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basement slab=vapor barrier+warm side of assembly...then why?

basement slab=vapor barrier+warm side of assembly...then why?

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Old 02-02-15, 05:03 AM
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basement slab=vapor barrier+warm side of assembly...then why?

I'm about to frame a sub floor over my basement slab and I've researched extensively the subject of using a vapor barrier. I've even asked a question on here on a previous thread.

Virtually every article, or forum post I came across says to place a barrier, i.e., 6 mil plastic sheet over the concrete first, then build the frame, then secure the deck, then the laminate wood,... except one authority.His name is Rod Gervais, He has authored multiple editions of a book on how to build a recording studio, and has 40+ years of building expertise.

Using the industry standard that the barrier go on the "warm side" of the assembly, he recommends strongly that the barrier should go OVER the frame before the decking is secured. That way the warm air in the living space is stopped before getting to the concrete, where it could condense. We discussed in great detail WHY this is correct, and the method of placing a barrier directly on the concrete is wrong.

Can anyone else attest as to why, if the aforementioned standard is in fact to place the barrier on the warm side of an assembly, that all over the internet people are placing a barrier on the COLD SIDE of this kind of assembly, e.g., the slab.

Please advise.

Marc
 
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Old 02-02-15, 05:11 AM
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Marc, although I respect anyone who writes books, experience will tell you it isn't the air....as a matter of fact, what air? There is no air movement under your framing. What the vapor barrier does is help keep moisture from your framing members. Enough to have the vapor barrier above, but what happens to your framing in the meantime?? Moisture will rot it eventually. In addition no fasteners in the sleeper system to puncture the vapor barrier.

Others will chime in here, so hang on.
 
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Old 02-02-15, 05:42 AM
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This certainly does get to be a subject about which theories abound.

If your house is of unknown construction with regard to vapor retarder placement beneath the slab, I would certainly place a diffusion retarder on the slab to keep upward migration to a minimum.

If you have a known retarder beneath the slab and have not had any dampness issues then I'm not sure placing one above the slab is of any great significance. You should perform the "test" to determine slab dampness, poly or foil taped or otherwise sealed to the floor in a 1 sq.ft.spot. Give it a few days and peek underneath to see if there are water droplets or the floor shows a darker spot which would indicate rising vapor.

You could install the entire assembly on top of foam insulation which would provide a semipermeable assembly and keep the concrete from attaining a dew pt., then you don't have to worry about condensation.

All of that could perhaps not even be required if you can make a determination of the dew pt. now, before you start any of the work. If you can take some readings and determine RH and general surface temp.of the concrete, you will be able to determine potential for an issue. Of course, you should do this at several points over the year to make sure you are getting a feel for the worst case. If you have a heating/air conditioning system that will always be able to control the variables then you can utilize that knowledge to help you keep control of the situation.

Need to do a little more investigating. By the way, even the most well noted pros change their mind. as building science advances.
 
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Old 02-02-15, 05:51 AM
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Why do you think you need to even build framing and not just use a product like Dri-Core?
 
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Old 02-02-15, 06:14 AM
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"Why do you think you need to even build framing and not just use a product like Dri-Core?"

Sorry, I didn't give enough details. My slab drops approximately 3" on one end. I had considered adding concrete, or a grout material to level, but I have no experience in concrete, nor can I afford to hire a professional. I do have a friend who is quite handy at carpentry. We've decided to build a frame, and cut the bottom runners on a slant, such that we can then create a level deck on which to attach my flooring.

Marc
 
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Old 02-02-15, 06:21 AM
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"Marc, although I respect anyone who writes books, experience will tell you it isn't the air....as a matter of fact, what air? There is no air movement under your framing. What the vapor barrier does is help keep moisture from your framing members. Enough to have the vapor barrier above, but what happens to your framing in the meantime?? Moisture will rot it eventually. In addition no fasteners in the sleeper system to puncture the vapor barrier.

Others will chime in here, so hang on."

