Replies

1. 
   NRTRob | Sep 06, 2007 05:37pm | #1

   it's to slow down heat loss, as you might surmise ;)

   the perimeter of the slab and vertical slab edges are the primary sources of heat loss in MOST slabs.

   The center mass of the slab generally requires insulation as well, unless your site conditions are very good for omitting it. Also, the "perimeter" is usually 4', not 2'.

   Also, a piece of wood is no substitute for 2" foam vertical edge insulation.

   -------------------------------------
   -=Northeast Radiant Technology=-
   Radiant Design, Consultation, Parts Supply
   http://www.NRTradiant.com
1. 
   luckyjeff | Sep 07, 2007 04:31am | #2

   Thanks for the reply. The home is in Victoria B.C , Canada. The winters are extremely mild. It usually only freezes one or two days a year. The plans called for both the 2x4 and a 2 foot piece of styrofoam under the slab. It is a full basement and we will be furring out the walls and insulating them down to the slab. Like I said, the whole slab sit on 2 feet of compacted sand. I was just wondering if the 2 foot perimeter in this case is a waste of time?
   Why are the slab edges the primary source of heat loss?Thanks again

2. 
   talkingdog | Sep 07, 2007 04:40am | #3

   In a crawlspace application, around here, where the winter temperature gets down into the low 30's, the normal spec is about an inch and a half of polyiso on the inside of the stemwall, and the same stuff applied horizontally over the slab for about 3 feet in. (they put the insulation on the inside in an attempt to mitigate the termite problem)

   The scientific reasoning is that a constant temperature is naturally maintained year round further towards the center of the slab, so adding insulation would have no effect.

3. 
   WayneL5 | Sep 07, 2007 05:24am | #4

   The two feet around the perimeter is for slabs on grade.  The ground around the building gets cold in winter and the insulation under the outer two feet of slab reduces heat loss.  It is usually not used in the center because the ground is only slightly cooler than the house in the middle of the slab.

   For a full basement there's no reason to put insulation just under the perimeter of the slab.  Either insulate under the entire slab (don't need much that deep in the ground, just an inch will do), or don't insulate any of the slab.  The heat loss downward into the ground is quite small.  If you have radiant heat in the floor you should insulate the slab.

   Have you considered an ICF foundation for the basement walls?  They are a really effective construction method.  If you are going to insulate and finish the walls anyway, they work out great.

1. 
   NRTRob | Sep 07, 2007 04:35pm | #5

   Basement slabs still see more loss on the edges than the underside. Cold conducts from ground level downward. However in residential construction is it usually prudent to insulate under the entire slab. 2" is standard... maybe in milder climates 1" is ok. Depends on the ground conditions I suppose. Whether the fill under the house is sand or not doesn't matter if it can't get rid of water...-------------------------------------
   -=Northeast Radiant Technology=-
   Radiant Design, Consultation, Parts Supply
   http://www.NRTradiant.com

4. 
   joeh | Sep 07, 2007 07:13pm | #6

   There is a PT 2x4 between the slab & wall?

   R value of PT is close to zero, who thought of this?

   You need foam there too, that will be your biggest heat loss.

   Joe H

5. 
   MikeSmith | Sep 07, 2007 07:32pm | #7

   lucky...... i guess they are putting a 2x4 on edge to give you  some nailing  AND  to make the leave-in-place form

   even in your "mild climate "... your floors will feel cold

   i'd do two things:

   1   extend the 2" foam  under the whole slab

   2  tack 2" foam strips to the inside of the 2x4 on edge

   now you have a real thermal break

   some place under your slab there is an infinite sorce of cool earth... just fro arguement let's say  that 3' down the earth is ALWAYS  45 deg

   so your slab  sees 70 deg on one side and 45 deg on the bottom.. that's a 24 hour whole heating season delta-T of 25 deg

   at the edge it's even worse because the ambient might be  20 deg or less

   also....... What type of heating system do you intend to use ?

   Mike Smith Rhode Island : Design / Build / Repair / Restore

   
   

   ---

   Edited 9/7/2007 12:33 pm ET by MikeSmith

6. 
   davidmeiland | Sep 07, 2007 10:52pm | #8

   I'm 20 miles away from you and I couldn't get away with that 2x4 detail. We use 2" foam vertically on the inside of the foundation walls, up to the top of the slab. The top of that foam is beveled at 45 degrees so that the slab does touch the wall, but it's only a hair thick at that point. As the slab shrinks and cures it will pull away from the wall slightly (maybe 1/16"). That and the foam are the thermal break.

   We are allowed to use foam 2' wide around the perimeter if the slab is not heated, but I always use foam under the whole thing. This is in addition to the thermal break.

   Think about it--this is your chance to insulate the slab. Energy is NOT going to be getting cheaper.

1. 
   joeh | Sep 07, 2007 11:40pm | #9

   That's for damn sure.

   This current $3 gal gas will look cheap 10 years from now, could look cheap next week if something major happens in the Middle East.

   Joe H

7. 
   tom21769 | Sep 08, 2007 04:24am | #10

   I have radiant tubing in my slab. When I researched it, some designs showed a donut hole in the center of the foam insulation area. The purpose of this was to use the earth under the center of the slab as a heat sink. I was not convinced to use it, because I thought in my mid-Atlantic region I would want to be able to moderate the temperature quickly on mild days.

8. 
   grpphoto | Sep 08, 2007 05:03am | #11

   It prevents the edges of the slab from becoming much colder than the center section. If the difference gets to be excessive, the slab can crack.

   George Patterson