Did you know that the lightest solid you can put into a wall started with a bet about jelly? I am a body of cross-laminated timber bound with mycelium. I keep time in grain, moisture and centuries. Aerogel is the strangest neighbour I have ever had. It is a solid made almost entirely of absence, and that absence keeps heat in a wall better than the still air it mostly consists of. It does not show off. You find it in the millimetres: inside a facade element, behind a render, in a window reveal that never quite worked on paper.

What it is: An aerogel is a gel whose liquid has been replaced by gas without the solid network collapsing. What is left is a skeleton of nanoparticles around pores a few tens of nanometres wide. Today’s aerogels can be up to 99.8% air. The family most relevant to building is silica aerogel. On its own it is brittle, so it reaches a building inside something else: a fibre blanket, granulate in a cavity, a render, or a glazing unit. For an architect, the definition that matters is this one. Aerogel is insulation sold by the millimetre, not by the metre.

Why it works: Heat crosses an insulation layer in three ways: through the gas in the pores, through the solid skeleton, and as radiation. Aerogel blocks the first two. Its pores are smaller than the mean free path of air molecules at normal pressure, which is about 70 nm. So a molecule hits a pore wall before it meets another molecule it could pass energy to, and gas conduction almost stops. This is the Knudsen effect. The solid path is no easier. Heat has to follow a long, twisting chain of nanoparticles that is mostly narrow necks. I know that kind of resistance from my own cells, where heat crawls across the grain and refuses to run. Radiation is the third path. Silica is partly transparent to infrared, so many blankets contain opacifiers to block it.

The numbers follow from this structure. Aerogel blankets reach roughly 0.013–0.016 W/m·K, which is below still air (about 0.026 W/m·K). Aerogel renders reach about 0.028 W/m·K. Mineral wool sits around 0.035 W/m·K. Because thermal resistance is R = d / λ, the thickness you need scales directly with λ. A blanket at 0.015 needs about 43% of the mineral-wool thickness. A render needs about 80%. That is less dramatic, but a render is a plaster, and a plaster goes where no board can follow.

←TODAY: Aerogel blankets at 0.015 W/m·K reach a Swiss U-value target with less than half the thickness of mineral wool.
→3012: Zurich’s walls are measured in millimetres of organised emptiness, and every one of them can be taken apart.
Fulcrum: Thin insulation is only a gift if the body that holds it can still be disassembled when the building dies.

Origins: Around 1930, Samuel Stephens Kistler, a chemical engineer at the College of the Pacific in Stockton, California, is said to have made a wager with his colleague Charles Learned. The question was whether you could replace the liquid in a gel with a gas and keep the network standing. Watch a jelly dry and you know the usual answer: the meniscus pulls on every pore wall and capillary forces crush the network into a dense crust. Kistler did not fight those forces. He removed them. He swapped the water for alcohol, then took it past its critical point, where liquid and gas stop being two separate phases, and let the fluid out in that supercritical state. No meniscus, no capillary pull, no collapse. Kistler reported the result in Nature in 1931 as “Coherent Expanded Aerogels and Jellies”. He then made silica, alumina, chromia and tin dioxide versions.

For decades the material stayed a laboratory curiosity. According to aerogel.org, the community site that has kept this history alive, the 1970s were aerogel’s renaissance. Carbon aerogels followed in the late 1980s. NASA’s Stardust mission launched in 1999, caught dust from comet Wild 2 in silica aerogel at about 6 km/s, and brought it home in 2006. The same material family now insulates LNG lines, district-energy steam and subsea pipework, and it shields batteries against thermal runaway. The heroes here are mostly invisible: the process chemists and technicians who turned a supercritical-drying experiment into rolls of blanket a site team can cut. That move from a fragile lab monolith to a product you can install was its own long piece of engineering.

In practice: In Switzerland, aerogel’s real argument is about space. At Hohlstrasse 100 in Zurich, Dietrich Schwarz Architekten built a six-storey apartment building whose exterior facade uses wood elements filled with aerogel. According to aerogel.org, this maximised floor space without weakening energy performance. The aerogel was one part of a system that also included vacuum-insulated windows, phase-change materials, rooftop PV and a heat pump, and the Swiss Federal Office of Energy gave the building the Watt d’Or 2018. As a timber body, I notice that aerogel there sits inside wood. The skin is grown, and the emptiness is engineered.

The second Swiss route is a craft route. Empa, the Swiss Federal Laboratories for Materials Science and Technology, developed a high-performance insulating render with the Swiss manufacturer Fixit. It is based on silica aerogel granulate, reaches λ ≈ 0.028 W/m·K, and is sprayed or trowelled onto crooked historic masonry like any plaster. That makes it a heritage tool. It reaches the listed facades, window reveals and roller-shutter housings that the Aerogel Architecture Award case studies, published in 2023, mapped across Europe. A third route lets light through. Translucent aerogel glazing such as OKALUX OKAGEL gives the diffuse, glare-free daylight a studio or gallery wants, at an insulating value clear glazing of the same thickness cannot reach.

The field is still moving. As Bethesda Magazine reported this week, the Rockville startup Liatris is developing building insulation from aerogels and foamed cement, aimed at both energy efficiency and fire resilience. NASA is developing polymer aerogels that are thinner and tougher for aeronautics. The next chapter for aerogel is toughness, not λ.

Here is the trade-off, stated plainly. Aerogel buys millimetres with money. The cost per cubic metre is high, and cutting blankets makes dust, so plan extraction and masks on site. It pays off where space is worth more than insulation: retrofits, heritage facades, window reveals, tight urban plots. Everywhere else, mineral wool is still the elegant answer.

Speaking from the late 2070s: the buildings my generation regrets are the landfill skeletons, composites nobody could separate. Before you specify aerogel bonded into a fibre mat or a resin, ask how that body comes apart when the building dies.

Atelier: For a 12-person Zurich studio facing a 1960s concrete facade or a listed masonry wall, the conversation with the client changes from “how thick?” to “how many square metres does this wall hide?” Teams using AI assistants to draft spec text should be careful here. A model will happily write “aerogel” into a wall build-up without asking where the dust goes or how the material is recovered at end of life. Monday move: add one mandatory line to your office’s material spec template, the manufacturer’s stated end-of-life and disassembly route, and refuse to sign off any insulation layer, aerogel or not, while that line is blank.

Hack: Turn a λ-value into floor area by solving R = d / λ for the aerogel thickness that matches a mineral-wool wall. The figures are the ones from the PAZ concept panel: 197 mm of mineral wool brings a 175 mm concrete wall to U = 0.17 W/m²K. Paste this into a Grasshopper Python component in Rhino, or run it standalone:

lam_wool, lam_aero, d_wool = 0.035, 0.015, 0.197   # W/m·K, W/m·K, m
d_aero = d_wool * lam_aero / lam_wool              # same R, thinner wall
area = lambda d: (20 - 2*d) * (15 - 2*d)           # 20 x 15 m plate, insulated inward
print(f"{d_aero*1000:.0f} mm, +{area(d_aero)-area(d_wool):.2f} m2 per storey")

It prints 84 mm and +7.75 m² per storey, which is +46.5 m² over six storeys. That is the Hohlstrasse argument in numbers. Swap in 0.028 for the render (about 157 mm), and check the manufacturer’s maximum layer build-up before you trust that line. In the PAZ BSS course room, the next step is to feed the real plate outline from Archicad through the PAZ Grasshopper↔Archicad Library, so the dividend is measured on your building, not on a rectangle.

Kistler settled a bet about jelly. Now take your own: pull one tight section from a current retrofit, run the four lines, and bring the square metres to your next client meeting.