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A large curious bear climbed up on his hind legs, jabbed his front paws into the eaves of the UtiYurt being tested in the Sierra Mountain range, and tried to rip open the side of the shelter.  All he left were some dirty swipe marks, a few bite holes, and one 3” (7.62 cm) gash in the side of the UtiYurt. There wasn’t any food inside so he probably didn’t try too hard, but nonetheless he didn’t get in. 

We’re not saying that the plastic walls of a FH Yurt will keep a determined bear out, but we are saying that these walls are tough! [See REPAIRING EXTRUDED PLASTIC, and EXTRUDED PLASTIC – WHAT’S THAT?]


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The Folded Homes® TekYurt and UtiYurt shelters were developed and field tested over a period of two years following the devastating earthquake in Pakistan in the fall of 2005. Engineers at Alliance Packaging and Folded Homes® collaborated in the development and testing of the structures in a process that saw nine major engineering revision cycles addressing about 50 different ECOs (Engineering Change Orders.)  The yurts were tested at three test sites chosen to stress the yurts in conditions of rain, snow, fire, high wind and intense sunlight.

Rain tests were performed at a test site in the Olympic Peninsula in northwest Washington State where average annual rainfall is 140 inches.  Snow load and wind-resistance testing was performed at a location in the Sierra Mountains of California at an altitude of more than 8,600 feet (2,622 m) where annual snowfall averages over 8 feet (2.44m) deep. Desert, wind, and burn tests were performed in the high desert country outside Christmas Valley, Oregon.

The LiteYurt was developed and tested during the first nine months of 2008.
Long-term testing of Folded Homes® shelters is ongoing.

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Polypropylene extruded plastic is impervious to damage from water and humidity.  Hot and cold temperatures don’t bother it, and it remains flexible in all but the very coldest weather.  It can take a lot of abuse and doesn’t tear easily.

The one thing that over time will hurt polypropylene is the sun’s ultra-violet (UV) light. Science Daily (www.sciencedaily.com/articles/u/ultraviolet.htm) reports that “Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. The Sun emits ultraviolet radiation in the UVA, UVB, and UVC bands, but because of absorption in the atmosphere's ozone layer, 99% of the ultraviolet radiation that reaches the Earth's surface is UVA.” Over time, this UV radiation causes plastic to become brittle and crack.

These two images show the surface of undamaged (left), and UV-damaged polypropylene (right) when viewed under the microscope.  The microscopic cracks on the right are what make the polypropylene brittle.  Any polypropylene surface that is directly exposed to the sun will eventually get brittle and crack.  Unless that is prevented, it will eventually cause a shelter manufactured from it to fail.

The manufacturers of polypropylene extruded plastic generally claim that opaque sheets of white polypropylene will last about two years in sunny environments.  FH Yurts are manufactured from opaque sheets of polypropylene.  (Translucent polypropylene sheets last less long and our experiments show that within thirteen months you can begin experiencing catastrophic failure in them.  FH Yurts are not manufactured from translucent polypropylene.)

A polypropylene shelter manufactured with UV inhibitors lasts roughly twice as long as a shelter manufactured from standard polypropylene.  Folded Homes UtiYurts are manufactured from standard polypropylene. Our TekYurts are manufactured from UV-enhanced polypropylene. [See UV-ENHANCED PLASTIC (FH YURTS)]

The easiest way to prevent the UV degradation of your shelter is to cover it with an opaque material that blocks the sun’s UV radiation from reaching the polypropylene. [See EXTENDING THE LIFE OF YOUR SHELTER (FH YURTS)]

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No, polypropylene extruded plastic is entirely odorless. [See EXTRUDED PLASTIC – WHAT’S THAT?]

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Folded Homes® shelters are typically manufactured from 4mm-thick extruded plastic sheets.  These sheets are stamped out and scored with indentations which define the necessary folds to transform the flat two-dimensional sheet into each three-dimensional part.  Once assembled, these parts are both strong and remarkably light weight as Wendy is demonstrating.

Extruded plastic has the same basic form as corrugated cardboard.  Structurally it has two parallel layers of flat plastic separated by fluting that runs perpendicularly between the two layers.  Unlike corrugated cardboard, extruded plastic is formed by extruding molten plastic out of a form to create the entire double-walled sheet in one go.  So unlike corrugated cardboard it cannot delaminate.

Folded Homes® uses polypropylene plastic.  This plastic has a number of great characteristics going for it. 

