Snow shelter are a great way to avoid carrying a tent in the winter. When built properly, they can be very comfortable, especially when the temperature is well below freezing. There are 3 basic kinds of snow shelters: snow caves, quinzhees and igloos. Also, hybrid snow shelters can be built that incorporate other equipment. The simplest of these is building a snow wall around your camp to keep out the wind. There are many configuartions possible that use tarps as the roof and snow as the side walls.
Snow caves are the easiest and fastest to build snow shelter, but a healthy snowpack is required. Snow caves are warmer and more wind resistant than a tent, but are usually more humid.
2m of snow is a good starting point, the more the better. A couple good shovels and probe are the only tools needed. Make sure to probe your potential snowcave site before digging to discover any rocks, stumps, ice layers, or crevasses that might be hidden under the snow.
Snow caves are easiest to dig if you start on a moderate slope. You can dig into the slope, instead of down, which make getting in and out easier. The slope makes it easier to clear the excavated snow away from the entrance. Don't pick a slope that's too steep, or else you will have to put a 90 degree turn in the cave to avoid hitting the ground. 20 to 30 degrees is usually about right. Digging into a slope also make it easier to construct a heat trap - read more on this later.
A good sized snowcave fits 2-4 people. Any larger and construction becomes a difficult and long process. There absolutely must be enough headroom to sit up inside without touching the ceiling, or you will be in for an uncomfortable morning. It's best to dig the ceiling into a high arch. This keeps the cave from sagging too much overnight. Don't go oveboard though - keep the thinnest part at least 30cm thick. Wide, low caves are the worst, and you will wake up with the ceiling 2 inches from your face.
The best method for excavation required 2 people working in tandem. One person is the mole, and goes inside the cave. The other person is the clearer, stands outside the cave, and clears away the snow that is sent out by the mole. The mole goes in head first, and the push the snow down past his/her feet, where the clearer shovels it away down the slope. Once there is enough room that the mole can get his/her entire body inside the main part of the cave, the job becomes a lot easier and this technique does not have to be used. The mole should wear complete waterproof clothing, the clearer can wear whatever they want. You can accelerate taking snow out of the cave by laying a tarp on the floor, have the mole carve the roof with the snow falling on the tarp, then pull out the tarp from outside at regular intervals. This saves a lot of snow shovelling out of the cave.
A good tactic for cold weather is to construct a heat trap. To do this, the floor of the sleeping area should be higher than the ceiling of the entrance, thus preventing cold air from flowing into the sleeping area from outside. A heat trap is easiest to build if your snowcave is dug into a slope, as it is very natural to dig upwards when excavating the entrance tunnel.
Putting s ventillation hole or two in the roof of the cave will keep the humidity down. If snow falls through the holes, cover the top of the hole with a pack or a shovel blade or a block of snow
Smooth walls don't drip, so try and make them very smooth and don't let your sleeping bag touch the walls. A well-placed waterproof jacket or garbage bag might mitigate some drippyness if your wall aren't totally smooth.
The biggest safety risk with sleeping in a snow cave is that someone will walk along and fall through the roof of the cave, so make sure to mark the area well. It is always a good idea to sleep with a shovel inside the cave.
Quinzhees are a Snowcave-igloo hybrid for shallower snowpacks. Snow is piled up to a suitable height, packed, and then dug out from the inside like a snowcave. Quinzhees are much easier to build than igloos, but take a lot more time than a snowcave because of the time required to make the mound. One trick to speed up the mounding process is to pile up all your gear, and then pile snow onto the gear. The gear can then be retrieved during the excavation. Be careful what you use for this, especially if the snow is wet.
Igloos are the mediterranean villas of snow shelters. They are roomy, confortable, and let in a remarkable amount of light. They are also notoriously difficult to construct compared to other snow shelters, and require you to have a snow saw in addition to the usual equipment. Smaller Igloos are easier to build, so start small and work up from there.
To start, a quarry must be packed out. The colder the snow, the more difficult this will be. The quarry needs to be vigourously ski packed (jumping preferred) and then boot packed (again, jumping preffered at the end) and the left to freeze up solid for about an hour. Once the quarry has been packed down, do not walk on it any more because doing so may crack the hardened snow. If there is hard wind slab somewhere in the snowpack, blocks can be cut directly from the slab instead and a quarry is not necesary. The quarry needs to be about 2m by 2m for each person the igloo will hold. This should allow lots of extra to account for the blocks that will inevitably break.
