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Community and Q&A

Can interior basement insulation cause moisture damage to my sill plate or frost damage to my stone/concrete foundation wall?

Charles Galgowski | Posted in Energy Efficiency and Durability on

Sources of technical information I have read on this are:
1. Retrofitting Basement Insulation, By Martin Holladay Issue 236 – June/July 2013
2. How to Insulate a Basement Wall, Posted on June 29 2012 by Martin Holladay, GBA Advisor
3. Hybrid Foundation Insulation Retrofits: Measure Guideline Building America Report – 1108, 13 January 2012 (Rev. 4/12), Joseph Lstiburek and Kohta Ueno
The overall goal for the house is to remove the oil boiler in the basement. Then install a domestic hot water heat pump in the basement and two air to air mini split heat pump compressors on the exterior of the house. The wood stove on the first floor will continue to be used.
The house is located in Storrs, CT in heating/cooling zone 5, was built in 1926, and added onto in 1955. The three attachments show the plan view and 2 cross sections. Cross sections CC, DD, and EE are not shown. The top of concrete slab at CC is 6 inches above the sill plate, the sill plate at DD is 5 inches above the ground, and the sill plate at EE is 1.5 inches above the concrete slab. The house sits on top of a small hill and 75 ft from the house the ground drops in elevation at least 6 ft to the north, east, and south and 10 ft to the west. The soil around the house as defined by the USDA NRCS Soil Map is a Gloucester Soil Series. These typically have sandy loam, loamy sand, and sand soil textures as defined by soil maps. The percentage of soil particles less than 0.02 mm are 3 to 12 %. So frost heave potential is on the low side. The water table is 17 feet below the concrete floor of the basement as measured down the 4 inch diameter well casing located in the basement. So the house has good drainage. The laid up stone with concrete interior wall shows some pink efflorescence. Over the last 18 years, I have never seen water dripping down the basement walls. There has been occasional water leakage down the hatchway and at the wall/ slab seam by downspouts that have leaked. It is not a high water table but very temporary drainage from heavy rainfalls. The newer 8 inch concrete wall has no efflorescence and possibly it was coated with tar when it was built. There is no visible water damage to the sill plate or rim joist as viewed from the interior.
I air sealed the rim joist 18 years ago with Dow Great Stuff and silicone. A recent blower door test showed I did not get all the leaks. A reputable air sealing/insulation company will be doing air sealing on the house and proposes to spray closed cell foam to a thickness of 1 inch on the rim joist, sill plate, and at my request out across the top of the stone/concrete wall which can be up to 16 inches wide. I then intend to cover the foam with Roxul (mineral wool). The basement interior walls will be covered with 1.5 inch thick Thermax for the top 6 ft of wall and leave the bottom 12 to 15 inches of the wall uncovered.
The concrete basement floor slab will not be insulated, because there is not sufficient vertical height from the floor to the basement girders to raise the floor. Also, when the foote valve of the jet pump with 75 ft of pipe has to be occasionally removed, water spills over the basement floor. The basement has and will be used for storage and a work bench. Temperatures ranging down to 48 to 50 degrees will be tolerated. As of October 10, the basement floor temp is 60 degree F. The basement presently stays around 50 to 57 degrees F. through the winter. It loses heat through uninsulated walls and air leakage into the basement and gains heat from the uninsulated ceiling and with the domestic oil boiler heat escaping from the tank less water heater. The bulk of upstairs space heat comes from the upstairs wood stove. In the future, the basement will lose the oil boiler waste heat and also be cooled by the domestic hot water heater. This will be offset by the newly insulated walls and deep ground heat from the uninsulated floor that should keep the basement from falling too much below 52 degrees. Depending on how this goes, I then hope to partially insulate the basement ceiling with rock wool to reduce heat loss from the upstairs to the basement and yet not allow the basement to get too cold.
Joe Lstiburek and Kohta Ueno very comprehensive report 1108 pages 8, 50, 51, and 65 raise some concerns with interior basement insulation increasing moisture in the sill plate. The articles and post by Martin Holladay raise less concern with moisture in the sill plate. For my house, would it be better to drop the Thermax down say 4 to 6 inches below the top of the concrete wall to allow capillary and diffusion moisture through the stone and concrete wall to help prevent the wood sill from possibly getting too moist?
As of October 10, the moisture content of the sill interior side ranges from 12 to 14 % on the east, south, and west and 14 to 16% on the north (north had been covered with reflectix insulation). Upstairs staircase banister was 13 %. So hopefully, this indicates no present moisture problem in the basement.
Joe Lstiburek and Kohta Ueno Report 1108 pages 9-12 and 65 raise some concerns with potential frost damage to rubble foundations when the interior wall is insulated. But page 65 states, “Overall, the research literature indicates that soil freezing behavior results in minimal likelihood of damage in heated (even insulated basements). How much of a heat source did these basements have and was the floor slab insulated as well? The heat source to my basement (below 52 degrees) will be the uninsulated floor slab and some heat from the partially insulated ceiling. The basement will be partially cooled by the domestic hot water heater. I am in the southern range of zone 5, have well drained soil, and the maximum extreme frost penetration is 50 inches. I do not think there is much chance of deep frost penetration, but some shallow adfreezing could possibly grab onto the lumpy protrusions of the stone wall foundation. The north outside concrete slab might be the most at risk point, because concrete has a low R value, snow is cleared off it, and it gets very little sun. This part of the basement wall could be left uninsulated if needed. I would appreciate your opinions on what frost damage risk I have.

