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Winterizing a stone summer cottage

Chuck Redpath | Posted in Energy Efficiency and Durability on

I own a 90 year-old stone summer cottage in the Pocono Mountains in NE Pennsylvania. I intend to gut it to the stone shell, and completely renovate it as our primary year-round residence.

The cottage currently has a dirt crawl space and the foundation depth is unknown, but the bulding has been stable for 90 years now, with no significant cracking. The stone perimeter walls are approximately 24″ thick and my plan is to dig out the crawl space a bit, pour an insulated ‘rat slab’. I plan is to expose some of the stone walls on the outside, frame out and spray foam the inside; and in some areas expose the inside stone surface and foam and ‘side’ the outside.

I intend to make the house as energy efficient as I can, within reason; my dream would be heating through a water source or ground coupled heat pump, that feeds radiant floors, with some wood supplimenting.

My major concerns are:

1. Will the stone walls wick moisture significantly and do I need to be concerned where the inside stone surface is exposed?
2. I plan to install a ‘radon fan’ under the ‘rat slab’ and piped above the eave line, but should I be concerned with radon seeping in, through the exposed interior stone?

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  1. GBA Editor
    Martin Holladay | | #1

    I strongly advise against your plan to insulate some of your stone walls on the outside, and some on the inside. It won't work.

    Your stone wall conducts heat. Let's say you insulate just one wall from the inside -- for the sake of discussion, let's say it's your north wall. On the other three walls (east, west, and south), you insulate on the exterior.

    The stone on the east, west and south walls heats up to room temperature. But at the northeast and northwest corners of the house, the 72 degree walls leak massive amounts of heat to the exterior, because your stone wall is exposed to the outdoors on the north side. Thermal bridge alert !

    So, Chuck, you've got to choose: insulate the wall on the interior or the exterior. But be consistent.

    If you insulate with closed-cell spray polyurethane foam, you shouldn't have any moisture problems. Just be sure to use enough foam, so that the R-value of your wall is at least equal to minimum code requirements.

  2. Riversong | | #2

    Ditto on Martin's advice, and I'll add that the only place thermal mass is efficient in a cold climate is on the interior of the thermal boundary, so I would suggest insulating, waterproofing and air-sealing all stone walls on the outside. Then you can still frame and drywall or wainscot of desired for finish and for creating a mechanical chase.

    Stone won't wick moisture, though mortar will, but will condense moisture if it's cold - which is another reason to keep it on the warm side.

    The radon vent should work without a fan, with just passive stack effect venting. If the radon count is still too high, then a fan can be added. But be aware that granitic stones (and well water) emit radon and you may not be able to completely eliminate it. So, once you tighten up the house, be sure to install some form of fresh air ventilation system, even if it's just an exhaust-only bath fan on a programmable timer with makeup air inlets in bedrooms and living areas.

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