TEC building would employ 2x12 studs, spaced 4 feet on center around the perimeter. Horizontal 2x4 girts are attached around the outside providing structure to which the siding is affixed. This creates a wall cavity 12-3/4 inches deep allowing for a generous quantity of friction fit fiberglass or cellulose insulation.
An open plate at the top of the wall, filled with non-combustible insulation to meet fire code, makes it easy to strap down the roof structure to the studs, and to pass electrical and other utilities up over the ceiling.
Roofs are built using trusses or rafters also spaced 4 feet-on-center. Naturally, trusses and rafters need to be engineered to handle the extra load. Contractors have found it to be economical to build trusses on-site, from engineer or architect plans, since the number required is about half the number normally used. Manufactured trusses cost, not counting delivery, around twice the cost of the lumber. This leaves three quarters of the amount you would budget for trusses to pay for the additional labor to assemble them yourself.
Assembling trusses takes little time using a blocked out ‘jig’ on the floor platform, and using glued and nailed 3/8 plyscore gussets both sides at all the joins. From the first set of pieces, identical sets for the final number of trusses are chop sawed, assembled in the jig, gussets attached one side, then tipped out of the jig and gussets attached to the other side while the next truss is being assembled. On a small building, all the trusses can be cut, assembled, and placed in position in the roof structure with a small crew of 2-3 people in less than a day.
The roof structure employs horizontal purlins over the trusses or rafters. If ribbed metal roofing is used, the roof does not even need to be decked with plywood. Of course, if other than metal roofing is desired, plywood decking goes on over the purlins.
Floors can consist of a well insulated slab. However, I prefer wood framing over a crawl space or cellar. Use floor joists spaced 4 feet-on-center. Over this structure of parallel joists and rim joists is built a second layer of 2x4 joists and rim joists spaced 1-foot-on-center, and at right angles to the system below. Dead-wood 2x4 blocking is placed between the 2x4 joists every 8 feet, every other primary joist below, and staggered to accommodate 4x8, straight cut edge plywood 7/8- to 1-inch thick applied on top using glue and nails around all 4 edges plus intermediate joists.
This single layer floor system with no floating edges does NOT require a second layer of plywood, and is ready, with perhaps a bit of sanding, for tile, carpet, or other covering. If a top layer of hardwood is in the plan, the thickness of the glued and nailed plywood can be reduced to 1/2-inch, allowing the hardwood flooring to take up the load.
Plumbing lines are easily installed immediately below this floor system, obstructed only every 8 feet by dead-wood blocking which can be drilled without compromising load carrying capacity. Then dropping a few inches lower, supplies and drains can run unobstructed on the other coordinate, all above the insulation in the floor.
If you would like to learn more, particularly about window and door openings, ventilation systems, vapor barriers, electrical and other penetrations through the inner wall surface, and other details, contact me at bentley@northnet.org or give me a call at 518-359-9300.