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Community and Q&A

Insulating basements and keeping them dry

Hammer 🔨 | Posted in General Questions on

Ok so read a lot about insulating and keeping basements dry on this website just wanted to ask questions for my particular situation.  My house is old built in 1903 so I don’t believe the walls are poured concrete, I think it’s masonry walls covered with a layer of stucco because I did some work at one point and found stone within walls.  I don’t believe they were pouring concrete foundations in the early 1900’s.  On one exterior wall in basement I have dampness on floor, the walls are dry.  I read that if you don’t have poured concrete you should damp proof, building a sump pump would not be a good idea.  Is this true?  Most of my walls are above grade except for wall in front of basement which is dry and the wall that is showing moisture on floor.  The side that has dampness is about 1/3 above grade the rest is buried in dirt and gravel.  At some point before I bought the house a drainage system was installed on this wall that alleviated a flooding problem.  So questions based on this set up.

1.  Since I’m mostly above grade would insulating the walls on outside be a better option, or can I combine it depending on what walls are above or below grade.  

2. Floor is unlevel in spots, can I break up entire floor and insulate it without underpinning or benching and re cement it.  Insulating under the slab would save precious headroom.  I heard it’s possible to drop an inch or two if you are still above foundation but how would I know how deep I can go.  I could test a corner but will I literally see the bottom of footing?

3. If water or moisture is only collecting near that one wall and wall is dry can I literally build a trench, remove dirt, and throw gravel in.  I actually suspect moisture is coming from underneath slab.

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Replies

  1. DCContrarian | | #1

    You need to figure out where the water is coming from. Is it rain water, groundwater, diffusion, condensation? The answer varies depending on which one it is. If it's bulk water intrusion -- rain water or groundwater -- you won't be able to stop it with dampproofing and will in fact make the problem worse.

  2. Jon R | | #2

    Yes, you can dig a trench drain and/or sump and install a pump.

  3. Expert Member
    Dana Dorsett | | #3

    There are poured concrete foundation wall basements that pre-date 1900, but site-made cinder block (made of coal cinder, sand, plus portland cement) foundations were quite common then.

    Both poured concrete and cinder block walls should be damp proofed, preferably on the exterior.

    >"1. Since I’m mostly above grade would insulating the walls on outside be a better option, or can I combine it depending on what walls are above or below grade."

    As a general rule insulating on the exterior is superior, since it put the thermal mass of the concrete or masonry inside the thermal envelope of the house. In most climate zones code even relaxes the R-value requirements of a so-called "mass wall", when half or more of the insulation-R is on the exterior. See the MASS WALL R-VALUE column in TABLE N1102.1.2 of the IRC :

    https://up.codes/viewer/connecticut/irc-2015/chapter/11/re-energy-efficiency#N1102.1.2

    If insulating on the exterior, installing it all the way down to the footing would be preferred, or at least down to the IRC's required depth for slab-edge insulation in your area (see TABLE N1102.1.2 )

    If using foam, EPS is preferred for it's high water tolerance compared to polyiso (which wicks and retains moisture when buried) and comparatively low CO2e footprint relative to XPS (by far the least-green insulation product in common use today in North America). EPS is an order of magnitude lower-impact.

    There are ways to combine sheet foam and a fiber insulated studwall on the interior without creating a mold farm, if that's cheaper/easier than insulating on the exterior, but your location (by IECC climate zone) would be necessary to advise on what it takes. See:

    https://www.greenbuildingadvisor.com/article/all-about-climate-zones

    >"2. Floor is unlevel in spots, can I break up entire floor and insulate it without underpinning or benching and re cement it. Insulating under the slab would save precious headroom. I heard it’s possible to drop an inch or two if you are still above foundation but how would I know how deep I can go. I could test a corner but will I literally see the bottom of footing?"

    In most climate zones it's uneconomic to break up a slab to insulate the basement floor. Depending on where you are that may have little or no impact on energy use or comfort. You may be able to seal any cracks & seams in the slab with an appropriate polyurethane caulk formulation and install a liquid-applied vapor barrier/finish over the top of the slab, unless you're insisting on carpeting or wood flooring.

    "3. If water or moisture is only collecting near that one wall and wall is dry can I literally build a trench, remove dirt, and throw gravel in. I actually suspect moisture is coming from underneath slab."

    Breaking out a strip along the full perimeter of the slab to install a French drain that runs to a sump for managing the bulk water may be necessary, but that doesn't usually require replacing the entire slab. Don't assume that a French drain only along the most obviously leaky edge would be the extent of it.

  4. Hammer 🔨 | | #4

    Dana,
    Thank you for answering my questions. Part of redoing the slab is to gain head room so I can add insulation, and make room have normal ceiling height. I’m in climate zone 5, basically suburbs of NYC. If I really don’t need floor insulation I can seal the slab with finishing coat as you mentioned. I don’t have a lot of headroom under my carrier beam but right now I can walk around without ducking. My only concern is something I can put on concrete that will make it softer for my kids without leading to mold. I don’t want to advertise any brands but they make a dimple mat that they claim you can ad carpet right to it but I don’t know if this will stop mold even if there is a carpet made out of inorganic materials. Any softer material that I can throw on concrete after sealer will do or kids will have to toughen up. This is all of course after solving dampness issue.

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