re: OT Engineers—How Insulating is Bubble Wrap?

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So basically air isn’t conductive but it moves easily, so it can convectively move a lot of heat if a lot of it flows past. Right?

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Basically.

The rate of heat transfer via conduction is a function of (among other things) the temperature gradient. In a “pure conduction” scenario the air molecules do not move relative to each other - meaning they basically behave like a solid. So you get a linear temperature gradient across the air gap where each molecule slowly transfers heat to the next, slightly cooler, molecule.

In a “pure convection” scenario, the air molecules can move freely including interacting with the environment. Heat is conducted from the pipe to the closest air molecules. However, the warm air right next to the pipe becomes less dense and basically floats away, being replaced by cooler air from the environment. This creates a much steeper temperature gradient and therefore a higher rate of heat loss.

You still technically have a conductive boundary layer but it’s very thin. So it’s kind of analogous to taking 1” of fiberglass insulation and cutting it down to 1 mm.

The “pure conduction” scenario isn’t real because air is a gas and molecules will always move around to some extent.

The “pure convection” scenario is what happens with no insulation at all.

All insulation materials/methods that use air as a barrier fall somewhere in between. Convection can occur within each cell, but (ideally) not between the cells and the environment. The more cells you have between the heat source and the heat sink, the more it approaches pure conduction. If you have gaps where air can flow freely between the environment and the layers of cells (e.g. bubble wrap that’s not sealed in the ends) then the heat can escape before passing through all of the cells.

The structural material that holds the air in place matters a lot. The smaller the cells get, the more structural material you have. The structure also conducts heat. So typically it’s important that the actual material of foam / aerogel insulation also be fairly non-conductive. Otherwise the heat will just flow through the solid structure and the air in the cells becomes ineffective.

No you dumbass. Read post above this one.

I think he meant “not sure. Air is a very good insulator”