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insulating basement joist bays

Post in 'The Green Room' started by saichele, Jun 2, 2006.

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  1. saichele

    saichele Minister of Fire

    Joined:
    Nov 18, 2005
    Messages:
    525
    As a matter of technique, what's appropriate here?

    I'm insulating the floor above my unfinished, unheated basement with unfaced r-13 fiberglass batts. Went with unfaced bacause it does tent to get a little moist down there, and I didn't want any (additional0 weird condensation issues. most of it is easy enough - push a batt in then bush in one of those wire clips every couple feet to keep it up.

    The two obstacles I encounter are 1) cross bracing, and 2) ductwork.

    Most of the framing has X braces every 8 6 ft or so. Should I cut the batt on either side and just butt up against them, or is there (as Tim Kaine might say) a "better way."

    Similarly, the joists are 2x10s primarily, and I have a lot of 8" ductwork that is either hung flush with the bottom of the joist or is very close. How do I insulate those? The duct itself already has the bubble foil wrap stuff.

    Thanks
    Steve

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  2. elkimmeg

    elkimmeg Guest

    Insulation is not an exact science. You should have installed vapor barrier towards the heating space especially,
    if you indicated a possible problem. Believe it or not. the cross bracing has a purpose of unifing the floor structure, deflecting the and helping distribute the load and preventing joist from twisting. You have it right cut the insulation tight to the strapping maybe over size the length and push it in tight from both sides, do the best you can. Over your duct work using a stick broom handle push it in over the duct work. Again not an exact science. just do the best you can. You will notice a distinct difference and your floors will seem quieter. R19 would have been better but r13 is a huge improvement, than none at all
  3. Turner-n-Burner

    Turner-n-Burner New Member

    Joined:
    Mar 21, 2006
    Messages:
    184
    Loc:
    North of Boston
    I'm faced with a similar project. 10" joists with cross bracing. No ductwork, but some plumbing etc... I have a pile of R25 waiting for me. I went with the R25 more for sound insulation than for actual heat concerns. Also went with the fully wrapped comfort therm. I'll be using part of my basement as a workshop and I wanted to deaden noise as much as possible. Also wanted to keep the sawdust from becoming a major nuisance and I figured the plastic would help. Though as Don mentions, the correct place for the vapor barrier is on the heated side. With vapor barrier all around, I figure I'm good to go...

    As for the cross braces.. I'm thinking of using some expanding spray foam... fill the cavity as much as possible from both sides with fiberglass bats, then foam the void. Again, my concern is sound as much (or more) than heat. not sure how many cans of $pray foam I'll need ..

    I will be running a dehumidifier as needed in the basement, but in the winter it should cycle rarely.

    -Dan
  4. Sandor

    Sandor Minister of Fire

    Joined:
    Dec 9, 2005
    Messages:
    917
    Loc:
    Deltaville,VA
    Insulate the walls and band joists in the basement and leave the floor uninsulated, unless its for noise.

    Go to buildingscience.com and read all about it.
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