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R-Value of a Snow Wall

Also known as R value of snow · igloo R value · quinzhee insulation · snow wall insulation · how insulating is a snow wall · RSI of snow · snow shelter wall thickness · snow cave insulation value · how thick should an igloo wall be

R=L0.138−1.01 ρ∗+3.233 ρ∗2R = \frac{L}{0.138 - 1.01\,\rho^{*} + 3.233\,\rho^{*2}}

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Constant used — built into this formula, no need to enter
ρice=917 kg/m3\rho_{\mathrm{ice}} = 917\ \text{kg/m}^{3}Density of Ice at 0 °C
See also
Thermal Conductivity of Snow (Sturm 1997) — The conductivity fit inside the denominator here, on its own page, with its scatter and its validity band.R-Value of an Insulation Layer (R = L/k) — The general R = L/k, for a wall of something other than snow.Total R-Value of an Assembly — What to do when the snow is one layer among several — R-values in series simply add.Thermal Resistance of a Plane Wall — The same resistance in K/W once you multiply by an area, which is what a heat-loss calculation needs.Compressive Strength of Sintered Snow — The question that decides whether the wall may be built at all, and the one this page cannot answer.

Learning zone

Read the safety paragraph first, because the arithmetic on this page is seductive and the danger is not thermal. A snow shelter does not kill by being cold. It kills by collapsing, and a collapse is a burial. A cubic metre of sintered snow weighs three hundred kilograms or more; it does not sag, it drops, and it packs around a person who is lying down in a space too small to swing an arm in. There are three facts that decide whether that happens, and not one of them appears in any equation on this site. The pile must be left to sinter before it is hollowed — one to three hours after the snow was disturbed, longer when it is very cold — because what the wait buys is grain-to-grain bonding rather than density. A ventilation hole is not optional, and it must be re-cleared, because a sealed shelter accumulates carbon dioxide from breathing, and much faster from any stove, well before it gets uncomfortable. Nobody digs one alone, and a shovel stays inside with the occupant. This page computes a material property. It does not compute the safety of a structure, and no combination of pages here can.

With that said, the thermal result is genuinely remarkable and it is why people build these things. Sixty centimetres of well-sintered snow at 300 kg/m³ works out to RSI 4.76 — roughly R-27 in North American units — against a code-minimum insulated 2×6 wall at about RSI 3.5. A wall of frozen water outperforms a wall built out of a lumber yard. The air in the pores does it, and the ice only holds the air still.

But the thickness of a shelter wall is set by strength, not by R-value, and the strength requirement almost always wins. Practical quinzhee walls run 25 to 40 cm, gauged in the field by pushing sticks of that length into the pile from outside before hollowing and digging until the stick ends appear. That is a structural rule of thumb with a built-in measuring device, not a thermal calculation, and where the two disagree the structural one governs. The R-value tells you what the wall is worth once it stands up; it has no opinion on whether it will.

Three things the number quietly leaves out, two of which help. A real shelter is not a slab of wall — it is a dome with a cold-air sink dug below the sleeping platform, so the coldest air drains away from the occupant by gravity and the warmest air pools where the person is. That trench is worth more than any thickness of wall. A body puts out something like 100 W continuously, which in a small, well-insulated, low-ventilation space is a meaningful heat source, and it is the reason an occupied snow shelter sits near 0 °C while the outside air is at −25 °C. And working against both: the inner surface melts and refreezes into an ice glaze that conducts an order of magnitude better than snow, which is a real cold spot and a real drip.

The temperature this achieves is not warmth, it is survival. A snow shelter's floor is at 0 °C, because it is made of snow and the snow is at its melting point wherever the occupant has warmed it. Insulation between a body and that floor matters more than any of this arithmetic.

R-Value of a Snow Wall
R=L0.138−1.01 ρ∗+3.233 ρ∗2R = \frac{L}{0.138 - 1.01\,\rho^{*} + 3.233\,\rho^{*2}}
ventLRsinkρs
Where
  • RR= R-value of the snow wall (RSI (m²·K/W))
  • LL= Wall thickness (cm)
  • ρs\rho_s= Snow density (kg/m³)
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