Wall System for Climate Zone 6B: Off-grid, wildfire and earthquake prone with dramatic temperature fluctuations
My wife and I are in the planning/design stages for a passive solar, solar-powered single family home of approximately 1,600 sq. ft., at an elevation of 6,500′ within a national forest in southwest Montana (Climate Zone 6B). The site is relatively remote and off-grid–nearest electrical hook-up is 8 miles distant. The area lies in a seismic zone and owing to dry summertime conditions, experiences periodic wildfire activity, the risk of which is made worse by the expansive area(s) covered by standing and fallen beetle-killed lodgepole pine trees. Local temperatures can range between 100+ degrees in the summer to -40 degrees in the winter, with annual heating days of approximately 8,500.
Because of the wide temperature variations, risk of fire, remoteness of the site, and stand-alone nature of the structure, we’re attempting to build in a manner that will minimize wintertime heating requirements (primarily), but also air leakage and thermal bridging, aiming for a whole wall R-value of 40-45, and sheathed in fireproof/resistant material (e.g. cementious board and batten siding).
What we are contemplating is something of a hybrid Klingenberg wall system featuring a 2″x4″ balloon-framed inner load-bearing structure clad in drywall, an Intello-Plus air/vapor barrier, and sheathed–for air barrier purposes–Zip system panels with taped seams. Secured to this would be Larsen trusses which in turn, be sheathed with Densglas (or comparable) panels (also taped), over which would be a wood lattice (drainage plane), and finally, cementious siding. The type of insulation we’re considering is either blown cellulose or rock wool battens.
My question, then, is two-fold: (1) Is this overkill in terms of complexity, price, and safety; and (2) will such a system (or another) be effective in preventing/retarding in-wall condensation while providing needed fire-proofing/resistance?
Many thanks in advance for your response(s).
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
What about something simpler:
Inside to out:
- Interior gypsum sheathing
- 2x6 @ 24" O.C. wall with dense packed cellulose or fiberglass
- Zip sheathing
- 4" Mineral wool
- 1x4 Furring
- Fiber-cement siding
That's roughly R-36 center cavity, and your house has a thick jacket of non-combustible mineral wool. You could up it to 6" of mineral wool to hit R-44 if that was critical.
Here's an example of this system: https://hammerandhand.com/field-notes/madrona-passive-house-wall-assembly-illustrated-fourth-of-four-analyses-of-high-performance-walls/
The ZIP structural sheathing on the exterior of the interior-side 2x4 wall is a sufficient vapor retarder to allow skipping the Intello Plus, given that most of the insulation is outside of the ZIP. For zone 6 it only takes about 1/3 of the R to be exterior to the sheathing layer for dew point control, and you'll have something like twice that. See:
https://up.codes/viewer/utah/irc-2015/chapter/7/wall-covering#R702.7.1
The high vapor permeance of DensGlas isn't necessary on the exterior skin under the lattice-vented siding either- plywood or OSB would be fine drying toward the exterior, given that it will be colder/wetter and thus more vapor permeable than the ZIP which is serving as the interior side vapor retarder. (That is, unless the DensGlas was selected for better fire resistance?)
Heat rejecting windows would be more resistant to shattering from the radiated heat of a forest fire, but would cut severely into wintertime gains. Exterior steel shutters might be worth considering as a fire-mitigation feature. Metal roofing and unvented insulted roofs have also proven more fire-resistant than vented roofs. In zone 6 at least half the R of an unvented roof has to be on the exterior for dew point control (a higher fraction than walls). Using polyisocyanurate would be preferable to polystyrene for the exterior of the roof in a fire zone, since it chars in place even when fully burning, rather than than melting, sending dripping burning polymer down the walls. Better still would be a layer of rock wool (which is completely fireproof), but that can get complicated in high-R roofs due to it's higher compressibility.
Jack,
You've been given good advice by Brendan and Dana. If you follow the advice on GBA, you won't have any condensation problems in your walls.
For fire safety, the most important factors are exterior cladding materials, and avoiding a vented roof assembly. (You don't want soffit vents.)
First-time designers of off-grid homes are prone to make several avoidable errors, so I advise you to read this article: How to Design an Off-Grid House.