This is the fifth edition of Efficiency Vermont’s “Thoughts of Home” series as written by Dave Keefe. It is intended for building science professionals in Vermont and not intended to be used as guidance or assistance directly for homeowners. Comments should be directed to firstname.lastname@example.org
Mother Nature is wet. If you dig a hole in a place like Vermont, chances are it will have water in it at least part of the time. That’s where we build our houses – on holes in the ground.
As Henry Gifford has noted, most of the reasons for basements no longer exist. We don’t need to dig below frost line anymore. We can move the frost line with insulation. We don’t need to gravity-feed our coal from the truck to the boiler in the basement. We don’t need to put the equipment downstairs so that the heat can rise up on its own from thermal buoyancy. We don’t need a cool place for the root crops (although it’s still nice to have). In short, most of the reasons for having a basement disappeared once we had electricity.
But we still have a lot of basements, and some of them are kind of wet.
This is a big deal for Home Performance contractors. One thing we can all agree on is that we don’t want to do harm. And we know moisture is important in buildings. It can have a big impact on the health of the occupants. And it’s the single largest killer of buildings. A few buildings catch on fire, and a few are knocked down to make room for something else, but most of the rest are killed by water. They are abandoned because they are rotting, and they are rotting because they are damp.
We typically make the houses more airtight. This is usually a good thing. But if there’s a pre-existing moisture issue, we can make it worse with air sealing. It’s pretty simple, really. The house needs to be reasonably dry before we can retrofit it. Of course, there’s no real definition of “reasonably dry”. That’s one of the complications. Unlike some other things that concern us, this is hard to quantify. We can’t just take a measurement and see if it “passes” or “fails”. With moisture, there’s rarely a hard line. It’s more of a continuum.
I only have a few houses I’ve retrofitted that I worry about. The one I think of most often I did in the late 1980s. I didn’t understand the moisture thing then. I knew, as the saying goes, enough to be dangerous. I went ahead and air-sealed the house even though the basement was pretty wet. I wouldn’t do that today, and I wonder sometimes whether I may have done more harm than good on that job. None of us wants to do that.
These days, I think if there’s any doubt we should err on the side of taking care of the water. Making the basement drier is going to be a good thing almost regardless of where you start. It’s certainly better to make it dry and then insulate as funds allow, rather than insulating and hoping that the moisture will be taken care of sometime by somebody.
The strategy is well known. You start by doing anything you can outside to reduce the water coming in. Ounce of prevention, pound of cure. If there’s still liquid water after that, we need to physically remove it by channeling a path to the sump or to a drain to daylight. If the basement has liquid water you shouldn’t be air-sealing it.
Once the liquid water is gone, we are left with the vapor. Any dirt floor should be covered, even if it looks dry. If the concrete (walls or floor) is damp it’s probably a good idea to get a vapor barrier in place. It could be a flexible film, a coating, or closed-cell foam. Ideally, the vapor barrier in the basement would be continuous from the bottom plate down the wall, across the floor, and up to the bottom plate on the other side. The seams and edges should be sealed with tape, caulk, or foam.
When we install an air/vapor barrier that way in a damp basement, the basement gets much drier. If we have insulated the walls, it also gets warmer. Homeowners love this. They also love that the basement seems much cleaner and brighter (choose a light-colored vapor barrier material!). I’ve seen this several times on QA inspections. They don’t generally think of this effect up front, so when their dank, dark cellar is suddenly dry and well-lit they are pleasantly surprised.
Don’t use standard poly. You can do better. Besides, the color doesn’t work. Black is dark and ugly, and if you use clear they will call you later because they are worried about the condensation collecting on the underside. They don’t understand that this is exactly what we want, and an indication that the barrier is working.
Be aware of the slipping hazard. You might want to use something (washed gravel, pallets, paving stones, etc.) if there is a traffic path to the clothes washer or whatever.
You can get direct energy savings from moisture control if they have been using a dehumidifier; Most of them use quite a bit of electricity, and there can be substantial savings if you can reduce or eliminate their use. You probably don’t want to guarantee that they won’t be needed at all, but the approach outlined above typically reduces the need for dehumidification substantially.
A physical structure can’t stand without an adequate base. You could consider an energy retrofit to be similar, because we’re not going to get good results without building upon a stable, reliable foundation. In this case, that means a dry basement.