Igloos - Guidelines on Building an Igloo

If you are interested in building your own igloo, here is what you can do.

Recently (January 2005) I was curious about how comfortable an igloo might be. So I located some books that claimed to describe how one is made. The authors must never have actually tried to build one, because some of their information was very wrong!

An igloo can have walls that are 6" or 8" thick. That is fine! But the books I had found showed the first row/layer of blocks being essentially vertical, and that is wrong. They neglected the fact that when the final center blocks are installed, they act like the blocks and keystone of a brick arch, they push out sideways really hard! Around 800 years ago, a lot of giant Cathedrals being built in Europe collapsed when the roof was placed on top, because, in the same way it made the walls buckle outward. (For Cathedrals, they eventually solved that with an architectural concept called Flying Buttresses!)

In any case, the bottom row of igloo blocks can NOT sit flat on the ground/snow. They must be slanted inward. So the snow underneath must be packed down at a slanted angle. I recommend around 15°. to 20°. The actual angle needed is dependent on the length of arch desired and the height of the interior. I have never seen truly huge igloos made, but this Engineering logic suggests that it should be possible, and that the completed structure should be incredibly strong, at least until substantial melting occurs! The only practical limitation then would be the compressive strength of the snow/ice blocks. If actual ice is used, which has far greater compressive strength than snow, the arch seems possible of a span of maybe 15 meters or 50 feet, with an interior height of maybe 7 meters or 22 feet.

I may be tempted to consider building just a hyperbolic arch of such ice blocks with those dimensions, except that the climate near Chicago rarely remains below freezing for long enough to make it very worthwhile doing here! If I should ever do it, it might vaguely resemble the Saint Louis Arch, but smaller! For reference sake, using any standard snow for such an arch would ensure immediate failure, as some part of some snow block would certainly contain a structural flaw where that block would disintegrate, thereby dropping the entire arch. I have used a simple method which converts a snow block into an ice block rather quickly and easily. Once I have compressed my snow block in my mold pattern, and stomped it down to be as compressed as is easily possible, I then (slowly) POUR a bucket of COLD water into the mold. The water quickly flows down through the open pore spaces between the snow crystals, and the cold of that snow then freezes the water into those spaces. The SPEED of adding the water is important! If it is added too slowly, the water tends to freeze too high up in the block, leaving the bottom portion to remain un-strengthened snow. If the water is added too fast, or if the water is more than a couple degrees above freezing, it can completely melt all the snow without freezing the water. When done at the right speed, the entire molded snow block becomes a solid block of ice. My experience is that it is NOT a semi-clear block which would form if water is simply frozen into an ice-cube. Instead, the blocks tend to become milky-colored semi-transparent blocks of ice. Note that a compressed block of snow might weigh 20 pounds but after this conversion to solid ice, it might weigh more than 50 pounds! (In addition, I am very aware that I will certainly need to have some scaffolding to support the two sides of my proposed arch, until the center keystone arch piece is added at the top, at which time the scaffolding will be unnecessary.)

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The outward force due to the weight of the middle top blocks is then applied to that slanted surface, and as long as the overall shape is close to being hyperbolic, those forces do not act to kink the walls outward. The only resulting force that then exists acts to try to crush the ice blocks, and so the compressive strength of the ice or snow represents the limiting factor.

I have received some criticism regarding the above thoughts, where the claim was made that a hemi-spherical shape of an igloo is better, allegedly because of the interior height which results. However, that argument is wrong! A hyperbolic arch can have nearly any imaginable interior height! The design of the angle of the base blocks is the important factor. If they are relatively vertical, then an extremely TALL and skinny igloo could be made (resembling the shape of the Saint Louis Arch). On the other hand, a hyperbolic igloo with an interior height of ONE foot is possible (but unusable!), which would resemble the hyperbolic shape of the roofs of many large sports domes. Few people seem to realize that the roof on such enormous buildings are all hyperbolic in shape! Such very low and flat domes REQUIRE an extremely strong (hidden) ring of steel cables to surround it, to keep the dome from immediately flattening out and collapsing.

