Microhome Addition

I’m building a 12x27 addition to our 320 sq foot microhome to provide a bedroom, bathroom, and living area. This is just about the bare minimum for us to live through the cold winters while we build a new home.

My desire is to use site-harvested materials as much as possible. The current design is comprised of roundwood Eastern Red Cedar post and beam supporting a 1:12 living roof. I’m leveraging wafati technology, included post-in-ground, rebar pins, wide eaves, and umbrellas of billboard tarp. Walls, I plan to use straw ale and cob, as well as ERC siding. To support the walls I am digging a rubble trench with some sort of cap.

I have two major questions - the posts are going lower than the rubble trench - 9 out of 10 post holes stay very dry, while one seems to be more damp. Although I am providing drainage in the trench, I am concerned that moisture will follow the post hole and hasten deterioration of the post. The base of the trench and the posts is in dense clay subsoil. I have redundant structural support in the post and beam and strawbale cob, so I am not concerned about collapse, but I would like to do all I can to preserve the structure for the long haul.

My other question is if there are site-harvested materials that can be used in place of gravel or road base, both for the post hole drainage base, and for the earthen floor base layer. I have almost unlimited sand at my disposal, and a number of 100-year-old cinder blocks. Is there an alternative to gravel for a long-term, stable drainage base?

Other info: site is nearly flat, depending maybe 3 inches in the 27-foot span east-to-west. Addition is on the north side of original slab-on-grade construction. Top-soil is compacted sandy soil. We have a diverse woodlot and sawmill available. I am focusing on ERC because of its merits as a rot resistant wood, it’s ease of use, and the fact the we have many, many standing dead or recently fallen due to crowding.  My main priorities are low cost, self-built, lasting, and fast. I know that mix of factors doesn’t really exist (per fast:cheap:quality) but you get the idea. It has to be usable by November, and I can continue working on finishing detail and interior in the winter.

Any thoughts appreciated! I’ll post progress.

I am NOT a fan of putting wood in contact with the ground in most situations, especially clay, unless you have perfect drainage.  The easy answer is a concrete pier.  If you are opposed to concrete, you can cut stone posts to go in the ground or build a rubble trench and build it above grade.  At least for the troublesome one.  

I agree with R Scott.
I would concrete a steel stirrup and mount the posts to each stirrup.
The stirrups can be fabricated to suit the posts.
What floor are you going to have?

Concrete piers were my original plan, until I got inspired by the bermshed etc. but I think you’re right, that this site is not a prime candidate for wood-in-ground. Piers it is!

I’m planning an earthen floor, either over gravel or road base, unless I discover another lower impact and local base layer alternative that promises good drainage.

You may need to lower the floor area if an earthen floor is used, they need a fair bit of depth for the layers.

John C Daley wrote:You may need to lower the floor area if and earthen floor is used, they need a fair bit of depth for the layers.

I’m currently 14 inches below the floor of the existing structure. My plan is to build up 4 inches of gravel or other drainage/stabilizing media, then add 3 inches in subsequent layers of earthen floor, to ultimately arrive at a plane 7 inches lower that the original floor. I’ll incorporate some wooden threshold at the connecting doorway to step down. That’s the plan, at least.

It does not look that deep!
Can you fabricate the stirrups yourself?Some hoop iron diagonally across the roof frame both ways should be a great help in creating lateral stability of the new posts.

John C Daley wrote:It does not look that deep!
Can you fabricate the stirrups yourself?Some hoop iron diagonally across the roof frame both ways should be a great help in creating lateral stability of the new posts.

Hoop iron reinforcement is an interesting idea, new to me. After a brief internet search - Is there a strong advantage over timber cross-bracing? I suppose they are easier and quicker, for one. And can be embedded into the wall and hidden if placed properly. I have some learning to do.

The flat metal stip lays flat and is quick to be installed.
We even can use a small device to tighten the strap, its 'U' shaped with a bolt and the 2 'U' shaped parts are pulled together by the bolt, very clever.

