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 WalkScore.com, 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. 

Materials

 

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.

Summary

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.

Building an Accessory Dwelling Unit In Portland from Kol Peterson

Oregon Clean Energy Works (Part II)

This week, I went through the Oregon Clean Energy Works (CEWO) program. I've wrote about the Clean Energy Works Oregon program in an earlier post, entitled Only the Worst Need Apply.

In it, I described how the program works. In a nutshell, it's a program that targets older homes in Oregon that are drafty or not well insulated and offers financial perks for homeowners to get their homes energy audited and weatherized. 

I am VERY impressed by the way that this program is funded and run. But, I wanted to reserve judgment on the program overall until I had been through it. Now that I have been through it, I want to give a strong endorsement to the program.

CEWO doles rebates for weatherization work based on how much modeled energy improvement a house undergoes in their program. Since the house had no insulation and leaked like a sieve, making major energy improvements to the house were relatively simple to achieve. The improvements were relatively cheap and resulted in substantial energy reductions. CEWO bases rebates on the level of weatherization improvements, and because my house was so leaky, I'll receive the full CEWO rebates (In the last month, CEWO has lowered the rebate amounts, so the rebates aren't as enticing as they had been.)

My monthly loan repayment for the weatherization work will be $30/month. In my case, since the energy bills are approximately $100/month, my anticipated 30% reduction will amount to $30/month. The loan repayment will be reduced by the same amount that is saved by the energy bills each month. So, essentially, for no upfront cost, my home was weatherized. After the duration of the loan repayment, the loan will be repaid, and the house tenants will benefit from the decreased energy bills forever more. The other advantage to weatherization is comfort; the house already feels less drafty.

For the energy geeks out there, the blower door test after CEWO brought the home's air sealing from 7,731 to 4,476 cubic feet per minute at 50 pascals of air pressure, a reduction in leaks of 3,288 cubic feet per minute, or from 21 ACH to 12.2 ACH at 50 pascals. This is a huge improvement in terms of air sealing. The R-value in the wall assembly went from R-2 to R-11, the roof went from R-15 to R-49, and the basement perimeter rim joist went from R-1 to R-15.

Here's a video of the blower door test:

Only the Worst Need Apply

Last week, I had a home energy audit conducted as part of the Clean Energy Works Oregon program. Although this energy program is not directly pertinent to ADUs, I was so impressed by the audit and the program overall, that I feel compelled to write about it.

Several years ago, in Washington DC., I managed to get a free home energy audit through a local grant-based, energy efficiency program. The DC home energy audit was not helpful at all; their findings did not result in me making any changes to my home, due to the program's inability to intelligibly convey the home energy loss findings to me, or to offer me any compelling financial incentives to take action. In contrast, this Oregon-based program is run brilliantly and it truly deserves a national spotlight.

I'll caveat this post by saying that I have not yet gone through the whole program, but my first 'touch' with the program has been fabulous.  If nothing else comes from the program other than the free audit that they conducted (which they value at $500) and the ten free CFL's, it was still totally worthwhile.

The Life Cycle of Clean Energy Works program funding 

You should visit their site for an authoritative overview of their brilliant program. But, here's my layman take on how it works:

The major power/gas providers tack on 3% to their customers' home energy bills; that 3% levy is pooled together and administered by the Energy Trust of Oregon, whose mission is to reduce the region's energy demand. This demand-side reduction ultimately saves the major energy providers (NW Natural, Pacific Power etc) from the costs of having to build another expensive power plant to meet the region's ever growing energy demands.

The Energy Trust doles out this money via incentives and programs to homeowners whose homes need energy improvements. In the case of the Clean Energy Works Oregon program (CEWO), one of Energy Trust's partners; you have to apply to get into the program, and only the big, old, clunky, under-performing homes are admitted into their program (thus, the title of this blog post).

