Finishing basement - 1 inch of height to spare for all flooring

Discussion in 'Remodel Forum & Blog' started by Miguelito, Dec 17, 2015.

  1. Miguelito

    Miguelito Member

    Joined:
    Dec 17, 2015
    Location:
    Massachusetts
    Hello,

    I've been reading this forum and it has been very helpful in planning my DYI basement finishing.

    I went to my (MA town) building dept, and after some reading was crushed to read that I need 7' to the ceiling and 6.6' to all boxes, beams, etc. While my (currently unfinished) basement has main ceilings of 7.5"+, the engineered wood beams are low enough to only give me 6'7". That means that I only have 1" to spare for all flooring and to wrap the beam. So much for my plan to use 1" XPS and 2 layers of 1/2" ply for the subfloor.

    Now the good news is that my basement has ZERO water or humidity problems and is level and bone dry. The house was built in 2009-2010 and the ground slopes rather well to the back of the property and it has soil that drains very quickly (both very much opposite to my last house that sat on clay!).

    I'm thinking of just applying drylok or similar and perhaps a layer of that thin foam used under PT wood when against concrete. Then simply put down carpeting (over carpeting foam for a bit of padding). Hopefully I can keep this to an inch or less.

    Thoughts?
     
  2. Reach4

    Reach4 Well-Known Member

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    Sep 25, 2013
    Location:
    IL
    You are saying that you want to insulate the main floor from the basement. It seems to me that if you insulate the sides of the beam, you could get by with little no insulation on the bottom of the beam. This is because while the beam does not have much R-value per inch, it has lots of inches vertically.

    I think you will want to consider where the vapor barrier goes in your system. In cold places, the vapor barrier goes between the conditioned space and the insulation. I don't know how you should consider this in your construction.

    I don't know much about this. These are just thoughts to add to your study.
     
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  4. Miguelito

    Miguelito Member

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    Dec 17, 2015
    Location:
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    What I mean is that right now, measuring from the concrete floor to the underside of my joists I have 70.5". But measuring from the same concrete floor to the few structural beams holding up the house, I only have 79".

    Per section 5305.1 on the linked PDF of the Mass Building Code below, that measurement from the floor to the lowest point of the beam can only be 78" then the basement is finished. As a result, I only have 1" for all my flooring including whatever I want to do to dress up that beam.

    http://www.mass.gov/eopss/docs/dps/inf/53051-basement-ceiling-height.pdf
     
  5. Reach4

    Reach4 Well-Known Member

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    You were clear about the heights.

    How about stain or paint on the bottom of the beam?

    How dressy do you want the basement? It is going to be largely uninsulated space, if I interpret correctly. You could insulate the basement walls.
     
  6. Miguelito

    Miguelito Member

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    Ah! The expectation is that this will be a nicely finished basement. I will do 2" of XPS all around with 2x3 framing and 1/2" drywall. I'd like to think I can get away with 3/8" drywall under those beams, but I think that is unlikely.

    I'm not terribly worried about insulating the beam. I don't think I need to.
     
  7. Dana

    Dana In the trades

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    Can I convince you to use polyiso on the walls instead of XPS? XPS uses HFC134a as the primary blowing agent, which has a 100 year global warming potential of about 1400x CO2. Most of that blowing agent is gone in 25 years, and as it leaves the R-value drops. At about 50 years XPS has about the same R-value as EPS of equal density & thickness.

    EPS and polyiso are blown with pentane which has a 100 year GWP of about 7x CO2. The blowing agent has already dissipated from the foam by the time the product is cut in to panels, and is usually burned on-site for process energy which produces some CO2 (which surprisingly, has a 10 year GWP of about 1x CO2 :) )

    At 2", the XPS on it's own would not meet current code-minimum for basement walls in MA, which is now R15 continuos insultion, or R19 in a studwall.

    The R-value of polyiso is a bit higher per inch than XPS, and in a basement wall application will pretty much meet it's labeled performance. 3" of foam would get you to code-min performance levels but so would 1"polyiso + 2x4 / R13, which is the same depth as 2" XPS and a 2x3 studwall.

    Reach4 has it a bit off, the LAST thing you want to do in a basement wall is to insert poly sheeting between the wallboard and insulation. In fact, it's better to design all wall assemblies so that it doesn't need any vapor barriers at all, and keeps all susceptible materials warmer (= drier) where ever possible. The ubiquitous and improper use of 6 mil polyethylene has created more moisture problems that it has solved in the lower 48 of the US, but it sometimes makes sense (with proper use) for some parts of US climate zone 8 (the interior of Alaska), or even the cold edge of zone 7 (International Falls, MN.)

