You have to take into account two things: 1. the porosity, meaning air fills the cavities. Can you really talk about open structures and say they're really lig

You have to take into account two things: 1. the porosity, meaning air fills the cavities. Can you really talk about open structures and say they're really light? Make an aluminum balloon filled with air and it's probably lighter than air as well. 2. the buoyancy, meaning that the fibers displace air. If a light material is weighed in air, the result does not give the mass of the object. With normal solids, this does not matter since they are heavy compared to air. For example, you can weigh wood with a volume of 1 liter in normal air pressure, and get a mass result of 0.500 kg. Then weigh it in a vacuum, you should get a 1 gram difference (air density is about 1 gram per liter) - a weight of 0.499 kg. In practice this is in the noise for normal materials. Cloced cell extruded foam is about 30 kg per cubic meter. So here the air mass 1 kg per cubic meter already has a 3% effect. I don't know if the weights are usually quoted with buoyancy or not. But this is closed cell, I don't know how well it stands up to a vacuum. One example of structures that can have a lot of mass but little weight is inflated ones. There have been some human powered aircraft that are really big but light on the scale. Yet they take a lot of time and energy to accelerate since all the air mass inside must be brought up to speed.