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  1. GBA Editor
    Martin Holladay | | #1

    In your case, the risk of frost damage to your foundation is quite low. Remember, in New England, plenty of barns have unheated stone and mortar foundations. If these barn foundations have deep footings and well constructed walls, they last 150 years or more without heaving. My basement has stone-and-mortar walls and no heating appliances, and my basement walls have no frost damage.

    When it comes to the risk of moisture accumulating at the sill, two facts lower the risk in your case: your soil is sandy and well drained, and there is a distance of 14.5 inches from grade to the lowest wooden components of your house.

    If you have any bushes near your foundation, trim them or remove them to encourage sunlight and air to reach the exterior of your foundation. This encourages drying.

    In your case, I would have no problems recommending that you install interior insulation. I would increase the thickness of the spray foam installed at the rim joist from 1 inch to 2 inches -- but even if you follow your plan (installing only 1 inch of spray foam), you will be fine. I would also extend the Thermax down lower, to within 1 inch of the floor slab, because otherwise you'll have thermal bridging through the concrete walls and stone walls.

    In your climate zone, the code requires at least R-15 of basement insulation. If I were you, I would install 3 inches of Thermax (about R-18) rather than 1.5 inch of Thermax (about R-9) as you plan.

    You'll find that your basement will be significantly cooler after the work is completed. My guess is that you won't mind, but it's quite possible that in late winter, the basement will be cooler than 48 degrees.

  2. Expert Member
    Dana Dorsett | | #2

    Storrs, CT isn't cold enough to accumulate enough frost from interior moisture drives to even remotely damage anything, even if you had a severe air leak bringing humid interior air out to the foundation sill. The daily average temps in Hartford in January are below freezing, but the average daily highs are above freezing. Any frost that forms during a cold snap melts within a few days- it doesn't keep accumulating over the winter, with increasing pressure the way it can in much colder climates.

    The measured moisture content of the wood is well below the danger zone. You don't have to worry about it unless creeps well north of 20% when the outdoor temperatures are averages 45F or higher- temps high enough for more rapid bio-activity. (Even 25% is fine if it's below freezing.)

    Most New England field stone foundations use hard stone with every little moisture wicking potential, and will not draw ground moisture or surface water moisture up to the foundation sill.

