Towards a better tinyhouse

Inventing to freedom?

Water building / gel building

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I had a really interesting idea today, with gellified water and reasonably strong plastic sheet, I think you might be able to make an all season habitat which would be relatively cheap.

Only the problem is that I am not so sure it is that great of an idea anymore, because it turns out plastic is a lot more expensive than I thought. Still, it’s doable.

Basically think of a sort of bucket whose walls are thin flexible plastic film. Make it a meter or so wide. Then if you added another bucket which was a bit smaller in the first, it would collapse from the surrounding water pressure, but if the walls of the inner bucket are connected to the inner part of the outer bucket, you could have the things all stay upright. You now have a sort of double walled thingamajig with water between the walls, which stays upright. The water pressure is the same on the outer part of the inner bucket as on the inner part of the outer bucket, but the area of the smaller bucket is smaller, so there is a net force keeping the inner plastic held outwards. Now suppose you eliminate the bottom of the buckets so it is an inner plastic tube, an outer plastic tube, connected together, and they are sealed at the top and bottom. Okay a diagram would be good here, but I’ll draw one later maybe.

The point is, if you follow this line of logic, you can see that you might be able to make a container out of thin plastic film, which when fully filled with water, would make a water filled dome that would stand up by itself, with an inner and outer wall of plastic, with the 2 layers of plastic connected to each other so they can’t get further apart from each other. Then add just a very small amount of gelling agent to the water, just enough to prevent convection (which would ruin the insulating qualities) and water from escaping through any small holes (since it can’t really flow) that would be inevitable.

Suppose the walls were 30 cm thick or so and the base was 5 meters wide.

It would be well soundproofed, unlike a tent or other cheap structures tend to be. I think 30 cm might get you more than 50 db, a sheet of 3/4 MDF is 23 db, and add 6 db for every doubling of the mass/unit area, that’s 4 doublings (mdf has density of about 0.8, water is 1), then it is visco-elastic, too ….. Remember, a normal wall is only 45 db STC, and a double glazed window 26 db. So it would be very quiet inside.

It allows sunlight through, so you would collect a massive amount of solar energy during the day, the whole side of the building is a collector. 15-20 sq meters or something of collector, and remember 20 cm of water gel gives about the same insulation value as a double glazed window. In a climate like Ottawa you might get an average in winter of 3 kw-hr per day of energy from sunlight per sq m, 45 kw-hr, which is a huge amount of energy. By simply placing a collector of some sort against the inside wall of the building, it could collect plenty of hot water.

So much heat is available, you don’t need a heat exchanger on the ventilation system, and it could be convection driven, making it very simple and cheap.

The walls of course provide a huge amount of thermal mass, though you could always add more inside the building with a tank of water.

It would be very well lit in the daytime, that’s for sure. You could add some dye to the gel if you wanted to tweak the spectrum of light that gets in. Remember that hong kong transformer apartment guy, he had this spectrum modifying film on the window so it looked like a sunny day all the time. Food coloring could be used as well as anything, I think.

No need for windows, you can see right through it. In winter the outer surface would freeze so you would want to use degassed water if you want it to still be transparent, or bubbles will form. If you did something clever during construction, maybe some areas of the wall could be made bubbly on purpose to provide a bit of redundant privacy (in addition to the curtains or whatever else is used) with degassed water being used in other areas of the wall so you get just a certain area of the window that does not get bubbly. That would reduce solar gain, but also increase insulation value, and there is a lot of gain to start with, you’d have to look at the details to see if adding bubbles would be okay.

Some structural strength in winter would be provided by the fact that the outer layer of water would be frozen, which could help for the snow loads. If there were some way of depressing the freezing point just slightly then you could be sure that if there is snow on it, it’s frozen… but that is not strictly necessary, it would just save some money by allowing you to use a thinner plastic film.

You might want to have a multiplicity of domes rather than one big one though depending on the size of the building, to keep the height down, I don’t know.

It would be relatively easy to set up/build, just need a water supply, a dosatron thing to mix in the gelling agents on the fly as the water flows through the dosatron, and then it goes into the dome shaped plastic bag thing, which might need to be propped up somehow with scaffolding, I think probably just at the center of it.

If the gelling stuff was $15 per kg and you used 0.2% and the wall is 30 cm thick then the water plus gel stuff is about $9.10 per sq m of wall. Hey wait a second, that’s not as cheap as I’d like.

For the plastic film, I think you would want to connect the inner layer to the outer layer with ribs of the plastic sheet stuff , running vertically. Consider a section of the wall in isolation which includes 2 of the ribs and the inner and outer plastic layers, which is just a circular arc, and it’s and 2.5 meters high, 5 meters wide. At the base of it the pressure exerted against the walls can be computed from the height of the water column, it is 24.5 kPa (kilonewtons per m2). I’m assuming that we are using the same thickness of plastic for the ribs as for the inner and outer layers.

Divide it by the height (m) and 24.5 kilonewtons per m, and if the tensile strength of our plastic film is 9 mpa then divided by the height (m), than it can withstand 9 meganewtons per m, and the amount that’s exerted has to be at least canceled by the amount the plastic can stand, so it takes 24.5/9000 0.003 of the column width that has to be plastic, or 0.0009 m , or 0.8 mm. Vapor barrier is available in 0.38 mm thickness for $2.5 per sq meter, both sides $5 , plus the ribs $7.5, multiplied by 6 (because it has to be a fair bit more than 0.8mm and let’s assume the price goes up linearly with thickness), $75 per sq m hey wait a second, shit.

Still, notice that the cost of the 16 mil is not any more than the 10 mil, even though it has 60% more plastic, and $2.5 per sq meter seems like an awful lot for plastic don’t you think? I have some garbage bags here that are 25 cents each, and if you cut it open and lay it out it’s 1.08 sq m, and this is a retail product. The bulk no frills LDPE plastic should be very cheap, though it would degrade in sunlight. In other words, surely we could get this price down by shopping around for a better deal on a more suitable material, but I guess vapor barrier wouldn’t work. Thick greenhouse plastic might do, maybe. Maybe whatever variant they used to make shopping bags out of, which I assume is the sheer cost/unit strength optimum, though it would need some anti UV stuff added. Still, I’m surprised that vapor barrier wouldn’t work, darn.

If it’s 0.38 mm thick and about a density of 0.9, that’s only $7.8 per kg, so I don’t know if finding cheaper ldpe is likely, it has to be a stronger plastic.

You could also make the dome shorter, reducing the height of the water column, making several smaller domes rather than 1 big one, or maybe make the walls straight up, and use some wood and rope to hold up the roof with. Also could reduce the thickness, maybe making 2 domes, one inner one and another outer one, that could easily get you the same soundproofing with a fraction as much water, and improve thermal performance a bit relative to the amount of water, but then strength to withstand snow might go down too much, I don’t know. You could certainly use thinner plastic as you go up higher, could save some money on materials at the expense of manufacturing costs. Also reduce the safety margin, of course. The way this is going though it would be pretty hard to DIY, but the price might be reduced to, what $30 per sq m, or $360 for the whole thing, which still is very cheap for what your getting, with the soundproofing etc. Any ideas?

I was happily envisioning a new form of super cheap housing here. It’s cheap but not super cheap.


Written by gregor

January 1, 2011 at 00:29

Posted in Uncategorized

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