Made a little progress last week before heading out of town. 7/10 posts up on concrete piers with rebar pins. Excavated another 3-5 inches to get down to 14 inch minimum depth so I’ll have more to work with when layering the floor. And lots of thinking and figuring.

My favorite idea for roofing method is to make structural panels with straw and wood chip infill, inoculated with reishi spores. The goal would be to foster the complete colonization of the substrate with mycelium, then dry it out and install, creating a biological insulation layer. But alas, I will probably have to wait on this until a future project, with time to experiment and develop. As it is, I’m still considering making insulated panels for ease of installation over the structure. To maximize headroom I’m thinking of a 1/2 in 12 pitch. I found some rolls of that durable, dimpled foundation barrier that I’m thinking of using over the insulation as a redundant moisture barrier atop my billboard canvas. It will add a pocket of air which I hope should solve for condensation. Over that, I’m considering options for UV protection. We have limitless creek pebbles/sand, so I’m wondering how suitable that would be as a living roof substrate - might welcome some very hardy perennials, but deter most other volunteerism.

Have any of you had any experience making your own insulated panels? Any insights welcome.

Here’s a quick sketch of the current plan. Still flexible, some aspects very much a work in progress.

I realize this is pretty late in the game.  When I lived in 12 x 32, I decided to go up with thec12 x 16 addition.  I avoided all the foundation questions. The ceiling joists of the original house were more than adequate to serve as the floor.

The biggest questions I have at this moment are regarding the roof cladding and insulation materials.

1) Is making my own SIPs a feasible and economical method? I’ve started trekking with Peter’s thread (https://permies.com/t/120/141249/Tiny-house-build-Hokkaido) and y’all have helped come up with some very cool ideas.

I don’t need them to be as high r-value as polyiso as we have a relatively short cold season and abundant wood. I also don’t need them to be water tight as they’ll be under a membrane. My father-in-law manufactures metal-clad panels that are very high performing and form a suitable roof in their own, so he is laughing at me right now. Different goals and philosophies.
2) If panels are feasible, what are some suitable materials (points for stuff I have or can harvest or obtain locally). Some possibilities are straw, wood shavings, and maybe millet hulls.

John F Dean wrote:I realize this is pretty late in the game.  When I lived in 12 x 32, I decided to go up with thec12 x 16 addition.  I avoided all the foundation questions. The ceiling joists of the original house were more than adequate to serve as the floor.

There’s an idea.

So are you saying you went up, adding a 2nd level?

Yes.  It was cheaper in the long run. Heck, for me, it was cheaper in the short run. Yes I know the roof is involved, but how long do you expect your current roof to last before it needs work? As for your current efforts. Make it a porch  designed to be enclosed in the future. But, much depends upon the ceiling joists.  If you have trusses the game may change.  Of course, you may be able to reuse the trusses, but the labor has to be considered.  And unlike a horizontal addition, you have to get it enclosed quickly.  Oh yes, I went with a spiral staircase to save space.  As a precaution, installed a large upper window that furniture could be moved in and out of ..... though I never used it.

John F Dean wrote:much depends upon the ceiling joists.

I can see how that would be a good way to go in certain situations.

My 2x4s at 24 centers spanning 12 feet are not interesting in becoming a floor, and I’d rather not in displace my family while I’m doing the work.

Minipost of today’s progress. Just one beam remains and some temporary bracing, then I’m thinking of moving on to the perlins.

Also, I finally got ahold of our quarry guy, so I can move into gravel and stem wall at some point.

Question of the day:
I’m wondering if 2x4 perlins on 16in center, spanning 7 feet at the max will be:
A) enough to support a modest living roof
B) enough to tie the timber frame together, with or without the aide of metal strapping or angular timber braces.

Anyone have 2 cents?

I cannot remember what your original plans for the wall cladding was. What ever you use that is conventional may clash with the round posts.

