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  1. #1
    Join Date
    Jul 2015
    Posts
    1

    Default 1860 support beams and dehumidification in basement - help !

    Our house was built in 1860. Basement is 5 ' tall, encapsulated with spray-on insulation over field granite walls, fire retardant over insulation, and 6 mil poly with tape over dirt floor. Great sump system with battery backup/charger, and drains feeding to sump pit. All works perfect ...... but just a little damp from condensation. Furnace with exposed and updated piping keeps it toasty when fall/winter hits here in Maine. Use this space for storage and utilities only (furnace, panel, plumbing, etc).

    So I placed a standing dehumidifier in basement, direct feed into sump pit. Humidity was 70% at start, and easily have it down to 55% now (digital readout on dehumidifier).

    But when I look up at floor supports running across ceiling of basement, the logs are all original (1860), some of which even have bark on them ! These timbers have seen 155 years of humidity ...... and I don't want to disrupt them if they have already lasted since the Civil War. Proper supports and joists have been added over last 100 years, so structurally very sound.

    Any thoughts on the appropriate humidity level to shoot for ? TC Hafford Basement Systems recommends 55%, but that is with a more modern structure with processed lumbar. The basement "feels" great at 55% humidity now, and it is easy to keep there (and there is no condensation anywhere.)

    Am I doing damage to the old timbers in some way ????

  2. #2
    Join Date
    Aug 2015
    Posts
    1

    Default Re: 1860 support beams and dehumidification in basement - help !

    ***! I have exactly the same question, except I'm in Pennsylvania. I guess the only solace I can provide at the moment is that you're not alone. If I find anything out in my searches, I will try to post to share. Good luck to us

  3. #3
    Join Date
    Oct 2015
    Posts
    2

    Default Re: 1860 support beams and dehumidification in basement - help !

    I'm in a similar situation but in NC with a crawlspace. What makes it funny is that the joists are all random sizes and sitting on huge rocks. They still are really strong though!

  4. #4
    Join Date
    Feb 2013
    Location
    Boston area
    Posts
    112

    Default Re: 1860 support beams and dehumidification in basement - help !

    I thought dry air is why wood relics from ancient Egypt survived in tombs for thousands of year. Cycling from moist to dry over and over can cause wood to crack, so I would think a steady 50% is better for the wood than cycling.

    I'm no wood expert but I can read tables and that 20% humidity change only causes a 5% moisture change. Assuming your basement is 50 degrees, changing from 75% humidity to 55% will only lower the equilibrium moisture content of the wood from 14.8% to 9.5%. http://www.csgnetwork.com/emctablecalc.html

    Personally my concern and experience is higher humidity allows mold to thrive, so I've been working to keep my basement dry. One of the biggest differences just came from regrading part of the lawn and getting downspout extenders, plus getting the mortar repointed.

  5. #5
    Join Date
    Aug 2015
    Location
    Houston, TX
    Posts
    75

    Default Re: 1860 support beams and dehumidification in basement - help !

    When I had my crawl space sealed off and lined, we installed a dehumidifier and set it to keep the air down there <40% humidity. That is significant in our humid, Gulf Coast climate. I now have no more mold or mildew on the joists, and have had no issues with warping or twisting, but they are tied into the structure enough that I don't think it would be possible to have noticeable warping.

  6. #6
    Join Date
    Jan 2008
    Posts
    2,737

    Default Re: 1860 support beams and dehumidification in basement - help !

    Wood in general has amazing qualities, able to withstand complete immersion or arid dryness for over a hundred years without any significant loss of it's strength. What destroys wood is rot which is usually moisture-induced, so lowering the ambient humidity level when it is high is a very good idea. With the numbers being discussed here there is nothing to be worried about once these levels have been reached although there may be some movement during the time it takes for the wood to acclimate to the new conditions. Once the moisture levels in the wood settle to the new norms, the wood will happily stay there until conditions change which they won't so long as the dehumidifiers are functioning.

    Phil

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