Blocks must be cut from the quarry with a saw in order to get them uniform. All blocks must be nearly exactly the same size, usually about 6"x18"x24". The blocks must be rectangular if they are going to balance properly. Boot packing the snow will usually pack down a hard slab about 10-14 inches thick, meaning the blocks must be cut out horizontally. Use a ski to help slide the blocks out of the quarry without breaking them. Save any pieces of broken blocks for later.
Thicker blocks are sturdier than thinner blocks, but are heavier and more physically challenging to lift into place. A really big, heavy thick block can weight 40 pounds. Whether you choose generally thicker or thinner blocks, try to keep them all the same thickness across the igloo. Once a thinner block has been placed, it's very difficult to place a thicker block on top of it. A slight amount of tapering from the base of the igloo to the celing it ok, but you don't want to be placing any 2" thick blocks anywhere.
The ideal igloo is a perfect hemispherical dome. To keep on target throughout construction, use a ski pole or a stick or something to measure from the centre of the igloo. Start the igloo by planting a ski pole in the center and drawing a circle in the snow using the measuring device. 2, 3 or 4 person iglooes are a good size, but the bigger the igloo the more skill and patience is needed. For 2, the inside radius should be about 115cm, for 3 use 135cm and for 4 use about 150cm. Make sure it is big enough to fit the required number of people before placing blocks. Check again after the first row is complete. The first few blocks must be cut off at an angle to make a ramp for the next row of blocks. To make life easier later on, the entire first row can be cut into a continuous ramp. The first row of blocks should lean in to the centre only very slightly.
Build the igloo in a continuous spiral, keeping at least one person on the inside. Each new block in the spiral is supported from 3 corners. 2 that touch the row below and one top corner that touches the adjacent block in the current row. It's important that all 3 corners of the block have good contact. If they don't, use the saw to trim the shape of the previous row of blocks to allow all 3 corner to touch. Each block should be gently tapped into place and held to allow the snow to sinter (stick) to adjacent blocks. Breathing on the joint (warm moist air) will speed the sintering process. Block corners can be trimmed slightly with a saw to maximize contact with adjacent blocks, making the sintered joint stronger, but make sure not to change the overall shape of the block. If the snow is fairly warm (typical for the coast), pack snow into any cracks around the block to help stick it in place. The only weak point in the igloo under construction is the most recently place block, because it's the only block that is not supported at all 4 corners. Be especially careful not to dislodge it. Once the next block in the row is placed, the block gains an additional point of contact and become much better balanced and stronger. Note that there will be triangular spaces between the blocks, as the square blocks lean in on each other. The spaces will get bigger towards the top of the igloo. Don't worry about filling these spaces until the end.
It is very important to measure the position of each block with the ski pole to make sure it conforms to the desired shape. If there are any deviations (especially in the first couple rows) the igloo wall will not be as strong, and the next row of blocks may be difficult to place. Each block must have more lean than the block below it, or there will be a flat spot in the wall which is a weakness - if you're using a ski pole to measure where to place the blocks, just keeping them perpendicular to the pole should naturally give each block the correct lean.
Towards the top of the igloo, as the angles beome more extreme, it may be necesary to cut some customized blocks. When making specialized blocks, keep them square and adjust the width and/or height of the block to fit the space better. Remember to always leave a corner sticking out to sinter the next block to.
Finally, when the hole in the top is small enough cut a cap to match the size of the remaining hole. The cap is an important structural element in the igloo, so don't skimp out by cutting a really thin cap. The middle person is critical for lowering the cap into place, and it usually takes some trimming it get the cap to fit just right. The cap should sit up against the edges of the final blocks, not on top of them, so that it completes the structure of the dome.
Note that once the dome is complete, the middle person will be built into the igloo. Now is the time to cut out the door. The easiest way is to cut a nice arch shaped hole into the igloo wall. Dig out the snow below the door on the inside and outside of the igloo to make getting in and out easier. If it's windy, you can make a tunnel over the entrance way using leftover blocks, but this is not always necessary.
A little more difficult way to get in/out of the igloo is dig a tunnel and leave the dome intact. This makes the igloo warmer since the low door creates a heat trap,
For large igloos, the height and lean of the wall can make passing blocks in from the outside impossible. Blocks can be stockpiled inside the igloo, but they aren't always the right size. In this case, it may be necessary to cut the door before the igloo is done to allow passing blocks into the middle.
When the dome is complete, there will still be lots of triangular holes between the blocks. Now is the time to fill the holes with pieces of broken blocks, or using any leftover blocks. After chinking the gaps, shovel snow onto the igloo from the outside and tap it down gently with a shovel to seal any remaining cracks.
How to build an Igloo NFB short film from 1949.