Must we install a "moisture, not vapor" barrier, room-side, on an insulated above-grade portion of an exterior basement wall

Can we forgo installing any " moisture, not vapor, " barrier on the INSIDE of our above grade portion of an exterior basement 6" wall that has been filled with fiberglass insulation (with no kraft paper)? If not, what do we use, the only "moisture, not vapor" barrier we've heard of is Tyvek; however, when we contacted Tyvek their Tech said "Tyvek is not for interior use"

The house is located in the forests of central western MA near the CT Border ( 01034 ). The walk out basement dimensions are 24x26. We have no knowledge about what moisture barrier may have been placed on the outside of the exterior walls in 1986/7 when the house was built.

We are considering closing up the wall using neither any moisture, nor "moisture not vapor", barrier. Would this be an okay approach, to close up the wall with no barrier other than the paneling (or sheet rock), leaving the fiberglass insulated wall, otherwise, unlined, unenclosed and unrestricted (air born particles) ?

The exterior stud wall rises to approximately six feet above the top of the 27" high concrete foundation wall and has seven windows, including the door's window panel, and 12 electrical outlets. A few years back the basement had a mold bloom (green) and all the exterior wall sheet rock and the previous fiberglass insulation with Kraft paper, were removed; and the basement was professionally cleaned. The floor was and still is bare (another problem, perennial dust). Since then we installed a powerful Sante Fe Classic dehumidifier and new fiberglass insulation (with no kraft paper). We are now ready to close up and finish up with, the room side of the exterior stud wall. We are considering installing pine T&G paneling on the stud section of the wall and we really need to decide what if anything to put between the insulation filled studs and the finish wall product, ie the paneling.

More on the matter . . . we also installed a mini-split and most recently a PH neutralizer tank was installed. This tank is uninsulated and sweats like crazy (but not onto the floor?) even with the dehumidifier running. Nothing else, that we can visually see, is sweating; however, this new tank's sweating is reminiscent of the sweat we used to see on the lally columns prior to the installation of the dehumidifier. This reminds us that the condensation threat is ever present at least in Summer. We are considering installing an insulation blanket on the tank.

Lastly, we have, on more than one occasion, been told by professionals that the basement is "dry" however, the concrete moisture level on the floor and the walls at the corners etc registers as high as even 90%, after a rain.

Okay, we've posted a lot of comments and questions. If you only get to answer one, please respond to the question of how to address the closing of the upper six feet of the inside of the exterior wall.

Please advise, as we would greatly appreciate any consideration you may give this matter.

  • 2
    Welcome to Home Improvement. You are correct that you've got a lot of questions here, and, unfortunately, that doesn't fit the mold of this Q&A site, as you'll see if you take the tour. Please edit this down to just one question and I'm certain you'll get a good answer. Do not hesitate to post your other questions each as their own question - nobody will look down on you for doing so, actually, it's expected! If you'd like, you can leave the background in this question and link to it in your other questions to save some typing. ;) – FreeMan Jul 21 '20 at 16:24

To me, “moisture barrier” is located on the exterior of a wall and “vapor barrier” is located on the interior.

If you are not sure there is a moisture barrier, you’d better check because without it you’ll have dryrot, mold and structural decay from rain leaking into the wall.

Vapor barrier keeps VAPOR from moving through the wall from the warm side to the cool side. Generally in the U.S. (including where you live) that means from the inside to the outside. (However, that can change at different times during the year.)

Vapor is a problem when it turns from vapor to moisture (water). This occurs at the “Dew Point”. I’ll let you Google dew point to learn about that, but basically it’s when the vapor moves from the warm moist side to the cooler side and turns into water. Walls are not vented like attics are vented. So, it’s important to keep vapor out of the walls.

Obviously if you have a lot of vapor on the interior of your house (basement rooms), then you’ll have more of a problem. If your basement is “dry”, you may not need a vapor barrier.

Recently we’ve learned a lot about the movement of vapor in walls. We use to just think in terms of winter. Now we know that if you live where it’s warm and moist on the exterior and you cool the interior, then the vapor moves in the opposite direction and you do not want a vapor barrier because it will “trap” any moisture in the wall causing mold...probably not dryrot because there isn’t enough moisture to do that kind of damage.

In northern environments, they use a plastic sheeting because the temperature never changes enough to have the vapor change directions. Likewise, in the south, they never use vapor barriers because the exterior is almost always warmer than the interior (due to air conditioning).

I live in a similar environment as you, but on the west coast. I never use a vapor barrier because 1) I don’t have that big of a vapor problem, and 2) I want the moisture to escape when the season changes.

However, you are different. When I paint my interior walls, that provides a substantial vapor barrier. You are going to install boards on the walls, so vapor will easily enter your walls AND you don’t have a vapor barrier on your insulation. Because of that, I’d use a vapor barrier. (No matter how good you install the vapor barrier, (even taping the joints and edges) vapor will still get in the wall through outlets, switches, etc. )

Your new mini-split system is good, because it will “move air” and help dry the room. The dehumidifier is essential and you should keep it running year-round.


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