[TheForge] Re: WAAAAAAAAY OT (while we're talking about

[wmullett at bright.net]

Many of the comments on this site:

http://www.practicalmachinist.com/cgi-bin/ubbcgi/ultimatebb.cgi/topic/ 
1/16725.html?

shows there is a lot of mis-understanding about the function of insulation, especially foundation insulation so I thought I'd point out a few facts.  

It would be great if insulation was a thermal isolator but it is not.  It is only a poor thermal conductor.  All materials will conduct heat but at different rates. Because quiet air is a good insulator, most modern insulations work by trapping air in small enough spaces that they resist thermal convection in the air.  Some insulations trap inert gasses when they are made and have a higher R value at that time but I believe that the gas in all of these eventually is replaced by air so you should always work with the aged value.

All open cell insulations must be kept dry in order to function.  Fiberglass is a very open cell insulation.  If you soak it in water, it no longer functions.  Unless there are proper vapor barriers installed, this is what happens to most houses over the winter months (especially with retro blown in insulation).  The vapor pressure in the house drives moisture into the insulation where it meets the dew point and condenses out.  This is why you will always find water stains in perimeter wood stud walls that have batt insulation.  Without proper barriers and as the winter months progress, batt insulation becomes more and more in-effective.

Not all polystyrene insulations will work below grade.  Bead board (white beaded styrofoam insulation) is an open cell insulation and does not work below grade if it is contacted with water.  Use the pink Foamular or the blue Styrofoam extruded polystyrene insulation for these areas.

Soil is a great insulator.  It's even better if it's dry.  Soil also has a high thermal mass that will hold heat. We normally insulate only the perimeter of buildings at the footing to below frost depth.  Below that, there is very little value because of the ground's insulating value.  

This perimeter insulation can be vertical or horizontal.  If it's horizontal, it can extend inside or ouside the building line and works equally well.  The Europeans use extruded insulation over water lines to protect them from frost and therefore don't have to run their lines as deep as we do.  

You can fully insulate a building slab if you wish to.  Extruded foam insulations have a high compressive strength but why do it if the money returns are not there?  

NEVER - NEVER put foam under an outside slab in freezing climates.  I know an engineer who thought this would be a great idea for his driveway.  He failed to realize that the foam did not allow the cold to penetrate below the slab thus keeping his slab within the dew point range. His driveway was a perpetual sheet of ice even when it didn't snow or rain!  He tore it all out and started over.

Walt