My ADU’s sexy wall assembly

Heating and cooling is the most energy intensive function of homes in the US. Accordingly, reducing heating and cooling energy demand is critical to reducing a home’s energy footprint. If roofs, walls, and floor insulation were as sexy as solar panels, the market's attention would rightly be focused more toward insulation and air sealing than on purchasing solar panels or building windmills on roofs. 

In the green building marketplace, there is a disproportionate level of interest and a disproportionate level of federal, state, and local funding focused on subsidizing distributed, alternative energy production, when that effort would be better spent on subsidizing insulation and air sealing. In the building sector, 30% of the energy we consume is wasted through leaks and breaks and poor building design.

By putting a little more thought and money into a building's wall assembly, the energy reduction gains will surpass whatever amount of energy one could generate by putting that same amount of money into purchasing solar panels. Eventually, when the cost of producing alternative, distributed energy will make financial sense for homeowners, but for now, the best gains will be made through weatherization. In this project, I am focusing my efforts on energy conservation and efficiency as a pragmatic design goal.

Therefore, this post will deal with my ADU’s sexy wall assembly. For the ADU, my goals are to achieve a high performance building shell at a low cost and adhere to the city’s regulation. 

Standard wall assembly detail

ADU Wall Assembly, explained step by step, from exterior to interior

According to Porltand’s ADU regulations, the ADU “Exterior finish materials, roof pitch, trim, eaves, window orientation and dimension must be the same or visually match the house.”  My existing house has asbestos siding however, which is no longer legal to install, so the city asked us to use a comparable siding such as cedar shingles, instead. 

Incidentally, the original siding under the asbestos is ship lapped siding, and I would love to be able to match that siding style instead of my asbestos siding. Without going through a $1,500, 10 week variance, I am required to match the existing, ugly siding. Ugh. I wish there were an easier or cheaper process to make a simple change like this that would be aesthetically appealing to everyone. Under the siding, there will be furring strips will be fasted to the plywood sheathing to produce a 3/8” air gap between the siding and the that will serve as a rain screen to drain water. 

Beneath the furring strips will be a weather protective wrap called Siga Majvest, which will be stapled to the plywood sheathing.  Like Gortex, the Siga wrap and sheathing tape are vapor permeable to allow the building to expire vapor outwards, but that will blocking driving rain from penetrating inwards. 

The plywood will be fastened to a staggered stud frame. Early in the process, I chose to use staggered studs as a relatively low cost framing method that would give me the flexibility to use a lower cost insulation and still achieve relatively a good R-value wall assembly since staggered studs provides two extra inches for insulation, and every inch helps. 

Staggered studs are basically two sets of 2x4 walls, placed parallel to one another. In this case, the exterior 2x4 will be the load bearing wall. Using two separate walls reduces 'thermal bridging', or heat loss, through the wall assembly. Several experienced builders and framers have discouraged me from using staggered studs, but their criticisms have were not persuasive enough for me to amend the wall assembly design. 

For insulation, I’m leaning heavily towards blown in cellulose.  There are many factors in this decision, but for me, cost and performance are the biggest factors, and blown in cellulose seems to be the best option here. In the end, the walls will be R30 or greater. The roof assembly will be R39 or greater (depending on the insulation we select), the slab on grade floor will be R-15 (3” XPS foam, then a concrete slab). 

Lastly, we’ll hang drywall on the interior 2x4 stud wall. I'm choosing drywall because it is the cheapest and simplest material to use right now for interior residential walls. 

SketchUp shot of the south-facing wall of the ADU- the majority of the window glazing faces south.

While the south facing wall has lot of glazed surfaces to maximize solar gain and natural lighting potential, the north facing wall will not have much glazing.

I felt encouraged about this wall assembly design after reading a Journal of Light Construction article about superinsulating new construction homes on a tight budget in Massachusetts. They have been using a similar wall assembly in their production, and apparently, it’s worked well enough that it’s JLC feature article for January.