John C Daley wrote:I cannot remember what your original plans for the wall cladding was. What ever you use that is conventional may clash with the round posts.

Straw bale and earthen plaster is the plan.

Pic post. Had guests in town for a week so just getting started again.

I did take some video of the final post and beam. I’ll upload and embed when I have reasonable internet.

Lots to update here.  

I acquired a bunch of structural insulated panel remnants from a nearby manufacturer.  The roof is up!  And tied in with the original roof!  And triple-waterproofed!  Amazing.  Feels good.

Did a lot of work during a very long cold spell, including taking a breather from the building to install a new wood stove (thanks Eric Von Raderson!).  Surrounded it in a protective shroud/thermal mass.  Kept us toasty in relentless cold!  I may replace this with a RMH in the new space, but I needed something quick and this fit the bill.

Now I'm working on finishing the stem wall so I can start putting up SIP walls and framing windows and the French doors.  Still debating final layout and plumbing details.  Thinking of incorporating vermicomposting system for gray and black water.  We shall see.

Images are in reverse chronological order, so scroll up from the end of post to see how the layers went on.  Captain Permie, feel free to change the order.  Rural internet prevents me from being too nitpicky.

It's been 11 months since my last update.  Once again, quite a lot to share.

We are occupying the addition! It is 3*F outside and it is cozy!  We are dry! I'm not sure how well it would have gone to spend another winter with 4 of us holed up in 300 sq feet for the winter, so we are VERY.  THANKFUL.  for the expanded living space.  

Things that have been completed since my last post, with italicized comments on what I'd do differently in hindsight.

1.   Stem wall went up, using 70-year-old cinder blocks from old, demo'd outbuildings.  Big, heavy blocks my progenitors made from locally mined cement and sand from our creek.  Stem wall retains a short berm, putting our finished floor height about 24 inches below grade.  If  I didn't already have these blocks, next time I will probably use local stone from the nearby Flint Hills.

2.   SIP walls went up - 7.5 inch thick, clad in OSB.  This was a big compromise from our ideals, which steer us away from both polystyrene and processed wood products, for both health and ecological reasons.  If I could do it again, I'd have gone ahead and used straw bales.  It is pretty near impossible in my area to source organic, so I was hoping to grow my own wheat and bail my own straw, but (I'm almost ashamed to say) expedience prevailed. I don't endorse OSB/Polystyrene SIP - but in the name of up-cycling and saving waste from a landfill, it is the way I went this time.  More on that, philosophically, in another post.

3.   Windows and doors on all 3 new walls.  So much light!  All windows and doors from Habitat for Humanity Restore.




4.   Packed road-base foundation went in over gravel. If I had it to do over again, I'd consider using our site-harvested sand.

6.   Cob Sub Floor went in.









7.   Finishing insulating with loose sheeps’ wool filling.  This was my first time trying a suet fermentation, and it worked great!  Again, worthy of another thread at some point.






5.   Interior walls and cieling plastered with standard 90-minute-set gypsum plaster.  This was an interesting development, probably worthy of its own post.  Here's the cliff's notes - I pursued lime-based plasters, but determined they don't play well with wood due to eventual dry-rot, catastrophic with intrinsic structural pieces like SIP.  Granted, it is also a no-no to apply gypsum plaster directly to OSB, for a number of very good reasons:
    i. the moisture can infiltrate the wood particles and cause separating, flaking, mold, or rot.  Exacerbated by impermeable poly on the inside.
    ii. the plaster doesn't have enough grip to stay on the wall.
So, I debated going to the time and expense of installing wood or metal lath.  While I was debating, I did several rounds of testing with off-cut SIP to see what would actually happen.  Turns out, this particular batch of OSB for these SIPs may be the exception - all plaster tests dried well, didn't flake, and keyed adequately to the OSB surface.  I WOULD NOT RECOMMEND THIS. But it worked this time, so hey.  Where I plastered some of the older OSB cladding on the original Cottage wall, there was some bubbling and separation - minor - but that gave me a great comparison to how disastrous this would be with most OSB.  If I had this to do again, it would have been site-harvested cob on straw or slipstraw, coated with lime-casein paint or similar.  I may still paint the gypsum with lime-casein when I get around to it, but the pure white gypsum's pretty nice in the meantime.

