I installed a bunch of blocking on the interior for bathroom and stairway grab rails, the shower partition glass, kitchen cabinets, the wall-hung microwave, and the heat pump head unit. Using 2×6 gives plenty of surface area to later play around with layouts.
I photographed and wrote-down the heights and locations of all the blocking so later I can still locate the blocking when the drywall is up.
I also installed blocking in one corner of the living room where I plan to install a wall hung table and chairs.
Because the blocking has to be quite precise I selected the following table/chairs and did the whole layout before adding the blocking. It’s an Irish company but they ship to the US. If I manage to find a local company then I’ll be constrained to my blocking.
I used Rockwool for both exterior and interior insulation. It’s slightly more expensive than XPS however not a big deal considering most of the cost is labor.
I was wondering how difficult it would be to work with, whether it would compress too much under the strapping, however overall I found it easy to cut and install.
Stud markup
I carefully marked out the location of all the studs on the exterior using a marker pen. This is important to get right as the strapping has to be fastened by long screws through up to 2-inches of rockwool and the ply sheathing to hit the center of the stud.
Exterior Rockwool panels
Rockwool cuts easily with any serrated blade.
I used 2-inch rock wool on two sides. I was limited to 1-1/2-inch rockwool on the two sides within the lot setbacks.
For the back of the ADU I ran out of rockwool and had to switch to XPS for half the wall as there was no 1-1/2-inch rockwool available locally. (2-inch seems to be a standard thickness.) I transferred the stud markings onto the rockwool as I installed each panel.
I used Round Cap roofing nails to hold the panels in place until the strapping was installed. It’s only necessary to use 4-6 of these per panel as the strapping is what will really hold them. You need to think about the length of the nails and whether you’ll be embedding them into a stud if you want to avoid penetrating the sheathing and creating a potential path for water and air.
Strapping
I used 1×3 strapping for the interior field, 1×4 strapping for the corners. Cedar happened to come in the length I wanted however this made the strapping actually quite expensive. It was overkill and I suspect some kind of spruce or fir would have made more sense.
Having carefully marked up the stud locations it was easy to drive in the long screws and hit the stud. Once again you need to think about the length of the screws to stop short of hitting any electrical wiring.
I used Heco Topix Therm Insulation Screws and they’re available from Small Planet Supplies.
These screws allow you to smoothly move the strapping in and out into alignment and create a flat wall plane for the siding. They cost quite a bit however they’ll save you a bunch of time and frustration trying to get the wall aligned.
It’s necessary to add small pieces of strapping next to penetrations like electrical outlets and vents so that the siding will be properly supported. It’s also needed on the gable ends where each course of siding will need a nailing base. This took almost as long as installing the main panel strapping.
Rainscreen
I nailed in Cor-A-Vent 3/4-inch strips at the bottom and top of each course of strapping. This will allow airflow from the bottom up and out the top at the gables and eaves and help keep the inside of the siding dry.
I nailed blocking at the top of the wall on the eaves and gables and stopped the strapping and Cor-A-Vent a 1/4-inch short of that to allow a gap for airflow.
Hammer & Hands best practices manual contains a bunch of detail on how to do this.
Before installing the rockwool panels I had tacked on insect guard around the perimeter of the base of the wall. After installing the rockwall, strapping and Cor-A-Vent, I then wrapping the insect guard up and tacked it to the strapping. This will prevent insects (ants) from getting up into the insulation which is where they would love to be.
The Completed Insulation and Rainscreen
Here’s the ADU with the exterior insulation, strapping, and rainscreen completed. You can see the gable-end strapping here that will support the siding courses. You can also see the Cor-A-Vent installed below and above the windows.
I used Rockwool ComfortBatt for the interior insulation.
Before installing the insulation I photographed the wiring to keep a record of this before it was covered over.
I also installed insulation in the interior walls for the bathroom. It’s a small space so anything the helps reduce noise coming from the bathroom, plumbing noise, is good.
During a previous inspection – I think it was for framing – the City had called me out on the anchor bolts.
Portland code requires the anchor bolts be installed with square washers to prevent pull-thorough under load. The builder had installed with no washers. I thought they needed the washers but the builder said it passes without.
Sometimes I have a feeling that the City comes down harder on DIY homeowners than the builders, although I’ve heard the opposite as well.
Anyway, I backed out the bolts – an good drill helps – also used a wrench where headroom was restricted. The washers are kind of spendy at around $5/each.
I was concerned the replacement bolts would not go in quite as tight so I epoxied any that were going in too easily.
A miscalculation here. I planned my window and door bucks and the metal cap flashing assuming the trim would be 3/4-inch thick. Wrong! Trim comes in 5/4-inch standard. If I’d left the trim at 5/4-inch then it would have protruded too far out forcing me to extend the metal flashing.
So I had to rip the trim down to 3/4-inch on a table saw.
I assembled the trim on a workshop table, glued and tacked them together using butterfly clips Then installed the trim in one piece like a picture frame. It’s easier to get the miters good and true this way rather than putting it together piece by piece on a vertical surface.
Gable End Detail
As it’s a historic structure I needed to add a gable detail back in.
I used “Alternative Drywall” and they did an excellent job.
I specified 5/8-inch Type X board as one wall was required to have a 1-hour fire rating. To keep things simple the drywaller used this material throughout. The contractor used Certainteed.
The wall behind the wall-hung toilet cannot be drywall; instead it needs to be cement board. The contractor used National Gypsum PermaBase 5/8-inch for this.
I was unable to source 5/8 Permabase a month later when I went to start on the shower construction. The only thickness available to a consumer like myself was 1/2-inch. So I used a combination of DuRock 5/8-inch and PermaBase 1/2-inch for the rest of the bathroom.
Permabase 1/2-inch is available from Florida Tile in NW Portland. They won’t sell direct to a home owner so I purchased through Campbell’s Tile Concepts. Durock 5/8-inch is available from GTS Interior Supply also in NW Portland and they will sell direct.
Drywall Installation
They completed the job by spray-painting with a primer coat.
The corner trim pieces (1-inch dimensional cedar) went on first nailing into the corner strapping.
Beveled Cedar Siding
I used primed beveled cedar siding. This was a major expense coming in around $2,600 for the wood. (Sourced from Parr Lumber)
I used the Gecko Gauge SA90376 Siding Installation Tool. This facilitates one-person siding installation and also helps ensure the boards go in level.
I left a 1/8-inch gap between the siding and the corner trim to allow for expansion and contraction. This gap was later filled with latex sealant.
I put metal flashing behind each butted joint.
I used Simpson S4SND5 4d Cedar Siding Nails 1-1/2″. These stainless steel nails are more costly than regular hot-dipped or galvanized but are relatively immune to corrosion from the cedar wood resins. I drilled a hole and hand drove the nails to reduce the risk of splitting the boards.
All cut surfaces were primed before installation. In this case I’m fitting the siding around a security camera mounting point.
Trimming out the Rainscreen
Above each window and above the door frame there is a ~1/4-inch gap to allow air to move into the rainscreen cavity.
I used a table saw to rip trim piece that go at the top of the wall at the gables and eaves. The 1/4-inch gap allows air to rise up inside the rainscreen cavity and escape out the top. I used insect screen to prevent bug intrusion through this gap.
Irvington Cottage 