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Advice for lowering the humidity in an old stone basement

I Live in a 19th-century home with a limestone foundation. I don't expect to make the basement into a furnished living space, but I would like to use it for an occasional workshop and for storage.

The grade is fairly flat, but generally slopes away from the house, so we never get bulk water in the basement. But as I have tightened up the rest of the house over the years, the basement has become damper and damper, to the point where mold grows on any organic material left down there over the summer, and the air is uncomfortable to breath. The space is dry as a bone in winter though. I'm not sure of the proportions, but I'm pretty sure the moisture is a combination of air-infiltration condensation and evaporation from the porous walls and the poorly built slab.

Currently, I'm working on cleaning the stone walls, replacing some crumbled lime mortar, and ultimately whitewashing with a lime wash. There are a lot of holes in the mortar where I can feel air coming in, so I hope that patching the mortar will solve a lot of my air infiltration. I'll also look for every other possible air gap and fill with either foam or caulk. Does this sound about right? I'd love to insulate over the limestone, but I'm scared to cover it up and not be able to inspect for spalling or other moisture damage that seems to be ongoing.

Much of my house has a single layer of T&G pine between the basement and first floor, so I plan to do some sort of air-sealing there. Perhaps I will cut sheet goods and fit them between the joists, then air-seal the edges (plywood? high-density fiberboard? rigid foam? painted homosote? or a combination?).

In the short term, my plan is to cover the floor with polyethylene sheeting and cover that with foam anti-fatigue mats (the kind that go together like puzzle pieces that you can buy at Harbor Freight). In the long run, I'd like to tear up the old slab and do a proper drained and insulated slab, but I can't justify the cost right now.

In the spring, I plan to do some grading, run longer gutter downspout pipes, and maybe even install an EPDM underground roof wherever I can get access to the foundation wall.

Finally, I believe I may still need to buy a new dehumidifier. Is there a reliable way to calculate or estimate the expected capacity I would need? The space is about 25-ft x 40-ft with 7-ft. tall walls.

Any advice for this project would be greatly appreciated!

Asked by Rob Wotzak
Posted Mon, 01/27/2014 - 17:46
Edited Mon, 01/27/2014 - 23:59


6 Answers

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You don't say where you're located, and both local climate and subsoil temperatures make a difference.

But the solution is to air-seal and insulate the foundation walls, and at the very least put down ground vapor retarder. Air sealing the walls reduces summertime infiltration reaching the rest of the cool basement, and raises the average temp both winter & summer.

Fiberboard/OSB/plywood are all mold-food, and have a very low insulating value. Forget about trying to air-seal the ceiling- even copious amount of spray foam would still fail. Instead, seal and insulate the band joist/foundation sill, along the foundation walls.

Use air-impermeable rigid foam (EPS is the most cost effective, and is highly tolerant of moisture) or closed cell spray polyurethane against the foundation, sealing the edges with can-foam (or FrothPak) and the seams with duct-mastic. You can cut'n'cobble the rigid foam (again, sealed with can-foam) at the band joist & foundation sill, sealing that to your wall-foam. You can then put a non-structural studwall butted up against the wall foam, insulated with unfaced batts. Put at least an inch of EPS under the bottom plate of the studwall & slab as a thermal break.

The thickness of the wall-foam required to do an insulated interior side studwall approach varies with climate, but if you stick with the IRC prescriptive values for your area you'll have plenty of margin:

Even an inch of EPS against the foundation wall will slow the migration of ground moisture through the foundation considerably, as well as stopping the air leakage if you seal it well. With the interior side of the studwall as unpainted or primered wallboard it won't trap any ground moisture that does find it's way in, and the average monthly temps of interior face of the foam would stay above the dew point of the interior air year-round, avoiding an accumulating moisture inside the studwall.

The foamy mats for the floor are somewhat insulating but not air tight, so if you took that approach you'd have a mold-farm under the mat. You can put a dent in the ground moisture load by using an acrylic or silane masonry sealer on the existing slab, but don't put ANY floor covering down until you can insulate it with an air-impermeable water-tolerant rigid board insulation (EPS or XPS- again EPS is cheaper.)