    All of MA is in comparatively temperate US climate zone 5, where it only takes R5 to provide sufficient dew point control at the foam/fiber boundary to use up to R15 on the interior side without using interior vapor barriers- latex paint on drywall is good enough. The vapor retardency of the polyiso (any facer, or even unfaced) is low enough that ground moisture wouldn't permeate through the foam into the studwall cavity any faster than it can leave through latex paint on wallboard. So as long as you use unfaced or kraft faced batts it will be just fine. The wintertime moisture drives can't accumulate due to the warmer temp of the interior face of the foam, and ground moisture won't be trapped in the cavity since it has a modestly permeable interior finish surface, with no vapor barriers. (Kraft facers have variable permeance, but are fairly vapor open at humidity levels high enough to support mold growth.)

    For the floor foam you can't use polyiso, since it can become saturated if there's much moisture coming up through the slab, either as liquid or water vapor. You DO want to insert a vapor barrier under the subflooring to protect against potentially VERY high ground moisture drives, which calls for a moisture-tolerant foam under that vapor barrier. That can be either XPS or EPS. With 1" of Type-II EPS you would have R4.2 under the subfloor, which is sufficient R-value that the subfloor stays above the summertime outdoor air dew points even with a rug above it. You don't want to go below R3 if you intend to put rugs on top of that much wood.

    If you make sure that the floor foam extends all the way to the wall concrete, and lap the 6-10mil slab vapor barrier up the foundation wall a few inches, you won't need to use pressure treated lumber anywhere. The vapor barrier is a sufficient capillary break to protect the wood from ground moisture, and the foam is sufficient thermal break to keep it from accumulating too much moisture when ventilating the basement with sticky summertime high-dew-point air.
     
  8. Reach4

    Reach4 Well-Known Member

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    I was under the impression that he was talking of insulating between the basement space and the main floor because he was talking about insulating his beam. I had not gotten the impression that he had considered insulating the basement walls, but I probably misinterpreted . I even wrote "You are saying that you want to insulate the main floor from the basement" to try to confirm my interpretation. So my comments about vapor barrier were with regard to the basement ceiling.

    It did seem incongruous. So was he talking about insulating the new basement floor when he said "subfloor" rather than the underside of the main floor? It now looks like it. So your interpretation is probably right. But in that case, why was he talking about insulating the beam? OK, maybe he wasn't. I had interpreted "wrap the beam" as insulating, which is why I wrote about R-value of the beam.

    So apparently I had totally misinterpreted what he was talking about, but at least I was not suggesting poly sheeting between wallboard and insulation.
     
  9. Dana

    Dana In the trades

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    The clue was: "So much for my plan to use 1" XPS and 2 layers of 1/2" ply for the subfloor." Nobody would be building a first floor or insulating a beam that way.

    With only an inch of headroom to spare there's no room to do anything but put moisture-tolerant flooring down and SKIP THE RUGS entirely. Even a decent ceramic tile job is going to add up to over a half-inch. A layer of 3/8" fan-fold XPS or half-inch XPS would need a thermal barrier against ignition to meet code on fire safety, such as half-inch plywood or OSB, and would not be sufficient for summertime dew point control with a rug above it. (R3 minimum.)

    Being built in 2009-2010 I'll assume jackhammering out the slab and digging down doesn't work here, the way it sometimes can in older homes with 1-2" rat-slabs.
     
  10. Reach4

    Reach4 Well-Known Member

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    When I stand in my basement, I look up and see the subfloor. That was where I went astray.
     
  11. Miguelito

    Miguelito Member

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    Dec 17, 2015
    Location:
    Massachusetts
    Thank you for all the information.

    Yes, this whole thread is in regard to how to cover concrete - be it the walls or the floor of the basement.

    1) As far as the walls, my town's building department just gave me a sheet two days ago and it (in effect) says R10 for concrete walls, and R13 for studded walls. Interesting what you mention about the new R15 requirement. I'm game with 1" of foam board plus R13 inside 2x4's. I'm also ok with Polyiso, except I thought those had to be faced for manufacturing reasons and that would introduce a barrier I shouldn't have.

    2) As for the floor, my basement, unheated, is always at over 50F even in winter and with unfaced fiberglass insulation between the joists on the ceiling. I was thinking that if I keep the basement at 60F+, I don't see how I could have condensation issues. But even then, (crazy idea coming) that's why I thought of using a thin (1/4") layer of polyethylene foam (the stuff used as a sill gasket). That way, in the rare event of a bit of condensation, I'd be protected if I use carpet.

    3) Yeah my basement slab is 4" and is really nice and level and totally dry, so I'm not touching it. All indications are that it handled the worst rains the region has seen in recent memory (April 2010 - 15" of rain in ~9 days - flooding everywhere) with no problems at all. I've kept a very close eye on all heavy rains and the record winter we just had, and you would think you are in a second floor. That's how dry my basement is.
     