Energy Trust sets the homeowner up with a local, small-business, weatherization contractor, who audits and assesses the home, and provides a bid to do the work to improve the energy performance of the home. The contractor gives a scope of work and a bid to the homeowner. The homeowner reviews the scope of work that they wish to complete with an "energy advocate" from CEWO. Then, they select a scope of work, presumably based on the bid and the likely payback period (let's say this scope amounts to $10K)

The contractor initially fronts the capital to do this work, and when CEWO says that the home improvements are complete, the contractors are paid ($10K) by a partner bank for the work. The bank then turns this capitol expense ($10K) into a 5.9% interest, 20-year loan, that the homeowner then pays back through a extra add-on expense in their monthly energy bill (~$50/month). The monthly loan payments are incorporated into the homeowner's energy bill, which makes the the loan repayment administratively simple for the homeowner (since I pay through autopay, for example, I'll never have to lift a finger to pay off the CEWO loan).

The weatherization work that has been performed on the home by the contractor will result in a lower montly energy bill. Ideally, if the contractor performs inexpensive weatherization improvements, such as air sealing and insulation, the monthly energy bill will be reduced by a dollar amount similar to the loan repayment amount. Ideally, this would mean that the homeowner would pay no more on a monthly basis than they would have paid on their monthly energy bills in the first place.

So, instead of paying $50 extra in my gas bills each month because home is poorly insulated, I'll instead pay $50 a month towards the cost of those weatherization improvements over the next twenty years through a monthly loan. (Presumably, as energy costs increase, this 20-year $50 loan repayment rate will be proportionally less expensive than the cost of inflating gas and/or electrical prices. In ten years, my wasted energy may have gone from $50/month to $100/month, for example) 

Furthermore, if a homeowner achieves 15% energy gains after going their program, CEWO gives the homeowner a $1,500 cash incentive. If you achieve a 20% reduction, you get $2,200. And if you achieve 30% in reduced energy consumption, you get $3,200K. Note that these incentives are not based on how much the homeowner spends on weatherization, but rather on how effective the improvements are at reducing home energy use--a crafty performance-based criteria.

The Audit

The audit that was done on my home lasted four hours; the inspections were extremely thorough. The weatherization contractor spent an hour on each floor, including in the basement and the attic. He conducted a blower door test, which in itself, is a service that would normally cost over $100 to administer. He found that the building envelope was 21 ACH @50 Pascals (they could only pressure it to 35 pascals before the blower maxed out). They estimated that the home gets one air change per hour under normal pressurization. For you non-energy geeks, that means that my house leaks like a sieve.

My leaky-sieve-of-a-house is THE target house for the Clean Energy Works program. For a house as leaky as mine on a gas combustion furnace, there's lots of low cost fixes that could reduce my energy load and monthly bill considerably. Their audit, model, and my gas bills, indicate to them that I have the potential to reduce my home's energy use by 33% by going through their program.

My Review of the Program

In my mind, from a policy perspective, the best benefit of this program is job growth opportunities for small businesses who do weatherization/energy conservation. The contractor who did my home audit owns his business with his wife and they employ one or two others for some jobs. Many of the contractors in the program are small businesses like his.

The Clean Energy Works of Oregon program is exactly what the doctor ordered. The US desperately needs job growth, it needs to stop wasting 30% of its buildings' energy use through careless building design, and it needs to find more creative ways to fund distributed micro energy-conservation projects like this. Citizens are eager to have an opportunity to be part of a solution to address a problem that is bigger than any one of us alone.

I have heard two valid criticisms of the the Clean Energy Works program, and there may be others:

• This 3% energy levy is a regressive tax that impacts poor homeowners/renter more than wealthy homeowners/renters. In other words, just like other citizens in the region, the region's poorest residents are subsidizing Clean Energy Works program participants. However, unlike the median and upper income residents, they are likely not the beneficiaries of this levy.
• Weatherization contractors may or may not have the capital to front the cost of expensive repairs for a homeowner in the program. This capital expense (up to $30K sometimes) is too large for some small businesses to afford, pricing them out of the program.

All in all, this is one of the most brilliant and holistic energy programs in the country and it is a pleasure to be a participant in it. My leaky sieve wants to express its gratitude as well.