    The poured concrete foundation can potentially wick moisture to the foundation sill, which could be an issue if there is no capillary break between the concrete & wood. But with 14" of exposure and reasonable surface drainage and reasonable roof overhangs it can still dry toward the exterior. If you're the type who won't sleep well if you don't do something about it, jacking the house up 1/4" a section at a time and inserting EPDM (membrane roofing material) between the concrete & sill works VERY well as a capillary break.

    +1 on going for 3" foam instead of 2", and bringing it down to within an inch of the slab. Don't rest it on the slab, since polyiso can wick ground moisture into the foam. It's fine to use an inch of EPS to insulate that bottom inch, but if the slab isn't insulated it's not going to make enough of a difference to matter.

    In NE CT you are withing driving distance of several foam reclaimers/recyclers, often at prices per R, below box store pricing of rock wool, and WAY below the cost of new rigid foam( 25-35% the cost of virgin stock goods.) Of the bigger reclaimers is Green Insulation Group ( ), which is only about an hour's drive from Storrs. A bit further afield, but still reasonable round trip (80-90 minutes each way) is Nationwide Foam in Framingham MA ( ) But there are others that may be closer or sometimes at better prices, eg:

    If the quantity is large enough to fill a flat-bed truck or a box van, the bigger vendors can quote you shipping to your site too, but you can also just show up with your own truck they'll sell you in quantities as low as a 4x4' or 2x8' halfsheet, if they have it in stock and that's all you need. Almost all of them will have quantities in stock of 2" & 3" fiber faced roofing polyiso in decent shape. I've never paid more than $20/sheet for 3" polyiso in good shape, but prices do vary with the market. The folks in New Haven are quoting $23. Brand new 3" polyiso runs about $60, so that's about 38% the price of virgin stock, which is still a substantial discount, but it doesn't hurt to call around.

  3. Charles Galgowski | | #3

    Thanks Martin,

    Yes, I understand the barn part. I grew up on a dairy farm with one of the barns having a foundation like you mention. Most years it had a great deal of heat from the cows inside. A side hay mow also trapped the heat against the ground and slowly released it to the wall all winter. However, there was a 10 year period when the barn was vacant and had no heat from cows and had no apparent damage. Does your house have interior basement insulation, so that the stored heat in the subgrade below the floor cannot flow to the walls. If that is so and if you are in Vermont, then I have no frost problem at all. Regarding the moisture concern, I do have sections on the northeast quadrant of the house where the sill is only 1.5 inches above an outdoor concrete slab, 5 inches above earth ground level. The east porch concrete slab is actually 4 inches above the sill plate. It has been this way since at least 1955 with no problems. The difference now is, I will cover the sill and rim joist, so a thermal gradient cannot drive moisture out to the exterior. Does this make a difference with these lower sill to ground heights? I do not fully understand these moisture and thermal drive systems. However, if many houses have had continuous foam from the rim joist across the sill and top of wall and down the interior of the wall for the last 10 or 15 years and moisture problems are not occurring than that is proof enough that I am okay. It was Joe Lstiburek and Kohta Ueno's report that raised my concern about this.

    Thanks, Charles Galgowski

  4. Charles Galgowski | | #4

    Thanks Dana,

    As my reply to Martin shows, there are places where the sill plate is only 1.5 inches above a concrete slab and also 4 inches below a slab. But I am thinking there is some sun exposure on both of these concrete slabs and that will set up a capillary drive that will tend to suck any moisture out of the sill plate in summer. Is that reasonable? Regarding the sources of foam supply, are any of these Thermax? Can they be used by code without sheet rock or plywood? The reason I am using Thermax is I need no sheetrock or plywood over it. My wall is pretty wavy and also has a sewer pipe only 1.5 to 2 inches from the wall. I can slide 1.5 inch Thermax behind it, the water softener and the electrical panel. Sheetrock and plywood will not fit or bend enough.

    Thanks for your advice.

