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Insulation beneath concrete slab---or not.

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Posted by fryster on June 8, 2006, 9:22 am
 

I've read conflicting opinions about this, so I'll toss it out to the
group.  Suppose I am building a passive solar house with south-facing
windows (in nothern hemisphere), and I would like to store the
accumulated solar gain in the thermal mass. Besides storing heat in the
walls, the house also  typically stores heat in a concrete slab in the
floor.  It is natural to insulate the exterior of the walls to keep in
the heat, but how about insulating beneath the concrete slab?

There seems to be 2 lines of thought. Some say, yes, insulate beneath
the slab, as wet earth in contact with the slab will suck all of the
heat out of the slab.  Others think that the slab should be in direct
contact with the earth, as the goal is to couple the thermal  mass of
the house to the nearly constant temperature fo the earth (about 55 deg
F 10 feet below grade). For example at

 http://www.dreamgreenhomes.com/plans/chezsoleil.htm

is a description of a design of a passive solar house.
------------------------------------------------
The lower part of the foundation is not insulated; it is coupled to the
warmer subsoil, which stays at about 70 degrees all year. This system
allows the home's thick thermal mass walls to dissipate heat during the
summer, and absorb and retain heat when it is cool.
------------------------------------------------

My intuition is that the lack of insulation beneath the slab works in
hot and dry locations, where cooling is more important than heating
throughout most of the day, but not in cold and wet places where heat
can flow the slab to the ground underneath.  I'd like ot hear more
informed and experienced opinions however.


Posted by Jeff on June 8, 2006, 10:09 am
 
fryster@burntmail.com wrote:


   Don't take my words as coming from someone that is either informed or
expert...

But, it seems to me that would be a very difficult place to store heat.
Hard to insulate well and with a very  large thermal mass.

   Look at the Seattle Building codes:

<URL: http://www.ci.seattle.wa.us/dpd/energy/res/Sec1003.htm  >

Take a look at the below grade slab F factors also.

Perimeter insulation appears to be a good idea:

<URL:
http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic 490
 >

   Jeff



Posted by rebel on June 8, 2006, 11:06 am
 I've read conflicting opinions about this, so I'll toss it out to the

   Don't take my words as coming from someone that is either informed or
expert...

But, it seems to me that would be a very difficult place to store heat.
Hard to insulate well and with a very  large thermal mass.

   Look at the Seattle Building codes:

<URL: http://www.ci.seattle.wa.us/dpd/energy/res/Sec1003.htm  >

Take a look at the below grade slab F factors also.

Perimeter insulation appears to be a good idea:

<URL:
http://www.eere.energy.gov/consumer/your_home/insulation_airsealing/index.cfm/mytopic 490
 >

   Jeff


/////////////
Here in the Uk. floating concrete floors are the norm. the concrete is
poured onto 100mm thick polystyrene foam blocks



Posted by Ron Rosenfeld on June 8, 2006, 11:45 am
 On 8 Jun 2006 02:22:51 -0700, fryster@burntmail.com wrote:


I have no personal opinion.  However, our home was designed by an
evironmentally conscious pair of licensed architects to be reasonably
energy efficient.  One was a former head of the Maine Solar Energy group.  

We have a full basement (common in the Northeast) and they specified
insulation below the slab; as well as external to the foundation walls.


-- ron  (off the grid in Downeast Maine)

Posted by News on June 8, 2006, 1:42 pm
 


That is my understanding too. I read of some research being conducted once
having externally insulated perimeter foundation walls running 20 to 25 foot
deep. The base had no insulation. The idea was to have the base of the house
on ground that had a stable temperature.  The heat would move up and down
the insulated square block of earth coupled to the deep stable temperature
earth beneath which has no, to little, influence from the surrounding earth.
I never did see any test results.



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