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Slab on top of slab: Building science?

I have an existing foundation which is a trench poured monolithic pour, slab on grade with the perimeter foundation walls extending 40 inches to the frost line. It has perimeter stem walls extending 24 inches above the floor slab. The room is unheated, 400 square feet, used 9 months of the year. It has 9 foot walls and vaulted ceilings.

Architectural plans for the room call for the floor to be elevated 14 inches from the existing. The monolithic pour is uninsulated and has no poly in place under it.

My options seem to be compact fill and pour a 5-inch slab or pour the entire 14 inches in place. Although the plan calls out that the room remains unheated, I want to provide an option to insulate the floor. What would be my options for a moisture barrier, insulation and concrete? And where should the components be placed? And insects are a concern also.

Home is located 50 miles outside of New York City.

Asked by Dennis Dipswitch
Posted Tue, 04/29/2014 - 18:04
Edited Wed, 04/30/2014 - 05:25

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9 Answers

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1.
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Dennis,
Here are two choices:

1. From the bottom up: 9 inches of EPS rigid foam, polyethylene, 5 inches of concrete.

2. From the bottom up: 5 inches of sand or crushed stone, 4 inches of rigid foam, polyethylene, 5 inches of concrete.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 04/30/2014 - 05:37

2.
Helpful? 0

I know that Lstiburek has addressed a few things regarding slabs,fill and poly.Am I correct in thinking the poly always goes directly beneath newly placed concrete?

My concern with using fill at all is that the moisture within it would always remain and never be able to dry in either direction...but I guess fill located in the ground in a climate that has rain will always have a fair moisture content?

Would termite treatment be essential to the fill in this sandwich?

Answered by Dennis Dipswitch
Posted Wed, 04/30/2014 - 07:14

3.
Helpful? 0

Dennis,
Q. "Am I correct in thinking the poly always goes directly beneath newly placed concrete?"

A. Yes. (Some concrete contractors have different opinions. Be prepared to stand your ground.)

Q. "My concern with using fill at all is that the moisture within it would always remain and never be able to dry in either direction, but I guess fill located in the ground in a climate that has rain will always have a fair moisture content?"

A. Of course, all layers under your poly can't dry upward. That's normal. In your case, there will be some sideways drying, because your fill will be in contact with your above-grade concrete stem walls.

Q. "Would termite treatment be essential to the fill in this sandwich?"

A. Termite issues (and termite infestation prevention details) are intensely local. Get local advice on this question.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 04/30/2014 - 07:26
Edited Wed, 04/30/2014 - 07:28.

4.
Helpful? 0

Would I want to continue the poly up the foundation wall?

Answered by Dennis Dipswitch
Posted Thu, 05/01/2014 - 17:12

5.
Helpful? 0

Dennis,
Q. "Would I want to continue the poly up the foundation wall?"

A. Assuming that the concrete foundation wall will extend above the height of the new slab -- and it sounds as if it will extend 10 inches above the new slab -- then the answer to your question depends in part on how you intend to insulate and finish the concrete wall that extends above the slab; what type of wall will go above the concrete wall; how the framed wall above the concrete will be insulated; and how you intend to detail the transition between the concrete wall and the above-grade framed wall.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 05/02/2014 - 08:52

6.
Helpful? 0

I intend to place foam vertically from the foam beneath the slab to the top of the concrete foundation.or higher,framed wall to be insulated with fiberglass batt insulation,1" rigid on outside of plywood sheathing on framed wall.Floor to wall transition(where vertical foam is) to get tiled and capped.

Answered by Dennis Dipswitch
Posted Fri, 05/02/2014 - 21:43

7.
Helpful? 0

Dennis,
There is no harm in bringing the polyethylene up vertically against the rigid foam on your concrete wall; nor is there any harm in omitting it. Either way is fine.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 05/03/2014 - 05:02

8.
Helpful? 0

Should there be any consideration on how to address how the top edge of the vertical foam is terminated/capped/sealed?

Answered by Dennis Dipswitch
Posted Sat, 05/03/2014 - 06:37

9.
Helpful? 0

Dennis,
You want to maintain continuity of your insulation layers. The insulation layer of your framed walls should be as continuous as possible with the insulation layer at your concrete wall, with no gaps or thermal bridges. If necessary, these layers can be overlapped at the area where these two insulation systems meet.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 05/03/2014 - 06:49

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