  • Polypropylene is really light: Light materials are cheaper to transport and can be packed in on men’s backs if necessary. [See TRANSPORT REQUIREMENTS – GETTING IT THERE (FH YURTS)]
  • Polypropylene has memory:  If you score it with an indentation, such as a fold line, that fold line remains in place for a prolonged period.  Nonetheless if you do not assemble your Folded Homes® shelter for the first time within a few months, the score lines can become less pronounced. So don’t delay too long to erect your shelter. Once the shelter has been erected and stays in place for some time, the plastic will “remember” the folds forever. [See PREFOLDING EXTRUDED PLASTIC PARTS]
  • Polypropylene is foldable:  You can use it for things like hinges since it can be repeatedly bent back and forth along a score line and will not easily break.  In fact, we recommend that you pre-fold your structure along all the score lines by folding them back 180 degrees against themselves before creating the three-dimensional part.  Such “prefolding” makes it easier to form the shape crisply.
  • Polypropylene won’t tear: You cannot tear a polypropylene sheet.  Neither can a bear; one tried at our mountain test site and all he managed to do was punch some holes in the wall with his claws and teeth.  But he gave up and went away.
  • Polypropylene is extremely strong and rigid: Over short distances it is quite difficult to bend making it ideal for Folded Homes® tab and slot technology.  Once you lock a tab into a slot, it is next to impossible to tear that tab out. The tab can be ‘unlocked’ by carefully pushing it back through, but it won’t tear out under tension.
  • Polypropylene performs well in hot and cold environments: The plastic will not crack or flow in any temperature your shelter is likely to encounter.  That said, we do not recommend assembling your yurt in extremely cold conditions since the plastic is harder to fold.  And please note that polypropylene will burn. So do not expose it to open flame under any circumstances.
  • Polypropylene is impervious to water damage. You could set your yurt up in a swamp and leave it there, and no harm would come to it.  Since it is 100% waterproof, it is an ideal shelter material for wet and humid environments.
  • Polypropylene is non-organic and safe: Unlike cotton tents, or wood structures, bugs, critters and mold won’t eat it.  And even if your baby tries to, it can’t get sick from it because the polypropylene is chemically inert.
  • Polypropylene is completely recyclable: If you ever need to dispose of your plastic shelter, it can be safely recycled.
  • Polypropylene comes in a variety of colors as well as in a translucent variety: Folded Homes® shelters manufactured out of translucent polypropylene are wonderfully bright inside.  You get the benefits of sunlight while retaining the privacy of opacity.

Coroplast (www.coroplast.com) is one of the major manufacturers of polypropylene extruded plastic sheets.  Here’s what they say about the material.

Coroplast™ is a high-quality polypropylene twin-wall profile sheet… Coroplast™ uses a copolymer resin in order to increase impact and low temperature performance. Copolymer resins are also used because they retain the ability to be flexed an unlimited number of times without breaking. We call this unique ability "a living hinge". Chemically, the sheet is inert, with a NIL pH factor. At regular temperatures most oils, solvents and water have no effect, allowing it to perform under adverse weather conditions or as a product component exposed to harsh chemicals. Coroplast™ twin-wall profile sheets can be modified with additives, which are melt-blended into the sheet to meet the specific needs that include: ultra violet protection, anti-static, flame retardancy, and color.

Health and Safety Aspect of Coroplast Products
Technical Bulletin - CSS-033-93

General Toxicity
The name Coroplast applies to a wide range of extruded corrugated plastic sheet products based on polypropylene copolymers. The natural polymer is chemically inert and is generally considered non-toxic and safe for use in contact with food. The base resin meets FDA requirements as listed in Food Additive Regulation Title 21, Section 177.1520(c), Item 1.1, covering food contact uses.

Pigmented or otherwise modified sheets are not considered to constitute any extra health hazard under normal handling and conversion. All additives are melt-blended into the polymer and encapsulated. Colors are available which will meet food contact approvals.

Effect of Heat / Combustion
Coroplast is made from a combustible thermoplastic material, polypropylene. While discretion would suggest that observation of precautions consistent with regulatory codes and standards should be followed when working with Coroplast, it is equally important to note that compared to other plastics on the market Coroplast is very low on a relative hazard scale.

Two areas of flammability hazard must be addressed: rate of combustion and toxicity of combustion products.

