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- Feb 1, 2015
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We just had our tank filled today. The first in two years. $1.75/gal.
So you need double the thickness of open cell to match closed in insulating value. So the economics are easy to calculate for heat flow. Next is moisture control. On one side you want to stop it, on the other side you want to get rid of it. It takes very little moisture at cold temperatures to condense and become water. Water brings with it mold, rot and loss of insulating value. Some way of stopping moisture from getting into the insulation is needed and if it does get in a way out before damage can occur. Where I live it gets moderately chilly in winter (at worst only -15F) so an interior vapor barrier is used. The assumption being that the outside of the wall or roof will be less moisture tight than the inside. There are places where the climate and direction of heat & moisture flow may be different. Closed cell foam is about as good of moisture stopper as you can get. Fiberglass batts have pretty good barriers but they are not continuous, seams, electrical outlets, poor installation ... Plastic sheeting is probably better but still leaks around electrical outlets a little.Closed cell has an R value of 7 per inch. Open cell has an R value of 3.8 per inch.