REMOTE modified for high wildfire risk
I plan to build a detached garage (heated, single story, simple gable roof) on wooded property near Missoula MT (IECC zone 6B). My strategy is to modify the REMOTE design to make it more resistant to wildfire. I will probably do almost all of the work myself – its going to be an expensive project so I might as well have as much fun with it as I can, right?
For the walls the plan is (inside to outside) 5/8″ DensGlass, 2×6 framing stuffed with fiberglass batts, plywood sheathing sealed along seams for air control layer, 5/8″ DensGlass, 3.5″ Rockwool insulation, housewrap, steel studs layed flat for 1.25″ drain cavity, and metal siding attached to the studs. The steel studs are attached at the top to the roof trusses (so 24″ OC) and at the bottom to a channel running along the bottom of the sheathing/DensGlass. It would be preferred to find a practical way to bend the metal studs into hat channels and reduce the drainage cavity to 5/8″ or so. I plan to use removable sections of XPS to insulate the slab from grade up to the Rockwool but only in the cold months so that I can inspect for termites and also avoid exposure to fire season. I was going to seal the drain cavity at the soffit to improve fire resistance. How does that sound? Any suggestions for material to screen the bottom of the drain cavity? My best idea so far is perforated (0.1″) aluminum sheet available from the orange big box store in 3′ square sheets.
The soffit is planned to be about 24″ from framing to fascia, so 17″ or 18″ from cladding to fascia. The outer layers of soffit and fascia will be 5/8″ DensGlass and metal soffit or fascia as appropriate.
The ceiling is planned to have a vapor barrier above 5/8″ DensGlass, and blown insulation of some sort above that. Another option is to omit the vapor barrier and have 2″ of closed cell foam sprayed on the top side of the DensGlass with blown insulation on top of that. I imagine myself sometime in the future poking holes through the DensGlass trying to find the roof truss so I can attach something to the ceiling. I think I can figure out that I’ve missed the truss before I poke through a 2″ layer of foam. At least on a good day. Thoughts?
I plan to vent the attic. One option is to use fire-resistant gable vents – one high near the peak and two low near the eaves. Is that likely to work, both for ventilation generally and mitigating ice dams? With this option the space from the top side of the soffit material to the bottom of the roof deck would be filled (as well as possible) with blown insulation. The roof has a 6/12 pitch with 24″ cantilever providing 12″ between the top plate and the roof deck. Fire resistant continuous soffit vents and ridge vents are available. But for maximum fire resistance one would cover the vents before evacuation and that seems more practical to do with gable vents.
Another concern are the intake and exhaust vents of the HRU and the high efficiency condensing tankless water heater used for in-slab heating. I am inclined to run them all up through the ceiling. The exhaust vents would go through the gable wall and metal terminations with dampers. That seems fairly fire resistant. I like the idea of terminating intake vents in the attic. Would there be problems with that? If the radon system needs to become active its blower would also be in the attic space so I would plan for that to be in one corner of the attic and the intake for the HRU to be in opposite corner right next to a gable vent (because of concern of leaks in the radon ventilation pipe downstream of the blower).
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Replies
Greg,
There are lot of details in there.
Densglass can be used as exterior sheathing and air barrier (there are roll on products for WRB and sealing the seams for it), you don't need the layer of plywood underneath. With exterior mineral wool it is better to put your WRB underneath the insulation, plus one less flammable item near the surface.
For interior, unless your code requires it, I would skip the interior densglass. For a garage, you can't beat the convenience of mounting things to a plywood wall.
Commercial drywall suppliers can get you brake forms bent to your specs in whatever gauge you want. I'm not sure about the 5/8", they might have a min bend size. Limit is 10' length around here.
The easiest would be to stick to light gauge steel studs, these can be made to your exact length, saves a lot of time installing it. You will need to brace the stud as you go up the wall, steel studs are very flimsy unless they a bridged at the appropriate spacing.
For the bottom of the insulation, the best is perforated J channel, roofing places can make this for you to your size.
Attics make a great storage place in a garage, I would keep it for that and insulate your roof joists instead. If this building won't be occupied all the time, there is very little moisture source on the inside, venting is not as critical. A simple unvented roof should work. It is still a good idea to air seal as much as possible.
A friend of mine did his garage with a gabled roof and a steel I beam for supporting the ridge. This was sized for the roof and also for a hoist. Super handy in the garage, this is detail I would highly recommend.
Floor heat is very slow. If you don't mind heating the garage all the time, it works, if you are there only occasionally, it will be frustrating. If you want the hot floor, I would recommend a secondary source of heat (mini split, hydronic air handler, electric baseboard).
Lot of tankless water heaters can be vented with steel pipes, this would save dealing with details around it. If you are side venting it, it would not cost much more then a plastic setup.
Akos,
Thanks for the suggestions on sourcing custom sheet metal parts.
I expect to heat the building all winter; it is my retirement woodworking shop. I don't expect to need the attic space for storage - the property already has a 2 car garage and a storage shed. I am not convinced that long term the exterior gypsum will provide enough structural strength. I plan on running cleats the lengths of the walls for hanging stuff to avoid accumulating holes in the walls as the shop organization changes. I expect a gantry crane would suffice if I ever needed a hoist - which I don't anticipate at this time.
My question on the water heater is taking the inlet air from the attic space rather than running it through the wall.
Greg
Greg,
I don't think any manufacturer will go along with getting the makeup air from the attic. One option is to go with a unit that takes the makeup air from the living space like the T-KJr2-IN-P.
Densglass is very effective for structural bracing, it is used all the time in commercial light gauge steel construction. I've built my house with densglass on the exterior for the firewalls.
One thing to watch for for interior use is the facing is quite itchy. I've found that rolling it with a thin coat of mud and then painting it works for sealing the fibers.
Hi Greg -
Best resource for how to/how not to vent cladding systems for wildfire management is https://disastersafety.org/fortified/fortified-home/.
Peter
Peter
Could you please be more specific? I went through that site a few times and could not find information on the issue of venting cladding.
Among the heater manufacturer's suggested installation options is getting intake air from a vented attic space if the unit is not direct vented, and that option is preferred over using living space air.
I will look further into Densglass as structural sheathing although I thought I saw that its strength for shear walls was far less than plywood. Thanks for the sealing tip.