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Old 01-13-2015, 01:57 PM   #1
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Vapor barrier in floor over cement pad


We have a room that was built over a cement pad that used to be a garage. We want to raise the floor and put insulation in the floor. Our question is does the vapor barrier go on the cement side or the top side? Any advice would be appreciated.
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Old 01-13-2015, 03:05 PM   #2
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Now that all depends on how the front wall, the one that replaced the garage door was built.?
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Old 01-13-2015, 03:21 PM   #3
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where the door was in now a large bow window. That wall is also not insulated and we will be doing that as well
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Old 01-13-2015, 06:56 PM   #4
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Was the wall built directly atop the apron or was block used.?
Will there be a chance for water to get into that area.?
Will the area under the new floor be vented.?

Got a picture.?
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Old 01-13-2015, 08:44 PM   #5
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Is there water perking up through the slab now?

The vapor barrier would have ideally been placed before the pour of the concrete.

The insulation board (i.e. Rigid XPS) you use should provide you sufficient vapor barrier effect.
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Old 01-14-2015, 06:51 AM   #6
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Assume cement/concrete is like a sponge and will let moisture/water through. First test your slab with 12x12 plastic sheet taped to the floor and see if you get signs of moisture in 2-3 days.
The vapor barrier should go on the slab. Think of your slab floor as a crawl space once new floor is installed. For a crawl space, the vapor barrier is installed and sealed against moisture coming up from the ground and start growing mold and rot the wood frames.
Even then you should design the floor framing so it will be possible to circulate air through entire subfloor space. This means holes through the framing.
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Old 01-14-2015, 10:59 AM   #7
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Keep in mind any insulation will aid in keeping the concrete surface from attaining room temperature and make it more subject to condensation temperature.

The plastic taped down test is a joke and should have been stated as such by the first person that tried it. Now it has been driven into so many minds it's locked in forever. That test is nothing but a dew point temperature test.
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Old 01-14-2015, 11:48 PM   #8
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Good point above, the moisture drive changes in different parts of the slab as does the seasons...

Use a liquid vapor barrier, not poly sheeting, before any rigid insulation/sleepers and keep your framing dry.

Gary
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Old 01-15-2015, 08:12 AM   #9
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About the dew point temp, would you, Senior and everybody else, explain the difference?
As long as the temp is constant in those few days, isn't it still a good, rough test? Does ambient moisture throw off these tests?
Thank you in advance.
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Old 01-15-2015, 05:40 PM   #10
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Are you going back with sheathing over the foam?

Staggered seams and sealed foam joints should do a good enough job of keeping room side air from getting to the concrete and condensing.
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Old 01-15-2015, 09:21 PM   #11
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Sounds like there is a step in elevations... Tar (or epoxy) the slab, install some tar paper, install some sleepers with concrete nails- on joist layout, install some rigid foam board-between the sleepers, install the joists, insulate with cavity insulation if foam was not thick enough per minimum code requirement for floor per your location; http://energycode.pnl.gov/EnergyCodeReqs/index.jsp (may be under a more current one)

I suggest a vapor barrier in addition to the foamboard as this will stop any moisture from outside, not the minimal moisture from the room. Most older slabs did not require a poly under them per code. Any flowerbed water, leaking downspout, or concrete stem wall being wet by the siding wall above will supply plenty of liquid water to the sponge (concrete) wall- unless painted, and it will move by capillary action (wet-to-dry) to your slab unless there is an impermeable break material between the two concretes. Some garage slabs are poured monolithic- even more incentive for a vapor barrier directly on the slab. Depends on the amount of surface/soil water, just foamboard may be enough though not with solid wood flooring. With your limited ventilation, it is cheap insurance, IMO.

Remember the furnace room may require its own supply air, and don't forget egress requirements if local AHJ requires. Though you will find out while obtaining the permit for "change of use"; garage to living space.

Gary
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