  12. Dana

    Dana In the trades

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    MA code is based on IRC 2012, and we're in zone 5, ergo R15 c.i. or equivalent. If you have a good sill gasket (EPDM is best) or at least a foot of exposure on the exterior you don't have to sweat the foil facers on polyiso. If your roof overhangs are less than one foot per story you probably have bigger wetting issues for the foundation sill from splash-back on the exterior than you do from moisture wicking up from the footing, in a basement that has "... ZERO water or humidity problems..." Roofing iso usually has fiberglass or asphalted paper facers, which are quite a bit more permeable than foil, if you're worried. And 2" roofing iso from reclaimers like Nationwide Foam in Framingham or Green Insulation Group in Worcester is cheaper than 1" foil-faced goods from the box stores. (I did my own basement with 3" fiberglass-faced reclaimed roofing iso, in a basement where the water table is above the slab during the spring thaw, with multiple sump pumps cycling to stay dry. I stopped the insulation a few inches above the high-tide mark.)

    The room temperature is not a good indication of the slab temperature. During the dog-days of summer outdoor dew points are north of 70F in MA, and I'll bet your slab isn't warmer than 60F at any time in the year. If you put down R1 of rug the slab (and thus the under-side of the rug will be a degree or three cooler on average. You'll never see condensation- but you will have adsorption events, and when the moisture content the mold-food (aka "rug") is high enough the mold to reproduce they'll do the mold-orgy thing. It takes about R3 of non-mold-susceptible insulation to keep a basement rug from being at risk at MA style deep subsoil temps, and you're not going to get that out of 1/4" of foam- you'll be lucky to get R1.
     
  13. Miguelito

    Miguelito Member

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    Massachusetts
    Thanks for the info, Dana - especially the reclaimers.

    Well, I guess I'll just have to use VCT tiles on the floor. That or try to get cute and see if I can pass inspection with just a painted floor and add the flooring I wanted after the fact (I much rather not).

    One more question, my stairs to the basement are in effect up against a concrete wall opposite to me insulated garage (and my master suite is also above the garage). Do I have to comply with the R15 on that wall also? That would require completely redoing the finished stairs and losing ~3" in stair width (currently about 37" wide).
     
  14. Dana

    Dana In the trades

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    You don't need R15 on the foundation wall if it's on the house side of an insulated attached garage. If that wall intersects an exterior wall it's good to insulate at least the first 4-5' from the intersection to at least R6-R8, since poured concrete is fairly thermally conductive. It's not worth ripping out the stairs, even if you can't quite get there, but you'll be able to feel the thermal bridging in that area on cold days.
     
  15. Miguelito

    Miguelito Member

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    Dec 17, 2015
    Location:
    Massachusetts
    That's great news! I will actually insulate the first 2 feet as the stairs will lead to a closet in front of the landing area, so the whole depth of the closet can be insulated on the side.
     
  16. WoodenTent

    WoodenTent New Member

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    Dec 27, 2015
    Location:
    Birmingham, Michigan
    I have the same basic issue with my basement. I was going to do the same as you wanted, floating subfloor over ridged foam. But would not have clearance under my main beam (Steel I-Beam, so not as bad for just a paint option). I also had the issue my floor is extremely un-even, I've built areas up 10 inches and poured more self leveling than I want to think of just to get it so it's not super noticeable. Thing is, after a few years working on the house, spending a lot of time down there barefoot on the concrete, the reality is it doesn't get that cold on the feet. Where there was Vinyl tile when I bought it was much warmer than the bare floor. So after working through every conceivable idea, including conceiving whole new products to solve the issue I concluded that just going back to vinyl in some form is the answer. It won't be that cold, especially since the basement will have insulated walls, and proper HVAC now. Just throw down rugs or carpet squares where ever, the rest stays a durable floor that is easy to maintain, and even though it is an extremely dry basement, and now have a whole house black water valve protecting it (which did kick in and save it during a freak storm when the sewer backed up for a few minutes), there is still the chance something goes wrong internaly. So having a floor that will just dry up with no issue isn't so bad.

    Drylock and such isn't meant for floors, I looked into these options, using thin material like sil gasket and then the PT as you mention. But you will never get a flat floor, it will just buckle and such. Without being able to use a good non-PT plywood/OSB in 2 layers to tie it together, it will just shift. Also if you end up wanting to do glue down flooring, you will have to grind the drylock off. Also such products are semi-worthless as the moisture is coming from the other side, so it would just peel in time.

    The thinest options are some of the rolled dimple material, and then run a floating click and lock floor over that. But it will probably make noise, and shift around, and still probably have moisture issues with time. Also with such systems, what happens when you spill stuff, it gets down in the layers of your floor system and may take a very long time to dry out.

    Sometimes the simple, basic, traditional answer is the one to go with. Like bad concrete in a garage floor, you can look at all sorts of options and in the end the best and typically cheapest is to just cut the floor out and pour a new one (did that).
     
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