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  1. GBA Editor
    Martin Holladay | | #1

    Did I miss it -- or did you forget to tell us your location or climate zone?

    In most areas of the U.S., one of the methods you describe -- "My upper 2x6 knee walls (above the foundation) have 6 inch unfaced fiberglass covered in polyethylene" -- is not a very good idea. The R-value is low; there is thermal bridging through the studs; and the polyethylene can cause problems, especially in summer.

    I would remove the polyethylene if I were you, and install a couple of inches of rigid foam instead -- protected on the interior with drywall, of course.

  2. Robert Metelsky | | #2

    Hi Martin - sorry about that. I'm in central Connecticut which is 4-5 marine.

    This upper portion of wall has quite a bit of junction boxes, lots of parallel romex - some plumbing. I don’t think I can cover that up…
    The metal studs are 10ft and go to the bottom of the joists, the high foundation is 8 ft and I was wanting to go with 8ft finish wall, the unfinished suspended ceiling above
    The joists are 12in oc and have unfaced fiberglass between them.
    I'm willing to do what work is needed but that north wall has tons of wires and stuff

    So, basically this 2ft area above the ceiling would be "unconditioned space", although I have the furnace, water heater in a 7x7 closet with bifold doors, located in the middle of the basement. This would definitely let "the room air" into above the suspended ceiling.

    Just for the record, I asked the building inspector about sheet rocking over the poly 2x6 wall and he said no problem. I'm very nervous about plastic vapor barriers as this is not my profession as well as being highly controversial. I certainly will take your advise. but... this 2ft area is above the foundation and the same detail as all the other above grade walls i.e. 2x6 with insulation +poly. I thought that was acceptable R value for this vintage home. late 80's

    So I was thinking the main problem is the concrete giving me the coolness and causing the condensation/moisture. The rest of the wood basement walls are 2x6 with unfaced fiberglass + poly

    Also fwiw this house was built in the late 80's is a contemporary somewhat passive solar design. A lot of glass on the south side. The house is surprisingly not hot in the summer

    Thank you for your time, much appreciated

  3. Expert Member
    Dana Dorsett | | #3

    IRC 2012 code min for basement walls in zone 5A is R15 continuous, not R12.

    Use a temperature rated foil tape for sealing the seams between the polyiso, not can-foam. Seal the edges with can-foam, and keep the bottom edge of the polyiso off the slab (it can wick moisture into the foam, and never dry with the foil facers.)

    Using 1" Tuff-R trapped to the wall with a non-structural 2x4 wall with unfaced or kraft-faced batts works. Put an inch of EPS (not polyiso) under the bottom edge of the studs as a capillary & thermal break.

    If you have access to it, lose the polyethylene sheeting, replace it with MemBrain before drywalling the place. If the exterior siding is back ventilated the poly might not be a problem, and you wouldn't even need the MemBrain. But in your climate poly sheeting will cause more problems than it solves. (Certainteed MemBrain won't be found at box stores in CT, but is sometimes available through distributors catering to the trades. Worst case you can buy it online.) Variable permeance "smart" vapor retarders do what they need to do in winter to protect the sheathing from interior moisture drives, but become vapor open if they ever need to, unlike polyethylene sheeting. In much colder climates (zone 7 & 8) it might make sense, but never in zone 5A.

    If the batts were installed in the '80s they're probably R19s, and wouldn't meet current code min, but adding as little as 1/2" of XPS or 1" of unfaced EPS under the sheet rock would get you to current code min performance. Just avoid foams with foil facers, unless you KNOW the exterior sheathing is back-vented. If adding an inch or more of interior foam, the vapor retardency of the foam would be tight enough to be able to skip the MemBrain.

    There are several vendors of reclaimed roofing foam in southern New England, which can take a bite out of all-foam solutions to foundation insulation. (Green Insulation Group in Worcester MA is one of the bigger ones but that's probably at least an hour away- see if there aren't some others closer to you.)

