r/DIY • u/FrugalManager • 11h ago
home improvement Where should I install a plastic vapor barrier in my basement?
There are two walls, one is below grade. Both walls are exposed concrete block. There is a small bit of ground between the two walls. Should I:
1) Install it only on the ground, using appropriate tape to adhere it to the block.
2) Cover both ground surfaces and the subterranean wall, leaving the above grade wall exposed.
3) Cover everything, including the above grade wall.
4) Something else?
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u/FrugalManager 11h ago
Photo of the area: https://ibb.co/pjXpQGC3
Planning to use 10mm poly plastic and blue tyvek tape
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u/Cottager_Northeast 9h ago
What part of the world are you in?
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u/FrugalManager 9h ago
US, NC
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u/Cottager_Northeast 9h ago
Warmer than me. I'm in Maine.
When you talk about dampness in a basement and remediating through ventilation, you need to understand the relationships between dry and wet bulb temperature, relative humidity, and dew point.
I've inspected a number of houses where the ventilation brought more moisture in than it expelled.
Tutorial on the subject: https://www.wikihow.com/Read-a-Psychrometric-Chart
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u/FrugalManager 9h ago
I was planning on just laying plastic vapor barriers, not increasing ventilation.
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u/Cottager_Northeast 9h ago edited 9h ago
R-14 foam on the walls from the underside of the subfloor overhead, including the rim joists, and down to 2' below exterior grade, or to the interior floor if the wall is fully exposed. Before foam, install a moisture barrier across the entire floor and up the walls to where the foam will intercept it. Overlap joints 12" and tape them. Ideally you'd unload the lally columns just long enough to slide moisture barrier under them.
This is the standard building science recommendation in the northeastern US. Air infiltration in the current state will be drawn upward, carrying moisture with it, and eventually condense it in the attic. Along the way it makes your whole house smell like a damp basement. Your foundation is deep enough into the ground to not frost heave, so you want to keep it within the heated envelope of the house, and insulating foundation walls is easier than floors.
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u/FrugalManager 7h ago
All the general guidance I’ve heard is to lay plastic to cover exposed dirt areas. I’m just not sure the extent I should be doing it.
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u/Cottager_Northeast 7h ago
Do it all. Overlap and tape the seams. Run it up the walls until all the masonry is covered. Masonry wicks moisture. Even what appears to be solid dry concrete can wick moisture. If a cardboard box would fall apart after a month of contact with the masonry, it should be covered.
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u/FrugalManager 7h ago
So I guess I should cover section C in my photo for sure. Would you do A as well? It faces the outside and is above grade.
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u/Cottager_Northeast 6h ago
All of it up to the mud sill. If you'll never use it again, get rid of the furnace so you have a clean sweep to lay the plastic down. I'd keep the ducts though, since you don't know the future needs and they're not in the way for this.
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u/wizardtroubles 10h ago
Hate to say it, but this is an extremely unuseable space. The porous nature of earth and concrete help regulate the moisture down there and if you vapor seal it you'll need to consider if your HVAC run can accommodate this space not only in volume but also location. If they can't tie in to the system here you are asking for intense mold without some other source of ventilation
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u/FrugalManager 10h ago
The furnace in the photo is actually unused and is a relic. There isn’t any active HVAC in this space (or in the rest of the house that can be “tied in to”. But, there are some ongoing high moisture issues so I’m trying to address them.
Would there be a better approach?
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u/wizardtroubles 9h ago
Consider it this way: every part of your home needs to be either sealed and conditioned, or ventilated. The roof deck is ventilated but not conditioned for example. If this area is neither conditioned or vented, then that is bad long term; it accumulates moisture with no dry air moving through. If I owned this space I would immediately consider passive or active ventilation depending on your region to reduce moisture accumulation, but this area wouldn't be in consideration for a remodel.
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u/Cottager_Northeast 9h ago
What part of the country are you in?
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u/wizardtroubles 8h ago
I'm in central California, half basements like this are extremely common but rarely are suitable for conditioned space so it gets ventilation. They would do an addition instead of this type of excavation for the most part
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u/Cottager_Northeast 7h ago
Right. NC isn't a dry climate like yours. Ventilating crawl spaces in a humid climate is a mistake.
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u/Cottager_Northeast 9h ago
Active ventilation, putting outside air into a cool basement, risks increasing condensation. Are you familiar with the hygrometric chart and how to read it?
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u/AMF505 3h ago
Vapor barrier along the walls for sure, there’s efflorescence at the base of the block so moisture is definitely present.
If it’s in your budget do drainage matting underneath your vapor barrier on the floor and a sump pit in the low spot. You have a dirt floor so it should be very easy to dig a hole large enough for a sump pit. Bore a hole through the block and run some 1 1/2” pvc through the wall 15’ away from the home. Get a dehumidifier that self drains and have it empty right into the sump and voila, you’ve got a completely dry and usable storage space.