When you're ready to take that step, dig down sufficiently to put 4-6" of clean gravel, and 1-3" of foam, and put a sheet of 6mil poly between the foam and the new concrete as a ground vapor retarder.

Answered by Dana Dorsett
Posted Tue, 01/28/2014 - 16:28

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BTW: Any standalone dehumidifier is sufficient to handle a volume that size unless your slab is only 1" above the water table, and you live in the tropics and leave the windows open. Prior to air sealing and insulating it could be quite an annual power use to keep it under 60% relative humidity (below the serious mold threshold), but after insulating and air sealing it'll be more reasonable.

It's worth the upcharge to go for a 65 pint/day or larger version though, not because you need the capacity, but because the required efficiency of units that size are significantly higher than that of a 30-pint version, and you'll more than make up the cost difference in power use over a few years of operation.

Answered by Dana Dorsett
Posted Tue, 01/28/2014 - 16:34

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Thanks, Dana. I'm in CT, so zone 5A.

That's what I figured for the dehumidifier--the larger of the stand-alone models are more efficient and worth the cost.

Those masonry sealers are usually not recommended for slabs, right? Do you suspect that's just because they aren't rated as a wear surface?

As for insulating the walls, do you have any experience with limestone foundations? Theoretically, if you seal up a foundation with rigid foam, it should reduce evaporation and hence reduce damage from hydrostatic pressure. But I'm just nervous about covering it up and not knowing for sure after seeing the spalling that has occurred over the past 10 years. I guess I could start by doing a test wall in an easy-to-isolate area and then inspect it at a later date.

Answered by Rob Wotzak
Posted Tue, 01/28/2014 - 16:42

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Masonry sealers are fine for slabs, but need to be re-applied every handful of years or so to remain effective.

Limestone isn't all that different from poured concrete- when you insulate it the temperature drops, and the moisture migration slows. When you also factor in the vapor retardency of the foam it changes the direction of the major flow from inward (toward the basement) to upward, drying toward the above-grade exterior.

A whole-wall sacrificial parge of lime-mortar on the interior of the foundation prior to insulating should give you another century or so, if not more. If the problem is mostly from moisture wicking up from the footing the spalling may move to the above grade exterior at or near ground level after insulating, but in the event that happens it can be protected with a sacrificial lime mortar parge every 20-30 years there too.

Active spalling is usually a symptom of freeze/thaw cycling on nearly saturated stone. If you dig down to put down your buried angled EPDM perimeter "roof", putting a 2" thick 2' wide "wing" of EPS insulation under the EPDM is further insurance against below-grade spalling issues.

At the warm edge of zone 5 you can get away with 1" of EPS + 2x4 studwall, but more is always better. Southern New England also has several vendors of reclaimed roofing foam, which can take the financial sting out of it too. Search the Hartford/Providence/Worcester/Springfield craigslist materials sections for "rigid insulation" on a regular basis and you'll find some of them, eg:

I ended up doing my (poured concrete) basement with 3" reclaimed roofing iso at ~$20/sheet for 4'x'8', but I've seen it even better deals since then. If you use iso, keep the cut edges off the slab (put EPS under it). You may also want to put a sheet of dimple-mat drain membrane (any vendor) between the wall and foam, since iso can take on moisture if the limestone gets truly saturated. (With reclaimed EPS or XPS that wouldn't be an issue.)

Answered by Dana Dorsett
Posted Tue, 01/28/2014 - 17:44

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Dana, I feel much more confident now. Your explanations cover details I suspected were viable, but just couldn't find documentation of specific successful examples. I'll go ahead with the slab sealer, the mortar repairs, the parging on the limestone, the air-sealing, and probably the dehumidifier. Eventually, I'll tackle the wall insulation and the insulated slab too. I'll be sure to take some photos and share the results once I've made more progress. Thanks for all of your advice. I always enjoy reading your feedback on GBA articles and forum questions.

Answered by Rob Wotzak
Posted Tue, 01/28/2014 - 22:47

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Oh, and thanks for the tip about the recycled insulation. I know someone who had good luck buying from Insulation Depot, and I'm a regular Craigslist shopper, so I'm sure I can find a deal when the time comes.

Answered by Rob Wotzak
Posted Tue, 01/28/2014 - 22:52

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