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Discussion Starter #1
I'm considering converting a carport into living space. Currently, it's just a concrete slab, but a very solid one, I'd say---lots of rebar, fiberglass in the concrete, 8" thick in the middle, at least 12" thick around the edge, professionally finished, not a crack to be seen after 3+ years. The only problem is that there is no vapor barrier underneath.

I'd like to put rigid foam insulation directly over the concrete and flooring on top of that (i.e., no air gap). Is this a good way to do it? Or do I need put a vapor barrier (6 mil plastic) on top of the concrete, under the rigid foam? Any particular type of foam that's better in this application (maybe insulation and vapor barrier in one)?

Any useful info is appreciated. Thanks.
 

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Tileguy
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Aren't you the same guy that asked all of these same questions about improving your basement a few weeks ago? If that wasn't you then search it out, it is all hear, the same identical nonsense ideas have already been addressed just recently here.:)
 

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Discussion Starter #3
Aren't you the same guy that asked all of these same questions about improving your basement a few weeks ago? If that wasn't you then search it out, it is all hear, the same identical nonsense ideas have already been addressed just recently here.:)
No, the previous questions you refer to were not from me. If you could point me towards those posts, I'd appreciate it, since I must not have found the correct search terms.
 

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Discussion Starter #5
I can't remember who/where it was but I'll look for it. Exact same deal.:)



>>>>>>>>Okay here ya go, my mistake, wrong forum, try this:
http://www.thefloorpro.com/communit...-how-to-prepare-basement-bathroom-tiling.html
I can't find such a thread. Anyways, maybe you could enlighten me as to why this is a "nonsense" idea. In searching, I've found plenty of discussions of people putting a vapor barrier and insulation over a concrete slab. Seems entirely reasonable to me. Of course, if there are potential problems, I'd like to know, and that's why I posted.
 

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Discussion Starter #6
I can't find such a thread. Anyways, maybe you could enlighten me as to why this is a "nonsense" idea. In searching, I've found plenty of discussions of people putting a vapor barrier and insulation over a concrete slab. Seems entirely reasonable to me. Of course, if there are potential problems, I'd like to know, and that's why I posted.
Oops. Now I got the link.

Certainly some things to think about there, but the suggestions I've seen always include sealing the concrete before putting the plastic sheet down. The idea is to keep the moisture in the slab, not trapped between the slab and the plastic.
 

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Discussion Starter #7
Come to think of it, there is no issue with head room or transition to another room, so I just as well do it right and not risk the possible mold issues. So, is this a good plan?

Plan B --- I'll put down PT 2x4s, with rigid foam and subfloor over that. And I'll vent it so the moisture can escape.

If this sounds good so far, should I also put a vapor barrier over the slab (below the 2x4s)? Any other things that should (or should not) be done?

Thanks.
 

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Pro Flooring Installer
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Plan B --- I'll put down PT 2x4s,

Thanks.
It's always been my understanding that pressure treated lumber should not be used indoors. I know some people say you can, but I would rather err on the side of caution.
 
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Discussion Starter #10
OK, so I've looked at the info on dricore and a similar product called platen, and I have to say that I just don't get it. Yes, these products do create a small air gap, and I could see that that would help keep the floor above warm. But, unless there is air flow, how does this eliminate potential mold/mildew problems?

Another question... It seems that today the recommended method of dealing with a crawl space is to put down a vapor barrier tight against the ground/concrete and then seal the crawl space (that is, no venting). A detailed description can be found here, for example:
http://www.vbinspect.com/crawl_space_1.pdf

So, my question is, if sealing a crawl space floor with a plastic sheet is a good idea, then why is doing the same to a slab with a floor directly on top of it a bad idea?

Thanks in advance for all constructive comments.
 

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The idea by leaving the gap is to leave an air space. You're not necessarily circulating air by any forced means, nor do you need to. By not having direct contact with the concrete you eliminate the wood product from absorbing or drawing up any moisture in the concrete (gr. 9 science: osmosis). If you're concerned about the floor being warm then drop your vapour barrier, strap 2" x2" to the floor (if that's what you decide), insulate between the strapping, and then cover with a tongue and groove plywood.

I hope this helps.
 

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Discussion Starter #12
The idea by leaving the gap is to leave an air space. You're not necessarily circulating air by any forced means, nor do you need to. By not having direct contact with the concrete you eliminate the wood product from absorbing or drawing up any moisture in the concrete (gr. 9 science: osmosis). If you're concerned about the floor being warm then drop your vapour barrier, strap 2" x2" to the floor (if that's what you decide), insulate between the strapping, and then cover with a tongue and groove plywood.

I hope this helps.
Yes, but a plastic sheet (vapor barrier) would also solve the osmosis issue. So, I'm still wondering what is the possible advantage of leaving a (sealed up, non-circulating) air gap?
 

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The only advantage to laying down vapour barrier then your floor covering is simply speed and cost. However, if this is a carport conversion with no vapour barrier beneath the slab I would be hesitant to go that route myself.