  4. Robert Metelsky | | #4

    My main reference page

    Link I found about foil faced polyiso

    Some environment data
    Today in central ct mid 80's temperatures
    Main floor of the non-air-conditioned house 79 degrees, 69% humidity
    inside the basement doors & windows shut 70 degrees 83% humidity

    Ive attached a few pictures that may help with my story

    Just for clarity - I'm not overly concerned with strict modern R values, this is a finished basement on an existing house. I'm only concerned with dealing with the moisture issue, so I don’t have adamp smell or excessive moisture in the air caused the excessivly cool north wall

    also fwiw - my wood exterior walls are just 2x6 with 1/2 plywood, tyvek and cedar clapboards. The 2x6 exterior framing in the basement is pressure treated.

    So - can I use the double foil face poly iso from the link I provided, or should I use another foam. I already ordered the polyiso but can return it.

    Note the pictures show my metal stud wall, i will remove and reinstall after I install the foam sheets

    Thank you

  5. Robert Metelsky | | #5
  6. Robert Metelsky | | #6

    Im reading this thread and seems like Im not describing it correctly. Im only questioning insulating the concrete walls - the other 2x6 walls I will just sheet rock over, again only concerned with moisture caused by the concrete walls - sorry for too much informations

  7. GBA Editor
    Martin Holladay | | #7

    To answer your basic question: Yes, you can insulate your basement wall with two layers of polyiso totaling R-12. More rigid foam would be better, but your approach will work, especially if you follow Dana's installation tips.

    However, your photos raise new questions.

    First, the fiberglass batt job is particularly sloppy. If I were you, I wouldn't cover up that mess. I would either pull out the fiberglass and do a better job with a different type of insulation, or at least pull out the batts and reinstall them with a little bit more care. And get rid of the polyethylene when you do that.

    Second, it looks like you installed your metal studs before the polyiso. I assume that you will be removing the studs so that you have better access to the concrete wall for your insulation job -- right?


  8. Expert Member
    Dana Dorsett | | #8

    At 83% RH @ 70F the wood and paper in the basement will be in the mold growth zone.

    83% RH @ 70F corresponds to a dew point temperature of about 65F, which is the approximate average outdoor dew point temp in central CT over the past week or so, which indicates that the basement's humidity level is probably tracking with the outdoor dew air's humidity. Pull up a dew point graph, and use the cursor to eyeball-estimate the average dew point temp over the past week to 10 days:!dashboard;q=hartford%20ct

    No amount of insulation or vapor barrier sealing against the concrete will fix this problem, since the major humidity source is ventilation/infiltration of outdoor air into the basement. Insulating against the concrete will raise the basement temp a few degrees, which in-itself lowers the basement RH a few percent, but to stay out of the mold zone you will need to air-condition or mechanically dehumidify that space down to 65% or lower.

    That's not to say it isn't worth insulating- it totally is, but it's not going to dramatically affect the basement RH. Even brand new perfectly built highly insulated high performance/low load homes have latent-cooling load issues like this.

  9. Robert Metelsky | | #9

    Note I’m not complaining here just asking straight questions, and looking for the correct answer. I’m not challenging anyone :-)
    and I greatly appreciate the responses

    Actually the foam will be a single layer of 1 7/8 polyiso, this foam board has foil on both sides. That’s the original question "Will it be an issue to have foil on both sides of the polyiso"
    Please see this link for the product

    Will painting the wall with drylock cause and issue with the foam. I’m trying to take every precaution before I cover up the walls.

    I was planning on using spray foam on the perimeter of every sheet and when dry - remove the excess with a knife then tape it with a ridiculously strong foil tape - you can’t remove this stuff
    Is that approach ok?

    Is it really necessary to raise the bottom of the foam sheet? I was planning on sitting it in a large bead of foam? If so... won’t I have to go back remove the spacers - then foam the gap? Otherwise there will be a gap at the floor / wall which I’m trying to avoid.. - Right? I was planning to space the mr drywall 1/2" off the floor.

