Saturday, June 18, 2011

ADU Building System Science and Building Program Design Overview

This long and somewhat technical post is intended for those who are interested in building an ADU themselves, or who are interested in building science. There's a 13-minute audio/visual presentation at the end of this post that covers some of the same information in presentation format. 

Building Overview
  • The goal for this project is to design and build personal housing that is financially sustainable. 
  • Projected cost for the design, permit, and construction is $92K, which equals $115/sq ft. 
  • 799 sq ft and its location is very walkable and bikeable.
  • Northwest Energy Star certified through Earth Advantage Institute with a EPS score of <40.
  • Notable design features: staggered stud wall construction, slab-on-grade,  92% efficient tankless water heater for radiant floor and potable water system, heat recovery ventilator, passive solar orientation; space efficient design accented by acid-stained concrete floors, exposed beams, and accent lighting.
Residential Sustainably – Living Small at No Cost

My concept of sustainability includes a financial component. I have explored a variety of low-cost, creative living ideas, and eventually decided on the idea of building an Accessory Dwelling Unit on a single family lot with a pre-existing primary residence. In this model, my financial goal was that the pre-existing house on the lot would pay for itself through rental income. Upon completion, I could live in ADU without paying the monthly mortgage on the main house. With this idea in mind, I purchased a single family home in 2010 with the intention of building an ADU on the property.

Conservatively assuming that the ADU would generate a $1K/month in rental income (if I were not living in it), a $100K investment roughly translates to an 8.3 year investment payback period, with an annual return of 12K after that payback period. Here are some variables that could alter this payback period:

    *         the construction could cost more or less than anticipated (+/- 15%)
    *         the rental income may be more or less than the mortgage payment (+/- 20%)
    *         there could be unforeseen maintenance costs on the primary house

I will not make much profit from the monthly rental income from the primary residence. However, I am confident that the investment will allow me to live at little to no cost for housing after this investment. Additionally, I am confident that the investment will eventually pay for itself and generate surplus income. This accomplishes the initial financial benchmark that I was seeking to achieve for this project.

Accessing tens or hundreds of thousands of dollars is the biggest barrier for those who are seeking to design and build an ADU. If one can surmount that significant financial hurdle, ADU's have a lot of potential payback.

Selecting a House with an Appropriate ADU Site
A critical factor in purchasing a house was the viability for the lot to gracefully host an ADU. The neighborhood was obviously important for many reasons, but so was the micro-neighborhood, and of course, the property and house itself was an important factor. When I found the property in NE Portland, it seemed well suited to the ADU design goals in these three categories.

Neighborhood Livability: The address scores a 71 on, but that doesn’t reflect how very walkable and transit oriented the neighborhood feels; house addresses 5 blocks away in either direction score 8 points higher. From an energy perspective, this urban location allows residents to easily reduce travel related emissions significantly for work downtown, as well as from running regular errands.

Micro-Neighborhood: The micro-neighborhood has a range of architectural styles—the next door neighbor lives in a yurt, and another home is sided with corrugated aluminum. Due to the wide range of architectural styles adjacent to this yard, the ADU is well-suited to this pocket of Portland. The homes in the immediate vicinity are oriented towards the street front side of their lots- in our 50’ x 100’ and 50’ x 50‘ lots, the yards are grouped together.

Property:  My primary house facade is only 11ft from the front sidewalk, leaving me with ample depth in the rear yard for a new 27’ length structure. I visited approximately 60 properties in NE Portland before finding this one. Overall, this house and the land purchase was competitive in terms of its current value, and even more valuable in light of its potential future use as a home with great ADU.

Site Selection: The ADU will sit on the north side of the lot. There is a gorgeous 60’ Cherry tree canopy on the south side of the lot, shading the ADU throughout the summer, and allowing light to pass through during the winter months. The extensive canopy provides a wonderful viewshed from the south-facing windows, so the majority of the glazing is oriented South. The ADU will have short eaves to match the trim on the primary house in accordance with Portland’s ADU regulations. In mid-summer, when the sun is above the canopy at noon, the short eaves will cast just enough shadow to block direct exposure through the clerestory windows. Most of the windows will receive dappled sunlight; the canopy will block direct sunlight 80% of the day.

The Building Envelope

The building envelope design goal is to achieve a high performance shell at a low cost. Rather than attempting to achieve Net-zero or LEED or Earth Advantage certifications, which require allocating funds towards particular materials, products, or renewable energy such as solar panels, I am investing proportionally more money into air sealing and insulating the building shell than in other facets of the building design. We’re using best practices for air sealing, adding redundant air barrier systems that are somewhat vapor permeable. Blown-in fiberglass is the insulation type for the 8 ¾” wall and 10 ¾” ceiling cavity.

