Re: which is better insulator, wood or styrofom

An interesting question. Heat can be moved or transferred from a hot object to a colder object by radiation (like the heat from the sun), by movement of a fluid (like air or water moving over a surface) or by conduction (when two objects are in direct physical contact with each other). When we talk about insulators, we are talking about moving heat by conduction. Most insulators, like wood, polystyrene, and fiberglass, rely upon trapped air for their insulating properties. Because molecules in air are much farther apart than they are in either liquids or solids, they do not conduct heat nearly as well as a liquid or a solid (since it is the bouncing of molecules next to each other that actually transfers heat.

When insulators are compared for their ability to restrict heat movement, we look at the “R” or resistance values for a particular material. Quite simply, the “R” value is a measurement of how easily heat passes through a material, with higher numbers preventing the movement of heat more effectively than materials with a low number. If one material has an “R” value of 2, and another has an “R” value of 4, then the material with the “R” value of 2 will let twice as much heat through as the one with the value of 4. If you are using two layers of insulation, like a wood wall followed by a polystyrene wall, the R values add together.

If you looked at a table like this one: http://coloradoenergy.org/procorner/stuff/r-values.htm you would see the “R” values for several materials. Please note that the R value listed for some materials is the value per inch, while for others the R value is for the typical thickness. So, on the web page, the R value for a typical 2 by 4 stud (that is 3 ½ inches thick) is shown as 4.38.

Expanded polystyrene (the white insulation made up of a lot of small beads bonded together) has an insulating value of 4 per inch thickness. Thus, if you used three one inch polystyrene boards together to make a wall that is 3 inches thick, the R value would be about 12, roughly 3 times better insulating than wood.

If you look over the table of insulating values, you might notice that the heavier a material is, the poorer it is at insulating things. For example, 4 inch concrete block has an R value of 0.8 compared with the R value for a 2 by 4 of 4.38. Heavy materials have less air space in them than lighter materials, so they have more molecules to transfer heat.

You are also right that being waterproof makes an insulator perform better. Water in insulation helps move heat from one side of the insulation to the other.

When testing insulation, it is important to design your experiment so that you are measuring just the performance of the insulation. For example, if you build a small dog house to test the insulation, make sure that you use a door of the same material, and that the door fits tight so that you don't have air flowing in or out that would add or remove heat. Also, you would want to test both dog houses at the same time, with each receiving the same amount of sun light and shade. Maybe one approach would be to put the same number of ice cubes in each dog house and see how long before all of the ice melts. That way, the temperature on the inside of the dog house is the same.

Good luck with your experiment.