    Can polyiso really wick moisture? If so, I don’t think its the right material for the job. I’d like to use it because it’s already ordered - but I can change it. I want to sleep at night - knowing I’ve installed the correct foam.

    Is it really necessary to put the foam under the bottom plate? I can and will do it - but seems overkill because I’m using metal studs. I'm using concrete nails now - I suppose Ill have to use tapcons? - yes?

    Yes Ill remove and reinstall the metal studs to properly install the foam, that's the great part of using metal studs - they come apart easy

    Yes I will neaten up and correct any issues with the fiberglass thats in the wood calvities and remove the poly.
    Q - I can sheetrock over the unfaced insulation without a "vapor barrier"?
    Q - Do I really need a vapor barrier on those walls?

    Q - If I have the bats in the ceiling joists would it be worth the time to foam the rim joist?

    Again - I just want to sleep at night and appreciate the input!!

  10. GBA Editor
    Martin Holladay | | #10

    Foil on both sides of the polyiso is not a problem.

    I don't know of any reason why DryLock would cause any problems.

    Yes, you can used canned spray foam and tape as you propose.

    Yes, it's a good idea to raise the polyiso off the slab and to fill the gap with another product.

    Polyiso performs fine on the interior of basement walls.

    You can get away with leaving the metal bottom plate attached to the slab, especially since that work is already done.

    Yes, you can install drywall over your wood studs without any polyethylene. Pay attention to airtightness and paint the drywall with vapor-retarder primer.

    No, there is no code reason or building science reason to install an interior vapor barrier; all you need is a vapor retarder. For more information, see Do I Need a Vapor Retarder?

    Yes, it's a good idea to install rigid foam or spray foam (not fiberglass batts) against the rim joist. For more information, see Insulating rim joists.

  11. Expert Member
    Dana Dorsett | | #11

    Polyiso can wick moisture through the exposed edges, but yes polyiso is an appropriate material for the job.

    In my own basement there is 3" of reclaimed fiber-faced polyiso on my basement walls, but the polyiso stops 6-8" from the floor, above the "high tide" mark of the deepest it's ever gotten when the sump pumps failed during the spring thaw. During the summer the water table is rarely more than 18" below the top of the slab, and in big snowfall years (like this year) the water table is above slab level, but with 4 sump pumps duty-cycling it stays dry. In some parts the polyiso transitions to EPS near the floor, but most of it is just air between the bottom edge and the slab.

    A sheet metal stud plate is a capillary break, and steel won't wick moisture the way wooden framing does.

    A vapor barrier would be prone to creating problems where none currently exist. The foil facers on the foam are vapor barriers, but the fact that it traps moisture in concrete doesn't create a problem. If you are putting the foil facers over the fiberglass on the wood studs it blocks all drying toward the interior, but it's better, not worse, than that poorly installed polyethylene sheeting with the huge air leaks/gaps/tears.

    Insulating the band joist is definitely worthwhile, whether cut'n'cobble polyiso scraps (foam-sealed into place) or spray foam.

  12. Robert Metelsky | | #12

    This is fantastic! thank you Martin

    Q- what is the recommendation of spacing the bottom of the foam board. I was thinking 2 - one inch x 1 inch blocks for each sheet, then coming back, remove the blocks and seal with another product - what product should I use for this gap?

    Q - How should I handle the detail of the top of the concrete wall. So, I have a 2x6 pt plate sitting on top of the concrete wall, which leaves about 4-5 inches of the top of the concrete exposed. I was going to put a strip of wall foam, up against the 2x6 plate and over the top edge of the wall foam, then thoroughly tape this outside corner.
    Ive seem some mention somewhere of a "capillary" requirement for this area.
    What would be the recommended detail?

