Basement insulation question

Discussion in 'Remodel Forum & Blog' started by snokel, Mar 12, 2014.

  1. snokel

    snokel Member

    Messages:
    45
    Location:
    Wi
    Hi,
    I am in the process of framing my basement and I see a lot of articles stating that you should put 2 inch rigid XPS foam board over all concrete in the basement.
    I am wondering if this is really needed if the basement has R5 on the outside of the foundation.
    Currently the basement has never been colder than 59 degrees even when it was -17 during the "polar" vortex ( I am in southeast wisc).

    From what I understand for condensation to form on the concrete, the temp of the concrete would have to be lower than the humidity of the air in the basement which during the
    very cold months is between 34 and 36 percent. I also have a radon sub slab suction system which really cuts down on the humidity.

    I also should add that I don't heat the basement and it stays around 68 in the summer and 63 during the day during the winter and the walls are not that cold.

    So I just don't see why I should put 2 inches of XPS over my basement foundatation walls when the exterior already has 1 inch XPS and my sill boxes are spray foamed.
    Also the framing will be at least 1/2 inch from the concrete so any insulation I put in the cavity wont actually be touching the concrete and I can always have it spray foamed after framing and mechanicals are done.

    The house is a 2200 sq ft ranch so I would need a lot of the XPS and at 25 bucks a sheet would add up as well as making all the rooms smaller.

    I am just not convinced that in this case it's needed.

    Oh, I should add that the cove joint all around the permiter is sealed as well as all expansion joints in the floor and the slab has a vapor barrier, so there is no moist air coming up from the cove joint.

    Comments?
    Last edited: Mar 12, 2014
  2. dhagin

    dhagin builder:anti-builder

    Messages:
    108
    Location:
    oahu
    Best to check locally and see what the best builders do. Local conditions vary and so do your options.

    How many sqft? Is the basement heated/cooled year around? Is the entire floor below grade? Are all perimeter walls more than 2' below grade? :)
  3. cacher_chick

    cacher_chick Test, Don't Guess!

    Messages:
    3,259
    Location:
    Land of Cheese
    I would be shooting for at least R-15 in the walls, and the foam is really the only way to go. The soil temperature below 3' will always be cooler than the inside, and the foundation below grade needs to be able to dry to the inside. Your basement is in the 60's during the winter only because of conditioned air from the heating system.
  4. snokel

    snokel Member

    Messages:
    45
    Location:
    Wi
    I emailedthe building inspector and he said that vapor retarders are not allowed below grade except for kraft faced he also said using the rock wool is good because it is hydrophobic (yes it's not a air barrier) and provides
    outstanding fire blocking ability. He siad if you have a newer home with the r5 on the outside and you keep at least a 1/2 inch gap between the studs and concrete your not going to have any issues as long as you don't already have a moisture problem. That's good enough for me, I will keep the option open to use spray foam even though it is freaking expensive.
    I also talked to a friend of a friend who does basement framing as part of his business and he said he never uses the foam board on the basement walls and siad that having the 1/2 inch or greater gap along with NO vapor barrier is the way to go, any condensation/frost that may form simply evaporates because the wall can breath.
  5. snokel

    snokel Member

    Messages:
    45
    Location:
    Wi
    I agree, but it's going to be a a lot warmer than the sill box areas which most builders simply stuff with fiberglass. This winter in my basement the sill boxes where ok until the polar vortex hit and then yes, with just the fiberglass I had a moisture problem and had to rip it all out and spray foam the sill boxes. and that fiberglass was tight up against the OSB rim joists because when I pulled it out the fiberglass had frozen to the OSB.
    Now if I insulated the framing with Kraft faced fiberglass I bet it would have been fine even when it was -17 outside becasue the kraft faced would be stapled which would not allow the fiberglass to touch the concrete.
    The sill boxes had the issue becaus the fiberglass was touching the concrete and acted like a sponge when condensation occured on the OSB.

    I already have a lot of the framing done, so I won't be gluing any foam board to the walls, but depending if I can find a spray foam contractor that is reasonable I may do that. Just getting ideas now.
  6. Dana

    Dana In the trades

    Messages:
    2,974
    Location:
    01609
    You never have to worry about condensation on the concrete in winter, but you sure do in the SUMMER. Summertime outdoor dew points in much of WI average well north of 60F, whereas your deep subsoil temps are 50F or lower. If you install even R1 of flooring or rug over that slab the RH under a wooden subfloor or rug will be well into the mold-inducing zone just from your summertime ventilation air.

