Plastic sheeting over basement dirt floor to combat moisture?

jlc102482May 18, 2010

I'm hoping some of you more experienced old home owners will be able to clarify something for me. I have an 1857 home with a stone foundation and a dirt floor in the basement. During the pre-sale home inspection, the inspector told me I ought to invest in some plastic sheeting or tarps and lay them over the dirt floor and also the stone walls of the basement to help keep moisture at bay. It seems to me that this would only trap moisture, though. Has anyone ever heard of doing this and if so, is it a good idea or a bad idea?

For the record, I haven't noticed any moisture problems in the basement. There is an addition on that basement from the 1940s that is poured concrete floor and foundation. I do notice dark (damp but not wet) spots on the concrete floor when I forget to keep the dehumidifier on, but otherwise it's dry and odor-free down there. There is efflorescence in a few spots on the walls of the 1940s concrete basement, but nothing at all in the 1857 stone/dirt basement area. We do have the dehumidifier set to go on automatically when the humidity reaches a certain point.


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I don't have a basement only a crawlspace. Eventually I will be installing 6 mil poly in the crawlspace, but only after I complete the rest of the updating on my 1946 bungalow.

If you have looked at a lot of old houses before you purchased yours you may have noticed that ones that had little to no restoration work done had no evidence of mold, while those that had an incomplete restoration done there may have been mold.

Mold is a newer issue in homes, created right around the late 70's when the mantra became energy efficiency. To make a home energy efficient you need to seal it up. When you seal it up mold has a much better chance to thrive. That's because when something gets wet it stays wet for a lot longer as there is no air flow drying everything out quickly.

So your answer is, yes eventually you will want to do something about that dirt floor, but personally I'd make it one of your last steps to be completed after you have sealed up the rest of your house. If that is even part of your plan.

    Bookmark   May 18, 2010 at 6:17PM
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I guess I'm just confused as to why the plastic sheeting thing is something that ought to be done, if there don't seem to be any problem with moisture. It sounds like it might create a mold problem to use the plastic, instead of leaving well enough alone. Are moisture problems a given with dirt floors, and I'm just not noticing potential/existing issues in my basement? Or is the plastic an energy efficiency thing instead of an anti-moisture thing?

I'm sorry if I sound contrary - I am just completely new to home ownership and know nothing about dirt floors and their maintenance! :) Thanks so much for your help thus far.

    Bookmark   May 19, 2010 at 9:22AM
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Moisture will pass from a wet area to a drier area. The earth is usually wet and the basement is drier so moisture usually enters at a slow rate.

Mold/mildew will grow in a damp, poorly ventilated area like a basement especially if there are organic materials for it to feed on. Test the floor joists carefully with a strong light and an ice pick.

If the dehumidifier is keeping the basement dry there shouldn't be a problem (be sure to test for radon!!) but don't store clothing down there or put anything directly on the earth. There doesn't have to be signs of water for something to get destroyed.

If the basement starts to smell musty you can add a poly vapor retarder on the floor but it will probably get torn up unless you cover it with smooth surface roll roofing, etc.

The only thing that works permanently is a concrete slab over a vapor retarder membrane.

    Bookmark   May 19, 2010 at 10:16PM
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"I'm sorry if I sound contrary..."

This is funny for a whole host of reasons.

Mainly because the answer is indeed contrary. Why do it? in a nutshell because it is the NOW thing to do.

The popular theory right now is sealed up is better. The ideal energy effcient home would be in a hermetically sealed box where every single bit of of air coming in and going out is monitored and controlled. While it is true that your heating and cooling bills would be minimized, the larger question remains, but at what cost?

Let's say in this perfectly sealed home you decided to have fish for dinner. Your house will now smell like fish forever, unless of course you install a ventilation system that also filters odors.

Why not just open a window you ask? Windows? even the best top of the line window is energy in-efficient. Never install windows in your home.

While this sounds rediculous, and is. Currently that is the trend, though not yet at that extreme thankfully.

Do you HAVE to install that plastic in the basement, not to sell your house to me, at least dependant upon what else you have done to your house. But to sell it to someone else? More than likely.

It really comes back to the last sentence in my first post. If your plans are to seal up the rest of your house and you do nothing with your dirt floor basement you have a potential of having an issue with the floor above your basement. Dependant upon how well you seal up the upper floors your de-humidification efforts may not be able to keep up.

If you do nothing to seal up the home, then you will be fine.

If you do some stuff but still allow your home to breath, dehumidifiers will more than likely work perfectly for you. This is what you should strive for, but it can sometimes be a difficult balance to strike.

Finally forget all of the above if radon is an issue, thanks macv, we don't have radon issues where I live and it completely slipped my mind.