I would be afraid of the ground becoming so saturated with water from a heavy rainfall that it wicks up through the concrete and then that cold dampness condensing on the warm side of vapor barrier and damaging whatever is sitting on it.

When you strap and insulate you are keeping the cold zone where it needs to be. The Dricore (not the only good product out there or only method) prevents your final surface from meeting the concrete where it is susceptible to damage over an extended period of time.

If you want to see how close contact will fair for you, your floor and climate you live in place a sheet of barrier on the floor. On top of that place a dry cardboard box weighed down with some towels on top of that barrier and leave it there for a month or two during your damp or humid season. If there is discoloration, deformation, or signs of moisture at the bottom of the box then you know you need to have an insulated space (or air gap) between the concrete and the final floor.

And always remember that if you're still unsure contact your local contractor, home inspector, drafts person, someone you trust and has a good reputation to come in and do a consultation. They will most likely guide you in the right direction.

Cheers and Merry Christmas
 

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Tileguy
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Another question... It seems that today the recommended method of dealing with a crawl space is to put down a vapor barrier tight against the ground/concrete and then seal the crawl space (that is, no venting).
Tho that website sounds official...I'll bet for every guy you find recommending that method I can produce three professionals that would disagree vehemently.

Venting and air-flow accommodation is always necessary under the circumstances that guy has drummed up.

You have plenty of information for you to act-on...it's your decision. You seem to continue to want to justify your intent no matter what.:)
 

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Discussion Starter #15
The only advantage to laying down vapour barrier then your floor covering is simply speed and cost. However, if this is a carport conversion with no vapour barrier beneath the slab I would be hesitant to go that route myself.

I would be afraid of the ground becoming so saturated with water from a heavy rainfall that it wicks up through the concrete and then that cold dampness condensing on the warm side of vapor barrier and damaging whatever is sitting on it.

When you strap and insulate you are keeping the cold zone where it needs to be. The Dricore (not the only good product out there or only method) prevents your final surface from meeting the concrete where it is susceptible to damage over an extended period of time.

If you want to see how close contact will fair for you, your floor and climate you live in place a sheet of barrier on the floor. On top of that place a dry cardboard box weighed down with some towels on top of that barrier and leave it there for a month or two during your damp or humid season. If there is discoloration, deformation, or signs of moisture at the bottom of the box then you know you need to have an insulated space (or air gap) between the concrete and the final floor.

And always remember that if you're still unsure contact your local contractor, home inspector, drafts person, someone you trust and has a good reputation to come in and do a consultation. They will most likely guide you in the right direction.

Cheers and Merry Christmas
That sounds like an excellent experiment---I think I'll give it a try.
 

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Discussion Starter #16
Tho that website sounds official...I'll bet for every guy you find recommending that method I can produce three professionals that would disagree vehemently.

Venting and air-flow accommodation is always necessary under the circumstances that guy has drummed up.

You have plenty of information for you to act-on...it's your decision. You seem to continue to want to justify your intent no matter what.:)
I've got no pre-conceived intent. I'm just looking for info so I can do it right. I've always had good luck getting useful advice from this forum in the past, and the best part is that I learn why things should be done in a particular way.

Maybe I'm being dense, but here's my confusion: In the case of a crawl space, you say that we need air flow and venting. But, as far as I can tell, products like dricore and platen don't require venting or airflow. So, it's bad to trap damp air in one case, but fine in the other? How does that make sense?
 

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Maybe I missed it but what kind of floor are you putting down you may not need a vapor barrier at all.
 

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This vapor barrier thing is being over thought as usual. How many millions of homes would you suppose have been successfully built slab on grade without any kind of vapor barrier and doing very well.

They have tile surfaces as well as wood, vinyl, bamboo, cork and plain stamped and polished concrete to name just a few and not one has ever wicked water up to the surface like a sponge as some would suggest.

If you ever see moisture on top of concrete within a building it's because of dew point temperature of the surface being reached resulting in condensation and not a sponge effect.
 

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Discussion Starter #19
This vapor barrier thing is being over thought as usual. How many millions of homes would you suppose have been successfully built slab on grade without any kind of vapor barrier and doing very well.

They have tile surfaces as well as wood, vinyl, bamboo, cork and plain stamped and polished concrete to name just a few and not one has ever wicked water up to the surface like a sponge as some would suggest.

If you ever see moisture on top of concrete within a building it's because of dew point temperature of the surface being reached resulting in condensation and not a sponge effect.
I'm sure that's all true. I've done lots of major home improvement projects over the last 4 years, and now whenever I walk around my neighborhood, I see all kinds of things that are seriously wrong. Those houses are still standing and inhabited.

Regardless, I like to do things the right way. And mold can be an issue here---we have a winter monsoon-like season where it rains more-or-less constantly for 4 months. That's why I want to be especially careful with this particular project.

I'm glad for all the responses, but I don't feel like I understand the situation well enough to act just yet.
 

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