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Thread: Radiant Barrier Insulation vs Fiberglass

  1. #1
    DIY Member Hillel's Avatar
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    Default Radiant Barrier Insulation vs Fiberglass

    Hello Everyone,

    My house is being rebuilt after having been gutted by a fire. The walls are built with 2x4 studs. Ohio building code requires R13 insulation in a 2x4 outer wall and R30 in the attic. The contractor is used to using the basic fiberglass batt with the minimal required R-values. I am thinking about instead using a radiant barrier insulation like the TempShield double bubble double foil at http://www.radiantbarrier.com/. The material is rated with R10 when mounted in a wall, and R16.8 when mounted in attic joists, so that would be two layers with a gap in both the wall and attic to meet code. (The company claims that the R-value as a measure is designed just for fluffy insulation and is practically meaningless for radiant barriers, and that the U-value is a better comparison.)

    Does anyone have experience with using radiant barrier insulation like the above product? Are there any gotchas with installing it in a northern snowy climate?

    Thanks.
    ---Hillel

  2. #2
    In the trades Dana's Avatar
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    Those "ratings" for radiant barrier are ASTM C 236 guarded hot box values at a fixed 30F (or is it only 25F?) nominal delta-T to use for comparisons, but based on very particular assembly installation criteria. For instance, to meet that performance in walls the RB must be installed at the mid-point between the exterior sheathing and the interior gypsum. In a 2x4 assembly if the gap is narrower than 1.5" on either side, it won't perform as well as advertised. In some localities those values are allowed to be used for meeting code, but in many not. If it's not air tight creating two pressure-tight cavities in each stud bay losses to convection currents at high delta-Ts are also VERY significant, so getting the labeled performance out of it requires METICULOUS installation, air-sealing it in each stud bay. But if you air-seal every seam, it'll work about as well as R11 batts, but it's still sub-code, and not recommended.

    In attic applications it's performance varies depending on whether it's warm side up vs. warm side down, and whether there are separate air barriers on both sides or just one. To hit 16.8 in a joist cavity implies that it's hot side up, which is not what you're looking for in Cleveland, where there are real winters when you'll be cold-side up.

    The bubble foil stuff is also problematic in that it is a true vapor-barrier, and once it's in the assembly you have to be VERY careful about the vapor retardency of both the interior and exterior material to avoid trapping moisture in the cavity. The cavities need to be AIR tight on both sides, but still be at least partially vapor-open from a water-vapor diffusion point of view (which sides and how much is climate specific), and that bubble-pack isn't vapor open at all, with a permeance rating of under 0.1.

    Low density fiberglass batts are not without problems, since air convects within the batt fiber at high temperature differences, causing loss of performance with big delta-Ts. (But the fiber at least retards that flow, unlike radiant barrier with air-leaks.) The also need to be tight the sheathing, gypsum and studs- you can get even bigger convective losses if air can move around the batt unimpeded, even if it's only short distances. This means you have to split the batt to tuck half behind, half in front of any wiring or plumbing, and cut carefully around electrical boxes, etc to that you have NO compressions and NO voids, it fits' tight to all features in the stud bay. The labeled R value is an ASTM C 518 plate test, measuring the heat flux 25F away from a center temp of 75F, in both directions. It gets colder than (75F-25F= ) 50F in Cleveland, so you really DO care about it's R value at 25F and at 0F, which will be lower than label. The air-tight Moving up to high density R15 "cathedral ceiling" batts is a big improvement- they're much more air-retardent due to the higher density, for lower convection rates, and have much better performance at the temperature extremes than might be implied by R13 vs. R15.

    Also, the test-fixture plate provides an air-barrier on both sides, which improves performance, the air-tightness of both the sheathing and the sheet-rock side can affect this. And in the attic joist there is RARELY a full air barrier on top, which in a warm-side-down configuration (like Cleveland in January), it's an efficiency disaster, since the buoyancy of the warm air on the ceiling gypsum causes convection loops that even go OUTSIDE the fiber layer. Again, with the higher air-retardency of high-density batts performance is much-improved- R38C high-density cathedral-ceiling batts still perform nearly to spec with a 25F attic, but again, any gaps or compressions really undercut performance.

    The better and more reliable solution: Blown/sprayed high-density fiber. With blown fiber it automatically fills in all gaps and voids, eliminating thermal-bypass convection completely, and installs nearly perfectly without obsessive trimming and tucking/sealing. At at sufficiently high density the loss of performance at the temperature extreme is very small indeed.