    Charles Galgowski

  5. GBA Editor
    Martin Holladay | | #5

    You wrote, "I do have sections on the northeast quadrant of the house where the sill is only 1.5 inches above an outdoor concrete slab, 5 inches above earth ground level. The east porch concrete slab is actually 4 inches above the sill plate."

    This information changes things.

    First of all, it's certainly a bad idea to have an exterior concrete slab that is higher than the wooden components of the building. That's a no-no (whether or not you have interior insulation on your basement wall or rim joist). Is it a fully enclosed porch, with no exposure to the weather, or an open porch that gets wind-driven rain and snow?

    Assessing the risk at this point becomes a judgment call that requires you to weigh the risk vs. the expense of fixing things properly.

    Ideally, the porch floor would be replaced by a wood deck.

    If cost is no object, it's always possible to jack up the house and slide a capillary break between the foundation and the mudsill, as Dana Dorsett explained. (Even a retrofitted capillary break, however, won't solve the ugly detail of the slab poured against wooden components of your building.)

    There are a lot of factors here: How much rain does the slab get? Does the existing porch roof do a good job of protecting the slab?

    Most important of all: How big is your budget?

  6. D Dorsett | | #6

    What Martin said, regarding sill plates that are below slab grade. Ideally the sill would be no lower than the top of the slab, and have a capillary break between the slab & sill. The theory that the slab will draw moisture out of the wood when heated isn't a sound one. In an ideal world with a perfect vapor barrier under the slab it might, but if it's poured on gravel or soil, not so much.

    Regarding reclaimed foam vs. Thermax,, adding layer of 1/2" Thermax would give the assembly the same fire rating as if the full 3" were Thermax. The fire ratings of Thermax are all about the characteristics of the facer, not the foam itself, but the bulk of the cost is in the foam. But a layer of 1/2" wallboard would be cheaper than half-inch Thermax, and would give the assembly an even better fire rating.

  7. Charles Galgowski | | #7


    Thanks for your further insight. The East porch slab that is 4 inches above the sill plate has a roof over it extending about 2 ft on both sides past the concrete and 1 ft past the concrete on the front. It has open sides and wind and rain can blow in. The slab slopes away from the house on about a 2.5 % slope. The wall from the slab up to the door sill is covered with as metal flashing. Possibly there is flashing between the house and the slab. It has been this way for at least 60 years and has no apparent decay as viewed from the basement. There are times when I use the "If it ain't broke, don't fix it philosophy". Maybe this is one of those times. When I think of it there are many concrete front door steps that open to the elements that have 6 to 7 inches step ups to the door sill. Hence, they must somewhat higher than sill plates. Possibly flashing is what is saving them. 10 years ago, I did replace about thirty feet of rotten sill plate, rim joist, and floor joist ends on a bank barn I own, because it had no flashing whatsoever by three drive in doors. If I jack up the house, I still cannot get at the sill plate, because the slab blocks it. The middle approach here is to not foam the 5 ft of sill plate by this porch, so it can be viewed from the interior. If decay is noted, then get a concrete saw and get at the sill plate/rim joist to make repairs then, before it eats into the floor joists. As I measured before, the sill plate sitting right on the concrete/stone foundation wall and blocked by the concrete porch has a moisture content of 14 %. Also, had 2 inches of rain 2 days prior to the measurement. If I do foam the rim joist, sill, and interior wall, am I raising the potential to make the sill wetter and rot it out? I believe I read on your site, that Joe Lstiburek felt basement walls needed to dry to the interior, but has recently stated this is not really needed. No easy answers here.

    Charlie Galgowski.

  8. Charles Galgowski | | #8


    Thanks for the tip about using only 1/2 inch of thermax on the top layer. This should keep costs down. As I said before, sheetrock will not bend enough on these curvy walls.

    Charlie Galgowski

  9. GBA Editor
    Martin Holladay | | #9

    I agree with you: "No easy answers here." But if I had to make the judgment, I'd say that the risks associated with your insulation plan are quite low.

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