Metal roof and wall assembly in Los Angeles
Could you advise on my roof and wall assembly? I want to do better than code and local building conventions (leaky stucco houses) , but don’t want to: (1) waste money since the climate is mild; (2) make inadvertent mistakes by diverting from what’s tried and true here, and by indiscriminately applying lessons that are meant for more severe climate.
What’s your opinion of the most cost effective spec for this climate?
The project: 1600 sqft addition (2 stories), Zone 3B, 5 miles from the ocean, California Building zone 6, HDD 1440 and CDD 730, winter 99% > 41 F, and summer 99% < 83F, average annual rainfall 14 inches. Yes, basically, sunny Californian paradise, mild, dry, ocean breezes all day long. Should I let my contractors do what they do by default? I.e. no serious air sealing beyond caulking around penetrations and windows/doors, no rain screen, no ventilation channel under the metal roof, no exterior foam, just fiber glass batt inside. My framing and OSB sheathing are already built. The roof underlayment is in, before I learned that our unvented cathedral roof needs foam insulation either under or above the roof deck. (The architect and the insulation sub both overlooked that. Maybe unvented cathedral roof with only well-packed fiberglass batts is ok in LA? This was specified in our city-approved plan. So the city seems to have overlooked that too.) Or, what’re your thoughts on the following assembly instead? Wall assembly:
(1) 2×6, 16″ on center
insulated with damp sprayed cellulose to R 19
(2) OSB 5/8
taped with Siga Wigluv 60 on the outside, and fentrim 12″ between concrete slab foundation and OSB
(3) Tyvex stuccowrap (or pay more for commercial D?)
(4) Benjamin Obdyke Slicker HP rainscreen with housewrap 6mm (1/4″ inch)
– only on the south west wall. At $1/sqft, I thought it too costly to do the whole house.
I figured that my south west wall is vulnerable, due to the prevailing wind from the ocean in the west, double wall height (up to 18 ft), minimal overhang (ranging from 6″ to 36″), and low roof pitch(half flat 1:12 and half sloped 4:12). My other walls are shielded by a row of 2-story-high tree hedges that are 4 feet away from the wall.
(5) and then hand over to the stucco contractor to do his regular thing: double building D paper, lath, 3-layer stucco with fiberglass mesh coat to prevent cracking, Santa Barbara smooth finish
And how about this alternative that I came up with?
– 5/8″ OSB (already in place, too late to tape with Wigluv)
– Titanium UL50 underlayment (already in place; I learned that I need foam for the unvented cathedral roof after the roofer already put this in)
– 1″ polyiso, taped for air sealing (I want to avoid sprayed closed cell inside. Is it ok that exterior air barrier is here? Or should I tape the underlayment?)
– another layer of underlayment on top of the polyiso (Roofer/manufacturer requirement)
– [if a good idea, either cedar breather 3D mesh or 1×4 to create a little ventilation channel]
– standing seam metal 26″ gauge, 12″ wide panels
(1) Can I get away without the cedar breather or any other kinds of ventilation gap under the metal roof in our climate?
(2) Can I get away without the rain screen even for that vulnerable wall? Is just adding the tevex stucco wrap good enough? (Most constructions here don’t even use and just have double D paper) My existing house was built in 1940s, and it has only felt paper under the stucco. The wood looks ok except a few termite-damaged studs. But the old house was really leaky with zero insulation. With the modern insulation and the wigluv air sealing, should I upgrade the moisture control measures too, even in this climate?
(3) Or I should just listen to the pros, and go with the minimal spec by default? What’s your opinion of a good minimum assembly or this climate?
Thank you so much!
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