Pine Tar, and Change of State


I was reading about a building contractor in the Carolina’s, who builds log houses.  Seems his customers preferred their house be made out of pine logs, rather than the hardwoods he would normally use.  Said a pine house is warmer in winter, and cooler in summer.  He wondered if this was true, and why it would be so, and he hired the university to look into it for him.  Here is what they found.

 Local pine there is called ‘long-leaf yellow pine’, and has a lot of pine tar in it.  It is the tar melting that keeps the house cool, and the tar freezing that keeps it warm.  This needs some background science. 

When something melts, that is, goes from solid to liquid, it soaks up a lot of heat but it stays at that melting temperature until every last bit is melted, then it can warm up.  Think of your cocktail last night, it stayed at freezing temperature until the last bit of ice was melted, then it started to warm up to room temperature.  Same when it freezes, water stays at 32 degrees until every last bit of liquid is gone, then it can go down in temperature.  When you get your head around that one, here is another, it takes a lot more heat to change from solid to liquid, or liquid to gas, than it does just to warm it up.  Think of a tea-kettle, it takes a lot longer to boil it dry (change it all to steam) than it did to get it up to boiling in the first place.  So there is a lot of heat involved in changing from solid to liquid, or liquid to gas.

 So, getting back to those log houses.  South Carolina is pretty pleasant, that is, it is usually in the 70’s in the daytime, and in the 60’s (Fahrenheit) at night.  The tar in those pine log walls melts at about 70 degrees F.  So when it warms up in the morning, and the sun shines on those pine logs the tar starts to melt and soaks up the extra heat, and keeps the temperature of the log (and the house) at 70 degrees.  When the sun goes down in the evening, and the outside temperature falls, that melted tar in the log walls starts to freeze, which gives off heat and keeps the house at 70 degrees. Of course if there is a prolonged hot spell the tar will all be melted and the house warm up, or if there is a prolonged cold spell the tar could be all frozen and the fireplace will have to be lit.  Melting the tar doesn’t affect the log, you can’t tell by looking at it, it doesn’t go all mushy or sag or slump or anything. 

So there you have it, a simple, elegant natural heating and air conditioning system. This is called ‘change of state’ heat storage, and if you google it up you will find commercialized versions of this natural system already exist.  A heat storage consisting of a tank full of tar or wax which melts at room temperature is a really green way to save on heating and cooling costs.  

Our jack pine here in Northwestern Ontario has a lot of tar in it, especially in the stumps and root, and it is not used to make paper, in fact it is mostly burned as fuel.  Maybe if this technology catches on we will have a whole new product coming from our forest. 



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