Building Science Professor Puts Theory into Practice

Allen Zimmerman is an associate professor at the Ohio State University, Wooster Campus, where he teaches courses in engineering technology and technical physics.

The deck on the south side of the building overlooks a private, forested lot.

The garage doors on the north side of the structure afford easy access from the street.

The sliding door, which opens to the deck, has low-e glazing and inert gas between the panes.

The conditioned garage space provides a comfortable location for washing clothes.

Figure 1. This apartment floor plan made the most use of the 576 ft2 living space.

Several years ago, my wife and I purchased a small lot in a town in Maine on which we plan to build a retirement home. We were not ready to build an entire house that would be vacant for several months of the year, but we wanted to have somewhere to stay during our vacations. Our solution to this dilemma was to build a garage with an apartment above it. Sometime in the future, we will build a main house on the lot.

First, we decided on the style, design, and location of the house that we eventually plan to build. We used this information and the lot characteristics to govern the planning of the garage/apartment. We selected a simple and functional design that permitted a straightforward approach to material selection and energy-efficient construction.

The building was oriented so that the garage doors and the half of the gable roof with a 12:12 pitch face north toward the street. A full shed dormer and deck on the back side provide living space, southern exposure, and privacy. With this design and orientation, all apartment windows could be located in the south, east, and west walls to take advantage of solar gain and cross ventilation.

A standard 24 ft x 24 ft two-car garage proved large enough to accommodate an upstairs apartment of sufficient size. In order to gain some additional headroom under the cape ceiling in the front side of the building, the 12:12 sloped rafters were mounted on a 1-ft wall at the eave. This construction technique also raised the ridge one foot and thereby increased the slope of the shed dormer roof from 4:12 to 5:12. For reasons having to do with security, energy efficiency, and cost, the garage does not have windows.

A frost-protected shallow monolithic slab serves as the building foundation. The slab rests on top of 2 inches of expanded polystyrene foam board, with an R-value of 10, and below that lies several inches of well-drained compacted gravel fill. The foam board extends up the outside of the foundation wall and has a fiberglass-reinforced cement coating where it is exposed. The site is graded to direct all surface water flow well away from the building. We had a diversion ditch dug about 20 feet away from the building that diverts ground water at the level of the gravel fill.

The exterior walls of the structure are 2 x 6 construction with unfaced fiberglass insulation in the cavities. The siding contractor who was selected for the building project typically installs 1 inch of R-5 extruded polystyrene foam under the vinyl siding. Although this amount of insulation was adequate for the garage walls, a higher R-value was needed for the apartment walls. Therefore, 1 inch of polyisocyanurate foil-faced foam board was installed on the inside of the exterior apartment walls. I calculated the composite R-value for the entire wall, including structural members, to be 29 for the apartment walls and 22 for the garage. The apartment floor was insulated with unfaced fiberglass batts with an R-value of 30.

About three-quarters of the ceiling area is 8 feet high with an attic space above. This portion of the ceiling was insulated to R-49 with 16 inches of fiberglass in the form of two layers of batts laid perpendicular to each other. It was not possible to get 16 inches of insulation in the attic space next to the wall where the rafters and ceiling joists rest on the top plate. Therefore, sections of 2-inch-thick foam board were cut to size and placed under the fiberglass batts in the attic along this wall to increase the minimum R-value at the top plate to 38.

The remainder of the ceiling is a cape style, with the ceiling components attached to the 2 x 10 roof rafters. This portion of the ceiling was insulated to R-30 by installing unfaced fiberglass batts in the cavities so that the vent channels maintain an air space and attaching 1-inch polyisocyanurate foil-faced foam board to the rafter faces.

The foil-faced insulation board serves as the major component of the air/vapor barrier system for the exterior walls of the apartment and the cape ceiling. Cross-laminated polyethylene sheeting serves as the major component of the air/vapor barrier system for the rest of the ceiling and the exterior walls of the garage. Caulk, tape, and expandable foam were used as appropriate to seal the rigid insulation board and poly film to structural components; to cover and seal seams in the materials; to seal holes inside the electrical boxes; and to seal around electrical, plumbing, and other penetrations through the foam board or poly film.

