Superinsulated Duplex: Technical Info

The goal of this project was to incorporate as many green features as possible while still being cost competitive on the rental market. Most green buildings which I have seen in books and on the web are either high-end custom homes or are self-built personal homes. With this project we hope to provide a low energy home for renters.

To keep costs down we did most of the work ourselves and minimized subcontractors. We did hire some labor along the way and also did sub out electrical, some plumbing, and some trim work. We didn’t hire out any design or engineering work, other than to calculate heating loads for winter.

We broke ground in March of 2007, completed one unit in May of 2008 and the second unit should be complete in October 2008.

This project was developed by RGM Properties, LLC which is run by my father, Merritt Lehman, and me. My company, Lehman Design & Mfg, LLC, designed the house and acted as general contractor.

Listed below are some features of the house:

Superinsulation: The cathedral ceiling is R70; dense pack cellulose. The gable end walls of the upper level are R36, cellulose + 3” XPS. The main level walls are R31, cellulose + 2” XPS, though areas above and below the solar heaters are up to R46. The lower level has 4” of XPS which extends down the bottom of the footers and is also under the slab for an R value of 20. Attention has been paid to minimizing thermal bridging. In addition to these high levels of insulation, the house’s very compact footprint with stacked levels and the two units sharing a wall results in less area exposed to the elements per square foot of conditioned space. What will the heating bills be? We won’t know until next winter.

Passive solar + thermosiphon solar air panels: Most of the window area faces south, but as Northern Indiana is very grey in the winter we kept the window size modest. The thermosiphon solar air panels have a fully insulated wall behind them (R26) and will have a back draft damper which means they collect heat when the sun is out, but not lose it when it is grey or nighttime. The panels have no fans or thermostats. In the summer, the registers can be closed to stop the flow of air. Also we have some overhang and 60’ tall oak trees to the south so overheating won’t be a problem. For some collector plans and much more solar information: http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm

High thermal mass design: Important when collecting solar heat, especially in spring and fall, to prevent overheating. The lower level slab is 4” plus has a 4” gravel bed under it (which is also part of the passive radon vent). The mass (concrete blocks) of the exterior lower level walls is exposed to the inside. There is a thin skim of drywall compound, then paint; all the insulation is on the outside. The rest of the floors are hard sufaces and the countertop weighs over 500#’s. Mass + solar case study see: http://eere.energy.gov/buildings/building_america/pdfs/32765.pdf

Air-to-air heat exchanger: Important for health and energy savings. This system takes stale air out of each bathroom and brings fresh air into the house. It will run on a timer on low speed at 86% efficiency about 6 hours a day to provide a controlled amount of fresh air to the occupants while transferring most of the heat from the outgoing air to the incoming air. For the model we are using: http://www.iaqsource.com/product.php/suncourt/he100/?product=111242&category=945

Tankless hot water heater: Water heaters without tanks save space and eliminate standby losses of a tank of water kept hot 24 hours a day. We are using a Steibel Eltron Tempra 24.

Passive radon vent system: Radon gas is the second largest cause of lung cancer in the U.S. Our part of the country has high radon rates and radon is more likely to be a problem in sandy soil which we have. The system consists of a perforated pipe in a gravel bed which is connected to a vent stack (separate from the drain waste vent system). Essentially it provides an escape route of least resistance before radon gets into the house. http://epa.gov/radon/zonemap.html

Compact fluorescent lights: All of the lighting is fluorescent. These lights use one-third the energy of incandescents.

No v.o.c. paint: V.O.C.’s are bad for you and bad for the environment

Low-e w. argon windows: On a tight budget, we went with vinyl Pella Pro Line.

Ceramic tile and wood floors; no carpets: Health and environment. Carpets harbor dust mites and end up in the landfill. In a rental house, carpets would end up in the landfill more quickly than average. If you must have carpets and have a larger budget than we did, check out Interface carpet tiles which can be recycled.

Air source heat pump for heating and air conditioning: Because the house is so compact and highly insulated, we eliminated the central heating system. The air source heat pump, which is like one found in a hotel room, will both heat and cool. Because we needed some help distributing heat around the house we put in some very small 400 watt baseboard heaters as well.

Energy star appliances: These are easily found and don’t cost much more than other models. The government awards Energy Star certification to products excelling in their class. http://www.energystar.gov/

Infill site; existing trees preserved; native woodland landscaping planned: Suburban sprawl on farmland with all new roads and and utility infrastructure is costly. This site is a lot subdivided off from the manufactured house sitting next door. We worked hard to keep the footprint of our house small to preserve the large existing oak trees to the south. We only had to cut one tree- a small crabapple. We are planning to add more street trees and establish a woodland landscape while minimizing the amount of turf grass.

Rough sawn untreated yellow poplar siding: Yellow poplar ages well, is fast growing, and grows in Indiana. The key to a long life (up to 100 years) is to keep it well off the ground, provide an overhang, and vent the back of the siding so when wetted by wind-driven rain it can dry out. Yellow poplar is not suitable for decks, or ground contact situations.

Standing seam metal roof: Standing seam steel roofs last a long time and can be recycled. Ours, in unpainted galvanized, will reflect much of the sun’s heat in the summer (though we are lucky enough to have shade on most of the roof until 1:30 in the afternoon)

Efficient use of space: The three level design makes the house feel larger than it is as the separation of rooms creates privacy. The elimination of a utility room due to downsizing of the heating system, and the tankless water heater save space as well. Bulk storage is outside the conditioned space and is under the front porch.