Roof and Wall Detail for Off Grid Cabin
New to the forum and have seen some of the topics covered but not to our exact conditions so please excuse me if this has been covered in previous posts/questions and if this question/s rambles a bit.
My wife and I are building a small (14′ x 20′) cabin with a single slope shed roof in the Blue Ridge Mountains of Virginia (climate zone 4A). We are doing this on a budget and understand that some of our planned details may or may not meet the suggested requirements for a typical full time residence. This cabin will be used as a get away and will likely be occupied only on some weekends and the occasional week vacation. Going to explain our current plans and am looking for feedback to see if we are headed in the right direction or if we are asking for trouble?
Cabin will be on concrete pier foundation and wood decking (top of pier is expected to be approximately 16-18″ above ground to allow room for future utility additions). Current plan is for framing to be 2×6 (@16″ oc) walls and 2×8 (debating 16″ oc vs 24″ oc) rafters with exposed tails on both the ends of the shed roof. Will have 3′ overhang on upper and 2′ on lower end for the roof. Wood burning stove will be only heating source. Will have power only when we run a generator other than a small solar package for venting purposes on the toilet. All electrical will be run in surface mounted conduit. No plumbing will be installed at this point but plan to put any future plumbing inside interior walls that will be installed but not finished at this time. Will have composting toilet. Water for exterior shower and wash station will be from a rain collection system.
Planned roof assembly is as follows but am open to other/better options if the cost is similar to this. 7/8″ corrugated exposed fastener metal roofing over roofing membrane (looking for advice on best membrane to use but assuming we will need ice and water shield at the bottom side). Utilizing 5/8″ osb sheathing for the roof structure. Site built 1.75″ deep vent channel created out of 1/2″ plywood (was considering creating this out of polyiso but am worried it would not breath). Will run exterior sheathing up to vent cavity (and sandwich a 1-2″ xps foam board behind sheathing) and air seal at sheathing to vent connection and around rafter tails. Working on detail for venting inlet and outlet but leaning towards a vertical frieze board with coravent or similar screen option trapped between the sheathing and the frieze board above the metal siding. Will use R-23 mineral wool batts between rafters. Not sure if we need a vapor barrier but assuming we do and would lean to use a fully taped and sealed smart vapor barrier (intelo plus or membrain) on the bottom of the rafters. Will finish with shiplap or plywood paneling for the ceiling finish (plywood would have open joints with black tape applied on vapor barrier at joint locations). Will have not roof penetrations other than a single spot for the wood burning stove vent/Chimney pipe).
Current plans for the Wall assembly are as follows. Use 7/8″ corrugated metal siding on 1/2-3/4″ vertical or diagonally installed furring strips. Plan to use plywood sheathing with taped seams and a house wrap (want advice on brand/type yet). Considering huber zip but breathability and water window and door details seem more difficult than house shingled house wrap. Will use R23 mineral wool batts between studs. Not sure if we need a vapor barrier in the wall assembly or not. Plan to use either shiplap or plywood panels (same detail as above or painted studs/blocking) as the interior wall finish.
Again sorry for the rambling but just want to make sure we are not planning for disaster down the road. Any input or suggestions would be greatly appreciated.
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Replies
The way I see it you would be better of skipping all the modern building science stuff and just build the old fashion way. Your wood stove will provide all the BTUs needed to keep the cabin warm and then sum even with zero insulation and doors and windows that leak lots of air.
I am afraid if you insulate and air seal the cabin while it is sealed up and unconditioned water will condense and things will get moldy and rot. It seems likely the mice will find a way into the insulated wall cavities and take up residence.
When you lay it out remember for safety nothing combustible should be within 3 feet of the stove. So, it is more or less an 8-foot circle in the center of the cabin.
Until you are ready to condition the space year-round, I think you do want the space to breathe. Keep the wall open on the interior even better if you can source ruff sawn lumber.
Walta
Walta,
I appreciate the quick feedback and am inclined to agree with leaving it as open as possible. Only follow up I would have is that if I avoid the air sealing steps (taped seams and liquid flash) during construction now won’t it be much harder when I do decide to condition the space?
Love the idea of rough sawn lumber but have not been able to find a place that isn’t charging an arm and a leg (mainly for decorative work not structural framing).
Thanks
JMJ
JMJ,
What you are planning makes sense to me. Unlike Walta I think seldom occupied structures are more forgiving, as there isn't the same occupant load causing interior moisture - which is where most problems originate.
