cinder block wall poly sheeting as vapor barrier


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Old 11-24-09, 02:30 PM
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cinder block wall poly sheeting as vapor barrier

Hi all, since I am in kindof a hurry I'll post this while I search the old threads.
I am preparing my basement for finishing and was about to waterproof the cinder block walls when the paint store guy told me I was looking at 20 gallons of paint at 75-100 sq-ft per gallon. (Zinsser Watertite was recommended over UGL DryLok) And, ALOT of brush work...But heres the thing, my basement is totally dry and has never shown signs of moisture on the walls (or floor), I just figured it was wise to waterproof it before I put up a wood stud/wallboard wall around it. I talked to my brother-in-law who was a contractor in NY state and he says they always used to just put up a 4 mil polyethylene sheet as a vapor barrier against the block and then put the walls up. This makes alot of sense to me as any seepage through the walls would then be forced to the drain in the perimeter of the floor and not cause a moisture problem for the insulation or sheetrock. But I wondered if it would be considered a fire hazard as it seems to me it would allow a flame to travel up the plastic to the ceiling. So I called the local building department to ask if they would consider it a hazard and he said he had never heard of anybody doing it...So he didn't know. He said since it would be covered by sheetrock, he figures it would be OK but could not be sure. He also said my foundation was waterproofed on the exterior so why would I even bother to waterproof the interior.
So now I am not sure what to do. I hate the idea of spending a couple of days and hundreds of dollars to seal with paint when poly sheeting would be so much quicker and easier to install. So I turn to this forum. What do you all think. Should I bother with this at all? Has anybody had any experience putting poly sheeting on the inside of foundation walls as a vapor barrier? All advice welcome.

Thanks,
Snarksdad.

BTW, I am cross posting this to "vapor barriers" also...
 
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Old 11-24-09, 02:54 PM
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Poly should never be placed on the wall. Poly on the wall will promote mold to grow.
 
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Old 11-24-09, 02:56 PM
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Hi dad,
There is a lot of misinformation out there about basements and some newer science that will need some time to be accepted,,,, or proven wrong which can happen. I'll post some links for reading to let you draw some of your own concerns. But a few corrections before I leave.
1. "He also said my foundation was waterproofed on the exterior" The tar that is sprayed on the exterior of a foundation does not waterproof it. Moisture will find a way through the tar and up from the footings below. Concrete or cinder blocks act like a sponge.

2. "my basement is totally dry and has never shown signs of moisture on the walls (or floor)" That's because they are not covered, so the moisture that is working it's way through is simply evaporating before you can see it. Moisture moves from wet to dry, so the moist soil is constantly delivering moisture to those blocks and into your basement.

3. "who was a contractor in NY state and he says they always used to just put up a 4 mil polyethylene sheet as a vapor barrier against the block and then put the walls up" Changing old habits is difficult .

I'll let you read from here:
BSD-103: Understanding Basements —

Energy Savers: Vapor Barriers or Vapor Diffusion Retarders

Bud
 
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Old 11-24-09, 03:55 PM
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Originally Posted by airman.1994
Poly should never be placed on the wall. Poly on the wall will promote mold to grow.
I assume you mean growth between the cinder block and the poly. Why is that? Is that because poly has such a low perm rating compared to other vapor barrier (er diffusion retarder) materials?

Thanks,
Snarksdad
 
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Old 11-24-09, 04:31 PM
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Not sure what you mean but will try to answer. Poly is a vapor barrier. I no of no other type of vapor barrier used in construction. Mold will grow because you will trap the moisture between the block and poly and it will not dry out.
 
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Old 11-24-09, 04:49 PM
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Originally Posted by Bud9051
Hi dad,
There is a lot of misinformation out there about basements and some newer science that will need some time to be accepted,,,, or proven wrong which can happen. I'll post some links for reading to let you draw some of your own concerns. But a few corrections before I leave.
1. "He also said my foundation was waterproofed on the exterior" The tar that is sprayed on the exterior of a foundation does not waterproof it. Moisture will find a way through the tar and up from the footings below. Concrete or cinder blocks act like a sponge.