So, step one have some radon testing done. Multiple ones at different times of the day and year. If that comes out fine then try and find and maintain that balance of allowing your home to breath while still making it more efficient.

    Bookmark   May 20, 2010 at 1:04AM
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The intended purpose for laying down plastic is to retard the transmission of soil vapor (which is in almost all soils except in extremely arid areas, and inexorably rising due to changes in barometric pressure) from coming up into the basement space. The fact that you are using a dehu means that you have a perceptible soil vapor problem (at least that's why you are paying the electric co. to run the dehus, which is comparable to running an A/C, in some cases).

Note however, that I said retard, not completely prevent the rising of soil vapor into the basement atmosphere. Nothing I know of will completely keep it out.

Because we now have a goal of sealing up our houses in the name of energy savings (and have modern, and different, whole-house heating systems) rising soil vapor can get stuck within the building potentially causing damage to the structure or contents or residents, if mold develops.

Of course your house, like mine, was built with long-tested systems that did not expect, or require, the house to be as hermetically sealed as we sometimes try for these days. In 1857 the vapor was a-rising just as it is today, but it was exiting the building much more easily.

Two other factors: people lived and worked at home and were more likely to be aware of and able to adjust the basement openings during the day, promoting additional passive ventilation to remove the moisture. And they may have highly prized that very same moisture if they stored food in the basement. Of course they also didn't have nifty stuff like cross-linked plastic molecules made into cheap (if you don't take into account any off-stream environmental costs), easily laid-out rolls of plastic to put to use, either.

Soil vapor is also coming from the basement's walls as your inspector noted. But blocking that moisture is as hard as, or harder, than blocking the soil vapor. If you had thoughts of converting your basement to modern uses (family room, media room, exercise parlor, etc.) you may be disappointed as this is rarely successful in old houses such as ours.

So the remaining issue seems to be: how tight is the house above the basement, and how might that tightness be interfering with the house's natural ventilatory patterns. (I think of it as how the house is "breathing", albeit in a passive way.) And of course, it also depends how much more you may be planning to do, in the name of energy conservation, to further tighten up the house.

You have run smack into one of the central dilemmas of old-house care: how do you integrate (without doing damage) modern building and energy standards with a structure that was designed and built to support a completely different system? Solving one problem can easily create a new, perhaps more difficult to manage, unforseen issue. There are many opinions and oxen-to-be-gored in this arena.

As a practical matter, if you want to lay down plastic (though I recommend special-purpose materials intended for vapor retardation instead of construction poly) and can seal it well with tape where sheets are overlapped, and along the bottom of the walls, you can susbstantially reduce the amount of rising vapor within the basement cavity. But you should know there are some who posit that doing so will shift this moisture (perhaps with damaging consequences) to adjacent soil spaces, including under and in the immediate, exterior vicinity of your foundation walls. This (according to some) may super-saturate the soil, disturbing the necessary firmness and creating settling or susidence problems. Does it? I surely don't know, but it does create - over time - a very moist soil under the plastic. (You can test this by laying a yard-square scrap of plastic on the dirt floor and see what happens.)

Others recommend various systems of forced, or air-pressure augmented passive ventilation to remove the moisture laden air from the basement before it can rise into house interior and replace it with (theoretically) drier exterior air. This method has the advantage of not monkeying around with existing soil mositure states, but it is complicated, and may take personal attention to adjust during each day and/or electricity to run.

If you have just purchased this house, I think you should observe the ebb and flow for a while (at least a calendar year), as you think about this. And keep the issue in mind as you contemplate any further sealing and insulation projects. It sounds as though the dehus are keeping things in check, so nothing (except cost of power and fuss of removing condensate) is lost by taking a go-slow approach.

And by all means, test for radon if you have it in your area. (Your local or state health department is a resource on this.) An upside of the necessary ventilation to control radon can be a reduction of basement moisture since the gas and at least some of the air vapor are removed at the same time.

    Bookmark   May 20, 2010 at 3:23AM
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You are confusing moisture migration from the earth with air infiltration in the superstructure. No wonder the OP is confused.

    Bookmark   May 20, 2010 at 7:21AM
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Thanks for all the info, everyone! I hadn't heard anything about radon testing before, nor did I know that was something that ought to be done. Like I said - home ownership, and especially old home ownership at that, is completely new to me! :)

    Bookmark   May 20, 2010 at 10:29AM
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"....the best crawlspace of all is filled with concrete and called a slab or dug out and called a basement." Dr. Joe Lstiburek

Here is a link that might be useful: New light in crawlspaces.

    Bookmark   May 20, 2010 at 2:21PM
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@liriodendron: One of these days I'm going to learn to write consise answers like that, thanks for clarifing what I was drying to tell the OP.

    Bookmark   May 20, 2010 at 5:40PM
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