insulating floor over crawlspace

Discussion in 'Remodel Forum & Blog' started by rick.a, Jul 10, 2010.

  1. rick.a

    rick.a New Member

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    Palmer, Massachusetts
    I need to insulate a small house that has a dirt crawlspace. We will be adding a concrete block foundation to create a sealed "conditioned" crawlspace with the dirt floor sealed with 6 mil plastic sheeting. I also want to add insulation between the floor joists but want this to NOT block the passing of moisture to prevent mold problems which are common in these situations. Is there an insulation available that has paper backing which allows stapling to the joists, for easy installation and will block airflow but NOT block moisture?

    thanks, Rick.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    You could use unfaced insulation. They make thin metal rods that are designed to fit between the joists to hold the insulation in place. You press it between the joists, then put one of the support rods up (no tools). See my comments in the heating area...if you consider staple up radiant, you'd want to do something other than fiberglass, probably.
  3. rick.a

    rick.a New Member

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    84
    Location:
    Palmer, Massachusetts
    But then I'd have to go over the whole thing again with an air barrier to eliminate the air penetration. Do you know if "house wrap" tyvek type material is air proof but NOT moisture proof? It would be good for me if it was.
  4. Dana

    Dana In the trades

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    Housewraps like Tyvek et al are highy water-vapor permeable (>>10 perms) but reasonable air-barriers. But if you're insulating between the joists, the crawl will run colder, and without a vapor-retarder on the warm side of the insulation, any time the crawspace drops below ~40F (and it will, in Palmer) you'll end up with condensation on the colder edge of joists making for a mold conditions- not ideal.

    Rather than insulating the floor with crappy fiberglass batts, it's less material & work to put vapor semi-permeable or semi-impermeable rigid board insulation (or spray foam insulation) on the crawlspace walls. You can go up to ~R16 with unfaced beadboard EPS, or ~R10 with XPS (pink/blue) before it's so vapor-impermeable that it drives moisture into the foundation-sill. If you call up the guys at Insulation Depot in Framingham (google 'em, or 411) and see what specials they're running. They usually have a good stock of used EPS (mostly salvaged from commercial construction- roofs mostly) for less than half retail, and if you're cutting them down to crawspace wall height you don't even need full sheets, and can deal with the real scratch & dent goods for even less (that they're more than happy to see leave). I did the walls of my full-height basement (in Worcester) in permeable fiber-faced isoboard to R20 for ~$500 in material, then used spray foam to seal & insulate the sill & band joist, taking ~20% off the heating bill.

    If you want to insulate the house from the ~50F subsoil in Palmer, 2" of EPS (~R8) either above or below your poly vapor retarder on the floor would be enough. If you're feeling rich you could then pour a 1.5-2" non-structural rat-slab on top to make it easier to move around in there without ripping the vapor retarder or crunching the insulation too much, but EPS can handle reasonbale crawling-around loads even without it. If you insulate just the walls to R12 or better, the crawlspace will run in the low-mid 60s F in deep winter if you keep the rooms above at 68F, but if you insulate both the walls and the dirt floor it'll run in the mid-high 60sF.

    Read this...

    ...and this...

    ...before proceeding.
  5. Dana

    Dana In the trades

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    I forgot to mention, since you're adding the foundation, take care to put a very good cappilary break (like 10mil poly, or better yet copper flashing) between the foundation and sill, not just a foamy sill gasket, which will effectively block the capillary draw of groundwater upward into the susceptible wood. This is important even if you're using pressure-treated wood for the sill.

    And, if you can, putting 2"+ of EPS or XPS on the exterior of the foundation wall extending up above the band joist the band joist & sill will run warmer (==dryer), and you don't need to insulate it from the interior side (as you would in most retrofits.)

    If a poured foundation is possible, using insulated concrete forms (aka "ICF"- concrete form systems made of EPS) is pretty straightforward, and they start at around R16, going well into the R30s if you like. (I did a ~250' addition in my house with ICF foundation, it wasn't bad.) It'll be more money than reclaimed EPS from Insulation Depot and a block foundation, but it'll be more substantial as well.
  6. rick.a

    rick.a New Member

    Messages:
    84
    Location:
    Palmer, Massachusetts
    Thanks for the info Dana. I lived in Worcester and Holden for many years before ending up in Palmer by a round about way. It appears that you live in Greendale.

    Since I am currently unemployed we need to keep this project as affordable as possible. I have a huge supply of 3 inch rigid foam panels. Not sure what their correct name is but they are bound on each side by black tar paper type material, and are nearly white stiff foam. They were stored under the house and screwed up onto the bottom of the floor joists. But a crappy job was done, with lots of gaps and no moisture barrier on the ground, thus the bottoms of most of them are moldy. They were also used make a sort of insulating foundation, being installed between the footings, but again a crappy job was done.

