Remove foil from one side?

Discussion in 'Remodel Forum & Blog' started by kcodyjr, Dec 11, 2013.

  1. kcodyjr

    kcodyjr New Member

    Messages:
    103
    Location:
    Chelmsford, MA
    I'm installing some Tuff-R polyisocyanurate sheets in the 2x3 exterior stud bays of the bathroom in my motor home.

    It's foil faced both sides. There is a 1/2 inch air gap between it and the aluminum outer siding - no sheathing. The aluminum is unfinished on the inside and therefore should function as a poor man's radiant barrier.

    I'm thinking I should remove the foil facing on the outside, to prevent moisture trapping within and around the insulation, and to allow the radiant barrier function - heat radiating from the insulation must reflect off the aluminum and then be reabsorbed by the insulation, which won't happen with the foil present.

    The inside surface will retain the foil, and there will be foil tape over the studs to create the class 1 vapor barrier.

    Thoughts?
  2. Dana

    Dana In the trades

    Messages:
    2,986
    Location:
    01609
    Removing the foil an only reduce the thermal performance. Radiant barriers work best when there are multiple low-E layers facing each other across an air gap, such as the exterior facer of the iso, and the interior face of the siding. The emissivity of iso without the facer is pretty high, radiating far more heat outward than with the facer. A trapped half inch vertical air space with aluminum on both sides is worth about R4-5 in average performance, but with the aluminum on just one side of the gap you're getting at-best R1.5-R2.

    The critical moisture-trapping issue rests on whether there is a poly or foil vapor barrier layer (or vinyl or foil wallpaper) on the interior side of the assembly. If there is, it has to come out if you're putting foil-faced iso on the exterior.

    Taping over the stud edges with FSK tape limits the ability of the stud to dry- don't do it. With the air gap on the exterior side it can dry into the great outdoors, and with a semi-permeable finish material on the interior the stud can dry toward the interior as well. The vapor permeance of wood is pretty low- it's it's own vapor-retarder (that's why it takes a year for freshly hewn cord-wood to dry enough to be burnable as fuel.) True vapor barriers cause more moisture problems than they fix, and if you have them, it matters where they are in the stackup, and the climate. In a motor home the climate could be anything from Key West FL heat & humidity to Fairbanks AK coolth, so tread carefully before introducing a true vapor barrier.

    If I understand your stackup correctly you have:

    interior air | finish wall surface of some sort | fiber insulation | foil faced polyiso | half inch of air | aluminum siding | outdoor air

    Or are you simply re-filling a cavity previously insulated with 2.5" of fiber insulation (or an empty cavity) with 2-2.5" of polyiso?
    Last edited: Dec 12, 2013
  3. kcodyjr

    kcodyjr New Member

    Messages:
    103
    Location:
    Chelmsford, MA
    I'm filling an empty cavity with 2" of polyiso, while the wall is torn apart from the interior due to a chronic moisture problem.

    Here's the stackup:

    great outdoors | paint | aluminum siding | half inch of air | double foil faced polyiso | 1/4" plywood | plastic shower surround | interior air

    The previous insulation was a thin sheet of fiberglass, not a batt, laying atop the studs, not in the cavities. There was a poly sheet over that, then two layers of wood paneling, then drywall, then the plastic shower surround. The rot went from the caulk around the tub all the way to the sole plate and rim joist.

    I thought that the essence of a radiant barrier is to create a "thermal diode" wherein the heat radiates from the insulation across the gap, is reflected by the barrier, and re-absorbed by the insulation. It seems like the foil on the outside of the polyiso would prevent that reabsorption, but allow less radiation in the first place. If the difference in emissivity is really that dramatic, then yeah, I'll leave it be.

    I feel certain the original design relied heavily on that one-way reflective behavior, because that flimsy 1/8" fiberglass and poly flatbread obviously isn't what's keeping the rest of this place reasonably warm. Not that I think the designers did that on purpose.
  4. Dana

    Dana In the trades

    Messages:
    2,986
    Location:
    01609
    Low-E surfaces like aluminum foil both reflect radiated heat, but also emit very little heat, so two low-E surfaces facing one another allow very little heat transfer. Unclad polyiso is about as emmissive as most common materials, and while polyiso is a pretty good insulator on it's own (from a conducted heat point of view), the temperature of the surface lets it emit more heat than when it has the foil facer.