    The value-leader here tends to be cellulose, but new-school fiberglass blowing wools like Spider and Optima can perform as well or better, but only if applied at a density of 1.8lbs per cubic foot or better. At lower density they're not sufficiently air-retardent to maintain performance at Cleveland's winter temps. By nature cellulose is denser and more air-retardent than fiberglass, even low-density (1.6-lbs open blow in an attic) cellulose is highly air-retardent, but can be made better/tighter if "dense packed" at 3.2-3.5lbs/cubic foot in wall cavities. In a rebuild it can be wet-sprayed at low density (with just enough water to activate a water-based adhesive) into open wall cavites, and trimmed flush with the studs and left to dry. In 2x4 construction you can close in with only 2-3 days of drying time, but with 2x6 you'd need a week, maybe even a space heater or dehumidifier to help it along. It will continue to dry through gypsum pretty rapidly as long as you don't paint it or put up a vapor retarder (avoid poly sheeting like the plague- it is NOT your friend in that climate, especially if you wet-spray.)

    Cellulose (and Spider, and Optima) can also be dry-blown behind mesh stapled to the studs, but if you go that route specify "borate only, sulfate free" cellulose, and specify that the insulation has to be rolled flat to the studs after blowing. With dry blown mesh solutions you can and should dense-pack, which increases it's air retardency, but also eliminates settling & compression within the cavity over the decades. (Settling is a function of density, and the degree of normal seasonal humidity cycling, but at 3.2lbs+ it should be good forever in Cleveland. In colder climates like Fargo or Duluth it's better to hold the line at 3.5lbs density.)

    If the house is getting new siding, it's worth putting at least an inch of rigid foam on the outside (I thought R13 + R5 foam was the 2012 code-min for northern OH!?!), in which case you don't have to put any sort of vapor retarder on the interior to protect the sheathing from accumulating moisture in winter- standard latex paint would be good enough, and BETTER than using a poly vapor retarder, since it allows the assembly to dry readily toward the interior. It also raises the "whole wall" R value (the average R value with all of the thermal bridging of the framing factored in) by about 50%, since it covers over the thermally bridging studs/plates/joists.

    At a fractional area of 20-25% for all framing (typical), an R13 2x4 studwall will average only R10 due to only have ~R3.5 wood over that framing fraction, but putting R5 brings it up to about R15-whole-wall, for a 33% reduction in U-value (heat loss/gain per square foot) compared to the wall without the foam. If you use foil-faced iso (nominally R6 at 1") and design-in at least a 3/4" gap (aka "rainscreen") between the exterior foil and the siding using furring through-screwed to the studs 24" o.c. it'll perform close to R17 (whole-wall) averaged over the season due to the radiant-barrier effect of the exterior foil facing the cavity. This would be more than double the whole-assembly "effective R" of a center-stud bubble pack solution, and nearly double a cellulose or high-density batt only solution.

    And that's what I'd do. YMMV.

  3. #3
    DIY Junior Member michielsemma's Avatar
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    Would love to put it on the East side but foundation is already in place complete with plumbing. Can't come in from the West wall because am limited on how far East I can go, and I'm unwilling to go narrower than 15' which is all the space I have. The building is a working shop with a center drive through, hence the East wall limitation.

    The apartment was planned from the time we built the shop 13 years ago...just now being forced to build it. I am a tax accountant and my office lease just tripled so need to use the space for a while as an office. We know having an independent apartment (i.e. income producing property) on our place will make it more marketable when we decide to sell so that's why the ultimate plan is an apartment.

    I didn't know what SIPS panels were so just googled them and looks like a good idea. If I can just get some pricing I can compare. It's a pain to be on such a shoestring budget but finances are tight and this wasn't an anticipated expense. We're doing the lion's share of the work ourselves so we'll see how it turns out...lol.

    I appreciate your input, it's given me another direction to look for energy efficiency.

  4. #4
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    Quote Originally Posted by Dana View Post
    At a fractional area of 20-25% for all framing (typical), an R13 2x4 studwall will average only R10 due to only have ~R3.5 wood over that framing fraction, but putting R5 brings it up to about R15-whole-wall, for a 33% reduction in U-value (heat loss/gain per square foot) compared to the wall without the foam. If you use foil-faced iso (nominally R6 at 1") and design-in at least a 3/4" gap (aka "rainscreen") between the exterior foil and the siding using furring through-screwed to the studs 24" o.c. it'll perform close to R17 (whole-wall) averaged over the season due to the radiant-barrier effect of the exterior foil facing the cavity. This would be more than double the whole-assembly "effective R" of a center-stud bubble pack solution, and nearly double a cellulose or high-density batt only solution.
    Sorry to dig up this old thread, but is this R17 estimate with insulation between studs?

  5. #5
    In the trades Dana's Avatar
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    Yes, you need a full cavity fill of fiber insulation between the studs to hit ~R17 whole-wall with an inch of exterior polyiso sheathing on a 2x4 wall. With an empty cavity you'd be at about R8 with only the inch of iso on the exterior.

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    Thanks, that's what I thought, but I was hoping!

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