    Q- Generally, do you think my plan is sound? anything else I can do to make for a moisture proof basement? My exterior grading is good, gutters are good. Condensation has been my issue

    Q - do you think when this work is complete that I can open the windows and doors in the basement without having every surface inside condensate? I believe the tall concrete walls on the south, east and west side, cool everything in the basement and keep it cool for a long time

    Q - what is a good floor covering that's, impervious to moisture, relatively economical, and looks half way decent? We plan to use large removable carpets like 10x12 etc.. over the "finish" surface.
    I was going to do vinyl tile in the bathroom and kitchen area
    The basement is about 900 sq ft about half vinyl tile, the remaining - something else

    I want to avoid any type of formal floor system, i.e vapor barrier, foam , pt sleepers, marine plywood and the like ;-)
    The concrete floor is very dry and fairly even / flat.


    I will implement all the recommendations given here and I truly appreciate the guidance. dealing with these questions is very controversial and confusing, I appreciate and trust the folks at GBA.
    Hopefully I can post back pictures or links to picture's on this thread

  13. Robert Metelsky | | #13

    Hi Dana thanks for the input

    My floor and basement has no seepage or obvious water, So I'm not really seeing the water wicking up on the bottom of the foam board, But I'll take the recommendation on my last response.

    I wont put foam board over the 2x6 walls, only mr sheer rock with vapor retardant primer.

    I will foam the band joist

    Please see my previous post as a summary , and please followup :-) - thank you very much!

  14. Expert Member
    Dana Dorsett | | #14

    Sealing the bottom gap between the cut edge of the polyiso and the slab with can-foam works.

    Doing a cut'n'cobble with wall-foam for that the strip of bare concrete to the foundation sill is fine. The foil facer is a capillary break. Tack it in place with foam board construction adhesive leaving a 1/4" gap between the foundation sill edge and polyiso, then come back and lay a bead of can-foam in the gap to seal it up tight. You can seal the foundation-sill polyiso to the wall polyiso with aluminum tape.

    Unless you insulate the slab putting down rugs is a BAD idea. The deep subsoil temps in your area are in the low-50s F, and if you put R1 of rug over the slab the RH under the rug is going to be much much higher than the room RH, and the mold risk to the rug is high. If you dehumidify to 65% RH @ 70F the dew point of the room air will be 58F. With 58F dew-point room air and a 60F slab, the 60F air next to the slab will be about 93% RH, well above the mold threshold. Anything you put on the floor should not be mold-food.

    Asphalt or vinyl tiles are cheap & effective. Any ceramic tile would also be fine. Don't put down any type of wood flooring or rugs unless you put at least R3/4" of EPS or XPS (and not polyiso) under it.

    With wood flooring you can sometimes use 1/2" OSB TapConned to the slab through the rigid foam, but if the flooring needs deeper fastener retention you may have to double-layer it. There's no point to using pressure treated sleepers or marine grade wood- there's no contact with the cold slab. The R3+ foam is your protection. If the slab doesn't have a vapor barrier under it, it's worth slipping a layer of 6-mil polyethylene under the floor foam.

    With the walls insulated you could still end up with condensation on or adsorption into the uninsulated floor during muggy humid summer days, but the walls would be at or above the dew point of the outdoor air.

  15. Robert Metelsky | | #15

    Thank you Dana, I'm feeling much better about this more work but a solid direction

    Q - I can consider the 3/4 foam and osb. Say I have the 900sq ft, open floor plan and will do the foam and tapconned plywood for actual "carpet" will that work? Basically I'm asking if the insulation is only required under the carpet area. The rest of the area will I've vinyl tile.

    Q - is there a reason for osb - I never liked that stuff for anything, Could I use cdx? Is osb better for this application?

    If I decide to do the foam & ply floor, I'm guessing the walls should go on top of the floor..?I would really prefer the floor system to go inside the wall plates to allow for removal if needed, otherwise I could have an ugly mess to deal with.
    Q - would it be acceptable to do the floor system inside the wall plates?