    But there is another reason to lose the foil faced polyiso concept anyway: Foil is extremely low-permeance to water vapor (a true vapor barrier), as is the plastic shower surround. You need to put something in there that is air-retardent, but allows moisture to leave, otherwise moisture will be trapped at the interior side, and the plywood will get moldy. Rather than polyiso, use either unfaced EPS (bead-board foam, like cheap coolers and coffee cups), or at worst, high density "cathedral ceiling" fiberglass or rock wool batts, split so that the air gap to the siding remains.
  5. kcodyjr

    kcodyjr New Member

    Messages:
    103
    Location:
    Chelmsford, MA
    I've already bought and started to rough fit the polyiso, so the options are keep the facers or peel them off.

    I went with this stuff because it's the only way to get anywhere near R13 (12.2 i think) within the 2" depth available. I think standard batts would have gotten me 5.6, and that's before splitting it. This is climate zone 5. With just half the panels in place, my right butt cheek is already very appreciative of the improvement.

    Eating up the air gap space isn't an option anyway; it's there because of two 4-inch furring strips notched in to the exterior side, plus the wiring bundle in a 3rd notch. Believe me, I wouldn't have designed it like this...

    So, let's say I peel off the outside-facing facer and just eat the emissivity gain. How about the inside? Should I peel it off entirely too, or would it be enough just to poke some holes in it? Would there be any sense in trying to retain just part of the facer? Should I remove only where the tub surround is going to cover it?

    I should note that there will be an open air channel underneath the tub, into the water heater closet at one end and underneath the kitchen counter at the other, so we really are just worrying about the space from the rim of the tub to the top of the surround.

    I do need a true vapor barrier in a bathroom, generally, right?
    Last edited: Dec 24, 2013
  6. Dana

    Dana In the trades

    Messages:
    2,986
    Location:
    01609
    Rock wool (Roxul) or "cathedral ceiling" fiberglass delivers about R8.5 @ 2"- it doesn't totally suck.

    You don't want to trap any wood between a foil facer and the plastic tub-surround, since the surround is a true vapor barrier.

    With the half-inch air gap for the plywood to dry into thorugh the permeable-foam or high density fiber, you don't need or want an interior side vapor barrier.

    As a general rule, you don't need true vapor barriers anywhere, but if you already have one (the tub-surround) you'll be shooting yourself in the foot by adding another in the stackup. Far more moisture problems in wall assemblies are caused by bulk water intrusions than anything else, and adding true vapor-barriers (anything with a permeance less than 0.1) traps that moisture, taking forever to dry. The next-biggest moisture issue in wall assemblies in cold climates is air-leakage moving moisture out to get adsorbed by wooden structural sheathing on the cold side of the assembly, which you apparently don't have in this assembly (your stackup shows the plywood on the warmer, interior side.)

    But if it did have plywood or OSB sheathing, with a vent gap between the sheathing and siding to dry into, you're still good to go without a strong interior side vapor retarder in a US zone 5 climate. In zone 6 or colder you'd have to go with a less susceptible/more permeable gypsum or fiberboard siding, plus the air gap, or put a class-II vapor retarder ( sub 1.0 perm) on the interior side to slow the vapor diffusion through the wall to a rate wood sheathing products can keep up with. (Half-inch to 3/4" OSB is about 1 perm, so the interior side needs to be lower permeance than that to keep up, but not a lot lower.) "Vapor barrier" latex primer runs about 0.4-0.5 perms when applied to standard wallboard products.

    If you lived in a climate where the mean binned hours weekly temp is below 40F for 9 months of the year, THEN a true vapor barrier like poly sheeting or foil facers start to help more than hurt.

    Bathrooms need ventilation to purge the high humidity peaks, but aren't dramatically different than any other room from a vapor-barrier need. Public baths, swimming pools, commercial kitchens etc where you get 8+ hours a day of high humidity need special consideration, but I'm gonna guess your shower gets well under an hour/day of high-moisture, the bulk of which gets purged via ventilation fans or open window within an hour of the onset of the high moisture event.
Similar Threads: Remove foil
Forum Title Date
Remodel Forum & Blog How much of the old fireplace can I remove? Aug 19, 2013
Remodel Forum & Blog Recessed Wood Framed Mirror -- how to remove caulk without damage? Oct 23, 2012
Remodel Forum & Blog Garage Door Wall Button Console - How to Open or Remove? (photos) Jul 30, 2012
Remodel Forum & Blog Attic Blown In Insulation Remove Or Blow More Over It ? Feb 5, 2012
Remodel Forum & Blog Permanently Remove Cast Iron Baseboard Radiator Dec 31, 2011

Share This Page