This is why it's so difficult for me. The other guy makes a very strong point on this. He says that IF I have no moisture wicking from the slab, a barrier their is a waste of time and money. It's the condensation that could occur, should the warm air from the room reach the cold concrete.

At this point I may just put a barrier on the slab AND over the frame. Would there be a reason to NOT do that, other than the time and money?

Marc
 
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Old 02-02-15, 06:47 AM
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MSR, if you are concerned about moisture (not vapor) from below you could use a sealant like "xypex" that will provide a water barrier. Xypex will reduce vapor transmission to some degree but is not designed to be the cure all of vapor flow. There are also liquid vapor barriers that can be applied to the concrete but I cannot think of the name of the one I have used, maybe someone will add that info..

Why not use a treated wood like "Bluwood" for the framing and you won't have to worry about decay organisms destroying your framing in the event you have some condensation.

If you have not had a moisture issue and you can determine that the dew pt. temp. will not be reached, you are overthinking the whole issue.
 
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Old 02-02-15, 06:48 AM
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Hi marc,
I made some notes, but have to run out, ahead of our 4th storm in 10 days, another 12"+ with this one. But the snow will definitely provide the time for a good conversation here and one that may be beneficial to many. The confusion you are trying to sort out is abundant in this relatively new energy efficiency world. Just hope the snow doesn't take down my connection.

Bud
 
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Old 02-02-15, 07:30 AM
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He says that IF I have no moisture wicking from the slab, a barrier their is a waste of time and money
Sure but do you know that's actually the case?
 
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Old 02-02-15, 10:00 AM
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"Sure but do you know that's actually the case?"

Well, all I can say is that I've had MANY heavy rains here over this past year, with no moisture making its way into the basement that I'm aware of. He also stated that the only time a barrier directly on a slab would work was IF I also have all the perimeter walls sealed with a barrier. If moisture makes its way in anywhere (other than the slab), then all of the expense and work is for nothing. I'm still considering putting a barrier on BOTH the slab and over the frame.

Marc
 
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Old 02-02-15, 10:27 AM
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Vapor coming up through the slab is not the same as water coming in when it rains.
 
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Old 02-02-15, 11:01 AM
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"Vapor coming up through the slab is not the same as water coming in when it rains."

Agreed. But couldn't vapor also come in at the perimeter floor area, thus creating issues even IF I had a barrier over the slab? I know nothing about vapor barriers, other than what I've learned thus far. Just trying to make sure I don't ruin over 2 grand worth of flooring. The walls have no barrier, and I'm not about rip them down just to put up a barrier.

Marc
 
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Old 02-02-15, 11:05 AM
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But you don't have framing trapped between the walls and the vapor barrier you're proposing to put inside of said framing.
 
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Old 02-02-15, 05:33 PM
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Just some quick bits to add to the thread.
1. There are three sources of moisture, from below (vapor pressure), from above (warm humid air) and your plumbing or a major leak. Where the advice to place the VB above the framing might address warm air reaching the colder concrete it ignores the moisture equalization that will occur from below. However moist the soil is below, everything from the VB down will become just as wet.
2. Also, there is no "one set of rules" for basements. What works in one home could be a disaster in another. I find the Building Science advice doing a pretty good job of covering many bases, but that doesn't mean all homes will fail if that advice is not followed. But who is going to want to ignore good science.
3. The "VB on the warm side of the assembly was not specifically targeting a concrete floor. Walls and ceilings will also maintain a direction for drying on both sides of that VB, the old rule of never having two VBs. Ceiling insulation is exposed to the attic or a ventilation path and houses are wrapped in a vapor open water resistant material. Basement walls and floors have the problem of needing to live with a constantly wet outer surface.
4. Warm humid air in a living space still needs to be managed, either through dehumidification or ventilation.
5. Then there is the need to mitigate Radon, provide a place for drainage, and a sump pit and pump, all normal features of a basement.

The real problem here is expecting to convert a basement into normal living space when that basement was never built from the start to do so. That basement smell that most people are familiar with is the byproduct of mold or mildew and unless extreme measures were taken during construction, is almost impossible to avoid in a retrofit living space down there.