2. "my basement is totally dry and has never shown signs of moisture on the walls (or floor)" That's because they are not covered, so the moisture that is working it's way through is simply evaporating before you can see it. Moisture moves from wet to dry, so the moist soil is constantly delivering moisture to those blocks and into your basement.

3. "who was a contractor in NY state and he says they always used to just put up a 4 mil polyethylene sheet as a vapor barrier against the block and then put the walls up" Changing old habits is difficult .

I'll let you read from here:
BSD-103: Understanding Basements —

Energy Savers: Vapor Barriers or Vapor Diffusion Retarders

Bud

I am reading the articles you provided links to and they seem to talk "around" this issue and do not seem to offer a solution. From my perspective what we want to achieve is minimal moisture penetration into the stud wall cavity/insulation from both the "room" side and the "earth" side. These articles seem to imply that you cannot have 100% vapor retardation since moisture will always find a way in. I can see that. They specifically advise against a poly wrap under the sheetrock directly on the studs. Airmen says that poly directly on the cinder blocks is a bad idea as it will promote mold growth. I assume that is because poly is too impermeable and traps the "earth" moisture between itself and the cinder block and causes mold on the block side of the poly. So I think what I am hearing is that a waterproofing coating is better since the exterior moisture never makes it into the stud wall cavity so it inhibits/prevents the growth of mold (does mold grow within the cinder block pores?) even though the poly is an order of magnitude better at preventing water transpiration?

So is the bottom line that I really do need to apply a coating as a vapor retarder to the cinder blocks? That is not what I wanted to hear... So much for my long weekend. Now let me see what threads there are on applying masonry coatings quickly and easily....Yeah, right.


I see there is a sticky thread on insulating foundations...Let me see what that has to say...

Thanks,
Snarksdad
 
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Old 11-24-09, 04:59 PM
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Originally Posted by airman.1994
Not sure what you mean but will try to answer. Poly is a vapor barrier. I no of no other type of vapor barrier used in construction. Mold will grow because you will trap the moisture between the block and poly and it will not dry out.
Thanks.
I think I am getting it now. I think what I was missing is that the poly will let the moisture escape from the block and then it will be trapped between the block with no place to go (unless it condenses and trickles to the drain) and therefore potentially cause a mold problem. Whereas a waterproof coating used as a vapor barrier does not provide a place for the moisture to collect (except within the block itself) so it does not promote mold growth...

Snarksdad
 
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Old 11-24-09, 05:28 PM
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I hear what you are saying, but my take on the reading and others if I can dig through them, is to allow the cinder blocks to breathe. Give the moisture a way to escape so it does not accumulate, behind the plastic OR within the block. You noted, your basement seems dry, continue that process by allowing a path for the moisture to escape. Where I'm headed is "no waterproof treatment".

The approach that is being promoted is to apply, say, one inch of rigid insulation tight over the basement walls. The perm rating of the rigid will be suficient to allow the moisture to pass from the walls to the basement right through the foam and your newly constructed walls. No oil based paints, no plastic anywhere, just let the walls dry.

Now, you can modify that approach to use rigid foam on the exposed wall area plus one foot below grade. Since there is less to no moisture up there you can use a thicker foam product and even one with a VB built in, like the foil faced polyisocyanurate. The wall area below this "plus one foot zone" is exposed to mother earth and experiences much less heat loss. In fact the heat you do loose provides some protection from potential frost pushing on the walls. Not sure what your frost potential is.

Here is another article and I think it speaks directly to the question at hand, the issue of allowing the walls to dry.
RR-0202: Basement Insulation Systems —

Bud
 
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Old 11-24-09, 06:15 PM
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Originally Posted by Bud9051
I hear what you are saying, but my take on the reading and others if I can dig through them, is to allow the cinder blocks to breathe. Give the moisture a way to escape so it does not accumulate, behind the plastic OR within the block. You noted, your basement seems dry, continue that process by allowing a path for the moisture to escape. Where I'm headed is "no waterproof treatment".

The approach that is being promoted is to apply, say, one inch of rigid insulation tight over the basement walls. The perm rating of the rigid will be suficient to allow the moisture to pass from the walls to the basement right through the foam and your newly constructed walls. No oil based paints, no plastic anywhere, just let the walls dry.

