Monday, April 4, 2011

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.
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