sweating concrete question

[herd]

I am doing an addon to my house which includes an expansion to my den. I had a 15x16 foot concrete deck off the back of this room so I built a roof over the deck and tied it back in to the house/den. Planning on taking wall out so will be one huge room. I had the deck built on concrete footers and block walls with brick curtain wall facing, carrying the new load should not be a problem.

I do have a problem tho with this concrete floor and condensation. This concrete deck is a "suspended floor" and not tamped/filled. I have noticed several times this past week for the concretes sweats really bad. Since the deck is 3.5'' below my den floor, I am planning to raise the deck floor with a 2x4 treated wood "sleeper system." Obviously, I need to solve this condensation problem before I do that.

Does anyone have any helpful ideas. A coupla options I thought of is to add a poly vapor barrier on the ground underneath the deck and then sealing the deck floor. I orig suspended this deck with 28 gauge steel decking with rebar.

 

[ultrarunner]

Saw something similar...

Plastic sheeting for a vapor barrier on the dirt and then they coated the underneath and sides of the concrete with Thompson's Water Seal...

It took many weeks for the concrete to dry-out

 

[EddieWalker]

When you build your wood floor on top of the concrete, fill the voids with fiberglass insulation. The insulation will create a barrier that will eliminate the creation of condensation on the concrete.

Eddie

 

[rambler]

When you build your wood floor on top of the concrete, fill the voids with fiberglass insulation. The insulation will create a barrier that will eliminate the creation of condensation on the concrete.

Eddie

Yea - let the slab go cold and be outdoors. Make the wood floor insulated & vapor barriored, and all should be good.

Heat flows through the concrete now, and that keeps it cold & condenses moisture on it - warm on one side, cold on the other side....

--->Paul

 

[herd]

So you guys are saying use the vapor barriers and not worry about sealing the concrete? Should I use like a tar paper (roofing felt) barrier against the concrete topside and then the insulation vapor barrier directly under the subflooring deck?

 

[SPIKER]

not sure if you can coat the concrete yet or not, not sure if it is cold out where you are and or raining... You can and should apply a good penetrating sealer on the concrete first, then lay down a vapor barrier, tar paper on top of that is not a bad idea, use treated material for the floor dim lumber anywhere that it can still gain moisture. I would use a layer of styro-foam board insulation between the treated stud floor boards. then some additional insulation between the floor joists up to level with the floor. The install the sub floor up to the level of the den...

depending on the size you could also install spray in foam closed cell insulation down after the stud/joists. and vapor barrier was down, costs are high for this though...
mark

 

[Ford850]

The sweating isn't coming out of the concrete, but from the warm air when it comes in contact with the cool concrete. Think of a can of beer or soda when you take it out of the fridge. The cold surface sweats when warm moist air comes in contact with it. Keep that cool surface insulated so warm air cannot reach it, as described in several posts above, and it will not sweat.

 

[DaveOmak]

Herd, All good suggestion from members. Vapor barrier on the ground is a great place to start.
I have put vapor barrier on concrete. I first put heavy tar paper. Second, 2 layers of 6 mill poly, then heavy tar paper on top of that. My thought was to protect the poly from punctures. Don't know if it's proper to do it that way but my philosophy is, "if you don't know how to engineer it, overbuild it".
Once the floor is down, it's expensive to re-do. Good luck and let us know how it goes. Someone will chime in who really knows what to do and I will learn something.

Dave

 

[randy41]

there must be some kind of paint on vapor barrier for concrete.

 

[SPIKER]

I forgot to mention that to test to see if the concrete is sweating from above or below, take a 1' square of plastic sheeting, tape it down to the floor with some good duct tape. wait 24 hrs and see if there is any condensation under the plastic. if there is then the water is coming up through the floor and will HAVE to have a vapor barrier installed and should also have a penetrating sealer installed after a few days of good hot heat in the room area.

then proceed as stated before with the vapor barriers and insulation and dont forget to use treated wood under the sub-floor anywhere it might come in contact with moisture somewhere down the line. either by condensation from above or below.

mark