Now, you can modify that approach to use rigid foam on the exposed wall area plus one foot below grade. Since there is less to no moisture up there you can use a thicker foam product and even one with a VB built in, like the foil faced polyisocyanurate. The wall area below this "plus one foot zone" is exposed to mother earth and experiences much less heat loss. In fact the heat you do loose provides some protection from potential frost pushing on the walls. Not sure what your frost potential is.

Here is another article and I think it speaks directly to the question at hand, the issue of allowing the walls to dry.
RR-0202: Basement Insulation Systems —

Bud
Interesting. Another DIY forum pointed me to the same link(s)...
While I am liking what you are saying since it saves me work and time, I am still unsure. If we allow basically the same amount of moisture to enter through the cinder block into the wall cavity as has been to date, and we have applied a vapor retardation factor of 2-3 in the form of wallboard covered in primer and latex paint, aren't we inhibiting the ability for the moisture to escape from inside the wall thereby creating a higher potential for mold growth within the wall? Seems logical that we would want to prevent the moisture from ever getting that far doesn't it? Wait, I think I am getting it now. We allow the moisture in from (roughly) grade level and below since this is in contact with moist earth. And we expect that in can transpire back out in the space that is above grade since this would likely be drier and, as you noted earlier - moisture moves from wet to dry. Is that the premise we are working with?

Another article I read discussed this "1 foot zone" also. They recommended hiring foam sprayers to insulate that portion to save the time of installing the rigid foam.

BTW, I have an 80 gallon air compressor in my basement. It has been running 3 years without ever having had its tank drained. I opened the draincock tonight expecting a small flood and not a drop came out. Very strange for a "moist" environment don't ya think?

Thanks,
Snarksdad (in central NJ)
 
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Old 11-24-09, 08:21 PM
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Any kind of paint you use on the wall will not be a vapor barrier. Water vapor will still come in. It is a moisture barrier. How much of one is questionable.
 
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Old 11-24-09, 09:15 PM
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Originally Posted by airman.1994
Any kind of paint you use on the wall will not be a vapor barrier. Water vapor will still come in. It is a moisture barrier. How much of one is questionable.
Interesting comment. One article referenced earlier by Bud:
Energy Savers: Vapor Barriers or Vapor Diffusion Retarders

Lists a chart with permeability ratings for various materials and states that any material with a rating of 1 or less is considered a "vapor barrier". Vapor barrier paint or primer is listed as .45. Is not moisture considered water "vapor?"
 
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Old 11-24-09, 09:57 PM
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The BS 103 explains it quite well. Here is another one per location: BSD-012: Moisture Control for New Residential Buildings —
Be safe, Gary
 
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Old 12-03-09, 07:08 AM
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OK, about the poly against the cinder block wall, this is current preferred code in Fairport, NY. I spoke with a friend last week about it. He just got a permit and apparently the town wants poly AGAINST the block before putting in batts. This seems a pretty rare approach as the old school approach was block, then studs filled with batts but then I think NOT a vapor barrier. I have not personally reviewed their specs but this is what he was telling me after talking to the town.

BTW, this was for a basement with the interior perimeter drain so that any excess water would leak down the wall or hit that poly and then into the per miter drain.

There would be no fire hazard of a little plastic, certainly not behind drywall, I can't imagine how it would be an issue.

In my basement, a town over, I have already spoken to my inspector about my approach, which is 2" ONLY of extruded foam, then my stud walls. I will not be using batts. I also have a perimeter drain.

I am no drylocking the walls. Even ignoring the fact that over time the stuff can fade away, I see no point at all to it since I have the 2" foam. I even went so far as to wedge/cut the bottom so that the part against the wall is lower than the front part and thus the water is literally forced to drain down into the floor drain and cannot find its way onto the floor. If I dryloked the wall I'd simply end up with the water behind the paint as opposed to in front of it but still behind 2" of foam, so why bother? Not to mention then I'm gluing to drylock. And if it's cured the company has said (based on another thread) that it's ok, but still I bet it's not as good a bond as to porous concrete block. I did use a few mechanical fasteners as well, though (3/16 tapcon screws with zinc-coated fender washers).

