Rustic Cyberpunk

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Cabin Life: Building Foundations

4 min read

The dearth of work lately has left me ample space to wedge in design exercises for my cabin. While I haven't finalized the design on the main living structure, I'm getting closer to finishing the others. I've found a good balance between space, cost, and building practicality since I'll be doing most of the work myself. I eventually decided that I'd like to standardize on just two structural sizes.

The main living cabin will remain 8 x 12ft as a base. I feel this is the largest size I'd be comfortable with to spend most of my non-outdoor, waking moments. All other structures will be 8 x 8ft including the library and solar power shed. This size was very carefully considered after looking at cost and space. I was considering under-cabin space, but decided against it due to maintenance concerns and the possibility of having to deal with critters building nests under the structure.

I realized that having larger spaces will mean entertaining the possibility of having more things than I will actually need. This is counter to what I hope to accomplish in the end. There's a difference between having "possibly necessary" things and "I may want that later" things. Extra space for "storage" will attract detritus and I want to avoid it if at all possible.

I was working on the design with OpenSCAD, the same as my wood stove. With it, I found getting the exact cut lengths for each piece of lumber a lot simpler than manually doing the calculations. I don't know if I'll continue using it for other aspects of the structures, but it has worked very well for me so far.

base foundation
Base foundation design. Aspects of this will be shared among all structures.

The structures will be at most just 1 - 2 feet above grade. This is probably variable depending on flood conditions, but that too can be dealt with when the time comes.

foundation skids
The structures will be built on three 4" x 4" treated lumber skids with a treated plywood base on top

I settled on creating a sealed underside for all structures. The treated lumber skids or rails will keep the structure off the ground while the plywood, which binds the skids together, will close the bottom to rodents and weather.

foundation skids
Above the skids will sit a treated plywood base, separating the ground from the floor structure.

A skirting around the foundation is still probably necessary either way, but I won't be running any utilities under the structures. This will give me the fewest number of problems should I need to move any of them in the future.

foundation structure
The foundation structure will be made of 2" x 6" treated lumber joists with staggred blocking in the middle. These will be anchored through the bottom plywood to the skids below with screws.

There will be "bubble foil" insulation strips stapled between the floor joists, leaving a small gap between the bottom treated plywood and the top surface. These are a tad controversial in the Tiny House and Cabin building community, but I believe that is due to a fundamental misunderstanding of how it works.

The insulation value is measured as part an assembled unit with an air gap between surfaces. This provides ample heat retention in the winter and, more importantly, doesn't attract rodents or insects unlike most other insulation materials. This is a bigger concern when building in a rural area as I plan to do. Even in rural New York, rodents like mice and rats are still prevalent. I prefer to make my home less attractive to them in the first place rather than having to hurt them later, once they become a problem.

I'll be adding another layer of the same material on top of the joists as a continuous sheet, taped together. I may follow a similar arrangement for the walls and roof since this solution is also significantly lighter than other types and that will be advantageous if I have to move or raise any of the structures at a future date. Since this continous sheet is on the warm side of the structure, I'll be gaining the benefit of a vapor barrier in addition to a thermal break.

foundation top
The final subfloor will be 3/4" plywood or other composite product, and will have a layer of bubble foil directly underneath to act as a vapor barrier and provide a bit of additional insulation value. The walls will be built on this assembly.

I decided that the entire structure will be built with screws. While the shear strength of nails is greater, that's only one parameter in the whole assembly system. The disadvantages of nails far outweigh the advantages for me when taken overall. Besides, the shear strength is not as applicable in a fastening system that's less likely to work its way out of the structure. Screws are also more convenient when building alone as the material comes together more tightly while being fastened.

The main cabin foundation will also be a variation of this, just adapted to the 8' x 12' size.

main cabin foundation
Main cabin foundation

I hope to have the walls and roof rafters designed soon.