    My current plan is to contain the perimeter of the house with a concrete block foundation and then insulate the inside of the block walls with these panels, one or two layers thick, cut to fit and glued tight to the block. Then cover the dirt floor with at least 6 mil poly film which would run up to the band joists.

    Do you think that this will suffice to insulate the crawlspace? CS height runs from 3 ft to 1 ft. If the CS is well insulated from the outside air, could I just not insulate the house floor? We may have insulated hot air ducting running through this space, if we go with this type of heating. I would think that if the space remained above 50 degrees that condensation on the bottom of the joists and subfloor would not happen.

    Will this work?

    Rick.
  7. Dana

    Dana In the trades

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    If it's a very pale yellow without an obvious sintered-bead structure (like foam coffee cups) it's most likely to be iso (polyisocyanurate), which will run ~R18-R20 in a 3" thickness. The asphalt-loaded paper facers is an indicator that they were probably marketed as roofing insulation at one time. If it's bead-board it's EPS (expanded polystyrene), and would run ~ R12-R13 in a 3" thickness. If iso, one layer should suffice, but if EPS you may want to double it up, lapping the seams by at least a foot for a better air-seal.

    If it's iso it can't be used in contact with the ground, but is just fine for insulating a block foundation. It's still worth using a paint-on/spray-on type of masonry sealer on the interior face of the blocks before putting up the rigid-board insulation if it's iso, which can retain some moisture. (The asphalt facers are very slightly vapor permeable.) Waterproofing the exterior face of the blocks below grade is also a good idea.

    Before putting up the iso, lap the ground vapor retarder at least 8" up the block wall an seal it to the wall with duct mastic (available at blue & orange box-stores) painted/troweled on about 1/8" thick. If it pulls away or has minor gaps (it's hard to make it perfect first-shot) come back and caulk or mastic the imperfections first. That way you'll have sheet plastic between the dirt and the foam, and it won't take on ground moisture. If your ground vapor retarder is in multiple sheets (probably is), lapping the seams by a foot and sealing the with duct mastic in the overlap works.

    Then cut the sections of iso to fit the wall with ~1/2" clearances to make it easy to install. Use walnut-sized blobs of foam board construction adhesive, not standard construction adhesive, which has solvents that mess up the foam, and the bond between the facer and the foam (also available at box stores) every 18" or so and just smoosh it in place. Cut & glue sections for the band joists etc as well, leaving ~ 1/4" gaps around them. If the seams between foam sections can be butted super-tight they can be sealed with foam-board adhesive, but it's often easier to leave 1/4" gaps to be filled with single-part spray foam (eg. Dow GreatStuff or similar) later. Use foam to seal all seams & edges around the band joists as well.

    If the crawlspace is all above grade (or less than 2' below grade) it's worth insulatinging the perimeter of the floor against the wall to at least 2 feet in, which will couple the building to the thermal mass of the soil, and the temperature of the uninsulated section will not vary much from month to month from weather variations, but might swing as much as 6-8 degrees between mid winter and mid-summer, but it'll always be above 50F (and may be above 60F most of the time) if you keep the first floor at 65F or above all winter. If it's more than 2' below grade (which it sounds like it isn't) the perimeter insulation isn't necessary, and just insulating the walls will be "good enough". But monitor the temp & relative humidity down there- particularly when outdoor dew points are in the 60s & 70sF (like this past week or so). (Cheap but decent temp & humidity meters from AccuRite are available even at Wal-Mart fur under $10.) If it stays above 70% RH for weeks on end in the summer a room dehumidifer should take care of it, but it's an indication of air-infiltration (maybe from outdoor air leaks in the rooms above, but the crawl will be at a lower temp==higher RH than the living space rooms in summer.) If it's below 55F in there during mid-winter cold snaps you may want to consider insulating the whole floor with InsulationDepot reclaimed stuff or Craigslist specials when the price is right. An inch or two of bead board EPS or XPS (eXtruded PolyStyrene) would be enough for the center-floor. With a plastic vapor retarder below you could also use iso, but crawling/walking around on iso will damage it (iso roof insulation has typically less than half the compressive strength of EPS or XPS.)

    If you're putting heating/AC ducts in the crawl, be sure to mastic-seal all seams & joints before insulating them, and insulate them to at least R6.

    Earth-coupled or fully conditioned sealed-insulated crawlspaces definitely work, and is absolutely the right way to go if you have plumbing or heating runs in the crawl. Like the guys at Building Science Corp point out, a conditioned crawlspace is really just a very short basement, and if you've insulated the walls you've protected the joists by keeping them warmer & drier, with little or no condensation potential for getting rot or mold started.
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