insulation questions and help for my basement

Discussion in 'Remodel Forum & Blog' started by mstemphoski, Mar 28, 2011.

  1. mstemphoski

    mstemphoski New Member

    Messages:
    2
    Location:
    Warminster, Pennsylvania
    I finished my basement last year. I have a french drain along the walls and about 6" between the finished wall and the concrete. I did not insulate the walls because i always felt that the temperture was comfortable down there prior to being finished while i was working out. I now realize this was a mistake for watching movies and hanging down there for the day. I am in a middle unit townhouse so i only have 2 outside walls. I can get behind one wall to insultate with no issue, but the other outside wall i cannot get behind to put insultation between the metal studs. My question is can i get blown insulation in there with the french drain? My basement has never had water in 26 years. I can also access behind the wall by removing the step down to my deck and put a hose in to blow in insulation. Would i see any benefit in insulating the outside walls only? Also my floor is not insulated, just pading and carpet. I do have one 6" hit in the back of the basement to heat/cool the basement and an electric fireplace to take the chill out of the room. It is ok for me, but my wife and son are too cold to be down there. Also in the summer, with 5 or more people it gets too hot. I was thinking of getting a portable AC unit or adding another hit in the basement. Basement is an L shape and about 700sqft. Thanks for any suggestions.
    Last edited: Mar 28, 2011
  2. Dana

    Dana In the trades

    Messages:
    2,919
    Location:
    01609
    Even if the firewalls to the adjacent town house basements are uninsulated, the biggest wintertime heat loss is to the exterior walls, and the difference in comfort you'll get by insulating them is very real.

    But doing it after the fact is a problem. What would have made sense is a variation on Case 4 (but with 1.5" of XPS foam, not 1" , or alternatively, 2" of XPS foam and no batts- See figure 37, page 49) found here:

    See: http://www.buildingscience.com/documents/reports/rr-1003-building-america-high-r-foundations-case-study-analysis

    Obviously you're not going to rip the walls apart to do the "right" thing and put semi-permeable foam against the walls, but pay careful attention to the hygric analysis in that document. Even though you used metal studs, not wood, if you just blow in fiber insulation you're guaranteed to have winter condensation/frost issues on the foundation wall from conditioned-space air/vapor intrusion. You can't just put up an interior-side vapor-barrier (or vapor barrier paint) either, since that would trap ground moisture in the foundation rasing the risk of rot at the foundtaion sill. The best-bet in your case would be to inject a somewhat permeable non-expanding foam such as Tripolymer, or Retro Seal, or even a masonry-type injected foam such as Core-Fill in the wall cavities from the interior, but it won't be cheap. Wherever you can access it, using 1-2" of closed-cell spray polyurethane foam to air-seal and insulate the foundation sill and band joist is probably a better first step, since that will better protect the structural wood from wintertime interior moisture than Tripolymer. With nonexpanding foam injection you can use relatively small holes drilled in near the ceiling or near the floor, and cover them with crown molding, border-paper, or kickboard. (Chair-rail molding would work too, for a mid-wall drill option.)

    The advantage to using foams is that they allow no convection, which would otherwise become a too-powerful moisture transport mechanism. With steel studs it'll be a much more severe thermal short than wood studs, but at least they won't rot from ground moisture & condensation the way wood studs will when insulated and placed against the foundation. DON'T attempt to use an expanding polyurethane pour in the cavites though. Risk of blowout is high, half-pound foam (open cell) would be too moisture permeable, and 2lb pours more than 2" thick will be too moisture-retardent- a near vapor-barrier.

    Carpet and padding on an uninsulated floor may make it more comfortable, but that's a recipe for mold growth in PA, where the subsoil temps are low to mid-50s. The carpet and padding is air-permeabable and slightly insulating, which means the temp at the slap will be 5-10F colder- close to the subsoil temps in summer. Even assuming you air-condition the place and dehumidify the basement to 60% relative humidity the bottom of the pad will close to or below the dew point of the interior air- prime for growing mold in the pad. To significantly mitigate that risk you'd need to keep the relative humidity below 40%. In winter that's not a problem, and in summer with the AC going it won't be, but there will be many days/weeks with no appreciable sensible (temperature) AC load where the humidity will creep up unless you're actively dehumidifying it. If it's not too hard to do after the fact, adding 3/4-1" of XPS and floating a nailer-floor of half-inch ply or OSB tapconned to the slab does a WORLD of good for both comfort, and mold-under-the-basement-carpet issues.
  3. mstemphoski

    mstemphoski New Member

    Messages:
    2
    Location:
    Warminster, Pennsylvania
    The problem with putting something under the carpet like foam with plywood on top is my ceiling height is only 7 foot right now. Adding 1"foam with 3/4 inch plywood, then padding then carpet would leave me with less than 6'9" ceiling. I have seen the spray insulation you mentioned. I wish I could find it to buy rather than have installed. I have a good 3' openning on the outside of my deck that i could get behind the wall to fill with the foam. Would that foam not be effected if wet? I am also thinking of not insulating and getting a ptac unit to heat and cool. I can get a 12000 BTU unit for $800 bucks which would be more than enough to handle my basement. Any idea on the cost of the spray foam? I would need 20' X 7' X 6"
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