Common Forms of Living Smaller

Much residential "green building" attention has been placed on material selection, alternative power sources, and mechanical efficiencies. These choices all play a role in building "green", but arguably, none of these charismatic green features are as important as the energy efficiency gains achieved through building and living in smaller spaces. 

Smaller houses can take many forms, and the three common forms that I'll mention in this post, all achieve notable reductions in terms of their environmental lifecycle impact reductions: Multi-family, ADU, and "Tiny Houses" 

Let's begin with the recent EPA-sponsored report about the energy impacts of residential housing choices. As stated on the introductory page, 

"Buildings and transportation together account for about 70 percent of energy use in the United States and are responsible for about 62 percent of U.S. greenhouse gas emissions"

The findings in this report illustrate that certifiably green buildings are not as significant in reducing that energy as where you build (Conventional Suburban Development or Transit Oriented Development) and the housing type (Single Family Detached, Single Family Attached, and Multi-Family). The report findings are summarized well in this rich graphic.

For me, I’m beholden to my innate desire to design and build something on my own terms. There are strong correlations between the size of average Single Family Detached, Single Family Attached, and Multi-Family homes. However, the impact of housing size perhaps has a more direct correlation to energy use than those building type descriptions.


From 1950, to 2008, the average sq ft per person has increased from 259 ft to 961 ft, a 372% increase. 

ADU’s are an accessible way for urban homeowners to make a big stand on issues that span community building, environmentalism, urbanism, and climate change. Done well, ADU’s present a compelling personal financial opportunity for individuals to create a regenerative, personal financial portfolio. I plan to discuss the economics of my ADU and ADUs at large in future posts. 



I have extracted some graphics from a presentation about the findings from the Oregon DEQ report, "Life Cycle Approach to Prioritizing Methods of Preventing Waste from the Residential Construction Sector in the State of Oregon". I anticipate this this groundbreaking report will prove to be pivotal in the green building movement and will help to instigate a shift toward an emphasis on living smaller. 

I believe ADU’s will naturally become more and more popular where there is sufficient housing demand and where they are allowed by zoning laws. With a few exceptions in the US, ADU’s are not legal.

The first ten minutes of this video is of Jay Shafer of Tumbleweed Tiny Houses addressing code barriers to small housing. Jay Shafer is the person who has popularized the notion of building tiny houses on wheels- which, as he explains, is simply a tactical exploitation of a common municipal zoning loophole to legitimize what would otherwise be illegally constructed tiny houses.

Views of the ADU

My architect has put together a great SketchUp model. I've put the model into the site baseline conditions model. Here are some view of the models.

Inspired by the Doers

It feels quite remarkable and rare to have such easy and immediate access to innovative, creative, industrial minds all around me.

I met today with one of the authors of this Oregon DEQ report that found that the single most important variable to reduce the life cycle impact of residential construction is the size of the house. We discussed the smaller housing, ADU's, regulatory hurdles, costs, and material selection.

It was a great meeting of the minds, and I hope to continue to think through the overall strategy of this ADU with him further. He constructed a gorgeous, timber frame detached bedroom in SE Portland.

My girlfriend has lived in two cabins that she has renovated. Her cabin in Portland is a refurbished garage; while not a legal ADU, it is an ADU nonetheless. And, I might add, it the most gorgeous ADU I've seen to date. From the outside though, you would think it was just a plain garage. She renovated it on a budget of $20K. It seems me that a good rule of thumb may be that complying with building codes doubles the cost of design/build.

Two of her and my good friends in NE Portland are in the early stages of converting their garage to an ADU, though they are still trying to assess whether their $40K budget is sufficient for the job.

My mortgage broker has an ADU that she is renting out. She built it for $50K. My Realtor intends to build one over her garage. My "Phase I" builder is hoping to build one at him home in NE Portland down the road.

My architect took on the challenge of packing a lot of function into a small space with vigor. He was extremely knowledgeable and generous with his time. I have no doubt that he put more time into this design than he would on most projects, and for a relatively modest sum. I'd recommend him to anyone.

