A material's ability to insulate depends on its "thermal effusivity". This is a measure of how quickly a material allows heat to be exchanged from one side of the material to the other. It depends both on the material's ability to store heat (its heat capacity) and its ability to transport heat (heat conductivity.) The chemical makeup of the material helps determine these as well as its physical structure, (how dense the threads are woven, layers or not, etc.)

In terms of an experiment to test a materials ability, one way would be to take a water bottle filled with hot water (use a thermometer) wrap the bottle in a piece of fabric, and put it in a freezer or refrigerator (also need to know the temperature of the freezer) for a certain time (~2 min maybe) Take the water bottle out and test the new temperature of the water. You could also do this in reverse (put an icy cold water bottle in the fabric and let it sit out in the warm air for a few min.)

Hopefully, different materials will give different changes in temperature, the smaller the change in temperature, the better the insulator.

The insulating properties of materials depend on the chemical type of the material and how it is constructed.Cotton is a type of carbohydrate. Wool is a type of protein. If the material is very thick, it will insulate better than if it is woven loosely. If the material is fluffy, it will hold a lot of air, and that will be a good insulator, too.

Another of the insulating properties of clothing is how well it pulls the sweat off our skin. When the sweat sits on our skin, it evaporates - and that makes us colder. But some materials pull, or 'wick', the sweat away from the skin onto the material, so the wet sweat doesn't stay on our skin, where it evaporates and makes us cold. I think polyester is good for wicking the sweat off our skin.

We can get better insulation by wearing layers of clothing when we want to stay warm. More layers give more insulation.

It would be fun to test different fabrics or materials for how well they insulate. Maybe you could have a thermometer that reads temperatures close to room temperature, and a table lamp with a light bulb that gets hot, like an incandescent light bulb. Then you could put the thermometer under different fabrics, under the lamp, and see how fast the temperature rises. If the material is a good insulator, the temperature won't rise as fast.

You will want to measure the room temperature each time you do experiments, and write it down. Then, you will want to measure how much the temperature rises for the thermometer under the light bulb, with no fabric covering it. That is your baseline. It tells you how much the temperature rises with no insulation.

You will want to do some experiments to find conditions, where there is a nice temperature rise. It would be nice to have the temperature rise a few degrees in about 10 minutes. Then you could measure the temperature rise in 10 min for different materials.
1. How far away does the hot light bulb need to be? Maybe the temperature change in 10 min is too small. You could get a faster temperature change by putting the material and the thermometer closer to the light bulb - maybe by putting some books under them.
2. How fast does your thermometer respond to temperature changes? For example, after it heats up, and you move it back to a place in the room that is at `room temperature, how long does it take for the thermometer to cool back down to room temperature? It would be nice to have a thermometer that responds fast, but that is not necessary.

You will want to be careful to make all your measurements under the same conditions. For example, after you make one measurement, the table will be warm where the lamp's light was shining; so, for the next measurement, you will want to move the lamp to a place on the table that is not warm.

You can measure the insulating properties of materials other than fabrics. How fast does the temperature rise if you put a piece of plastic wrap or aluminum foil or glass or bubble wrap or wood under the light bulb and on top of the thermometer? What happens if you test a flat zip-lock baggie, and then put some air in it and test it again? What about bubble wrap with little bubbles, as compared with bubble wrap with big bubbles?

Another experiment with insulation is to measure the temperature change for different numbers of layers of the same material, such as: 1 layer of material, 3 layers, 10 layers, etc.

For a few experiments, or when you are finding good conditions, you can measure the temperature of the thermometer under the material at the beginning - "time zero" - and then at 5 minutes, 10 min, 15 min, and 20 min later. Then you can make a graph of the temperatures and the times.

I like your project. It sounds fun and interesting and useful.

The more air that a material is able to trap, the better it is at insulating. Down traps air very well, and is very warm; nylon, not so well. However, nylon is also air-tight, and so blocks the wind, which down does not.

Put coolers or water bottles loaded with ice inside of pieces of clothing and leave them out during the day and see which ones have the most melting inside of the bottles. The ice needs to be contained, so that when it melts, it won't soak through the clothing (which alters the insulating properties).

Oddly, the most important factor in a material's ability to insulate is usually its ability to trap air. Air is a better insulator than most solids, so the most insulating clothing tends to have lots of air in it. To test out the insulating ability of a piece of clothing, you could try putting a container of hot water (or bags of hot water) in the clothing with a thermometer, and monitoring the decrease in temperature with increasing time.