Another Puzzler

Another question I’ve been pondering:  is it worth the trouble and expense to insulate the ceiling of our (essentially unheated) cellar– i.e., under the floor of our kitchen/living room/guest room?At first this seems like a no-brainer:  we have nothing but a couple layers of wood slats between the (relatively) warmest part of the house, and the potentially coldest part.  But that doesn’t necessarily mean it’s worth doing.

Consider: unlike the outdoors, which around here has the potential to get down to -15º at various times, the cellar last year never went below 34º.  Whether that’s because the cellar is 2/3 underground and draws in geothermal heat, or because it draws some heat down through that uninsulated floor, or both, I don’t know– but if the cellar ever did start to head towards the freezing mark, we’d have to start heating it intentionally– so, conceivably, a result of insulating the top of the cellar might be that I have to turn on a heater in the cellar.

Consider also:  unlike most homes, where the living room might often be at 68º, 70º, or higher, ours is rarely above 64º, and averages closer to 55º.  So, the differential between cellar and living space is typically going to be less than 30º.  Consider further:  the boundary between the cellar and living space, being a horizontal plane, exists at the highest point of the cellar and the lowest point of the living area– thus, likely, at the warmest level of the cellar and the coldest level of the living area.  So that probably narrows the maximum temperature differential still further, maybe to 25º (I could measure this, I suppose).

So, after considering all that, is it worth buying 600 sq. ft. of insulation ($250?) and spending a day of my life getting covered in fiberglass to install it?  I think the answer may be this:  insulate under the guest room only.

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13 Responses to “Another Puzzler”

  1. C Neal Says:

    The old house my wife and I bought this spring had batt insulation on the cellar ceiling – and it was absolutely gross, covered in mold and mildew. We tore it out. In a damp basement, insulation is just one more thing for moisture to condense onto.

    How about a carpet instead?

  2. Patrick Says:

    I was insulating pipes in my basement recently and had a similar thought – “should I do the ceiling, too?

    That question led me to this: http://www.thisoldhouse.com/toh/asktoh/question/0,,1131829,00.html

    To paraphrase what they say: You can, but it seems like more trouble than it’s worth – take the money you would have spent on the basement and make improvements elsewhere in the house.

  3. Patrick Says:

    I haven’t had the pleasure yet, but I just read that they’ll be at Great Lost Bear next week, and I plan on being there!

  4. Leif Says:

    If you’re gonna insulate the ceiling/floor, I’d recommend foam instead of fibreglass:
    Seals
    Quiets
    Light color – which makes the cellar easier to light.
    No mouse habitat (fibreglass is ideal mouse habitat)
    No itchy fibreglass raining down in the cellar forever
    No mold habitat
    Thinner for same R – may be easier to leave wires & pipes exposed.

    Two foam techniques that I know of:
    1 Loosely fitted rigid sheets/pieces, sealed around the edges with canned foam.
    2 Real spray in foam, like that from http://www.tigerfoam.com/gogreen.php which qualifies for 30% tax rebate for the next few weeks.

    The first is the most work, the latter is the most expensive, but easist & best.

    I’ve hear good results of putting an inch of spray foam upper few feet of cellar walls, rim joist, and right up to subfloor.
    Forms a very good seal, no windblown air drafting around in the cellar.
    Insulates the cellar from its coldest exposure – the sill area and the area of earth above the frost line. While preserving thermal contact to the 55 degree floor area.

    In the winter you should find the cellar temps to increase = lower relative humidity. In the summer you might find that ingress of 90 degree humid air is minimized – easier to keep dry with a dehumidifer.

    • coldhousejournal Says:

      Leif– will have to check out your foam next time I’m at your place. I wonder about what chemicals might drift out of sprayed-in foam… makes me a little hesitant. Or at least, I think I’d want to install it in summer with the windows open, rather than in winter.

      Ooh. Mouse habitat. Hadn’t even thought of that, but it’s important. I’m not sure if our cats are net reducers or increases of our indoor mouse population.

  5. Jane Says:

    Wow, that spray foam is expensive! (Zero VOCs, btw, according to the Web site)

    Solid foam, ie blue board, is pretty inexpensive, but boy is it a pain in the butt to work with, especially if you’re bothered, as I am, by that screeching sound it makes cutting through it or just rubbing against things. Aieeee.

  6. Patrick Ledwith Says:

    Whatever came of this?

    • Cold House Journal Says:

      Never did anything about it : ) Realistically, I think there’s very little heat transfer going on between the cellar and the house. I would consider insulating the cellar ceiling in one situation: if I planned to hole up in the cellar for the winter! Which, if my cellar was less grungy, I might almost consider. (Note: a friend just renovated his entire cellar, including putting insulation UNDER the concrete floor. I chided him for this, since in my house, the floor draws heat IN during mid-winter…)

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