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Does the height of a house affect air leak problems?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by EatenByLimestone, Feb 15, 2013.

  1. EatenByLimestone

    EatenByLimestone Minister of Fire

    Joined:
    Jul 12, 2006
    Messages:
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    Loc:
    Schenectady, NY
    Taller chimneys draft harder. Does a 2 story house draft harder than a single story? When walking into the Leeds certified 14 story building I work in every morning I can hear air whistling past the closed doors. To get the certification the building has to be fairly energy efficient.

    On my own house I recently finished off they attic. This year I'm noticing air leaks I had not in the past when I only heated the single story. By this I mean I have had to run a humidifier that I didn't have to run in the past. To be fair though I replaced the bathroom vent so the shower is not adding to the house humidity and the larger fan would be pulling more makeup air into the house.

    So, are the dryness issues related to the increase in the heated volume of my house?

    Matt

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  2. DickRussell

    DickRussell Member

    Joined:
    Mar 1, 2011
    Messages:
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    Loc:
    central NH
    What you are talking about is the "stack effect." The pressure difference from bottom to top of a chimney, the driving force for flow of air or flue gas up the chimney increases with the height of the column of air & gas in the chimney and with the difference in densities of that column of air/gas and the air outside of the chimney. So tall stack and hotter flue gas both make for more pressure difference and thus more flow (better draft).

    Now as to air leakage into/out of a house, that is much like a flue stack. You have some height over which there is a difference in density, and you have the difference in density; air at 70 F inside the house is much less dense than outside air at, say, 0 F, by about 14%. Thus you have the heavier air outside trying to force its way into the house at lower levels, via any tiny crack it can find, while the lighter inside air is trying to leak out of the upper levels through similar tiny cracks. Somewhere in between is the "neutral pressure plane." This link provides a couple of photos of this effect, where you can see the flexible temporary wind screening puckered inward down low and billowing outward up high: http://www.greenbuildingadvisor.com...ral-questions/24286/i-need-photo-stack-effect.

    Sometimes those "tiny cracks" where air leaks in a house aren't so tiny. An older house might have a rather large gap between an interior masonry chimney and the framing around it, as code requires, but completely open at attic level. Can lights in the ceiling of the top story also are notorious air leaks, even those rated AT ("Air Tight"). But cracks at ceiling-wall joints, holes in framing where wiring passes, cracks at the sill, improperly installed and flashed windows, and electrical boxes at exterior walls all contribute to the leakiness of a house. Cold outside air is exceedingly dry, compared to inside air, and excessive leakage does make for dry inside air (and makes it harder to heat the house). Just throwing humidity at the dryness isn't a good idea, as all that added moisture will be leaking out through wall cavities and the attic space, where it will contact cold surfaces and possibly lead to mold and rot over time. The solution to excessively dry inside air in winter is to tighten up the house, going after any and all cracks and holes that are accessible, even if it means an uncomfortable afternoon crawling on your belly around unfinished areas of your attic to locate and plug leaks with can foam or other sealant. Tightening up a house really does go a long way toward reducing dryness in winter, although it's unlikely you'd get to the point where an older house would become so tight that mechanical ventilation would be required, unless extreme measures had been taken. With new construction, a house built to be extremely tight does need mechanical ventilation, both to provide fresh air for the occupants and also to keep interior humidity down (yes, down). Human activity produces moisture, and if it can't leak out then it must be removed by deliberate ventilation. But all this is beyond what you asked in the first place.
  3. EvilRoySlade

    EvilRoySlade New Member

    Joined:
    Jan 4, 2013
    Messages:
    10
    Loc:
    Midland, MI
    I like it. This is where my head has been all winter. I know I will never get my house to passivehaus standards but the closer I get the more my little insert will heat my house and when the time comes to replace my furnace I will be able to drop about 20,000 BTU off the size.
    From my studies I agree with you, seal you attic from your living space first and foremost. The goal is to have zero conditioned air entering your unconditioned attic, like said this means everything not just attic doors, can lights and bath fans.
    Remember a drafty house is not because air is coming in, it's because the warm air we so enjoy is finding it's way out. Once you reverse your thinking on this it gets easier to find your leaks. The cheapest way is to get some incense sticks, light them and walk around your upper floor when the house is nice and warm and see where the smoke goes, seal those spots up and keep going. You will find yourself considering roof replacement that will then allow you to remove sheathing just so you can access spots otherwise unobtainable.
    In reality this is not beyond what he asked, if we all could make our houses as air tight as possible our wood usage would go down and the heat retained would be very nice. On the extreme side some of these super tight homes in areas like MN may only have 20,000 BTU for heating purposes in a 3,000sq.ft home.
  4. EatenByLimestone

    EatenByLimestone Minister of Fire

    Joined:
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    Messages:
    4,809
    Loc:
    Schenectady, NY
    I have started airsealing. Luckily I haven't really trimmed it out up there yet. One bad thing is I now realize the top of the only internal wall I have upstairs is leaking air. I'll have to open it up to seal up the top of the wall. I have noticed an improvement from what I've done so far though. It just takes time and money. Unfortunately they never come at the same time.

    Matt

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