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Community and Q&A

Attic baffles–My ProVent modification. Or not . . .

Mark Waldron | Posted in Energy Efficiency and Durability on

As detailed in previous posts, I am now working to improve the efficiency of a 1950 Cape Cod style house. One major “challenge” of this style of house is the heat transfer through the sloped ceiling area upstairs. The summer heat can be especially problematic. My house has rafters that are 6″ deep, which doesn’t leave much room for insulation. I wanted a vented roof assembly, and decided to compromise and use the approx 1.5″ deep ProVent baffles/chutes in this space. Site-built baffles with polyiso trays would have offered better insulation and a potentially deeper vent channel, but I am concerned about flammability issues.

My two initial qualms about ProVent:
Cost: The 14.5″ wide vent costs $2 for 48 inches, so about $5 for each of my rafter bays in the “cathedral slope.’ I know this may not seem like big bucks, but it’s considerably more than the cost of the fiberglass insulation that will fill the rest of the bay. Seems crazy.
Color: Why are these darn things black? In the summer they will absorb IR radiated energy very efficiently from my hot roof deck and transfer it directly to the top of the insulation.

To deal with the second issue, I installed a layer of aluminum foil loosely in the channel, suspended from the sides and drooping into the trough. Reflective on top (but would eventually get dusty there) and low emmissivity on the underside, so low radiated heat transfer to the Provent (and my insulation). Cheap, and easy. I got about 10 of them installed, and was pretty proud of my modification. Then a slight breeze outside set the foil to rattling and rusting. I don’t know if it would have been audible inside after the insulation and drywall was in, or if it could be heard outside. Anyway, I couldn’t take the chance–there’d be no going back once the FG, polyiso under the rafters, and drywall is up. So I ripped out my great invention.

I still wonder why ProVent isn’t shiny, or at least white.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mark,
    The difference in performance between black baffles and shiny baffles might be R-1 or R-2 -- in other words, not much. And the difference in performance disappears when the baffles get dusty.

    There are lots of things in this world to worry about. In my mind, the color of your ventilation baffles isn't one of them. You need to learn to relax. Maybe take a yoga class, or take up meditation.

  2. Mark Waldron | | #2

    I'll freely admit that black vs white vents may not result in a big difference in energy use. But I've got just 6 total inches between my optimallly-inclines-solar-collector south roof deck and the inhabitd space, so it alll counts. (and, even if it's just R1 or R2, that's as much as an inch of polyiso changes when it ages, and people have written entire articles on that!) But for a product specifically designed and sold to do a niche thing, it would seem logical that it would come in a color that does that well.
    Anyway, just having some fun here. No one will live or die based on the color of these baffles, that's true.
    Do I need to relax? Probably. Noted. . . .

  3. John Clark | | #3

    Colors absorb heat from light so the color of the baffles is irrelevant since they're not exposed to sunlight.

  4. GBA Editor
    Martin Holladay | | #4

    John,
    Your statement is incorrect. A low-e surface (for example, aluminum foil) facing an air space raises the R-value of the air space, even in the absence of light.

  5. John Clark | | #5

    @Martin,

    Correct, I misread the OP comments. However unless the surface is also polished I doubt the OP would notice a significant difference between black and some lighter shade.

    Edit: Out of curiosity here's the emissivity of plastic: Black .95 White .85
    For comparison aluminum (shiny side) is less than .04

    http://www.infrared-thermography.com/material-1.htm

    For comparison aluminum (all types) is less than .09

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