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Old 04-05-2014, 09:51 AM   #1
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Floor framing directly on existing slab


Planning to build a deer shack on an existing 16' x 20' concrete slab. The slab is at least 4" thick and hasn't cracked for the 40 years it's been there. There was a small shack there that rotted out due to failed roof.

I want to build a wood framed floor over the slab for insulation purposes and comfort. Since I presume there isn't a moisture barrier below the slab, I'm thinking of build up as follows:

1) poly over slab for moisture
2) 2 x 10 pressure treated joists 16" OC
3) batt insulation
4) another layer of poly ??? Or is this not really needed?
5) 3/4" T & G plywood

Any thoughts would be appreciated.

Thanks!

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Old 04-05-2014, 12:14 PM   #2
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Floor framing directly on existing slab


Ayuh,.... I like it, Except,...

#2,... I donno's ya need 2x10s,... 4s, 6s, or 8s,...
Sittin' "On" concrete, there's nothin' to span,...

#3, I'd go with styrene foam over fiberglass,...
abit more "Critter"- proof,...

'n skip #4,...

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Last edited by Bondo; 04-05-2014 at 12:16 PM.
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Old 04-05-2014, 12:49 PM   #3
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Floor framing directly on existing slab


Quote:
Originally Posted by Bondo View Post
Ayuh,.... I like it, Except,...

#2,... I donno's ya need 2x10s,... 4s, 6s, or 8s,...
Sittin' "On" concrete, there's nothin' to span,...

#3, I'd go with styrene foam over fiberglass,...
abit more "Critter"- proof,...

'n skip #4,...

I have to agree almost completely with Bondo. I would have said "instead of fiberglass", over, "over fiberglasss".

Ambiguity Police

Last edited by Robpo; 04-05-2014 at 12:53 PM. Reason: missed something
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Old 04-06-2014, 12:29 PM   #4
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Floor framing directly on existing slab


Thanks for the replies.

I should have been clearer on my use of 2 x 10s. I plan on two bump-outs and will have a 4' x 16' on one side and a 4' x 12' on another. Also plan on using PT plywood on the bottom of bump-outs to keep out critters. Should end up with a fairly critter proof floor.

Last edited by papas88; 04-06-2014 at 12:40 PM.
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Old 04-06-2014, 01:40 PM   #5
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Floor framing directly on existing slab


Instead of poly over the slab, I would use "ice & water shield" (as in roofing). That way it will be self healing for any fasteners thru the slab.

Any plumbing in the floor?
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Old 04-07-2014, 07:21 AM   #6
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Floor framing directly on existing slab


Thanks rjniles.

No plumbing in the floor.
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Old 04-07-2014, 10:27 AM   #7
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Floor framing directly on existing slab


I would not worry about putting any thing on the slab now. Vapor barrier is to be under it not on top of it. go with rock wool insulation 100% better then foam board don't need any poly over the insulation. your good to go. I would use pressure treated for the bottom plates that way you won't rot out there.
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Old 04-10-2014, 03:53 PM   #8
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Floor framing directly on existing slab


Poly (or epoxy coating, drainage mat;http://www.buildingscience.com/docum...ms?full_view=1) on the slab is acceptable though will do nothing for the the thermal bridging of the joists/ply from the slab/earth temperature there- same as outside temps. Foamboard will address the thermal bridging and add cavity insulation of your choice- FG is the cheapest/worse, makes a dandy furnace filter though... rock wool will lose R-value when wet, same as FG, though I'd use it 10/1 over FG any day; http://www.google.com/url?sa=t&sourc...W6TAjQ&cad=rja

Think thermos bottle- FB on the slab (outside insulation) and the framing/cavity insulation (liquid in thermos) is warm due to room temps; http://www2.dupont.com/Tyvek_Weather...20Bulletin.pdf

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Old 04-11-2014, 03:25 PM   #9
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Floor framing directly on existing slab


Bondo has it right -
Quote:
Originally Posted by Bondo View Post
Ayuh,.... I like it, Except,...

#2,... I donno's ya need 2x10s,... 4s, 6s, or 8s,...
Sittin' "On" concrete, there's nothin' to span,...

#3, I'd go with styrene foam over fiberglass,...
abit more "Critter"- proof,...

'n skip #4,...
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Old 04-11-2014, 05:34 PM   #10
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Floor framing directly on existing slab


If you are going to pour concrete under the bumpouts I would lay 2bys on flat and put 1 1/2" foam between them and then 1" foam on top of that and then plywood. That should keep your feet warm.
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Old 04-12-2014, 12:38 PM   #11
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Floor framing directly on existing slab


Quote:
Originally Posted by Gary in WA View Post
Poly (or epoxy coating, drainage mat;http://www.buildingscience.com/docum...ms?full_view=1) on the slab is acceptable though will do nothing for the the thermal bridging of the joists/ply from the slab/earth temperature there- same as outside temps. Foamboard will address the thermal bridging and add cavity insulation of your choice- FG is the cheapest/worse, makes a dandy furnace filter though... rock wool will lose R-value when wet, same as FG, though I'd use it 10/1 over FG any day; http://www.google.com/url?sa=t&sourc...W6TAjQ&cad=rja

Think thermos bottle- FB on the slab (outside insulation) and the framing/cavity insulation (liquid in thermos) is warm due to room temps; http://www2.dupont.com/Tyvek_Weather...20Bulletin.pdf

Gary
Gary Mineral wool or rock wool is hydrophobic meaning it repels water. water and rock wool don't mix. it won't absorb water like two other forms of insulation does.
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Old 04-12-2014, 06:43 PM   #12
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Floor framing directly on existing slab


Rock wool absorbs water, but at a much lower rate than fiberglass..and dries quicker when direct water source is removed. Ron
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Old 04-14-2014, 10:21 PM   #13
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Floor framing directly on existing slab


I should have said, "not as much", thanks guys; http://www.google.com/url?sa=t&sourc...W6TAjQ&cad=rja

Gary

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