As a Physicist, I generally describe such shapes as being a hyperbola-of-revolution. This is technically only true if the THICKNESS of the walls is constant all the way up. Since igloo walls tend to get thinner as they go up, the ideal shape is therefore slightly different from a perfect hyperbolic shape. The hyperbola turns out to be the best shape for such an arch where gravity is providing all the forces involved in the structure. Each block is subjected to a similar compressional force with this shape. Once the entire hyperbola is completed, the shape is immensely strong! (I personally doubt that Eskimos were ever very aware of hyperbolas or the Engineering or Physics behind their homes!)

I decided to make a "brick mold" so all my blocks were the same size. This works great for the first two or three layers.

Here is what I made:
I started with four 2x4s 26" long. I used a table saw to cut one side of them to a curve of 36" radius. So these pieces were the full 3.5" wide in the center but only around 1.5" wide at the ends (with one edge still straight. I laid these down as ribs (which were OUTSIDE the box where the snow would be thrown), with the flat side down. Then a piece of 1/4" plywood 12" wide by 21" long was nailed down on top of the curved edges. This formed a surface that is bowed up in the middle. (This would actually eventually be the inside surface of the block.)

Next, two pieces of 1x12 about 28" long will become the sides. The top edge of these were marked and table saw cut to a radius of 44" radius. (This radius was because I decided to make an igloo that was around 7 feet in diameter.) They were nailed into the side ribs, such that the top edge curve is 8" above the curved floor, but only with two big nails right now! Those sides will be spread out at the top (which will be the outside of the igloo) so that each higher block is tilted a little more inward.

I got two short pieces of 2x12", around 15" long for the end pieces. Both ends are cut at a slight angle so that one edge (the top) will be the 15" while the other (the bottom) will be 12". So a triangular piece 1.5" in maximum width is cut off each end. These pieces have to be pushed down between the sides to spread them out, all the way down to the bottom. Now, these end pieces will NOT be nailed straight across! They will be nailed at the farthest extreme of the floor (to one side piece), but they will be tilted in 1.5" before being nailed to the other side. Both ends are angled in like that to make the top of a block 3" narrower than the bottom.

This is a rather complex shape! The curve of the floor of the mold will be the inside wall of the igloo, and they are nice and smooth. The blocks start out at the ground 15° or 20° and the top of each block is tilted inward a little more (due to the form sides being flared out. Each block has a top that is 3" smaller than the bottom, because as the walls slant in the total circumference of a layer gets less.

It is a great idea to line the entire inside of the form with black polyethylene (like a tarp) or Tyvek or some other plastic sheeting, so the blocks come out easier. Otherwise they tend to want to stick inside, and getting some out can be a real annoyance!

Making blocks is really easy! Using a shovel, pitch loose snow into the form. Then step on it to pack it down, and even jump up and down on it! Pitch more snow in and pack it again. I found that three fills usually was enough snow for a full block that was really packed and solid. You can use a straightedge to smooth out the top, but I usually just left my footprints visible (on the outside of the igloo)! The filling and packing takes about 3 minutes per block. Once the form is filled, it is just rolled over and slightly shaken. The block MAY slide out! You can't shake too violently or drop it without possibly breaking the block in the process. When blocks did not fall out (which was common when I did not use the plastic liner), I found that a standard hand saw could loosen the block from all four sides and the block always then dropped out. Then just start laying them down for the first layer, and then the second.

By the time you get up to the third row, a new problem occurs. The top of the second row is so slanted that the third row blocks will want to slide inward! Until you get the entire row around, where they can press against each other sideways, temporary supports are needed for each of the blocks. These don't have to be very strong, but there have to be around ten of them. Scrap pieces of 2x4 work fine, but all should be the same length. Once the entire layer is in place, the supports can be removed, and the blocks will slightly press together. This third row of blocks is above my entry tunnel, and so the fact that the circle of blocks is complete allows them to press against each other to actually support themselves, but only AFTER the entire layer is in place.

Each row above does the same. Above the third row, I found it desirable to make a different "brick mold", much narrower in width (circumference on the igloo), but still 8" thick and 12" in height. The curvature of the bottom of the mold needs to be of tighter radius, although the smaller blocks makes the radius less important.

The very top piece gives you a choice. It could be left out completely, remaining a 12" diameter open space as a chimney for a fire. It could be a cork-shaped (tapered) piece to close it up completely. Or it could be a cork-shaped piece that has a rather small hole through the middle. Even if that keystone block is left out and open, the structural integrity of the COMPLETE CIRCLES OF BLOCKS in the upper layers, ensures great strength if the igloo.

