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Community and Q&A

Regarding cellars, retrofits, and capillary breaks

efficient_builder | Posted in Energy Efficiency and Durability on

The house is in the process of being purchased and will undergo a complete gut rehab and deep retrofit. It was built around 1910 located in upstate NY in climate zone 5A. The basement has a dirtfloor and a height of 6′ from the dirt to the bottom of the 1st floor, floor joist. There was an addition put on that has a crawl space attached to this area. There was another addition that was was partially dug out- it slopes rather steeply from grade to 6′. This part is also attached and happens to have a well or a cistern in it. It’s 15 feet or so to the water level. I am thinking it is a well, but will have to confirm. The foundation is brick and extends approximately 1 foot above grade. The only sign of moisture issues I found was mold on the an object on the floor near the well. From what I remember (I was last in the basement a few months ago) the bricks showed minimal signs of damage or discoloration, although there had been a few repairs.

I would like to treat this area as a cellar and completely isolate it from the house with access through (existing) Bilco doors. I would like to add 2-4″ of rigid and fill the joist cavities with cellulose. However, it seems the overwhelming majority would say treat it as semiconditioned space and insulate and seal the foundation walls and floor.

The cavities between the floor joist, over the foundation, and against the rim joist will require further thought. In the deep retrofit I may only be able to use 2″ of polyiso which will be installed on the exterior of the rim joist. The capillary break between the foundation and wood is another issue. I am unsure that I will be able to jack up the house to install a membrane of some sort.

So my questions:
If liquid water is properly addressed, i.e good exterior and interior drainage practices. Is it possible to get away with just insulating and air sealing the floor? If so is closed cell spray foam more or less the best way of addressing the cavities along the rim joist? Are there other acceptable alternatives?

Again assuming proper water management, am I asking for serious trouble if a capillary break is not installed between between the foundation and wood?

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  1. GBA Editor
    Martin Holladay | | #1

    In upstate New York, you have two problems:

    1. You need to keep your heating equipment within the home's thermal envelope. Unless you have an upstairs mechanical room, that usually means that your basement will include a furnace or a boiler.

    2. You have to keep your pipes from freezing.

    In your climate, it makes much more sense to insulate your basement walls and bring your basement within your home's thermal envelope.

  2. user-788447 | | #2

    I live in a home with similar conditions. I can commiserate.
    At the turn of the century there was no concept of 'thermal envelope' and in most cases establishing a proper thermal envelope in these homes is financially unfeasible if not next to impossible in some cases.
    The basement/ cellar is a gray area. Ideally you jack up the house and completely reconstruct a proper insulated basement. Maybe $80k more or less?
    My second recourse which I have not tried yet is to 'mildly' insulate the basement ceiling and add a heat source to the basement on its own zone. You can then keep the basement as low as not to incur mold problems and have less heat loss through your main level floors (which in part makes my first floor uncomfortable during the winter.)
    DYI folks who install radiant floors and insulate under them are essentially doing this. I've seen a case where the homeowners were able to maintain relatively low heating bills.

  3. efficient_builder | | #3

    All mechanicals will be in a utility room on the first floor. There should be minimal if any runs in unconditioned space. Only concern would be the water supply line, but addressable.

    I am wondering how much the lack of the capillary break will impact the building. Am I going to overwhelm the drying capabilities of the wall or cause other moisture issues? I plan on taking the outsulation approach (foil faced polyiso on the outside), blown in cellulose in the stud cavities and airtight drywall.

  4. GBA Editor
    Martin Holladay | | #4

    It's quite possible to lift your house a half an inch so that you can slip a capillary break between the top of your foundation wall and your sill beam. If I were you, I would investigate the integrity of your sill with an awl. If your sill is sound, you are probably good for another 50 or 100 years. If your sill is soft, it's time to make repairs.

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