July 8th. Over 2′ of cob built since Sunday. Now its time to let the walls dry before I add more height.
The wall will be around 2′ thick, but since cob has a very low insulation value, I am building it as a cavity wall with two layers of 3.5″ Thermafiber UltraBatt mineral wool in the middle for an R30 of insulation plus around 16″ of cob encasing it for a total of around R36 with almost no thermal bridging. Mineral wool is much more substantial than fiberglass, so does not compress much at all as the cob is built next to it. 70% of its content is from recycled slag from iron production. Thermafiber is based in Warsaw, IN, only an hour and a half from here. The wall ties are strips of 1/2″x1″ white oak and black locust fastened with deck screws. Has anyone seen a cob cavity wall (other than bale-cob) before or wood ties? If so please let me know. Another option would have been a cob mix of 50% perlite…does anyone know if this kind of a mix can be mixed with a machine without crushing the perlite?
White oak gable end wall built from 5×5″ rough sawn and 2×2″ for the mullions and transom. will have wattle and daub or light straw clay infill. The window is a tudor style 10 lite window, which wouldn’t be historically accurate for a cob house. The 2 car garage in the lower level will have an awning roof which is not framed yet.
Some more progress today with a new mix of cob with less straw.
June 4th. First day of cobbing. The high straw content of this first batch is challenging. Tony came by at the end of the day and helped fork some cob onto the wall.
Pile of cob. And a toad who I discovered when I pulled back the tarp.
June 3rd. Mixing the cob. Solomon is flaking straw into the mix. We went with at least 1 two string bale of straw per cubic yard of material. As we split the intial pile to reserve some of it for the next batch, it was an estimate, so we ended up with a straw heavy mix which will boost the insulation value, but make the workability a bit more challenging.
Cob test cookies. Got a 22 ton load of material from Yellow Creek Gravel. 1/3 of the load is clay and 2/3 is a sand gravel mix. Cost $8.75/ton. All three test cookies came out nice and hard.
Fieldstone. And dad standing in the reclaimed 8″x8″ hand hewn barn beam door frame.
Fieldstone stem wall.
Gable end wall section made from white oak with large 10 lite window:
April 26th: Gathering field stone from a local farm:
April 5th: Curved mulberry logs cut in half. They will be incorporated as braces in the gable end wall which will be mostly built of 5×5″ white oak.
Just finished up with some framing details, and putting another coat of beeswax and mineral oil on the frame:
Dec 3. Radiant heat in the basement:
Framing the half hip:
August 24th: Working on eyebrow arch over front door area.
Framing for the front door eyebrow arch.
Shiplapped ash ceiling which also acts as a subfloor for the loft areas.
July: standing seam metal is now installed on the roof over the upper level bedroom. The rest of the roof will be slate.
May 31st. We got some rafters on today. The curved wall which faces south is conventionally framed and will be covered completely by a flat plate solar collector which will heat water for for hot water and space heating. The part of the house to the right in the picture above will be field stone and cob.
January 10th. I worked on the house for the first time in a month as we had unseasonably warm weather. The first framing section went up in December.
A truss made from log and roughsawn ash 5×5’s. Steel insert is to strengthen truss. The butt end of the log is around 13″ diameter. Mom finishing logs.
We tarped off the house for the winter so I can still work when it is wet or snowy. After 40 mph gusts in late November, less than a month after the tarp went up I will have to retarp it with a heavier tarp due to grommets getting ripped out and tears starting.
Foundation arches: The post and beam frame will bear on the piers. The arches will hold up the inner leaf of the cob cavity wall. This all turned out to be time consuming and expensive.
Sept. 30th concrete pour. The foundation uses conventional materials. There will be a 2 car garage in the basement under the main floor of the house.
June 1st. Working with dad stacking blocks.
May 27: Poured the footer with help from Dad and Bob.
Big D stripping bark off of a log.
Started excavation with Bobcat
Broke ground May 16.
Trees cut to clear the site will be used as posts the house.
