Non-Insulating Weight Blanket

The Idea

When I sleep I like to have a medium to heavy weight blanket on me. I'm also a little quirky because I also need to cover as much skin as possible, so I typically sleep with socks, long pants and long sleeves. On warm nights I obviously get too hot with all that insulation, so I have to toss the blanket and/or the clothes. This makes it more difficult for me to get a good night's sleep. I'm sure I could learn to live without all my securities, but I like to increase my quality of life when it's convenient. Some people grew up in more difficult circumstances where such securities are practically a necessity. These types of dependencies are commonly tied to autism and Down's syndrome. I identify with people on the spectrum and often suspect and joke that I, myself, am on the spectrum. I've remember one parent's story about how their high needs child would come home at the end of the school day and burrow into a pile of blankets to recover from the unpredictable social jungle that is called Junior High. So I've thought to myself, "Wouldn't it be nice to have the security without the added discomfort?" That's how I came up with my idea of the non-insulating weight blanket. It's a blanket of equivalent weight without the insulating properties. I want one of these blankets, but I can't find one sold anywhere, so I'm going to have to develop it myself.

 Progress

 My favorite blanket right now is a denim blanket made by my wife. It's about the perfect density and provides a secure level of pressure, but it probably falls on the lighter side of a good security blanket. It's 56 inches wide, 70 inches tall, and weighs 4.8 pounds. That's a pressure of 0.0012 psi (or 2.5 feet of air, 0.9 kg/m^3, 84 microbar, 8 pascal, etc.). I found some blankets made for people with sensory processing disorder. A blanket of similar size can be up to 25 pounds, but I think those are designed for temporary therapeutic relief and not meant to be used for an entire night. In any case it seems the acceptable range of pressure is fairly large.

Critical Ratio

My first inclination in such a design is to modify the thickness to possibly exploit the critical insulation ratio. When you insulate a round object the added insulation slows down conduction but speeds up convection. There is a critical insulation thickness such that when your insulation is below the thickness you speed up heat transfer by adding insulation. And when your insulation thickness is above critical adding insulation slows down heat transfer. So my first thought is to estimate the critical ratio for a blanket around a person.

Without going through a bunch of explanation (here's a good link for that) the critical ratio basically comes down to the rate of conduction vs the rate of convection, or k/h. I estimate the thermal conductivity (k) of a denim blanket to be around 0.04 W/m*K. Estimating the convection coefficient (h) is a little harder, but to just get in the very rough ball park let's think about a human body. The internal temp is 37 degrees C, and the room temp is 20C giving a delta of 17C (or Kelvins if you wanna be technical). Using a guess of 2 square meters of surface area, 80 Watts heat generation that gives around 2 W/m^2*K.

So that gives a critical radius of (0.04 W/m*K)/(2 W/m^2*K) = 2cm. So, um, you might be able to cool down a finger...but your whole body ain't gonna fit inside 2 centimeters.

Mesh and Material

If I can't win by changing the thickness of the blanket then I will have to remove some of the insulating material and turn the blanket into more of a mesh. That will cause a hit in density, so I'll have to modify the material composition to increase density. The ideal material would be thermally conductive, easy to wash, easy to sew or press, flexible and cheap.

Material	rho (gm/cm^3)	k (W/m*K)	Washable	Sew/Press	Flexible	Cost ($/lb)
Dirt / Earth
Polypropylene