I am taking a class with Stephen Aiguier, the founder and president of Green Hammer. He is the most knowledgeable person I've met on green building issues. Incidentally, he seems to be increasingly focused on Passivehaus construction. What I respect most about Stephen is his deep understanding of building science, and his pragmatic and driven idealism. Learning from him is humbling.

My immediate neighbor is a craftsman and fabricator who specializes in reusing material, next door to me is the owner of a custom bicycle construction shop.

The ADU concept is ablaze in my circles in Portland. That is due in part to incredible range of needs and purposes that ADUs serve, but it is also a testament to the fact that here, in Portland, they are legal to build.

And that is rare. I'll discuss this much in more detail later.

The Permit

Two weeks ago, my architect submitted the drawing to the City of Portland for review. Two weeks later, we got our plan review meeting. I asked my architect and builder to both be there. I told them I would keep my mouth shut for fear that I may something stupid. The architect was to take the lead, since they were his drawings and he knew them better than anyone. My nerves caused me to shake.

This meeting was three months and $3,500 in the making. I probably shouldn't have been so nervous. If there were problems, we only had to address them in a permit revision to be approved. Nonetheless, I felt nervous.

The lead planning reviewer had first reviewed the permit that morning. Having had them for two weeks, I  had thought that everyone would have already reviewed them. But, the plans couldn't be reviewed by the structural engineer until the planner signed off on them.

At 1pm, we sat down in with the reviewers, who ushered us in; no smiles. The planner reviewer looks over the brims of her glasses, and said that the ADU was 860 sq ft. It needed to be downsized to 800 sq ft to pass her planning review.

In describing their math, she mentioned quickly that the stair space was counted twice, once for each floor. I had to open my mouth to ask whether I understood her correctly. She confirmed what she had stated. To this, I explained that we had been told differently. My architect then said that he had an email from a planner stating that stairs were not counted twice. He seemed to brush off the critique.

Though she found this apparently glaring error, she said that she saw no other problems with it. This was a relief, because I felt pretty confident that we would be able to prove that the structure was very deliberately designed to be under 800 sq ft according to the stated rules.

Next, if/when the plans past planning, they still needed to be reviewed by the structural engineer. Once they're through structural, we'll get the permit.

So, it had been two weeks already, but even if we had passed planning immediately right then and there, they would still need about ten more working days to complete the permit process. We hadn't gotten a straight answer about that one, so this was news to us. I had assumed that we could potentially walk out of there with a permit in hand, but this wasn't the case.

That evening, last night, my architect sent out a brilliant, humble and short email with a dimensioned drawing to show the sq foot calculations. He attached the email that he had received from the city stating that the stairs would not be counted twice.

This morning, the planner replied to my architect in a e-mail, "I concur with your measurements", she wrote , "So I can sign off on the permit."

It was music to my eyes.

Structural, Bring it. :)

Still designing

The design process has been incredibly enlightening. And time consuming. I plan to write about it extensively, but needless to say, I've been humbled by the amount of aspects of a well built home that I didn't understand.

Meanwhile, I am taking an intensive 96 hour class called the Sustainable Homes Professional course, which has added an entire new perspective to my process.

At this stage, the architect and I have had at least 10 hours of meetings and phone conversations, 15 schematic drawings, 200-300 email exchanges, and countless hours of playing around with the floor plans and models on our time.

I'm super psyched about what we've come up with for the floor plan. The plans are currently being worked on by the structural engineer.

In five days, I'll meet with the architect, and the builder that I've selected, and we will pass the design build torch.

Next week, I'll be submitting the drawings for a permit through the City of Portland.

Design #12

The design process is taking much longer than I had anticipated. I thought that with my SketchUp model complete, it would just be a matter of translating that model's dimensions into CAD.

It's a good thing that I have a good architect. It's been frustrating for me, but he has altered and refined the floor plan again and again. We're now on the twelfth iteration of the floor plan, and he considered this to be the preliminary set of designs.

Shortly, he'll put these into SketchUp, where I'll give him more feedback, and then we'll hand over the plans to an engineer, who will do the post and beam gravity sizing. Then, once the engineer has signed off, my architect will create drawings for the city that we can get stamped and approved.

Hopefully.