About the house:
Form: The house is designed to be an archetypal cottage of northern European form. It will be a three-aisled hall house, a form used for millennia, with four bays. The side bays form niches–the benefit and use of which are described in Christopher Alexander’s “A Pattern Language,” pattern 179 “Alcoves”. The alcoves define the space while not separating it completely into different rooms. Furniture is in the niches, not primarily in the center of a room like a dining room table would typically be. This leaves the central hall clear of obstructions which helps a small house feel bigger. (This is the same layout as the straw bale and cob duplex featured on my website.)
Frame: The frame will be a post and beam frame of rough sawn ash, with some round posts of oak and cherry. The frame of my first house incorporated round tree trunks as posts. The original reason for this was simply to save money and not have to buy milled beams. The round tree trunks turned out to be a primary aesthetic element of the house and received many positive comments by visitors. The large posts about which my structural engineer commented: “You realize these posts are grossly over sized…” lend a feel of solidity to the house. The organic form of the posts is a delight to run one’s hand over–unlike the feel of a rough sawn square post. The new house will have its main posts sourced from on site, several are trees cut down to clear the building site. Due to there being a 2 car garage under the main room, the main log posts do not bear directly on the foundation, but form part of a truss that bears on the concrete wall–this keeps the garage open and free of posts.
Walls: The walls will be cavity wall cob. Solid cob walls have a very low insulation value which is inappropriate for the cold, grey winters we have in northern Indiana. Using straw bale walls was my first inclination, however bales are square, bulky, and have an irregular surface which requires much labor to plaster. Straw bales also harbor dust mites which are an allergen which is a consideration for my wife. The cavity will be filled with mineral wool. One of the largest manufacturers of mineral wool is in Wabash, Indiana only 60 miles away. Mineral wool is 70% recycled content made from steel slag. It is fireproof to over 2000 degrees, is vapor permeable, does not rot or take on water, and pests do not like to live it in. A double layer of 3.5″ mineral wool batts will give R30 with the bulk of the cob sections raising the total wall R value to around R 36, as good as straw bale and more than twice that of a 2×6 framed wall. Building with cob allows for gently curved, sculptural spaces. The exterior render will be a lime-sand plaster and interior finish will be gypsum plaster painted with a traditional milk paint.
Roof: The roof structure is framed up with rough sawn native lumber visible to the inside coupled with stock 2x construction lumber above it to create a space for insulation. The roof will be superinsulated with dense pack cellulose to R 72. It will be roofed with Vermont gray slate. The pitch of the roof is 15.25:12 or 51.8 degrees. This proportion makes a triangle in which the base is in a Golden Section ratio with the hypotenuse. The high pitch will allow winter snow and ice to slide off the south facing roof solar collector.
Foundation: The footers are conventional with poured concrete and rebar. Below grade is concrete block. Above grade the foundation will be natural stone up to the base of the cob walls. The floor slab will have insulation under it and will have radiant heat. There are concrete columns connected by arches on which the wood frame and inner leaf of the cob wall bears on.
Size/Proportions: The size of the house is determined by the size of a three-aisled hall house with 4 bays which optimizes the bay size for various uses. Each side bay or niche will be 6.5×4′ which can be a bathroom, couch niche, office niche, dining booth, niche for a bed, or main entry area. The house’s proportions will be based on the Golden Section which has been historically considered to be aesthetically pleasing. While the Golden Section is not a magic number, the repetition of any proportion in the composition of a space gives a feeling of continuity. There will be an upstairs level on half of the floor area, while the other half will have a cathedral ceiling.
Energy: The house will be insulated at twice the level of a conventional house and will be quite small with a footprint of 750 square feet. Total finished square footage with the loft will be around 1100 square feet. In addition to south facing windows, the south face of the roof will be covered with solar heat collection panels which will heat water which will circulate to the floors of the house for radiant heat. Back up heat and cooling will be supplied by one or two mini split air source heat pumps.
Where is the house located? Look at the Goshen Ecovillage part of this website. The house will be in the lower area of the site with the driveway off of Steury Ave. The front door and porch will be facing north into the future ecovillage commons.