Article 1756 of alt.architecture:
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From: david.singer@housenet.com (DAVID SINGER)
Newsgroups: alt.architecture
Subject: Re: Insulating a slab
Date: Sun, 20 Feb 1994 09:15:00 GMT
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MR>From: richman@reepicheep.gcn.uoknor.edu (Mike Richman)

MR>I'm trying to figure out the best way to keep a concrete slab warm
MR>enough in winter to allow it to be confortable to walk on barefoot.
MR>The slab will be finished with ceramic tile and the heat is forced hot
MR>air (so no hope of radiant tubes).  Does anyone know if insulating the
MR>slab with styrofoam under the full room (as compared to 2 feet of
MR>perimeter insulation the code calls for) would help, hurt or make no
MR>difference.  Any recommendations?  House is in Oklahoma, so winters
MR>are not too severe.  Would it tend to overheat more in the summer?

Mike, my experience is from Maine, and several working and non-working
attempts under my belt.

Here's the deal:  The soil temperature is going to be lower than the
slab temp, so you want to drain the water away from the slab by building
on crushed stone.  Then you want a layer of blue foam (waterproof is
still important) on top of which you pour your slab.  You also want to
insulate two feet down at the edge of the slab, and, if possible, two
feet out from the edges.  This traps whatever heat is rising from the
ground, keeping the relative temperatures ground/slab as small as
possible, and maximizing the effectiveness of the insulation.

Oh, notice this configuration presumes a free-floating slab.  Your local
codes may require a poured footing, which complicates the heat retention
problem somewhat because the footing will be a significant heat sink.
With a footing, you attempt to insulate *all* of the footing surface,
with the exception of the load-bearing surface.

Continuing with the free-floating slab concept, it will actually lose
most of its heat at the edges exposed to the wind etc.  Insulate heavily
here, and design the house to be backfilled at least a foot above slab
level, if you can. This will help to keep the house cooler in the
summer, as well.

What's left is the task of heating the slab.  You have two options,
which can be used in tandem, if you so design.

The first is to reverse the air flow from the forced air system.
Typically, warm air is driven up the walls, and allowed to flow over the
ceilings to sink toward the center of the room.  This, in theory,
maintains a uniform air temperature across the room (temperature
stratifications will still exist).  By flowing the air over the floor
first, and presuming you have a well-insulated floor, some of the heat
will be forced into the slab.  Pretty slow heating process, and if you
are looking for a warm floor for morning tooties, you will have to have
the furnace going a few hours before you get up.

The second is to absorb sunlight.  Roof design, window placement, house
orientation, and floor composition all have to be factored in for this
to be effective.  For discussion purposes, lets presume the first three
are optimal (that's up to you to find out in another posting, I
suppose).  Floor composition can be as simple as admixing a dark
substance (one approved for the type concrete being poured) so the
sunlight is soaked up.  You can lay slate.  Or, you can lay tiles with
eutectic salt centers.  Roughly speaking, the results are about the
same, more dependent on the first three factors than the floor material.
However, if admixture is a 1, slate is a 3, then eutectic tiles are a
10 in a price scale of 1 to 10.

But, the most important factor of all is usually overlooked by the
designer.  I refer to that as the SO factor.  The Significant Other
suddenly decides that rugs would be nice, and all your investment is
litterally covered up.  Then the SO says, one morning, "Wow, are these
floors cold!"

There has been only one case that I know of where rugs were appropriate,
and that was on a solar house in which the *entire* front was glass, and
the *entire* floor was black.  Summertime, rugs when down real quick!
However, these same people seldom took up the rugs for winter, because
where the heck do you store a house-worth of rugs?

Any little kids to concern yourself with?  There is a reason they are
called "rug rats."  Their heads find hard spots like magnets find nails.
One fall, and it's rug city, courtesy of the SO factor.

David


