I have completed removal of suspended 1/2 pex tubing and rehanging the tubing with Thermofin C extruded plates.  The basement is unfinished, but will be used as a play area and storage for kids.  What is the best way to finish the job.  I planned to put 2" airspace/foil bubble insulation, than fiberglass R30, but I don't believe there is actually space for this in the 9" deep joist bay.  Do I need the airspace?  Can I put the fiberglass directly against the extruded plates and foil underneath to protect the fiberglass from the kids?    Will there be any moisture concerns if the temp in basement drops from insulating the ceiling so well and no other heat source?

Thanks

skip the foil and the airspace.

What Rob said. If you already have the foil, putting it on the underside won't hurt- conditioned space air from above wouldn't condense on the foil in winter until the foil was well below 40F in the basement, a temp at which you should be thinking about pipes freezing (or even frost-heaving the foundation), not the moisture content of the joist edges. Whether that's a realistic risk depends on your climate and how air/heat leaky the basement is.  If you don't have foil, housewrap type air-barriers would be good enough to keep the fiberglass dust from spreading.

If the basement leaks outdoor air like a sieve it's something of a "solution problem" for the moisture levels in the basement. The cold air is drier, but it's freezing down there. (In summer outdoor air brings IN moisture in much of the eastern half of the US, but in winter it would take moisture out.) If you air-seal the basement the basement is warmer in winter, but interior moisture or wicking of ground moisture can raise the humidity. The best solution is to air seal and insulate the basement walls, and at put at least a concrete sealer on the slab. You may still need to run a dehumidifier down there to keep it under 60% RH, which is usually the boundary above which mold hazards increase. There's no rationale for keeping the basement under 50% RH though.

Basement walls can be insulated in several ways, but the particulars are both climate-specific and building-detail specific.

Make sure that you include an air barrier - air can flow right through fiberglass and this will hurt its effectiveness. Is exposed fiberglass even allowed? - maybe you need a layer of drywall, in which case you might as well blow the enclosed joist bays full (edit: partially) of cellulose.

Tyvek or foil RB is a good enough air barrier for the fiberglass, if properly detailed, but in conjunction with a real ceiling it's even better.

Putting in cheap R19s (or even R13s) under the floor and applying the savings toward insulating/sealing the foundation is probably the better solution overall. R30s or blowing full-depth cellulose fill under the floor is a bit overkill from a zone-isolation point of view, and only makes the basement colder without addressing the infiltration issues. It's damned-near impossible to fully air seal a basement ceiling even with a full-foam fill between the joists. See:

http://www.buildingscience.com/documents/reports/rr-1108-hybrid-foundations-retrofits-measure-guideline

I should mention there is a radon mitigation system in use and the basement moisture is lower than I've had in previous basements as a result. I would likely still run a dehumidifier in spring/fall due to some dampness noted on cardboard etc. (My old one broke down before moving in this June).
Lowes sells an insulation that is prewrapped in plastic all the way around. Has anyone used or would recommend this?

http://www.jm.com/insulation/building_insulation/products/bid0007_comforttherm.pdf

When you say an air barrier, I assume you mean under the insulation vs. between the plates and the insulation? I believe the paper backed Kraft insulation is ok to use, but I was concerned with possible moisture/mold issues with it.

Would I use a vapor barrier insulation and if so, is it placed against the Thermofin plates or toward the basement?

Before you decide look at mineral wool bats, (Roxul) a little more costly than the cheapest fiberglass but they are stiffer and because of that attribute fit better and require a bit less support.

Posted By Liebler on 30 Sep 2012 11:16 PM
Before you decide look at mineral wool bats, (Roxul) a little more costly than the cheapest fiberglass but they are stiffer and because of that attribute fit better and require a bit less support.

Thanks, I did see this product.  I noticed I can get a higher "R" value with Roxul with less depth which would allow R30 in my 9" bay.  Only thing is it doesn't have a vapor barrier?  I believe one is necessary between the warm upstairs and cold downstairs closest to the subfloor.  I could use the foil radiant barrier first I suppose. 

Posted By spolleypt on 27 Sep 2012 06:22 PM
When you say an air barrier, I assume you mean under the insulation vs. between the plates and the insulation? I believe the paper backed Kraft insulation is ok to use, but I was concerned with possible moisture/mold issues with it.

An air barrier is exactly what it sounds like- any surface that stops the free flow of air. This is distinct from a vapor barrier, which blocks the free permeation of water vapor through the materiral.

For low-density fiber insulation to perform anywhere near it's tested-rated values it needs air-barriers to prevent convection and infiltration losses.

Kraft facers are vapor retardent (~0.4- 0.5 perms), and in a semi conditioned basement ceiling application would have very low risk for creating a mold issue.  ( If it were a crawlspace vented to the outdoors it might though- depends on the climate.)  But even a true vapor barrier like foil or 10 mil poly wouldn't be a problem here.  The basement temperature would have to average below the dew point of the conditioned space air for weeks/months for the vapor barrier to be much of a threat. In winter that temp would be under 45F.  In summer it depends, but air with dew points above 65F are downright miserable-sticky, and air conditioned buildings never reach that humidity level (which would be bordering on a mold hazard even in the conditioned space.) But rather than "needing" a vapor barrier, NO vapor barrier would be the safer option.

