Replies

1. GBA Editor
   Martin Holladay | Apr 27, 2015 06:29am | #1

   Howard,
   You describe a foundation with stem walls at the perimeter that are 4 feet high -- right? And the bottoms of these 4 ft. stem walls sit on a concrete footing below the frost line -- right?

   One thing you didn't tell us is whether the volume inside of these 4-ft. walls has been filled with gravel and topped by a garage slab -- or whether the volume inside of the stem walls is still empty.

   If I understand correctly, the builder has installed 4 inches of vertical rigid foam on the interior of the stem walls, and has also installed 4 inches of vertical rigid foam on the exterior of the stem walls. Is that right? If so, it's unusual -- unless these are ICF walls.

   I can't visualize the polyethylene you describe, because you haven't told us yet whether we are looking at a garage slab, or just a foundation consisting of 4-ft. stem walls.

2. 
   user-3368124 | Apr 27, 2015 07:24am | #2

   Good Morning, picture a 4' wide trench, square in shape, 5' deep, stone at the bottom, then big footings for a 10" thick wall, then perimeter drain. Foam against foundation, inside and out. the backfilled with stone sand mix. inside and out. Footings are below frost.

   The interior of square is undisturbed original sub soil with stone on top, aprox 4" thick.

   There is no concrete floor yet - plan for floor was plastic over the stone, then 4"' foam, then concrete.

   It is a bit of a hybrid - end use will likely be heated workshop..

   To keep it simple the plastic literally drapes the concrete and foam assembly, then there is sill seal and the pt plate. Plastic may be there just to keep the whole thing dry and was going to be cut away? at least on the outside ?

   Easy to take plastic out entirely or cut out at PT plate edges...

   Does that help...? Thanks, H

3. GBA Editor
   Martin Holladay | Apr 27, 2015 08:25am | #3

   Howard,
   1. The polyethylene is doing no harm. You can leave it. The condensation under the plastic doesn't matter -- after all, it's normal for the soil to be damp. The polyethylene keeps this dampness where it belongs, and prevents the moisture from diffusing upwards.

   2. When you get around to pouring your slab, the polyethylene should go between the rigid foam and the concrete -- not under the rigid foam. Here is a link to an article that explains why: Polyethylene Under Concrete Slabs.

4. 
   user-3368124 | Apr 27, 2015 09:27am | #4

   thanks Martin, .... your help is appreciated.

5. Expert Member
   Dana Dorsett | Apr 27, 2015 10:52am | #5

   The short answer on the slab stackup is:

   With the polyethylene under the foam it will have potential reservoir pockets that can accumulate moisture in a flood or high-water table situation, which would then take forever to dry through the slab foam, with potential degradation of the finish floor (depending on what that is.)

   For the sub-slab foam you can cut significant cost using reclaimed or factory cosmetic blem EPS or XPS sourced locally & cheaply from Green Insulation Group (Pullman St. Worcester) or Nationwide Foam (Waverly St. Framingham), at 1/4-1/2 the cost of virgin stock goods. Using reclaimed goods is substantially greener in every respect.

   With R30+ foundation walls in a heated garage it's worth taking it to at least R15 under the slab, maybe even R20 if the slab is the radiation (depending on the temperature you intend to maintain) even with virgin stock goods. But when using reclaimed goods dropping in 5" of foam becomes almost a no-brainer.

6. 
   user-3368124 | Apr 28, 2015 05:53am | #6

   Thank you for the detail, it is helpful. I am aware of the recycled foam. It is great.

   You both have also answered 2 other inquiries for which I owe you a thanks.

   1) Cape Cod moisture and cold concrete floor. Project to be done early this summer,
   2) Radiant slab (insulated) and using excess PV energy to power it. This project I have flip flopped on 4x. As I also have access to cheap propane, via Selkirk. And I really, really want Minisplits as you get AC built right in.
   The flaw in question 2 is that it is my own house, makes everything harder. ... and the home is really 2 distinct systems, being almost 100' long, but easily separated.
   If you guys encourage me, I will start a new thread, it is the wisdom of HVAC, overlaying the nuts and bolts mechanical that is likely necessary. (ie smallest power plant and which fuel, to heat a thick insulated slab, and using hyper heats mini splits at the other end of the house. Since both propane and mini's are "Trade" installed, the pricing has been all over the map)

   I went to your NE trade show and really came away dreaming and scheming about all the options.

   I enjoy the site, the give and take of information, Thanks again, H

7. GBA Editor
   Martin Holladay | Apr 28, 2015 06:11am | #7

   Howard,
   Yes, you should start a new thread about Question #2.

   If you have a grid-connected house with a good net-metering agreement, you don't need to find a way to use up your extra PV power unless your PV system generates more electricity on an annual basis than your home uses. That's possible, but rare.

   It's always going to be less expensive to use electricity to run ductless minisplits that it will be to use electricity to heat water that is pumped through PEX tubing.

8. 
   user-2890856 | Apr 28, 2015 11:16am | #8

   How will you heat your domestic hot water ? There may very well be an opportunity there .

9. 
   user-3368124 | Apr 29, 2015 05:10am | #9

   Good morning, Hot water has either electric or propane option. If that gets touched, likely switch to heat pump water heater.

   I will start a new thread on this project, before it goes any further.

   Thanks again for discussion on foundation and slab.