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