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What’s the best way to adhere polyethylene vapor barrier to block Foundation?

What’s the best way to adhere polyethylene vapor barrier to block Foundation?


Old 09-24-03, 01:23 PM
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What’s the best way to adhere polyethylene vapor barrier to block Foundation?

Is there an adhesive product out there that you can get for a caulking gun? I am trying to make things watertight under the sub floor; it is dirt with gravel and vapor barrier that is loose. I plan on taping the seams and adhering it to the block foundation.


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Old 09-25-03, 06:36 PM
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Water tight is a big word........you will never achieve it with sheet plastic, but are you sure you want too. if you've got a serious amount of water under your house, you need to attack it at the source. trying to glue your vapor barrier to the concrete block walls in the crawl space is like trying to put a band aid on wet soapy skin.
Old 09-25-03, 10:38 PM
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Hi crosswind

I actually got this idea from a T.V. home show. They were doing this very thing and it seems to make since to me. Especially considering what happens to the vapor barrier every time some one goes under the house. The problem is that I took for granted that it was no big deal and didn't write down the adhesive they were using or the name of the show.
The local hardware store sent me home with what they thought would work, well it doesn't and they really aren't sure what will.
I have never had a problem with moisture. It's just that I'm installing some hardwood and want to be on the safe side. I had to do some structural work to the foundation and thought I would address the vapor barrier.

Thanks everyone for your help,

Old 09-26-03, 06:07 PM
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you can use a heavy gauge wire to pin the plastic down tight to the ground, even to hold the seams together. Tape wont work, wont stick to the plastic. The most ground that you want to cover is 80 to 90 percent. it is almost as bad to have a crawl space that is too dry as it is to have one that is too wet .
Old 10-02-03, 08:30 AM
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Crosswind - Now you have made me curious. What is the downside to completely covering up the dirt in the crawl space? Why do you not want it as dry as reasonably possible?
Old 10-02-03, 09:39 AM
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thiggy, spoony, crosswind,

Here is the scoop on what you are discussing as the issue of adhesives and the like is not what is done. There is tape available for doing the poly if so desired but again this is not necessary but overlapping is.

There is a simple rule when it comes to moisture control that one should follow. That is warm air condenses against cooler surfaces. This makes crawl space susceptible to condensation. With that in mind venting a crawl space is determined by which area has the greater humidity level. For example, if the air outside contains more humidity than the crawl space, you would not ventilate the crawl space. On the other hand if the air outside contains less humidity than the crawl space, then you would want to ventilate the crawl space. Naturally this will vary from time to time. You base this on the majority of time. For example if you lived in an area where it was hot and humid most of the summer, then you would not want to ventilate the crawl space and vice-versa.

Install a polyethylene vapor retarder, or equivalent material, over the dirt floor. Tape and seal all seams carefully. You may also cover the polyethylene with a thin layer of sand or concrete to protect it from damage. Do not cover the plastic with anything that could make holes in it, such as crushed gravel. Be sure the headroom of the crawl space meets local code regulations if you are considering pouring a concrete slab.

If the crawl space is to be unvented, seal all holes in the foundation where outside air can enter. Install the plastic ground cover as described above for an unvented crawl space. Run the plastic up the walls and attach it to the first piece of wood (the mudsill). Install rigid insulation foamboard against the foundation from the subfloor to the plastic (or concrete slab) on the floor of the crawl space. Do this all the way around the perimeter of the foundation. An alternative to foamboard is to drape fiberglass roll insulation down the foundation walls with the edges butted tightly together. This is an acceptable alternative to foamboard insulation as long as the crawl space stays dry.

Insulating the crawl space is determined by the way you decide to ventilate the area. For example, if you don't ventilate the crawl space, then insulate the walls of the crawl space. If you ventilate the crawl space, then insulate the ceiling of the crawl space.

There are instances when both are done. Meaning to say the walls and ceiling in the crawl space are insulated. An example here is lake shore property in an area that experiences cold winters and hot humid weather in the summer. In the summer the crawl space is not ventilated and in the winter it is. Hence the need for insulating both the walls and ceiling in the crawl space.
The majority of the water vapor that accumulates under a structure comes from the earth itself.

Applying 6-mill. Ployurethane sheeting over as much dirt as possible reduces the evaporation rate into the crawlspace. I have never seen a crawlspace that was to dry, so cover it all and lap the plastic by several feet and then tape the loose edges. I like to take gutter spikes and stake the corner down so the plastic does not move around. You can live with a few holes. By the way, the water that accumulates under the plastic is a good thing, it is not in the air.

Just food for thought
Old 10-02-03, 05:14 PM
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Doug Aleshire

Great info. I think you must have a masters in crawlspaceology. I sincerely appreciate your knowledge and I think I understand it. Since I live in the Willamette valley near Eugene Oregon I have decided to insulate the floor with R-19 and make sure the vapor barrier is complete and tight as you suggest. The only reason I thought about bonding the plastic to the foundation is because I saw it done on a T.V. home show. We had central heat installed last year and the workers made a mess of the barrier and it seemed like a good idea to seal it to the foundation and tape it together just to combat any future displacement. We have good ventilation. The winters are mild with plenty of rain and the summers are mild with little humidity.
D.A. do I understand you correctly in that this is probable the best approach for my area? Again thanks so much.



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