    Follow up - when I was saying rug - I meant 8x10 or throw runs on top of vinyl tile

    I'm not really interested in doing any kind of actual finish wood flooring, I was thinking synthetic but not that gastly allue type flooring, any decent synthetics your're aware of? I wouldn't rule allure out if you had good experience with it

    Thank you for your time!!

  16. Robert Metelsky | | #16

    tI finally started insulating my concrete basement walls with 2 layers of foil faced polyiso. 2inch + 1inch boards. Will be a total of R18 I've gone through great detail and expense of making the entire install of polyiso airtight. I'm confident it is. All layers are taped and foamed sealed to the extreme

    My concern is this foil coated foam seems like a perfect condensation surface.

    My wall detail is
    8ft below grade concrete foundation wall
    2ft 2x6 knee wall above that.
    My metal stud wall is 10 foot from slab to ceiling joist.
    My finish wall will be @8ft, with suspended ceiling above that.

    So, there will be open airspace in front of the polyiso insulated wall and the 2x4 metal studs, this will connect with the 2 foot unfinished area above the ceiling.

    My concern is, If I leave the windows open in the summer, warm air can enter, get above the ceiling and condensate on the front of the foil faced poly.

    Is that likely? If so what can be done?

    I did some searching about this topic anc found this

    Can you please provide me your ideas?

    Thank you

  17. Robert Metelsky | | #17

    extra house details
    the 8ft foundation wall is the longest and north side aprox 40ft long
    west side is steped fouldation with wood wall and windows
    south side, walk out sliding glass doors and windows
    east - crawl space

  18. GBA Editor
    Martin Holladay | | #18

    You wrote, "My concern is this foil-coated foam seems like a perfect condensation surface."

    It isn't.

    A condensing surface has to be cold. Think about a drinking glass sitting on your kitchen counter. If the glass is empty, and at room temperature, there is no condensation. But once you fill the glass with ice cubes and water, you can get condensation on the exterior of the glass. Why? Because you made the glass cold.

    The foil facing on the interior layer of polyiso should be at room temperature, so it won't be a condensing surface.

    By the way, most building codes require rigid foam to be protected with gypsum drywall for fire safety reasons. If you have 10-foot walls with a suspended ceiling at 8 feet, it would be a good idea to have your drywall be 10 feet high, so that the rigid foam above the suspended ceiling is protected by the drywall.

  19. Robert Metelsky | | #19

    Hi Martin, thank you so much for your response. Your support here is greatly appreciated.

    Originally I believed the inspector would accept the open area above the foam. Technically the foam height is about 4 inches lower than 8ft wall so you wouldn't really have any exposed foam. I'm aware of the ignition/thermal barrier but wasn't planning on covering the 2 ft with sheet rock. Now that I'm thinking about it - doing that may be good idea to keep any "warmer" air from the open windows getting behind the stud wall and "possibly" condensating. So Ill most likely do that. Plus it will be a correct thermal/ignition barrier.

    My other insulated 1/2 and 2/3 high concrete walls will be completely enclosed with metal studs, sheet rock and a wood shelf on top, so I was never concerned about warm air getting in there and possibly condensating.

    Your analogy about the glass is a good one. I was thinking along similar lines where if you had a granite counter top - you wouldn't expect that to get condensation on that

    It seems folks in the north east like to keep the windows open in the basement to provide a natural air exchange and avoid basements having "stale air". The humid, warm outside air contributed to my condensation problem big time - mainly I believe because of the cold concrete wall.

    One phenomenon I experience in this basement was - I had a roll of 3.5 In wall insulation, factory sealed in plastic, sitting in the middle of a concrete floor, in a crawlspace that was impacted by condensation, the plastic on the outside of the roll has actual weather on it from condensation. I haven't understood how that roll could be so cold that it created that much water on the outside of the plastic.

    Anyways, I really have to keep the understanding of cold/warm and how it creates condensation. I know its a very simple concept but this basement has had significant issues with it that were that obvious, mainly from the concrete wall getting slightly damp from condensation. So I'm very leery about warm air coming inside because the basement is inherently cool.