    Similarly, if you just throw up a finish wall of gypsum the back side of the gypsum will run cold in winter due to the ~R0.5 of the wallboard, especially if you haven't meticulously air-sealed the foundation sill and band joist. With R5 on the exterior of the foundation all the way up & over the band-joist you can get away with less foam-R on an insulated interior side 2x4 studwall, but you'll still need some to keep the fiber from getting moldy.

    If you don't have an air barrier on the exterior side of the fiber insulation it will seriously under-perform it's rated R due to convective air transfer between your air space and the material.

    You do NOT want to install a vapor barrier into the wall stackup, since you have both ground-moisture saturation and room-air condensation issues to deal with. You need the R-value of the air-impermeable foam at the top of the wall to be sufficient that you don't have chronic wintertime condensation (or frost) events at the interior surface of the foam. The ratio of foam-R/fiber-R needed to meet that will vary by climate- WI is in climate zones 6 & 7, and the prescriptive values in the IRC for a 2x4 studwall is R7.5 for zone 6, R10 for zone 7 (ergo the 2" of XPS recommendation of some.) But it depends exactly which zone you're in.

    By virtue of the fact that most of the foundation is probably below grade and the fact that EPS outperforms it's R value at cold temps, putting 1" of unfaced EPS (the cheaper bead-board stuff) against the foundation, and putting the studs tight to the EPS, and using only UNFACED batts in the studwall will get you there. Unfaced EPS has a comparable or slightly lower water vapor permeance of latex paint, so the foundation won't saturate to the point that it compromises the foundation sill (if you have an EPDM or metal sill gasket it's not a problem, but a foamy or fiberglass sill gasket is barely better than nothing).

    If you use open cell foam against the foundation you risk saturating the foam with ground moisture, unless there was a metal or EPDM capillary break installed between the footing & foundtation wall. If you use closed cell foam it's more expensive than XPS. Both closed cell foam and XPS use fairly environmentally damaging blowing agents too, with a lifecycle greenhouse gas potential more than 1000x CO2, whereas EPS uses pentane at ~7x CO2. (And as XPS slowly loses it's blowing agent over the decades it eventually hits the same R/inch of EPS at the same density.) Between it's ability to moderate the moisture in the concrete, lower cost per R, and lower environmental hit it's the right stuff for the application.

    Similarly, you'll need at least R3 on the floor to get you out of summertime mold issues under a finish floor, and it'll make it more comfortable to boot. An inch of EPS runs R4.2 and gets you there at a lower cost than an XPS solution. Since the studwall is not structural and doesn't need to support the house you can just run the floor foam (and any subflooring) to meet the wall-foam under the bottom plate of the stud. That way you have a full capillary & thermal break between the wood & concrete, and the cold-edge of the bottom plate & subfloor will stay above the dew point of the summertime air.

    Find yourself on this map to figure out which climate zone you're in, but since an inch of EPS runs ~R4.5-R4.7 when it's warm side is 40F, and the concrete is at least R5, you'll still have R10 between the great outdoors and the batts, and be OK:

    [​IMG]
  7. snokel

    snokel Member

    Messages:
    45
    Location:
    Wi
    "If you use open cell foam against the foundation you risk saturating the foam with ground moisture, unless there was a metal or EPDM capillary break installed between the footing & foundtation wall."

    I don't think I will have that issue as the vapor barrior under the slab extends over the footing and I completely filled the cove joint around the entire basement with polyurathane crack filler and backer rod where the gap was large. Not to mention I has sub slab suction from the radon system pulling all the moist air out of there.
  8. Dana

    Dana In the trades

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    Location:
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    With the 6mil poly as capillary break between the footing open cell foam is going to be lower-risk, provided the bulk water is managed well at both the surface and the footing to limit ground moisture getting in to the wall. With the inch of XPS on the exterior the vapor diffusion through the foam to the concrete from the soil-air will be miniscule.

    The summertime condensation issue on walls is less likely with air-impermeable foam on walls than with a finish floor over an uninsulated slab, and not likely to be enough to compromise o.c. foam.
  9. snokel

    snokel Member

    Messages:
    45
    Location:
    Wi
    it should be OK as the form a drain seems to work really well even during super heavy rains, spring of 2013 we go 7+ inches of rain in a few days and never had a drop of water, sump pump ran almost all the time though :)
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