This is a design for a tarp wrapped straw bale home. The thick tarp protects
the straw from moisture and provides tensile strength, so that together it
forms a straw / plastic stress skin panel. The roof is made of styrofoam, also
bonded to thick tarp material to make a light weight styrofoam / plastic
stress skin panel. The roof is flat so that it can be used for a garden or
balcony. Any excess styrofoam after cutting can be used in the base of the
straw / plastic panel foundation, to protect the straw from water if there is
a leak in the foundation tarp. The straw bales can be handled with two
people and the styrofoam roof panel, although large is relatively light and
can be installed by about a dozen people so a crane isn't required.

This round bungalow has a floor pad 11.3 m in diameter or a radius of 5.64 m.
The straw bales are assumed to be 23" X 16" X 42" ( 0.575 m X 0.4 m X 1.1 m )
and may weigh from 75 to 85 pounds ( 36 kg ). These are the sturdy 3 string
bales preferred for home construction. They cost from $ 0.50 if available in a
farming area to $ 3.50 in a city.

Laid flat, a bale has an area of 0.575 m * 1.1 m = 0.58 m^2
This bale has a volume of 0.575 m * 0.4 m * 1.1 m = 0.23 m^3
To cover the floor pad of ( 5.64 m )^2 * pi = 100 m^2 / 0.58 m^2 = 173 bales
The perimeter is 11.3 m * pi = 35.4 m
The middle perimeter is ( 11.3 m - 0.575 m ) * pi = minus 4 times the
bale width = 40 m - 4 * 0.575 m = 33.6 m Which means 33.6 m / 1.1 m = 30.6
about 31 bales per layer To make the walls 2.4 m high, 2.4 m / 0.4 m = 6 bales
high So 31 bales * 6 = 186 bales for the walls
Total = 186 bales + 173 bales = 359 bales
At city prices, 359 bales * $ 3.50 / bale = $ 1256.5

A typical styrofoam sheet of 8' X 4' X 2'' ( 2.4 m * 1.2 m * 0.05 m ) is about $ 10
Laid flat, a sheet has an area of 2.4 m * 1.2 m = 2.88 m^2
This sheet has a volume of 2.4 m * 1.2 m * 0.05 m = 0.144 m^3
To cover the roof of ( 5.64 m )^2 * pi = 100 m^2 / 2.88 m ^ 2 ~ 35 sheets
To make the roof 0.4 m high, 0.4 m / 0.05 m = 8 sheets high
To fill the roof, 35 sheets * 8 = 280 sheets
To make the roof stiffer, the top sheet should be extruded polystyrene, the
blue or pink colored heavier styrofoam which costs about $ 20 / sheet of the
same size as the white styrofoam sheets.
At typical prices, 245 sheets * $ 10 sheet + 35 * $ 20 / sheet = $ 3150
1 sheet of styrofoam at the bottom of the straw bale foundation would
be useful to keep the bales dry in case water leaks in through the tarps and a
thin layer of extruded styrofoam would be useful on top to provide a soft floor
surface. Thin extruded styrofoam of 0.012 m thickness costs about 1 / 4 as
much as 0.05 m thick styrofoam, so about $ 5 a sheet.
At typical prices, 35 sheets * $ 10 / sheet + 35 * 5 $ / sheet = $ 525
The total styrofoam cost is $ 3150 + $ 525 = $ 3675

The floor should be made first, then the walls, then the roof. The overlap should always be pointing down, so that water is directed away from the straw. The wall tarps should be as taut as possible so that the tarp contributes tensile strength to the wall. The floor straw should be built with two round tarps, the bottom one gets wrapped up around the side, a rope is put through the grommets then tightened and the top one overhangs down the side, a rope is put throught its grommets then tightened. The walls should be built with two rectangular tarps, the inside tarp should be about:
( 2.4 + 0.575 * 2 ) m ~ 3.6 m high with the part of the tarp that is under and over the walls cut apart between the grommets so that the inside can be taut. The top and bottom of the tarp should be connected with ropes to tighten it. The other rectangular tarp, this one about:
( 2.4 + 0.575 + 0.2 ) m ~ 3.2 m high should be tied at the top of the wall a rope going through all the grommets, above the inside top of the wall. The tarp should hang down below the wall, to around the middle of the floor and tied there with a rope going through all the grommets. The top should be built with two round tarps, the bottom one gets wrapped around the top, a rope is put through its grommets then tightened. The top tarp should hang down below the roof, to just below the top of the wall and tied there with a rope going through all the grommets. This way, any rainwater will drip down the outside of the tarps and the inside will stay dry.