As long as none of your blocks disintegrate, the igloo will be complete and very solid. I am pretty sure you could climb on top although I did not test that. Keep in mind that if ANY block disintegrates, then the circumferential pressure loading would immediately be released, and that row would likely all immediately collapse in, along with all the layers above it! So it pays to really stomp down on the snow in the form, to try to produce the greatest compressive strength in each block that you can!

I found that when the temperature was extremely cold (below 10°F) the snow did not pack very well, and some blocks were questionable. When the temperature was around 20 to 25°F, the snow seemed to pack extremely well, and the blocks were all impressively strong.

I placed a piece of 1" thick blue styrofoam insulation on the floor and my sleeping bag on that. It was surprisingly warm and cozy, even without any fire or heater. It turns out that a human body creates and releases around 400 Btu of heat every hour, and the excellent thermal insulation of the air-spaces inside the snow blocks provides really good thermal insulation. So even with NO stove at all, just the heat from my body, trapped inside the igloo, made the interior temperature surprisingly tolerable!

I closed the top piece of the igloo completely, so I couldn't use an open fire, but I used a campstove for some cooking, and very quickly it got up to around 80°F inside, so warm that I started sweating! I turned the cookstove way down after the cooking was done, and it was great inside. I wound up wearing a tee shirt and no jacket or sweatshirt at all! Without the cookstove, a sweatshirt was necessary, and the sleeping bag at night.

The heat inside is a good thing. It causes the inside surface to briefly melt and freeze back into a thin layer of ice, which made the igloo even more airtight! A little melting occurred (because I like it pretty warm!) but the water ran down the walls and never bothered me. The walls and ceiling did not seem materially thinner after a night of sleeping bag camping inside it. But I guess it would eventually weaken the walls and roof if it was kept really warm inside.

I was surprised that the light of my camping lantern did NOT make the whole igloo glow brightly from the outside. Only where the seams between the blocks was any significant light visible from outside. Even tents get lit up by a lantern, but I guess that 8" of packed snow blocked the light.

One person could use such a form and make an entire igloo in a couple hours. I worked more leisurely and took parts of three days. If several people work together, just an hour should be necessary.

Observations

I was amazed at how warm and cozy it was inside the igloo. The slab of foam insulation on the ground (R-5) eliminated any cool effect of the ground, and the air inside the igloo was at whatever warmth I wanted. Truly impressive! Of course there was no wind inside, even though outdoors there were some strong winds roaring.

I was surprised at how soundproof it was! Apparently the air-spaces between the snow crystals were sound insulation as well as thermal insulation. The normal sounds of civilization of 19 miles from downtown Chicago were simply not heard inside the igloo.

I stayed in it overnight, and used a campstove and a camp lantern. I wasn't sure how much heat I would need, and was amazed that the two burner propane campstove very quickly got it so warm that I was sweating! Pretty weird to be laying on TOP of a sleeping bag, in underwear, sweating, in an igloo! So I turned off one burner and greatly turned down the other, and kept it at a nice 72°F or so all night. As cozy as being inside my house!

However, I do not see how Eskimos deal with the very cold ground. If I didn't have my slab of insulation on the ground (like along one side), the ground was so cold that it was very penetrating. Actual igloos are built on very deep snow in extremely cold temperatures. Yes, there is some insulation level due to all the air-spaces between the snow crystals, but still! Seems like it would be extremely uncomfortable to be an Eskimo, but only because of the coldness of the floor!

In Chicago area weather, there were mostly days in the 20s and lower 30s, with around 15°F each night. So I was surprised that much of it collapsed after about a week. I realized that the structure was essentially like a brick archway. If even one brick would crumble, the entire arch would immediately collapse, because they are all squeezed together.

It might have been that one of my blocks had some flaw in it, which allowed it to collapse. Weeks later, most of the blocks still survived, with some of the lower walls still standing. Clearly, extreme attention is needed to make sure that not even one of the blocks could crumble!

But it makes me wonder just how long a real igloo lasts in Alaska or northern Canada. Does the heat they have from their fires inside gradually make it disappear? Do they have to build a new igloo each year? Interesting questions that had not occurred to me before my adventure!


This presentation was first placed on the Internet in January 2005.

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C Johnson