Air leakage was also an important consideration in the selection of the windows and doors. High-quality casement windows and exterior doors with low air filtration rates were specified. The windows and sliding door contain low-e glazing and inert gas between the panes. They have an overall R-value of 3.2. The garage doors have a double-track system that ensures a tight seal. The exterior doors and garage doors are metal with foam insulation and have R values of 10 and 14, respectively.

The design heating loads were calculated to be less than 2.5 kW for the apartment and 2.2 kW for the garage at a design temperature of 70°F. The annual heating loads for the apartment, if occupied, and to keep the garage above freezing were estimated to be about 2,500 kWh and 700 kWh, respectively.

Given these small design and annual heating loads and the fact that natural gas is not available in the area, the advantages of electric baseboard heat--space savings, zone control, simplicity, and low installation costs--made it the obvious choice for our heating needs not met by the apartment's solar and intrinsic heat gain. Combined with an energy-efficient electric water heater, electric heat also eliminates any health and safety concerns associated with combustion appliances.

Moving from design values to actual baseboard installations proved interesting. The veteran electrician I chose to work with had never seen such a tight, well-insulated structure. As we discussed tentative heater sizes and locations, it became clear that he was very concerned about undersizing the heaters. I had to be diplomatic as I attempted to educate him about the need to downsize heaters from what he was used to. Fortunately, since electric heaters are 100% efficient and the increased cost of a one-foot longer heater is negligible, oversizing electric heaters is not the important issue that it is with gas furnaces and other heating systems. This fact made reaching a compromise not as difficult as it might have been.

For the apartment, I wanted a 5-ft heating unit installed in the living room, a 2-ft unit in the bathroom, and 2-ft units in the two bedrooms for a total capacity of 2.7 kW. I ended up with 3-ft units in the bedrooms and 3.2 kW of total capacity. To enable me to heat the garage sufficiently so that I could work there in winter, I wanted two 3-ft units and one 4-ft unit. Instead, I got 5-ft, 4-ft, and 3-ft heaters installed along the three garage walls for a total capacity of 3 kW.

I designed a simple controlled ventilation system for the apartment. A 70 CFM exhaust fan with a 0.5 sone sound rating and a 15.4-watts electrical input was installed in the bathroom to provide both spot and whole apartment ventilation. A short length of duct runs under the attic insulation and connects the fan to a vent cap mounted in a gable sidewall. Fresh air comes into the apartment through a 5 inch duct running from a vent cap mounted in a gable sidewall to a ceiling grille located in the bedroom closets. The air inlet duct runs under the attic insulation and contains a backdraft damper that opens when the fan is operating. The closet and bedroom doors are undercut to allow the incoming air to move throughout the apartment. The mixing of the air in the closets and as it moves under the doors provides some tempering.

A spring-wound timer is used to activate the fan when spot ventilation is needed in the bathroom. We can also control the fan with this switch when we want whole-apartment ventilation. The fan is wired in parallel to a dehumidistat mounted on the wall of the living room. Whenever the humidity in the apartment gets too high, the dehumidistat automatically operates the exhaust fan.

Fortunately, we were able to team up with a local remodeler/ builder who has an excellent reputation for quality and craftsmanship. Michael Bickford had the expertise to do the rough and finish carpentry, install the insulation and drywall, and do the air sealing. We felt that having one person responsible for all of these functions would help to ensure that the integrity of the thermal envelope was maintained.

We assured Michael that we would pay the added labor cost associated with the extra time, features, care, and attention to detail required for the insulating and air sealing. He estimated this added cost to be about $200.

The period from November through March provided an indication of the thermal performance and energy efficiency of the building. During these months, the amount of electricity used for heating the apartment to 45°F and the garage to 35°F was 1,305 kWh. Although the amount of electricity used for heating the apartment when it is occupied will obviously be closer to the estimated annual heating load of 2,500 kWh, it is apparent the building is performing as expected in terms of energy efficiency.

Building a comfortable and energy-efficient apartment or house does not require far-out designs, exotic materials, or complex assembly techniques. By using building science and engineering fundamentals and adhering to the building as a system concept, we got a well-insulated, tight, and mechanically ventilated apartment that is healthy, quiet, comfortable, durable, affordable, and energy-efficient.
 
 

Home Energy can be reached at: contact@homeenergy.org
Home Energy magazine -- Please read our Copyright Notice