Rate of Combustion
There are many flammability tests in use for different applications in the various parts of North America. A test valid in one area is not acceptable in another. We will try to offer information which will allow the relative hazard of Coroplast to be evaluated. If a flame retardant sheet is needed, ask about our Firewall FRB Brand Product.

In the United States the most generally accepted Surface Flame Spread Test is the ASTM E-84 Tunnel test. This test involves suspending a test specimen in the ceiling of the tunnel and igniting one end in the presence of a forced air stream.

This test is not applicable to thermoplastic material because they will melt out and fall to the floor, prejudicing the result. By this test Coroplast received an unrealistic low 25 (Red Oak is 100).

In Canada, this test has been modified to take thermoplastics into account and samples are tested in the same tunnel but on the floor (ULC-S102.2-78). By this test the 4mm natural sheet exhibited a more realistic Surface Flame Spread of 178 with Smoke Developed of 200 and Fuel Contributed of 100.

One of the factors not visible in the test figure is the length of time to get the fire going.

Until the sheet reaches a temperature of approximately 600 degrees F, it will not release flammable - low molecular weight hydrocarbons.

Should a fire occur, any available fire extinguisher may be used. In a limited fire situation dry chemical powder extinguishers have proven very successful. In a larger scale fire, water sprays/ sprinkler systems are very successful because they quickly cool and damp down the fire.

Toxicity of Combustion
In a fire situation the amount of smoke and the toxicity of the smoke is perhaps more serious than the burn rate. Smoke evolution is dependent on the available oxygen present, but polypropylene copolymer normally generates little smoke. This is not true for many other thermoplastics. The compounds of combustion of polyolefin plastics are not highly toxic except for carbon monoxide which will be formed when any organic matter - e.g. paper, wood or gasoline burns. In fact, the carbon monoxide given off by burning Coroplast is less than for cardboard or hardboard.

% Carbon Monoxide (CO) W/W evolved from Coroplast, cardboard and hardboard.

Temp. Celsius

% Oxygen v/v

Percentage of Carbon Monoxide & CO W/W Evolved from





















While Coroplast will burn, it does not have a flash point or an uncontrollable flame spread rate like some acrylics or styrene. It responds very much like paper. Should a fire start it is easily extinguished by any type of extinguisher. Burning Coroplast generates combustion products with very low toxicity. This is common to polyolefin polymers.

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Human beings rarely do things perfectly.  And they often don’t read the assembly manual… For example, shelter designs that rely upon perfect or near perfect assembly to be waterproof tend to leak.  If any critical characteristic of a design, like structural integrity or keeping the elements out, depends upon perfect assembly or maintenance, those characteristics will be candidates for failure.  

Folded Homes® yurt technology has been designed to be fault tolerant. Making the structure waterproof does not depend upon perfect assembly.  If you leave out a few bolts, the structure is unlikely to collapse.  And if there is a problem, it is generally easy to fix. [See ASSEMBLY TECHNOLOGY (FH YURTS), ASSEMBLY – WHAT TOOLS ARE REQUIRED? (FH YURTS), and KEEPING DRY - ROOF DESIGN (FH YURTS)]

Keeping it simple is a Folded Homes® company mantra. UtiYurts in particular are assembled from only six different plastic parts and three pre-assembled metal parts. TekYurts with their fancy latching and locking doors and windows, connector kits and so forth have a few more parts.

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FH Yurts will burn; they are made out of polypropylene plastic. [See EXTRUDED PLASTIC – WHAT’S THAT?] Open flames should never be permitted near to or inside a FH Yurt.

The good news is that the rate of combustion of polypropylene is not explosive.  Even in the presence of direct flame, it takes a while for standard polypropylene to catch fire.  Folded Homes®  conducted burn tests on the polypropylene used to produce standard FH Yurts.  See for yourself.

Fires like chimney-like structures; places where a current of hot air can suck the flame upward, generating heat and expanding the flame.  The fluted structure of extruded plastic can be thought of as a host of small chimneys.  The wall and roof segments of the FH Yurt can be thought of as a set of twenty large chimneys.  These chimneys create ideal conditions for flame propagation.

This 90-second video clip shows the effect of exposing the flutes of polypropylene plastic to direct flame.  It is significant to note that after 90 seconds it was easy to blow out the flame with one breath.

It takes longer to ignite a material if it is not a chimney structure.  This 180-second video clip demonstrates that it takes twice as long to generate the same flame when the non-fluted surface of the extruded plastic is exposed to flame. The entire exterior surface of a FH Yurt and most of the interior surface of a FH Yurt is not fluted. Note that flame spread is not rapid and explosive.

