Replies

1. 
   MartinHolladay_GBA | Apr 12, 2012 09:52am | #1

   Polyisocyanurate under a concrete slab

   Stealman,

   I am astonished that an Rmax rep would recommend the use of polyiso under a concrete slab. In this application, I would choose either extruded polystyrene (XPS) or Type IX expanded polystyrene (EPS). Polyisocyanurate absorbs moisture, and should therefore never be used below grade (unless it is on the interior side of a dry concrete wall) or under a slab.

   I have reviewed the manufacturer's application recommendations for Rmax Thermasheath; here are the web links:

   http://www.rmaxinc.com/wall-thermasheath3.asp

   http://www.rmaxinc.com/downloads/DataSheets/Thermasheath3.pdf

   These documents list recommended applications -- and these recommended applications do NOT include use below a concrete slab.

   For more information, see the Green Building Advisor encyclopedia:

   http://www.greenbuildingadvisor.com/green-basics/rigid-foam-insulation

1. 
   glacierfergus | Apr 12, 2012 11:29am | #2

   I'll second that...

   and add that pink or blue foam boards are usually cheaper as well.  and they will withstand the pressure better, not absorb water, and there is no real benefit to having foil under a slab. 

1. 
   rich1 | Apr 12, 2012 11:41pm | #5

   trust me pink or blue will absorb water given enough contact time.

2. 
   DubixCube | Feb 08, 2021 10:20am | #13

   referred to this article by a client of mine, which means it is still referred to for information. That means I feel like i should update some of the info in here. The pink and blue material have had to make changes in their chmistry according to new laws. This has made their price go up significantly in some states, making polyiso and equivelent price choice, if not better. (even when comparing thickness, not just R-value)

2. 
   stealman | Apr 12, 2012 10:52pm | #4

   MartinHolladay_GBA wrote:

   Stealman,

   I am astonished that an Rmax rep would recommend the use of polyiso under a concrete slab. In this application, I would choose either extruded polystyrene (XPS) or Type IX expanded polystyrene (EPS). Polyisocyanurate absorbs moisture, and should therefore never be used below grade (unless it is on the interior side of a dry concrete wall) or under a slab.

   I have reviewed the manufacturer's application recommendations for Rmax Thermasheath; here are the web links:

   http://www.rmaxinc.com/wall-thermasheath3.asp

   http://www.rmaxinc.com/downloads/DataSheets/Thermasheath3.pdf

   These documents list recommended applications -- and these recommended applications do NOT include use below a concrete slab.

   For more information, see the Green Building Advisor encyclopedia:

   http://www.greenbuildingadvisor.com/green-basics/rigid-foam-insulation

    You are confusing me. First you say not to use polyiso under a slab and then you say unless it is under a slab on the dry side. That is what I am talking about. Dirt, 6" base rock, vapor barrier then insulation and then conc.

   I talked to the Rmax technical support twice and he insists that is is ok. He says it is closed cell and can hold 1% moisture and is rated at 20psi. I am still Leary because I cant seem to find any other source that will back it up and Rmax web sight does not mention it except in there faq where it says it can be used in this application.

   I actually talked to a person that works at the insulation depot. They sell second hand ridged and he says some people swear by polyiso under a slab. I have asked at pima.org, which is a trade group or something like that, about it and they have not gotten back to me. I will bug them some more.

   The only reason that I am even considering it is because I can get factory seconds for a very good price.

1. 
   ktkcad | Feb 08, 2021 03:43pm | #15

   Reread his comment and take out the statement in parenthases. It would read like this: "...and should therefore never be used below grade ... or under a slab."

3. 
   DubixCube | Feb 08, 2021 10:22am | #14

   Perhaps the Rmax rep knew that polyiso foam isn’t the culprit in drawing moisture into the product, but that embedded glass fibers were, as they wick water. Only a couple brands of polyiso embed glass in their foam... the foam itself is virtually impenetrable.

2. 
   joeh | Apr 12, 2012 04:20pm | #3

   Type II EPS with borate

   Type II is plenty for residential construction. R-control with borates so the bugs don't use it for a next.

   Joe H

3. 
   lbatko | Apr 13, 2012 08:24am | #6

   The foil will deteriorate if its in contact with concrete. The reason for the insulation under the slab is for a thermo break. You will also want to insulate between the slab and foundation. It prevents conduction between the concrete and the ground and or foundation. The R-value is not as important as the break in contuction for heat transfer. Heat rises

1. 
   davidmeiland | Apr 13, 2012 09:51am | #7

   Say what??

   lbatko wrote:

   Heat rises

   NOT TRUE!!

1. 
   KDESIGN | Apr 13, 2012 01:12pm | #8

   General reply to no one in particular:

   Warm air rises.  Heat transfers in any direction it can by conduction and radiation.  With convective transfer, heat rises with the convective medium. 

1. 
   davidmeiland | Apr 13, 2012 02:51pm | #9

   Warm air rises

   when displaced by cold air

1. 
   KDESIGN | Apr 13, 2012 03:21pm | #10

   or liquid.

2. 
   Clewless1 | Apr 16, 2012 11:35pm | #11

   Air IS liquid. Heat rises, but heat travels equally in all directions in terms of conduction (not convection).

4. 
   Clewless1 | May 10, 2012 11:03pm | #12

   I don't think polyiso is  recomended for under slab/below grade ... use extruded polystyrene ... blue, pink or grey board.