How to Detail a Concrete Basement Floor to Lower Humidity
Our 1990 cape in New Hampshire has an unfinished basement, with concrete floor and concrete block walls, and a drain. In the basement, there is a heat pump hot water heater. The exterior does not appear to insulated, and there is no evidence of a water barrier/membrane on the outside. There is a perimeter drain installed and the soil around the foundation is sloped to channel water away. Our standing seam metal roof has no gutter and a lot of water does fall onto the soil 14″ away from the foundation walls.
Our basement has always been humid (43% in winter, 61-73% RH in summer). If you put a sensor on the concrete floor in summer, it reads 80% RH.
This April we had our basement walls (and attic space) closed cell spray foamed as part of the HPwES program in New Hampshire. Before the concrete block wall was spray foamed, I framed out a 2×4 stud wall with a PT bottom plate, and floated it 1.5″ off the wall, so the spray foam technician could spray between and create a capillary break. The spray foam is now getting covered in drywall to meet fire code.
The humidity level has not dropped since the spray foam was installed. There is no way that I can lift up the foundation and install a water membrane (though maybe one was installed to meet 1990 code and I just don’t know it?).
I was thinking that I should install a water/vapor barrier over the concrete floor to stop the humidity from entering the basement and in turn molding the drywall and affecting IAQ. And/or, I could do stuff on the outside.
My options are to:
– Install gutters
– Install french drains
– Install water membrane on the concrete outside the walls
– Apply some kind of paint on the concrete floors
– Put down some kind of inorganic material (foam), tape the edges, then plywood twice, to cover the concrete floor
– Run a dehumidifier and go broke with a large electricity bill
-Install insulated R sheathing for concrete basement floor, tape it, and cover it with some kind of cheap flooring (even rubber horse stall mats)
None of these options seem straightforward or cheap.
Can you answer these 3 questions?
1. Which are most common solutions in New England?
2. What is the cheapest and environmentally friendly way to go?
3. If I can’t afford the big projects right now, what would you recommend I do right now to prevent any damage that could lead to large expenses later?
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Replies
> 61-73% RH ... humidity level has not dropped since the spray foam was installed
Because contrary to popular opinion, not much water vapor diffuses through concrete. What little that does doesn't require much dehumidification to remove (eg, single digit pints/day). Bulk water would be different - dehumidifier usage would be high after rain.
> stop the humidity from entering the basement
Focus on better air sealing, with blower door testing to verify it. You will need to run a dehumidifier, but air sealing plus an Energy Star model will make the operating cost reasonable.
You want to seal the floor against moisture wicking out of the ground. Plastic sheeting will work, a coating of paint is more durable.
In New England you have high humidity in the summer. This can cause condensation on cool surfaces. You need to either lower the humidity or eliminate cool surfaces. A dehumidifier or air conditioner reduces humidity. A layer of insulation on the floor removes cool surfaces.
If the house is air conditioned you can direct some of the air conditioning into the basement.
If the basement can be sealed from the rest of the house you can close it off and run a dehumidifier.
If you don't have air conditioning and can't seal the basement, your only solution is to insulate the basement so there are no cool surfaces. This is the riskiest approach.
In any case you want to close off all other sources of moisture. It sounds like you're well on your way with the spray foam. Unless you see liquid water I wouldn't worry about french drains or gutters.
In any case, you want the basement to be as tight as possible against air infiltration. If you can't ventilate with dehumidified air do not ventilate with outside air, that just brings in a never-ending stream of new moisture.
>"Our basement has always been humid (43% in winter, 61-73% RH in summer). If you put a sensor on the concrete floor in summer, it reads 80% RH."
The temperature to which the humidity is relative (it's RELATIVE humidity after all) matters. The same air will have different RH numbers a different temperatures.
It's the coolness of the slab (often 55F or lower in NH, even in summer) that makes the RH near the floor much higher. There is no practical solution for that once the slab is poured. In new construction INSULATING the slab and foundation walls keeps the summertime RH lower by raising the temperature of the slab & walls closer to the room temperature.
>"This April we had our basement walls (and attic space) closed cell spray foamed as part of the HPwES program in New Hampshire. Before the concrete block wall was spray foamed, I framed out a 2×4 stud wall with a PT bottom plate, and floated it 1.5″ off the wall, so the spray foam technician could spray between and create a capillary break. The spray foam is now getting covered in drywall to meet fire code."
A thickness of only 1.5" or even 2" is below IRC code performance, but sufficient for wintertime dew point control on R11 - R13 batts to be used in the studwall, bringing the assembly up to above code performance.
Kraft faced "contractor roll" R11s & R13s can often be had for dirt cheap even at box stores. While unfaced is preferred (if there is any history of even minor flooding), kraft facers are fine - just strip the facers to a level a foot above the prior high-tide mark and it can still work. Fiberglass won't wick moisture very far up the wall, but paper facers (on wallboard or batts) can. Kraft facers do not create a moisture trap- they are "smart" vapor retarders that become fairly vapor-open when moisture levels in the cavity are high enough to support mold.
>"– Put down some kind of inorganic material (foam), tape the edges, then plywood twice, to cover the concrete floor"
Rigid foam works, if it's EPS or XPS. As little as 1.5" of EPS (R6-ish) does a good job at raising the floor temp enough to where rugs won't get moldy without needing to dehumidify to Atacama Desert levels. It would have been better to install the foam UNDER the bottom plates of your studwall as both a capillary and thermal break, but with PT bottom plates it's not a big deal.
A single layer of 5/8" plywood TapConned to the slab through the foam is usually enough of a nailer/subfloor for most types of flooring, even half-inch CDX works with some types. Unlike joist mounted subflooring, the plywood over the foam isn't carrying much dynamic load, since it's supported by the foam & slab everywhere rather than on 16" o.c. 1.5" wide strips. It only needs to be a sufficient nailer for the flooring material, and fastenened to the slab in enough places to not warp & curl. Double-layered half-inch plywood can be floated, with no fasteners to the slab.
Whether double or single layer, leaving a 1/4" gap between the plywood & studs is prudent to accommodate seasonal dimensional changes due to humidity.
To take the wallet-damage out of the floor-foam , use reclaimed 1.5-2" roofing EPS / XPS (but not polyiso) or factory seconds foam. Used foam is typically 1/4-1/3 the price of virgin stock goods, factory seconds run about half-price.
Both reclaimed & factory seconds foam and wholesalers/distributors of batt insulation can be found running this search, hopefully somewhere near you:
https://nh.craigslist.org/search/sss?query=insulation
This hit (from Nationwide Foam/Insulation Depot) for Type VIII EPS (1.25 lbs nominal density) is in Framingham MA and as-usual, reasonably priced:
https://nh.craigslist.org/mad/d/framingham-eps-xps-rigid-foam-insulation/7343183789.html
At $500/ pallet stacked 96" tall a pallet of the 2" or 1" goods gives you 1500+ square feet @ 2" foam for about 33 cents per square foot, plus shipping. That's about 4 cents per R per square foot, less than half the cost of new Type-VIII EPS.