Sunset Hill ADU — Zip System, R-5 Rockwool, Lunawood
The best kind of exterior
This Sunset Hill ADU is the kind of wall we’d build on every project if budget allowed. Every layer was chosen for a specific job, and every layer works with the next one rather than against it. The result is a high-performance, high-durability exterior that will outlast the building it’s wrapping.
What we specified, layer by layer
This is our best-in-class Seattle siding contractor build — a modern siding installation in Sunset Hill on a new ADU. If you’re looking for a wood siding installer in Seattle who can specify a full high-performance wall, not just the cladding, this page walks through exactly how we built it.
1. Huber Zip System sheathing
Zip System replaces conventional OSB plus a separate house wrap. It’s structural sheathing with the WRB bonded to the face, and taped seams make it a continuous air barrier in one install. Fewer layers to get right, and the WRB is mechanically durable rather than a fragile fabric. On an ADU where every detail is visible and the crew is small, Zip is simpler and more forgiving.
2. R-5 Rockwool Comfortboard exterior insulation
Rockwool mineral wool does three things that rigid foam can’t. It’s fire-resistant (doesn’t contribute fuel). It’s hydrophobic (doesn’t hold water). And critically for the PNW, it’s highly vapor-open — around 30 perms, compared to 1–2 perms for XPS or polyiso. That matters because any moisture that ends up in the wall can dry to the outside through the insulation rather than being trapped behind a vapor-impermeable foam layer.
Continuous insulation outside the stud line is what actually breaks the thermal bridge. A wall with only stud-bay insulation loses 20–30% more heat through the framing than the nominal R-value would suggest. Adding R-5 of continuous exterior insulation jumps the effective assembly R-value dramatically — and the homeowners feel that every winter.
3. Two-layer cross-batten rainscreen
The cladding is vertical Lunawood, which means the cavity behind it needs to run vertical too — otherwise air can’t move from soffit to eave to dry the boards. But the battens that support vertical boards have to run horizontal to create fastening. Solution: install both. Horizontal battens first, fastened through the Rockwool into the studs with long screws. Vertical battens over the horizontal ones, creating a continuous vertical airflow channel behind the cladding.
This cross-hatched cavity is what gives the wall its drying capacity. Water that makes it past the Lunawood has somewhere to drain to. Air moves continuously behind the boards. The cladding dries from the back 24/7, even during Seattle’s six-week winter rain stretches.
4. Inset windows with custom jamb extensions and sills
The homeowner wanted an inset window look — windows recessed from the cladding face rather than sitting flush. That’s a clean, modern detail that also improves weather protection because the cladding shadows the window. With a 2.5+ inch thick insulation-and-rainscreen build-up outside the sheathing, the window can’t install from the outside the traditional way. Instead, the window mounts on the Zip System plane, and we fabricate custom jamb extensions plus a custom sill to bridge out to the Lunawood face.
The sills are the critical part. They have to drain water out of the opening and away from the wall, with a back dam, side flashing continuity, and a drip edge that sheds clear of the cladding. We mock-up the sill on every window before setting it, and we integrate the flashing tape back to the Zip seal so the water path is continuous.
5. Lunawood thermally-modified cladding
Lunawood is Scandinavian pine heat-treated at roughly 215°C in steam. The process drives out resins and changes the cell structure so the wood absorbs far less water, moves less with humidity cycles, and resists rot without chemical preservatives. In practical terms, it behaves more like a tropical hardwood than softwood cedar in the PNW climate, and it weathers to an even silver-gray if left unfinished.
Fastened with stainless only. Anything less will leach and stain the wood within a few Seattle winters.
Why this wall will last
The reason this exterior is so durable is that every layer is vapor-open, and every layer has a drying path. Zip System is around 12–13 perms. Rockwool is 30+ perms. The rainscreen cavity is effectively infinite perms. Lunawood is a traditional wood cladding, also vapor-open. Any moisture that ends up inside the wall has somewhere to go — out through the cladding, out through the cavity, out through the insulation, out through the sheathing, or back in to the conditioned space.
Compare that to the typical Seattle wall: OSB sheathing + Tyvek + composite siding installed tight to the wrap. That wall has no drainage plane, no ventilation, and very limited drying capacity. Water that gets in stays in, and damage accumulates until it shows up on the inside or until a siding fails outright.
This ADU will outlast any siding we would have spec’d on a tighter budget. And it will heat and cool for less. That’s the “best kind of exterior” trade.
Frequently asked questions
What does “vapor-open” mean for a wall assembly?
A vapor-open wall allows moisture vapor to pass through each layer at a reasonable rate (typically 10+ perms per layer) so moisture can dry to the outside rather than accumulating. Rockwool is about 30 perms, Zip System is about 12–13 perms, and the rainscreen cavity is effectively infinite. This assembly can dry in either direction — critical in the PNW where weeks of continuous rain prevent outward drying through the cladding alone.
Why Rockwool mineral wool instead of rigid foam?
Three reasons specific to the PNW climate: Rockwool is vapor-open (~30 perms vs 1–2 perms for XPS or polyiso), so it doesn’t trap moisture behind the insulation layer. It’s hydrophobic — it does not hold water, even if the rainscreen cavity is saturated. And it’s fire-resistant. Foams perform well thermally but can trap moisture at the sheathing interface in wet climates.
How does Zip System compare to OSB + house wrap?
Zip System is OSB sheathing with the WRB bonded to the face. Taped seams make it a continuous air barrier and water-resistive barrier in one install, rather than two separate layers. It’s mechanically durable (not a fabric that tears), easier to inspect, and simpler to integrate with window flashing tape. Costs modestly more than OSB + Tyvek but eliminates a significant source of field errors.
What are inset windows and why do they need jamb extensions?
Inset windows are recessed from the cladding face rather than flush with it — a modern European detail that also helps shed water because the cladding shadows the window. With 2.5”+ of insulation and rainscreen outside the sheathing, the window mounts on the inner sheathing plane, so custom jamb extensions bridge the depth between window frame and cladding. Custom sills drain water out of the opening and over the cladding.
References & manufacturer resources
The finished ADU
Sunset Hill’s newest high-performance exterior. Zip System, R-5 Rockwool, two-layer cross-batten rainscreen, Lunawood, and inset windows with custom jamb extensions and sills — all working together.
Building a high-performance exterior?
Whether it’s an ADU, a new build, or a reside done right, we’ll spec an assembly that performs and tell you honestly what each layer costs and what it buys you.
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