Crawl Space

Discussion in 'Remodel Forum & Blog' started by ASG, Mar 30, 2012.

  1. ASG

    ASG New Member

    Mar 30, 2012
    So I have a house, it is older, built in 1909 and is a bungalow. Under the living room is a crawl space accesable from the basement open earth floor. I am remodeling, gutting all the way down to the joists and studs. What is a less expensive way to offer some insulating factor for the floor above without choking out the air flow needed to prevent rotting? Can I run some fiberglass insulation inbetween the joists? If so, what would the reccomended type be, faced or unfaced or foam sheets? And is there a best R rating I can use in this case, or am I completely on the wrong track here?
  2. Dana

    Dana In the trades

    Jan 14, 2009
    Better solution is to air-seal an insulate the crawl space converting it to a conditioned crawlspace, closing off ALL venting, which puts all of the wood inside of conditioned space where it's warm & dry, and rot-free. More details here.

    Any approach that insulates between the joists leave the joist edges colder, more likely to be below the dew point of the crawl-space air.

    At a minimum you'd want to put a 10-mil poly vapor retarder on the floor, but if it's nice and level and you can get full sheets in there 2-3" of rigid EPS roofing insulation would work for both the dirt floor and foundation wall insulation. (Reclaimed goods are dirt-cheap at The Insulation Depot in Framingham MA, if you can pick up- they don't deliver small quantities. From them and similar sources it's usually cheaper per unit-R than virgin-stock fiberglass batts!) If it's a fieldstone foundation it may be easier to insulate the both crawlspace walls and the band joist, with ~2" of closed cell foam (about $2 per square foot of coverage, end result is ~R13),and EPS on the floor, with the vapor barrier between the dirt & floor.

    If you still want to leave the walls uninsulated, at least crudely cut'n'cobble in at least 3" of EPS or fiber-faced iso (sometimes available in Framingham, or search craigslist/materials for other sources) onto the band joist & foundation sill, air-sealing it and filling in the edges/seams with 1-part spray foam. You can then put in ~2.5-3" of rigid foam (any type) up on the bottom of the joists to insulate the floor, holding it in place with furring every 24", though-screwed to the joists with screws that penetrate the joist by at least 3/4". The seams & edges would need to be air-sealed with 1 part foam or duct mastic, etc. That way you'd have a comparable or greater R value than the exterior walls, all the susceptible wood stays at the indoor temp & humidity, and it would be air-tight for minimal infiltration.

    If you use iso on the walls, be sure to lap the floor vapor retarder between the foam & foundation for the first 12" off the floor, or it may slowly pick up moisture.

    If in spite of this advice you insist on going with a batt solution, go with unfaced batts thick enough to fill up the cavity completely, and use Tyvek or similar for a bottom-side air-barrier. If you went with kraft faced batts with facers on the bottom for ease of installation it would all but guarantee a mold problem within 10 years in most MA locations.

    FWIW: My central MA bungalow is only14 years newer than yours, and has a section of crawlspace under the master bedroom- I've starred in this movie. :) Someone had previously installed kraft faced batts, facer-side down, and yes, it was more than just musty in there! I've since done a DIY- R20 in rigid foam on the walls, and removed the moldy batts, as well as putting a vapor retarder on the floor. Access is only via a 15" x 18" port , so getting in full sheets to lay down on the crawlspace floor or underside of the floor joist isn't possible without some dirty demolition/rebuild, so for the time being it's vapor-barrier only on the floor. The average floor temp over the crawlspace section is still a couple degrees less than the part over the more-conditioned basement, but it doesn't stink of mold (anymore) and it's never frigid.
  3. Sponsor

    Sponsor Paid Advertisement

  4. ASG

    ASG New Member

    Mar 30, 2012

    Thanks Dana, that was exactly what I was looking for. I am doing a full remodle, so I think it will be easier to do all this after I pull the subfloor up. Up towards the front it reduces all the way to about 2-3 ft from ground to joist and I am not a small guy and the wife refuses to do it!
Similar Threads: Crawl Space
Forum Title Date
Remodel Forum & Blog Floor joist insulation in crawl space Nov 21, 2014
Remodel Forum & Blog crawlspace/under deck insulation Feb 26, 2013
Remodel Forum & Blog insulating floor over crawlspace Jul 10, 2010
Remodel Forum & Blog Spray Foam Insulation for Vented Porch Crawlspace?? Oct 26, 2009
Remodel Forum & Blog Crawl Space Beam and Joist Support Questions Jun 22, 2009

Share This Page