Wall Assembly (from exterior to interior)
  •     Cedar siding shingles- a naturally water repellant composition.
  •     Rain screen from strips of dimple board, through which the shingles will be nailed.
  •     Weather Protective Barrier- Siga Majvest weather protective wrap and Siga Wigluv tape for the plywood sheathing joints. The Siga wrap and tape are also vapor permeable.
  •     5/8” Plywood sheathing to which the shingles will be nailed.
  •    Staggered studs framing. Selected because it provides a 8 ¾” insulation cavity and has less thermal bridging.
  •     Loose fill blown in fiberglass- R33
  •     Drywall
  •     Paint
Roof, Slab, and Glazing: The roof assembly will be R44 and the slab on grade floor will be R-15. The slab is thermally broken from the foundation walls (using 1” EPS foam) and the gravel (using 3” XPS foam). The windows and French door will be vinyl, with a U-value of 0.30 to comply with Energy Star standards.

Heat Loss Calculation: Earth Advantage conducted an energy use calculation using a RemRate analysis and found that the ADU will use 39.6 Million BTU/year. Based on an assumed performance of 6.5 air changes per hour @ 50 pascals, the Energy Performance Score (EPS) will be 40 (EPS is a simple numeric score that correlates with the Million BTU/year for a given home. The average new Oregon home has an EPS score is 89, so the ADU will use less than half that amount of energy. The ADU will likely beat that 6.5 ACH air sealing benchmark, which will in turn lower the EPS score by several points.

For the heat loss calculation, I assumed that 83% of the wall assembly will be R-33, 17% will be R-6 due to thermal bridging at the corners, door and window jams. The ADU will have 0.15 air ACH under ambient conditions. Because the ADU does not have ducts and will have a heat recovery ventilator (HRV) to balance pressure and moisture, there will be very little air pressure differential to drive air loss through leaks. The HRV will provide 60 cubic feet per minute of fresh air and capture 65% of the heat energy from the air vented.

Since the ADU is going to have tankless water system to heat potable water due to space constraints, we have overlaid that function with providing hot water for the radiant floor system using a heat exchange. We will be using the Navien condensing tankless combination water heater, which is 92% efficient natural gas water heater.

The heater will be situated inside the thermal envelope to eliminate any heat loss leak potential. There will be three radiant zones: one in the concrete slab on the first floor, one in the bedroom in a vacated air space under wood flooring, and one zone in the upstairs bathroom. 

I had notions of using mostly salvaged construction materials as a mechanism to reduce upstream waste and reduce costs, however this proved to be impractical. While it may have saved some money, it would have taken too much time to find used lumber and other building products. And, I’ve learned that many constructional materials must be new, such as the foam, concrete, nails etc. As a rough guess, pound for pound, this project will be 95% new materials, 5% used. This ratio is disappointing to me, as I would have loved to reuse materials to a greater extent. However, many of the finish materials and fixtures will be salvaged: flooring, railings, sink, tub, doors, and possibly other finish materials such as paints. 

The building mechanics are designed to be relatively simple. The mechanical systems are limited to the water heater, heat recovery ventilator, and the appliances. The new appliances will all be Energy Star certified and several of them will be eligible for substantial rebates from the Oregon Department of Energy. 

Indoor Air Quality

There will not be much ductwork in this ADU except for a couple short duct runs associated with the HRV. The HRV will vent the bathroom, and the gas stove will be direct vented. The HRV will bring in a constant supply of fresh air based on a weekly cycle. Additionally, the open, lofted floor plan will allow me to leverage the stack effect and winds to air condition the ADU through operable windows on East and West sides of the structure. All of the floor and wall surfaces in the house will be hard; there will no carpets to collect dust. We’re using a high-quality weather resistant barrier in additional to cedar shingles, so I don’t anticipate any moldy conditions in the walls.

Quality Control

The ADU will get Energy Star certification through Earth Advantage to help ensure that the builder is using the best available framing, sealing, and insulation practices. The builder must build to Energy Star standards and is contractually obligated to use the ‘Thermal Bypass Checklist’ and relevant ‘Critical Details’ and ‘Tech Tips’ from the Northwest Energy Star website. To help ensure good water protection and air sealing quality, we are using SIGA Majvest weather resistive barrier and Wigluv sheathing tape. We are getting technical drop-in visits from the Energy Trust subcontractor outreach specialist. Earth Advantage will be conducting a blower-door test to check the air pressure and to help identify any sealing gaps.


This ADU project is an attempt to design and build a small, beautiful structure, and develop a zero-cost residential financial model. So far, it has been very successful. The ADU is under construction now and it is looking wonderful. The costs have increased from my initial goal of $100/sq ft to $115/sq ft, but this is still a relatively low cost for the product and building performance that I am expecting to achieve.

My parallel goal for this project is to document and write about the design build process with a strong focus on ADUs, and the nuances associated with their development. This project has sparked a new passion for me, blending my interests of green building, environmental and urban planning, and policy. I hope to use my personal building experience, and green building background, and governmental experience to help other individuals and communities who are seeking to build ADUs, and to help others design better, tighter, and greener.

Here's a slide presentation that covers some of this same information.

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