6.   Flagstone bathroom floor laid, mortared with lime-sand-clay.

7.   Lime-earthern floor laid.  I went for a lime-enhanced floor because I loved the idea of a shorter cure time required by typical earthen floor cured with linseed oil.  And I had first-hand experience of the headaches inducing off gassing the can come from linseed-cured floor.  Allerton Abbey had very similar woes along the way to its currently, beautiful state.
I did a test patch, which came out beautifully - perfect actually.  It was something like this, by volume:
   -1 part slaked hydrated type N lime (slaked about 3 weeks)
   -3 parts sand from our driveway (which comes from our creekbed.  It happens to be well-graded, semi-sharp sand.)
   -1 part beautiful black mystery clay from our creek.  Had some sand mixed in.
   -1/4 part crushed, sifted sheep manure.  Beautiful, finely chopped straw.
It was so beautiful that I scheduled helpers to come mix while I laid the rest of the floor the following week. What followed was a tiring week of heartbreak, attempting to control the curing process as I watched hairline cracks spread over the entire surface of the floor.  I mean, talk about a gut-punching learning experience.  And I made it worse trying to smooth them out as it cured.  I have spent heaps of time wondering what factors cause this to come out so different than the test batch.  What I have come up with is that hydrated lime can vary drastically from one batch to another, and also does not age well at all. I would say I am 70% sure it was the lime, which came from a different source from the test batch.  The other possibilities are: inconsistent mixing ratio, too much water in the mix, moving too fast to be able to compact well.  Anyway, we are living with it as it is for now, and maybe in the spring I'll take a square at a time and lay a new finish coat of clay or lime-clay.  This one has no "if I had to do it again," because it's just one of those things.  I think if the whole floor had laid as well as the test patch, I'd be ecstatic.  












8.    Lots of minor shovel earthworks outside to convince water to do what I want.  

9.    Lots of site-hewn Cedar trim work.



Now here is probably the number 1 thing I've learned.  It is often said around here that mixing modalities doesn't often go very well.  I think that's right.  I hope this is the last project I do on our farm with so much compromise regarding natural v conventional modalities.  

The number 2 thing I've learned is to just go ahead and do it.  Do the best you can, make as many mistakes as you have to.  The building may or may not perform as I desire over the next 100 50 20 10 years, but listen - I am the work in progress.  And the next one will be so much better and go so much more smoothly.

The number 3 thing is - it is not a bad idea to gorge yourself on all the natural building endeavors at Wheaton Labs and beyond.  If it weren't for the Berm Shed Youtube Videos, I would not have used rebar, butt-and-pass style roundwood timber framing, and this would have probably been a lame-ol stick frame.  Would I do it again?  Maybe.  On sheds and outbuildings, absolutely.  On another residence, I'd like to think I'll take the time resources to do some nicer joinery.

But hey, this worked.  It worked fast.  And I pretty much built it with an electric chainsaw, power drill, ratchet straps, and hand tools.  Using 75+% site-harvested materials, with most of the balance consisting of reclaimed/upcycled materials.  And 95% by myself.  (related - number 4 thing I hope I learned - ask for more help.)

I think if someone were to ask me what they should do to start thinking about undertaking a project like this, I'd recommend visiting Wheaton Labs, taking a tour, staying for a while.  Short of that, hit up the Wheaton Labs Tour Movie
 to get some exposure.  A lot of really excellent people contributed to all the structures, gardens, and tech shown, as well as in the movie itself.  Worth it ten times over.