    Ill definitely be aware of keeping the windows open in the summer too much - we've acquired a nice dehumidifier that we plan to use when the humidity goes up. Also... since all the problems with condensation I have 2 temperature & humidity meters in the areas, so I know what those numbers are and keep track of them.

    If you have any other comments of feed back Id love to hear it. If not I understand - Again, I have to say how much I appreciate you guys prompt support with these type of questions. I have tons of pictures Ill be posing links to in the relatively near future. I'm just finishing up the 2 layers of foam


  20. Expert Member
    Dana Dorsett | | #20

    "I had a roll of 3.5 In wall insulation, factory sealed in plastic, sitting in the middle of a concrete floor, in a crawlspace that was impacted by condensation, the plastic on the outside of the roll has actual weather on it from condensation. I haven't understood how that roll could be so cold that it created that much water on the outside of the plastic."

    The deep subsoil temps are below the summertime outdoor air dew point. When you plop a roll of insulation on the floor, the slab under the insulation and the bottom of the insulation fall toward the soil temperature.

    Since there's nothing to stop air from getting in there, the air under the roll near the slab becomes saturated as it cools, condensing on the roll of insulation. Surface tension will cause that moisture to spread until it reaches a point on the plastic that is above the room's wet-bulb temp, and the moisture returns to the air.

    If you keep the roll a couple inches off the floor the temperature at the slab may still be below the room air's dew point (or not) but the temperature on the insulation roll won't be. If you had an insulated slab this would not be a problem.

  21. Robert Metelsky | | #21

    Hi Guys, I'm at the sheetrock phase of my project and have a question again


    all of the concrete walls have 1 layer of 2" tuff r poly iso fastened with drill in plastic mushroom anchors (see image)
    another 1" layer of polyiso glued over the 2"
    all seams 100% foram sealed and foil taped on both layers

    So Im 100% confident of an airtight foam seal on the concrete wall

    I have 10' metal studs, Glued and screwed using green sheetrock

    Im noticing the wall sounds extremely hollow -

    This can be remedied by stuffing in a 6" X 2ft ppc of unfaced fiberglass insulation on the upper portion of the wall - see image
    Since I have nearly r19 with foam and the material was very expensive, I don't want to fully insulate the wall with fiberglass just for sound, but I need to resolve the sound problem

    Do you see any potential problem of me adding this 2ft piece of insulation on the upper portion on the wall. I don't think so - but figured Id sanity check here before things gets closed up

    As always thanks for your time!

  22. GBA Editor
    Martin Holladay | | #22

    The fiberglass batts shouldn't cause any problems.

  23. Robert Metelsky | | #23

    Hi Martin, thanks for the reply. I just wanted to double check. Much appreciated and I promise to set up a photo gallery of all the work when done - I think it would add to this thread Im doing all this work myself so time is tight

    Thank You again

  24. Robert Metelsky | | #24

    Guys - my main vapor barrier insulation has been completed, Im in one of the other sections of the basement and need some advise.
    Please see the attached images show upstairs finish which Id like the basement to eventually look like and lower levels in question
    I have this 1/4 round stair detail, concrete wall that I'm concerned may need some type of vapor barrier to avoid condensation as describe in the above post.
    The area is approx 6ft wide x 9 ft tall exposed concrete
    I'm limited to how much foam I can put on the wall due to the stair and hallway building code width - currently less than code but the inspector understands its existing
    My idea was to wire lath and stucco but Id like advise on the actual need or benefit to provide some type of vapor barrier/insulation
    I had an idea of thin reflective insulation 1/4" bubble foil which possibly seems like it would be just the thing - but looking on GBA - I see you guys aren't fond of the product. But it seems to be good for my need - i.e. thin and at leas something…

    What do you guys think about this situation?

    Thanks Bob

  25. Robert Metelsky | | #25

    Also should point out this is on the north wall and about 8ft below grade

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