- Consider this floor assembly, as even with an 18" space under the floor, it's very hard to do a good job of insulation and sheathing from below. I used it this summer on a similarly sized building and it works well. https://www.southmountain.com/wp-content/uploads/2019/02/19-Small-Home-Harmony-Article-PDF.pdf
- Any underlay rated for use under metal roofs is fine. Poly, Intello or Membrane on the ceiling will all work.
- For the walls I like Tyvek Commercial. It's slightly lower perm, but very tough and hard to damage. Stick with plywood for sheathing. It's a variable perm vapour-retarder, and deals with prolonged moisture better.
- Install your wood stove according to the manufacturer's instructions, not rules of thumb. Ours is rated for an 8" clearance to combustibles in the rear, and 14" at the sides.
I agree making the old fashion cabin I described into a tight well insulated building of today will be almost impossible. If you keep a sound roof on that drafty cabin it seems likely to stand for a hundred years unattended.
There is an old fashion saw mill almost everywhere. Green ruff sawn 2x4 lumber will cost 1/3 of the store bought 1.5x3.5 The down side is it is not stamp and certified for construction and would not be approved by your inspector. Seem unlikely you will have in inspector looking over your work. The 2 and 4 inch dimensions of the green lumber will shrink about 7% as the green lumber dries but the length will not change. Some board will want to twist as they dry but if nailed in place they can’t move much. Sometime they will have dry lumber, or buy it 12 months early and dry it yourself. Some percentage will twist beyond use.
There are a ton of old guy with portable mills that will cut your downed trees into your lumber.
https://woodmizer.com/us/Find-a-Local-Sawyer
Walta
Some thoughts from a hobbyist:
- It won't cost you much extra to provide a little extra height underneath. Future you (or whoever you contract) will be cursing up a storm trying to crawl under there to do any work with 16" of space. Make that space a little taller. Provides better protection against snow drifts against the door/walls in storms. Makes it easier to use the space for storage. Makes it less attractive to pests. etc. If you have easy access to concrete (are you getting a truck out to pour footings), you might even consider pouring a thin slab so that you can easily keep that area clean (hose it down) and discourage pests from using it as a den.
- How thick are you making the floor? Are you going to consider additional insulation below the joists?
- Moisture is the enemy in a building like this that is not heated continuously.
Walta described one approach. Here is another:
1) Take your interior vapor barrier seriously to prevent the moisture from boiling water, breathing, wet clothes, etc. from getting into your walls. Place it as close to the inside as possible.
2) Plan on having a good way to get fresh air into the building when you are occupying it. Your stove will need it. Perhaps a vent or window near the stove.
3) Take all reasonable steps to prevent bulk moisture from getting into your wall assemblies from the outside. Your large overhangs sound good.
4) Ensure your walls dry easily to the outside. Be wary of any coverings that reduce breathability.
5) Leave a bunch of fresh moisture absorber (like Damp Rid) every time you leave the building vacant for a while.
Or maybe here is another:
1) Put the vapor barrier on the outside of your wall so that you aren't getting any moisture in the wall when external conditions are moist (100% relative humidity with dew forming on everything.)
2) Focus on dehumidifying from the inside. Moisture absorbers but perhaps even something more active. E.g. Can you run a small dehumidifier on a low duty cycle with a solar powered battery bank? You might get some moisture in the wall assembly while you occupy it but you will then dry it out to the inside.
And another:
1) Build for moisture. Use products like cedar, pressure treated wood, and tile. Assume that when you aren't in the building, it will be as moist as the outside air (e.g. on a foggy night, it will be moist.)
Walta might have a point but I would take a slightly different approach.
Air sealing is near impossible after a build, so something you want to get right the first time. Critters follow air leaks to get into the house, they don't just randomly chew at walls. If you get your place air tight, chances are you have eliminated most of the critter issues. The rest is any spray foam around doors, windows and services, there you can push stainless wool into the spray foam before it sets. Something simple like taped plywood under your floor joists and over the wall makes an excellent air barrier. You can even reinforce corner and lower edges with some hardware cloth if the the local critters are extra persistent.
After that if you insulate or not, is up to you. I've been in cottages with no insulation and as soon as the weather even remotely turns colder, they are very uncomfortable. Wood stove helps but that won't get you through the night unless somebody wakes to feed the fire.
If you are going to take the no insulation route, I would still recommend some exterior insulation (say even something like two layers of fiberboard sheathing or an inch of polyiso) on the outside and leave the stud bays open without any interior finish (back side of painted plywood actually looks pretty decent). This would let you run the services properly and insulate down the road without having to take any interior finishes apart.