I would be concerned about any advice Rod provides in the face of much science and testing now available. I find to old experts with a lot of work behind them would be ill advised to now admit they did it wrong.

Bud
 
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Old 02-03-15, 05:06 AM
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Thanks Bud...a couple of things here:

I have to do something,... this is a 1200 sq/ft basement with a lot of potential. Regarding your comment:

"The real problem here is expecting to convert a basement into normal living space when that basement was never built from the start to do so. That basement smell that most people are familiar with is the byproduct of mold or mildew and unless extreme measures were taken during construction, is almost impossible to avoid in a retrofit living space down there."

This makes sense, however, this house was built into the side of a hill. I only have one wall of the basement that below ground level. All other walls are at ground level. I have to wonder if this place perhaps would be somewhat easier to convert than basements that are completely below ground level?

That said, when I moved in, it wasn't long before I found MAJOR external water issues, which lead to my having to completely re-do the basement. There were large voids beneath the slab, which caused lots of cracks and heaves(which I found under the existing carpet). All of these issues have been corrected, and as stated, I've seen no evidence over the past year+ to think differently.

So, my questions are:

Is using a barrier on the slab AND over the subfloor frame a bad idea? Rod did say that IF I still wanted to put a vapor barrier on the slab, that I should use a sealer. Stating that using 6 mil plastic wasn't a good idea due to the movement that is still going to occur. He says that over his 40+ years he's had to remove old slabs, and seen the plastic sheathing underneath in complete ruins due to aforementioned "movement" that will most definitely occur.

I don't think I want to use a sealer due to the fact that the cracks have been sealed with SIKA self leveling sealant, and I'm not sure how a liquid sealer would react with the SIKA product.

What about the xps rigid insulation I've read about? More expensive it would seem, but would it perhaps be a better option for mitigating any vapor/moisture that MAY come through the slab?

I've also read that one should put down an asphalt mastic FIRST, then the plastic sheathing. That way, when the framing is fastened to the concrete, the mastic will "seal" the hole created in the plastic.

I cant see why the whole unit couldn't be floated, and not fasten the frame to the slab at all. Bad idea?

Whew, that's all the time I have for now. Thanks for your discussion, and I appreciate the input.

Marc
 
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Old 02-03-15, 08:17 AM
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Is using a barrier on the slab AND over the subfloor frame a bad idea?
Yes. Not many absolutes but never use two vapor barriers is one of them.

What about the xps rigid insulation I've read about?
Vapor retarder, not a vapor barrier and is often completely appropriate.
 
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Old 02-04-15, 06:40 AM
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OK guys, a good discussion so far and thanks for all of your input. Still researching and still can't decide which would be the best method for placing a subfloor frame on top of a basement slab. I was even watching "Holmes Makes It Right" last night. He was working on a basement that had a huge sewage flood. When all was said and done, he simply topped the slab with what looked like a rubberized pad, which he called a "thermal break", then carpet pad and carpet.

So, over the past few weeks I've read many different ways to complete this project, some of which, but not all are:

1-placing vapor barrier over frame only
2-6 mil sheathing on slab, then float frame or secure frame through plastic
3-seal slab with asphalt primer, then put down asphalt mastic, then anchor frame through all, and the mastic will "seal" the holes as they go into the slab.
4-glue xps rigid foam on slab and tape seams, place the frame on that,... floating or secured through foam into slab.
5-the above, but add 6 mil sheathing on top of foam
6-seal slab with Dry Loc masonry paint, place frame directly on that.
7-seal slab with Dry Loc, or asphalt primer, then butter the bottom of frame runners with mastic, then secure into slab with ramset or tapcons, then place rigid foam between runners, then 6 mil sheathing over all of that. (this was allegedly recommended by the Hardwood Manufacturer's Association)... but wouldn't this method be using TWO vapor barriers, e.g., the sealant and the last layer of 6 mil sheathing,...which is wrong, no?

At any rate, IF you were going to do this,...which method, or combination of methods would you guys use?

Looking for a consensus here??

Thanks,
Marc
 
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