Insulation over a concrete slab on grade

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  #1  
Old 05-12-11, 02:40 PM
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Insulation over a concrete slab on grade

I'm converting my detached 20'x23' garage into a studio apartment (with permits for everything). It was built in the 1930's. My contractor is suggesting that we put a 6mil vapor barrier on the old 4" concrete slab, followed by 2x6's on-edge, then fill the cavities with R-21 kraft covered insulation batts and then plywood subfloor over that. The local city inspector says this will be fine.

Also, the footprint of the building is being extended about 2 feet beyond the old concrete slab (in other words, the wall where the original garage door was is going to be 2' further out). He has poured a new foundation wall at that point but he wants to just leave the interior floor area with dirt and put 6 mil plastic over that section before installing his floor joists.

Does all this sound correct? I'm concerned that moisture will migrate up from the concrete (and dirt in that one area) and stay inside the floor joists, getting the batts wet over time.
 

Last edited by fromhollywood; 05-12-11 at 03:19 PM.
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  #2  
Old 05-12-11, 03:09 PM
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You do not want to use kraft faced batts with plastic already down. The plastic is the vapor barrier and the kraft faced would add a second one. You never want two vapor barriers. That will result in trapped vapor. Plastic vapor barriers seems to be the common way to seal out moisture in crawl spaces with dirt floors. I don't see a problem with it. Make sure the plastic is sealed against the walls with acoustic sealant. Don't forget the seams as well.
 
  #3  
Old 05-12-11, 03:25 PM
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Most of the research I've seen for insulating garage floors seems to be using rigid insulation panels like XPS or ISO. I've seen lots of postings from Britain about doing garage conversions---I guess they do it more than we do in the US. Most of the time I've seen them use foil-faced ISO (Polyisocyanurate) but its very expensive.

Is R-21 fiberglass batting ok over a concrete slab?

I thought there needed to be airspace underneath a floor but if I do it the way the contractor is suggesting, then it will be almost completely filled with fiberglass. He was telling me that "slab on grade" foundations don't have any airspace under the floor. I think they do usually have rigid foam boards put under the concrete though but as this is an existing old garage, we can't do that so we have to put insulation over the concrete instead.
 
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Old 05-12-11, 03:57 PM
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The batting is over plastic. If the plastic keeps the moisture out, where will the moisture come from that you are concerned about?

Slab on grade will only have insulation under the slab if they are going to put radiant heat into the slab. Otherwise it is only around the perimeter. They also put the plastic down under the concrete for a vapor barrier. And that would be on new homes. Old homes do not have that.
 
  #5  
Old 05-12-11, 06:28 PM
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Its just hard to imagine that a thin sheet of 6 mil plastic will forever keep moisture out of that space. It will, of course, be overlapped ten inches or so and wrapped up the several inch foundation wall but it would be unreasonable to think it won't have holes or mice & insects chewing it or something in the future. Maybe the plastic will just degrade over time. I would think the overlaps would allow a bit of moisture inside the flooring joists. There won't be any ventilation of any kind so, if moisture ever does get inside, I would think its certainly going to create mildew issues.

I have never lived in a home with a slab foundation. I'm used to dirt crawl spaces with 16" or so of space between the ground and floor joists. That makes sense to me and you put the vapor barrier over the ground.

It's living right on-top of a 4" concrete slab right on-top of dirt that seems odd to me. I understand the newer poured slab homes have rigid insulation under the slab but this is a 1930's concrete slab so whatever moisture and thermal control is done has to be on-top of it.
 
  #6  
Old 05-14-11, 12:35 PM
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If you were to place and secure 2" rigid foam on top of the pad, tuck tape all the seams, and then sufficiently tapcon screw the plywood over that you would have both a vapor, and thermal barrier. This would of course mean that you will need to extend the pad the additional 2 feet. With the rigid foam there would be no need for the 6mil poly as well.
 
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