The denser thinner versions of fiber insulation don't need an air barrier to get decent performance, but you don't need anything LIKE R30 under the floor to a semi-conditioned or fully-conditioned basement.  If the basement is normally heated R11 would even get you there, but if you let it "coast" in winter, even it's only 50-55F down there, cheap low density R19s are fine.

thanks Dana, that helps.
My plan so far is to do the following from the subfloor down.... plates/tubing, foilfaced insulation 1" below plates with foil facing plates, then finish the bottom of the joist bay with bubble foil insullation (which was already there and I have saved). This would give me a vapor barrier against the warmside, insullation, and an air barrier to aide the insulation in reaching proper R value.

If I understand this correctly, the vapor barrier is more important between the outside wall and inside, than between my upstairs/basement. In theory, the vapor barrier would protect moist air from my heated uptairs from traveling into the basement and condensing in the colder space. In reality however my basement is likely to be more "damp" than my upstairs. Without a vapor barrier agains the warmside, would the air from above travel through the insulation and condense on the inside of the bubblefoil insulation covering the joist bay?

Posted By spolleypt on 02 Oct 2012 09:15 PM
thanks Dana, that helps.
My plan so far is to do the following from the subfloor down.... plates/tubing, foilfaced insulation 1" below plates with foil facing plates, then finish the bottom of the joist bay with bubble foil insullation (which was already there and I have saved). This would give me a vapor barrier against the warmside, insullation, and an air barrier to aide the insulation in reaching proper R value.

If I understand this correctly, the vapor barrier is more important between the outside wall and inside, than between my upstairs/basement. In theory, the vapor barrier would protect moist air from my heated uptairs from traveling into the basement and condensing in the colder space. In reality however my basement is likely to be more "damp" than my upstairs. Without a vapor barrier agains the warmside, would the air from above travel through the insulation and condense on the inside of the bubblefoil insulation covering the joist bay?

Putting the foil facer with a 1" air gap the plates leaves a thermal bypass- air moves freely in the space, and any air leaks at the ends of the bays can allow convection & infilration that you DON'T want. Better to flip the batt, snug the fiber right up against the plate & subfloor with as little gap & void possible making the subfloor/plates the topside air barrier.  Use the facer as the bottom side air barrier, and skip the bubble pack.

You do not NEED and it's better NOT TO HAVE a vapor retarder between conditioned/semi-conditioned space.  If you want to use a faced batt as the air-barrier, use kraft faced (~0.5 perms) not foil faced (~0.05 perms, sometimes lower).

The bubble foil adds about R2-3 the stackup, but only if you leave an air gap on one side, and it is a true vapor barrier of comparable to (or even lower permeance than) 0.5 mil foil batt facers, a vapor barrier that you neither need or want.  Bubble foil has a negligible effect on the heat transfer when you have fiber insulation in the bay, or if it doesn't have air gaps on both sides.  With a trapped 1" gap on both sides it might come close to R7 performance in an idealized setup in a lab, but not so much in a real-world app.  Look up the actual ASTM tests in different orientations and air gaps required to get any performance out of the stuff- it's not worth anything in this application, and is of dubious value in most.  The best an only real world use of any consequence is as a rafter-mounted band-aid for houses in cooling dominated climates with little to no attic insulation, uninsulated ducts in that attic, and little or no room add insulation. (Even there the money is usually better spent applied to a different approach.)  It's also an allowed (but more expensive) path to California Title 24 compliance on "cool roof"  solar reflective index (SRI) when roofing materials with a lower-than-code SRI is installed. But under radiant floors in combination with batts it's barely worth the effort to put it up, even if the material cost were $0.

If you use unfaced batts and you already own the bubble foil you can use it as the bottom side air barrier, and if you leave at least 1" between the foil and the gypsum you get at least some benefit from it.

It's pretty safe to assume that if you air condition the house even slightly the dew point of the air inside the fiber will never be lower than 65F and the seasonal average would be well above the temperature in the basement.  The highest risk day occur only during the muggiest of summer days if no AC is being use.  When the basement & bottom-facing facer is averaging 65F or above there will be no condensation. But even if it were  colder than that at the ceiling in the basement (unlikely), with the basement is being dehumidified even if high humidity were allowed on the first floor the bays can still dry toward the drier air in the dehumidified basement through a kraft facer, but not through foil or poly.  In winter the conditioned space air is usually much drier, with much lower dew points, and even lower condensation risk in the basement than in the summer.

Dana, I can't tell you how much I appreciate the time you put into that explanation. Not to mention I understand the principals involved much better and can make an educated decision about what to use and how to do it. As a DIY, I don't want to spend the time and money without doing my homework.
Thanks Rob from NRT here in Maine, Dana, and the others that posted for spending the time to share your knowledge.

One more fact to consider.... Kraft paper facing basement, left unfinished would be a fire hazard and technically would have to be finished with gypsum. Would there be something I could use with unfaced insulation besides that would allow vapor through but not be a fire hazard?

I with Rob;

Better to put the foil in the dumpster lest someone find out you went for the biggest hoax in radiant floor heating...just behind the blanket thing.