Here are the technical details.

Toxicity of Combustion: “In a fire situation the amount of smoke and the toxicity of the smoke is perhaps more serious than the burn rate. Smoke evolution is dependent on the available oxygen present, but polypropylene copolymer normally generates little smoke. This is not true for many other thermoplastics. The compounds of combustion of polyolefin plastics are not highly toxic except for carbon monoxide which will be formed when any organic matter - e.g. paper, wood or gasoline burns. In fact, the carbon monoxide given off by burning [polypropylene] is less than for cardboard or hardboard.”[1]

Flame Spread: FH Yurts are Class C structures from the perspective of flame spread.

“Flame Spread [is] the measure of a material’s relative burning behavior. Both the flame spread and smoke developed are measured in accordance with ASTM E84.” [2]

“The Flame Spread Rating [is] a measure of the relative flame spread, and smoke development, from a material being tested. The flame spread rating is a single number comparing the flame spread of a material with red oak, arbitrarily given the number 100 and asbestos cement board with a flame spread of 0. Building codes require a maximum flame spread of 25 for insulation installed in exposed locations.”[3]

“Flame Spread Classes: the UBC and BOCA codes use the I-II-III designation, and the Standard code uses A-B-C.  The flame spread categories are as follows per ASTM E-84/UL 723:

  • Class A or I: Flame spread 25 or less (Fire Retardant Treated Wood, some Fire Retardant surface coatings)
  • Class B or II: Flame spread 26 to 75 (other Fire Retardant surface coatings)
  • Class C or III: Flame spread 76 to 200 (untreated lumber and plywood)”[4]

[1] Health and Safety Aspect of Coroplast Products - Technical Bulletin - CSS-033-93
[2] www.usg.com
[3] www.electromn.com
[4] www.uslumber.com


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Although extruded plastic is not biodegradable, it is clean, non-toxic and recyclable.  Over the long run, unless it is protected from the sun’s UV, sunlight makes it brittle and it disintegrates into a fine powder.


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Folded Homes® expects that the useful life of a standard FH Yurt will substantially exceed the FH Yurt warranty period.  [See WARRANTY (FH YURTS)] How much longer the FH Yurt will perform satisfactorily is dependent upon a number of factors: environmental conditions, plastic composition, and whether additional steps are taken to mitigate the known degradation characteristics of polypropylene plastic. [See UV-ENHANCED PLASTIC (FH YURTS)]

Folded Homes® believes that properly cared for FH Yurt structures can have a long, multi-year life span.

Environmental Conditions: Since the primary degradation mechanism for polypropylene plastic is UV-degradation, the amount of sun that the FH Yurt is exposed to has a direct impact upon the rate of degradation.  A FH Yurt set up in the middle of the Sahara desert will get more sun than one set up in the woods in northern Canada. [See EXTRUDED PLASTIC – WHAT’S THAT?]

Plastic Composition: FH Yurt plastic material comes in two flavors; with and without UV-additives.  Use of the latter roughly doubles life-expectancy by slowing but not preventing the gradual degradation of the plastic. [See UV-ENHANCED PLASTIC (FH YURTS)]

Preventing UV-Degradation: If the sun’s UV can’t get to the plastic, it can’t degrade it.  Any steps you can take to block to UV prolong the life of your FH Yurt. [See EXTENDING THE LIFE OF YOUR SHELTER (FH YURTS)]

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Ventilation results from the pressure difference across an opening. The pressure difference can be produced mechanically (e.g. by a fan), by the action of wind, or by temperature differential since warmer air tends to rise and cooler air tends to fall.  Mechanical ventilation systems and wind action are said to produce positive airflow.

Passive ventilation is ventilation by other than mechanical means. A key benefit of passive ventilation systems is that they rely upon the ambient energy of the environment and do not require some additional power source.  Passive ventilation systems can be disadvantageous in climatic environments subject to both high heat and humidity together.  In the absence of positive airflow such environments can favor mildew growth.

Folded Homes® yurts harness passive ventilation. [See KEEPING COOL – PASSIVE VENTILATION IN FH YURTS]

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We tested a FH Yurt at 8,600’ (2,620 m) altitude in the Sierra Mountain range over the 2006/2007 winter. At one point there was an accumulation of 4’ (1.22 m) of snow.  The FH Yurt had no trouble supporting that load.  The only damage the FH Yurt sustained that season was from a bear attack…


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