I am finishing my radiant floor heating and have a question about the insulation I have to put. My setup is with joist track, I need to insulate the underneath of the tubing of course and that mean it is on the basement. Part of it is finished and another is not meaning that no heating on this part.

My question is that during 'hot' days, like most basement, it gets damp so I am debating what insulation I would need (and how to place it). Fiberglass bat is interesting from a cost perspective but I am worried that it could get moisture and therefore develop some mold.

'Solid' pink insulation looks great but will add a significant cost to the installation. Same goes for blown rigid foam.

Any suggestion ?

Thanks,

Geoff.

I'm assuming you're talking about retrofiting radiant floors for the first floor, with the tubing & insulation between the joists?

First, if you use fiberglass, staple radiant-barrier (either the aluminized bubble pack or aluminized polyester variety) ~1" below the PEX tubing, since fiberglass is somewhat translucent to infra-red heat. The air gap isn't supercritical, as long as it doesn't contact the tubing or heat spreaders. (If you're concerned about the radiant-barrier forming a vapor trap, use the perforated aluminized polyester versions- there are several vendors out there.) Then install UNfaced batting below the radiant barrier, holding it in place with wire bails or semi-rigid netting (or gypsum board, if you're semi-finishing it.) If the basement is unconditioned space go with at least R19. If it's heated, R11 will do.

If your basement is damp enough to grow mold in the fiberglass, it's WAY over the level that would cause it to grow on any structural wood, and you need to deal with it (a topic too big to address here, with too many variables. Search the web for: Building Science basement insulation -for documents to get a handle on the issues.)

I've never done it, but wet-spray cellulose applied in netting underneath the radiant barrier would do a better job than fiberglass since the fit would always be perfect, but it's not a DIY job. Batts are a lot easier, and more effective when the warm side is on the top than conversely, due to lack of convection currents within the fiberglass in that configuration. This is about the only application where standard-density batts work pretty well- they're not so great whenever there's an air convection potential (walls, attics, sloped ceilings). High density "cathedral ceiling" versions are pretty good, but not necessary to maintain R-value in a "warm-side above" installation like this.

I would generally not advise gambling on long term radiant barrier upward-facing effectiveness, and I'm not too worried about fiberglass being "translucent' either (though other insulation methods that seal for air infiltration are certainly more effective if you are over unconditioned space). I would pick your insulation based on your moisture/vapor issues, achieving higher R-values than Dana recommends, depending on your area. If you're in a cold area and the basement is not insulated, I'd recommend an R30. If it is, an R19, and if it's heated, R13 is my recommended minimum over heated area for a joist mounted application.

More insulation is always better no doubt! Where the zero in the cost/benefit curve is depends on your climate, basement infiltration level, and whether it's completely unconditioned & leaky vs. semi-conditioned vs. heated space.

If your basement isn't insulated, it SHOULD be (the next project?), since uninsulated basements typically account for 1/4- 1/3 of the annual heat loss. It's often an easier/cheaper project to insulate basement walls effectively than basement ceilings (often less material, and usually much less labor, fewer mechanical details to fit & seal around.)

The effects of dust on upward facing radiant barrier has been well studied over the past 2 decades. While it's efficacy may be impeded by as much half in a ventilated attic, it can take as little as a year, or as long as 10 years to approach that. But that's in attics, attics with air-transported particulates. With VERY-low/no air infiltration situations like radiant staple-ups the slope of the deterioration curve is surely an order of magnitude or two longer. (Who really cares if it turns out to be only half as effective in 50-100 years? All things are transitory... :-) )

The translucency of fiberglass to infra-red is well documented & long understood- borates are even added in the glass spinning process to make it more IR absorptive to counteract that effect. It's the reason foil-faced batting was poplularized in the 1960s, the reason fiberglass heating-pipe insulation has a foil layer, and the reason why radiant barrier has proven efficacy in reducing cooling loads in fiberglass-insulated attics (and less effect in foam or cellulose attics.)

To be sure, if you take NRT.rob's advice and insulate to R30 it won't be much of an improvement (you'll need some pretty deep joists for that- high density R25 cathedral-ceiling bats may be the best you can do). With R11-R19 low-density cheap stuff, radiant barrier is well worth the effort.

While it's always dangerous to talk about "R-value equivalents" when talking about radiated heat (R-value is primarily a conducted-heat measurement, wherease radiant barrier is only effective in controlling radiated heat), the radiant barrier becomes effectively another R6 or more, depending on the difference in temperature & emissivity of the radiating element (the PEX/heat spreader plates) and whatever is on the low-temp side of the barrier. The higher the temperature of the heating water, the bigger this effect is.

and also the temperature of the joist bay, and it's about more than just raw cost/benefit, it also affects performance for the radiant panel above.

the only studies on upward facing radiant barriers that I have seen is the ORNL study you are referencing regarding the attic (i assume). If you have other studies about joist bays, I'd love to check them out! I've been starved for info on that and choosing to act "conservatively" in the interim. I don't regard joists as clean spaces as dust can be smashed down from above over time, degradation occur, mice happen, etc, but then again, I also just don't see the point of spending the cash, time and energy on the radiant barrier when regular R value does a very adequate job in most cases, when you insulate to (what I consider to be) appropriate levels under radiant floors. Most people would be way better off putting the money into plates or something.