Heavy duty UV resistant tarp material goes for about 1 $ / m^2. To cover
both sides of the a surface and to provide for overlap, tarp material with 3
times the surface area is needed. The floor and ceiling both have an area of
( 5.64 m )^2 * pi = 100 m^2, their total area is 100 m^2 for the floor plus 100 m^2
for the ceiling = 200 m^2.
This multiplied by the coverage factor of 3 is 600 m^2.
The walls have a perimeter of 35.4 m and a height of 2.4 m for a surface area
of 85 m^2.
This multiplied by the coverage factor of 3 is 255 m^2.
The total material needed is 765 m^2, which should cost around $ 765

About $ 1000 of adhesives and sealant are needed to bond the plastic to
styrofoam on the roof and seal the gaps between the floors and walls and
ceiling.

The total cost for the shell of a round straw bale bungalow is:
$ 1256.5 + $ 3675 + $ 765 + $ 1000 = $ 6705.5
the inside surface area is ( 5.64 m - 0.575 m )^2 * pi = 5.065^2 * pi = 80.6 m^2
the cost per square meter of living space for the shell is about $ 83 / m^2

Assuming a comparable cost for the interior, a complete round straw bale
house would cost about 2 * $ 6705.5 = $ 13,411
the inside surface area is ( 5.64 m - 0.575 m )^2 * pi = 5.065^2 * pi = 80.6 m^2
the cost per square meter of living space for the house is about $ 166 / m^2

Compared to the square straw bale house, the round one is:
83 / 88 = 0.94 times as expensive and it requires less land for an
equal amount of interior space. On the other hand, it requires more skill to
build the round house. In general, if somone has experience building straw
houses or the land is expensive, it's better to build the round one.

Straw bales have an average thermal resistance of 16 m * K / W and
styrofoam has an average thermal resistance of 28 m * K / W. The wall bales
are 0.575 m thick, so their thermal conductivity is:
1.0 / 0.575 m / 16 / m / K * W = 0.11 W / m^2 / K
Walls 2.4 m high have a thermal conductivity of:
2.4 m * 0.11 W / m^2 / K = 0.26 W / m / K
With a middle perimeter of 33.6 m, the wall's thermal conductivity is:
0.26 W / m / K * 33.6 m = 8.8 W / K
Windows and doors have a thermal conductivity of about 1.2 W / m^2 / K;
which is higher than the conductivity of the walls by:
( 1.2 - 0.11 ) W / m^2 / K = 1.09 W / m^2 / K
With around 10 m^2 of windows and doors, the extra thermal conductivity is:
1.09 W / m^2 / K * 10 m^2 = 10.9 W / K
So the total wall thermal conductivity is:
( 8.8 + 10.9 ) W / K = 19.7 W / K

The styrofoam roof with a height of 0.4 m has a thermal conductivity of:
1.0 / 0.4 m / 28 / m / K * W = 0.089 W / m^2 / K
With a middle of wall surface area of ( 5.64 m - 0.4 m / 2 )^2 * pi = 5.54^2 * pi = 93.0 m^2,
the roof's thermal conductivity is:
0.089 W / m^2 / K * 93.0 m^2 = 8.3 W / K

The thermal resistance of the floor per unit area is:
thermal resistance of 0.4 m of straw:
0.4 m * 16 m * K / W = 6.4 m^2 * K / W
plus the thermal resistance of 0.062 m of styrofoam:
0.062 m * 28 m * K / W = 1.74 m^2 * K / W
for a total thermal resistance of 8.3 m^2 * K / W
or a total thermal conductivity of 0.12 W / m^2 / K
The ground underneath the floor decreases the thermal conductivity there by around a factor of two, so the approximate total conductivity is:
0.12 W / m^2 / K / 2 = 0.06 W / m^2 / K
With a middle of wall surface area of ( 5.64 m - 0.4 m / 2 )^2 * pi = 5.54^2 * pi = 93.0 m^2,
the thermal conductivity of the floor is:
0.06 W / m^2 / K * 93.0 m^2 = 5.6 W / K

The total thermal conductivity of the round straw house due to conduction is:
( 19.7 + 8.3 + 5.6 ) W / K = 33.6 W / K
in houses, the heat loss due to air movement is roughly the same as the conduction losses, so the total thermal conductivity is:
2 * 33.6 W / K = 67.2 W / K ~ 67 W / K

Compared to the square straw bale house, the round one has a thermal conductivity per unit area:
( 78.3 / 80.6 * 67.2 / 67.8 ) = 0.96 times as great

If long straw bales could be used, at least 5 m long, preferably 12 m or more
long, they could substitute for the styrofoam in the roof. The bale would be
used as a beam; oriented with its wide side vertical, it should be able to
span at least 5 m.

When straw is compressed and heated, the resins in the straw bind the reeds
together, forming a composite. Sometimes, glues are used to further strengthen
and waterproof this composite. The main use of these panels is currently for
interior walls; but, they could also substitude for the styrofoam at the
bottom of the foundation and if they are available in large sheets, they can
also substitute for some of the plastic. Unfortunately, there is currently
only one plant in North America that makes these panels and few stores carry
them.
 
 

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