If mold can grow in the water-alone between the foam and block, well that could happen at all times since you cannot really get a dry wall, but at least it's pretty effectively sealed away from the interior.

I should add that in all the readings I've done recently, all of which say to use moisture-resistant (i.e. foam) insulation, at the very most one has said to pre-paint the wall. The rest are all foam right up against raw cinder block.

Check out insofast.com as an example product, it's 2" foam right against walls. Or owens corning insular panels, they are 1.5" foam for basements held against raw block with studs. Also DOW has its wall barricade system
 
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Old 12-17-09, 06:39 PM
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Fairport huh. Not too far from where my brother in law used to build (Waterloo/Geneva area). So I guess there is some credence to the poly on block argument. But that area has a very clay based soil whereas I have sand down to 15 feet...Excellent drainage.
Do you think they read the links that were posted here :-)
BSD-103: Understanding Basements —
BSD-012: Moisture Control for New Residential Buildings —
Besides giving you more information than you'd ever want on the subject, they even contradict one another on some points.
I am believing airman in that I think the plastic sheeting is probably a bad idea in my case. I can see the logic in the case for mold growing between it and the block wall. Now if one does have a water problem, plastic might not be such a bad idea. Better moldy than wet.

I am now thinking that since I have to leave an inch to 1 1/2 gap between the block and the stud wall (to allow access to the perimeter drain) I will not put anything in that gap. Let the block breathe and let convection in the "air gap" allow the moisture to dissipate. Then just put batts in the studs as a normal wall. No vapor barrier at all. Hows that sound?

Snarksdad
 
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Old 12-18-09, 04:29 AM
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Lets try to look at this again.
First of all concrete absorbs water from the ground and that is then released into the air over time as water vapour,usually into colder air, ground sourced water rises within concrete to four feet above ground level.
Warm always moves to cold.
Second as the concrete is damp all the time and therefore usually cold compared to the surrounding air then a lot of the time the water vapour in the air condenses into the concrete, this happens about two hundred days a year.
Third while very few things are water vapour proof (water vapour having such tiny molecules they are like comparing tiny ball bearings with football sized air molecules) with most things they merely skip in and out of the loose molecules near the surface of the product.
Forth mould spores are everywhere, across the entire world from the North Pole to the South Pole.
They are in the air looking for a home and place to grow. What they need is water and food. The water can come from the air as condensation and the food comes usually from wood, leather, paper etc.
In most instances the amount of water in concrete and the lack of food causes them to look elsewhere.
Very often mould spores will start to grow and then die as they run out of water and or food.
Keeping a room at the same temperature will avoid problems with condensation as this happens when warm saturated air meets a cold surface, the air temperature drops, the air can no long hold the same amount of water vapour and the water vapour turns into condensation on a smooth surface like glass or just finds its way into the surface on rough things like concrete.
 
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Old 12-18-09, 05:43 AM
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I would like to point out that Skoorb's comments relating to one towns requirements has to be taken in context, they are slow to keep up with the changes. Last seminar I attended, my town's code compliance officer was there. He and perhaps a couple of others were the only ones from over 50 surrounding towns who were invited. They are busy, they have their degrees which do NOT need to be updated in their opinions, and the little details we are dealing with rarely need their attention. Would I expect them to be 10 or 20 years behind the times, absolutely. Can you change their mind, probably not, unless you are very determined. I would be, before I put up that poly.

And don't feel bad if you are having trouble deciding what is best, we all don't agree right here on the board. The experts don't agree. And there are thousands of different combination's of issues within different homes that can make the same suggestion pass or fail despite whether it is a good or bad approach.

Do your best. If a town requires a plastic barrier, I would get it in writing so an insurance adjusted 5 years down the road doesn't blame you.

Bud
 
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Old 11-12-10, 06:46 AM
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There is no food for mold in concrete. It is inorganic.

Any mold that would grow there would probably die quickly. If you were really worried about mold behind the vapor barrier I would spray the wall with bleach first. I'm not an expert but my first 2 sentences can't be disputed.

What did you end up doing? Was it a good choice?
 
 

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