Your point about operating temperature is a very good one though and one I rarely remember to consider! I nearly always design for low temps, but there are times that is not possible and I should remember this as an option in those cases. Thanks!

The Lawrence Berkeley Nat'l labs, University of Texas, and Florida State University have all done pretty good studies of radiant barriers in attics, but not dust-accumulation in joist bays so far as I know. The model & temps are pretty applicable to staple-ups- interior attic rafter/roof decking @ 110-170F would have roughly similar infra-red emissions characteristics to 130F joist bays. Hopefully you'll never have to run your staple-ups at 150F, but 110-130F is pretty normal.

It's impossible to get good data from radiant barrier vendors- they all feel like snake-oil salesmen selling phantom R-values with deeply hidden fine print. Proper installation makes all the difference in actual performance. Most wood floors (even really old ones) have a dust barrier between the finish & sub flooring (rosin paper is the classic), so if you do a decent job of the RB installation, it'll take very VERY long time to accumulate much dust, and with a bit of air space it should do pretty well. It IS a separate step though- the stuff is cheap, but the labor isn't. Figuring out how to to it labor-efficiently is key.

R30 against a 66F basement?

If your basement is 66 deg F, it's heated space. that's a common fully heated room temperature. In that case with a joist heating system (at low temp) I would use an R13

Otherwise, the basement is 66 deg F. because of all the heat it's sucking off of your main floor. If you want a heated basement though you are way better off actually heating the basement ;)

Dana, I don't know, I'm still skeptical that dust accumulation is as low as you think it is in joist bays. You very well may be right, but then why risk it? A little rigid foam and a can of spray for the seams, no worry about quality of insulation, better air sealing characteristics, and a reliable R-value long term.

However I am, more and more, starting to think that fiberglass needs to go the way of the dodo... but my education level of all of the issues involved is weak (and I have a sneaking suspicion that some of its problems are overstated as well, masking the real issues). Is there a good summary discussion on the topic you are aware of?

The vast majority of heat "loss" from the basement is the uninsulated perimeter (frost line). Downward flux through the floor is minimal as the delta T between earth and floor is nearly always quite low (not allowing for typical heat generating appliances. Once frost is accounted for, there are dramatically deminishing returns on basement insulation. More is just more, not necessarily better.

All,

Thanks for all the advice, opinion ... etc.

Some clarification based on the comments.
The basement gets humid during hot days but we are not talking enough to grow mold so far.
The basement is composed (or will be when done) :
* 1 family room (250 sq/ft) : this room will be fully insulated with spray foam on the wall which will act as a very good vapor barrier and give me the best R value per inch (closed cell foam). In addition to reduce the humidity level all walls and floor will be coated (before foam is applied) with epoxied fiberglass and that should do it pretty good (otherwise fiberglass boat will sink ...). Of course this room will be heated ... it use to with baseboard but I had to remove them for the radiant and I am still debating what to use knowing that I can't do radiant floor over the current slab for head room purposes (already 7' only).
* Laundry room : will be heated indirectly by the radiant system as the control room/closet will be in it. And yes insulated as well.
* Entrance hall (20 sq/ft) : will be heated ultimately but same as the family room I am debating what to use ...
* 1/2 bathroom : same as entrance hall
* large storage area (80sq/ft) : will be close but I don't plan to heat it.
* Wine cellar : you can guess that this one will not be heated but refrigerated at 55 F and controlled for 70% humidity

So what I plan do to is :
* use radiant barrier everywhere: it is nor really for insulation or radiant barrier perspective but mostly to make sure I can have the proper gap between the tubing and the insulation. I don't need the top quality for the reason above.
* for heated room : R15-R19 non-faced fiberglass
* for non-heated room : R25-R30 non-faced fiberglass if I can make it easily with the depth of the joist I have
* For the wine cellar : It will be R45-R50 using spray foam insulation. No compromises on this one ;-)

If you think that will not do it, you are welcome to comment.

Thanks again,

Geoff.

It the entire perimeter of the basement is insulated and reasonably air-sealed it becomes semi-conditioned space- probably never drops as low as 55F even after a week of cold weather(?), and anything over R20 is probably overkill (fine, but has diminishing returns.) If the basement gets moist during hot humid days (and only then) it's an indication that there is air infiltration either between the basement & the floor above, or directly to the outside- insulation isn't going to fix that, but the severity of the problem may not be all that great. Drying that air mechanically (dehumidifier or AC) to keep it under 65% RH is usually sufficient to keep mold from getting a foothold.

Testing for radon after you've sealed & insulated is probably a good idea. If your radon level needs remediation, consider heat-recovery ventilation HRV) as your first level of abatement, since it will control humidity to some degree as well, and usually uses less power than a slab-depressurizion + dehumidifer. (It's possible to control their cycle with radon-detectors, humidistats, or a combination to keep operating costs at a minimum.) Then re-test to verify- worst-case you may have to add a slab-depressurization system- if that happens insist on the lowest power fan available- it's a 24/365 background load, and you'll spend more on power than the cost of the fan in short numbers of years.