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Community and Q&A

Strange venting and ice dams

user-1135248 | Posted in General Questions on

I visited an acquaintance’s house last night for another “informal
energy audit”, as he was complaining of ice dams bad enough to
bring a little bit of water drip into the top of a couple of window
frames. Boston area, zone 5, and our thermal rollercoaster of
the winter we’ve been having. The house is typical early-nineties
vintage split-level, simple 6-in-12 or thereabouts roofline with
maybe 5 – 6 inches of blown-in fiberglass on the attic ceiling.
Ridge vent most of the way along. The rafter bays have those token
styrofoam vent channel pieces sitting at the eaves but not nearly
enough insulation piled up against them to deal adequately with the
top plate area — in fact the insulation basically ends at the top
plate because it would otherwise fall overboard into the soffit area.
It looks like there are some small chunks of pink batt stuffed in at
the junctions, but a quick crawl of the “roofing nail scalp massage”
sort couldn’t reveal all of what had been intended there. Many
of the vent channel pieces aren’t up against the deck, but lying
at random angles on top of the end of the insulation — totally
typical; I don’t think I’ve ever seen a *good* installation of
those things and in several attics I’ve seen them having been blown
away from the roof/wall junction completely.

The oddest thing is that his soffit vents aren’t in the soffit
underneath, they’re in the fascia board. Above the gutters. The
soffit pieces under the foot-wide or so overhangs are completely
closed off. No particular evidence of water problems visible along
them, but when one considers how ice dams pile up in the gutters and
start backing up the roofline this “side vent” topology seems like a
profoundly bad idea.

So the question is twofold: first, how to improve the insulation
over the top plates without rebuilding the entire roof — I’ve
done a bit of searching and gone back over the advice and drawings in
Joe’s attic air sealing guide and the piece on ice dams, and have a
pretty good idea on how one could firm up the venting and then wedge
more insulation into the crevice below. I realize that approach has
its limits, so what would best-practice be here? Is it advisable to,
say, hang the end of a batt over the top plate a few inches into the
soffit space as long as sufficient vent slot is left open above it?
Maybe even tack it down so it doesn’t get blown back up by wind [which
would be hard to do from inside, but keep reading]?

The second question is what to do about the side venting. Is this a
common approach? I haven’t looked in detail at enough roofs to get
a feel for that. Wouldn’t it bring much more risk of water ingress,
especially with wind-blown rain? There’s no way for the soffit box to
drain, which is what we’re thinking might have helped conduct snow-melt
water in toward the wall, but it’s hard to tell if his drip [gone now]
was from that or higher up in the more typical ice-dam scenario.

The owner is willing to throw some money at this problem, so it doesn’t
seem beyond reason to pull off all the old gutters, soffit and fascia,
install solid fascia and F/J channel for a run of real perforated
vinyl soffit stuff underneath along front and back. Which might also
give better access while opened up to the top plate area, for a better
and more consistent job on the vents and insulation. This of course
requires finding a clueful contractor who understands what’s needed.



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  1. GBA Editor
    Martin Holladay | | #1

    GBA has a really good video showing what to do (with Mike Guertin). Here is the link: How to Ventilate Rafter Bays When Adding Insulation.

    There is a limit to how much R-value you can get between the top plate of the wall and the underside of the ventilation baffle -- which is why two-component spray foam is a good insulation for this location if the homeowner can afford it.

    The old-fashioned way to install ventilation in a solid soffit is to buy those round aluminum soffit vents -- I think they are about 2 inches in diameter. You just have to drill a lot of holes in the soffit with a big bit or a hole saw, and then the round aluminum vents are friction-fit in the holes. These vents don't look great, but they work.

    Remember, ventilation is overrated. See All About Attic Venting.


  2. GBA Editor
    Martin Holladay | | #2

    Lots of manufacturers sell the round aluminum soffit vents. Here is a link to some made by Owens Corning: Round mini soffit vents.


  3. user-1135248 | | #3

    I can't see that video, evidently due to membership type.

    I realize the under-soffit-vent solution possibilities are numerous;
    is this all implying that the owner *should* close up the fascia-side
    vents above the gutter and get rid of that obvious water-ingress risk?
    Why would it have been originally done that way, do ya think?


  4. GBA Editor
    Martin Holladay | | #4

    Evidently, the reason that you can't see the video is that you are not yet a member. You can sign up for a free 10-day trial GBA membership at no cost. If you want to help support GBA -- in other words, to pay for all the advice that I give online, and to help pay the bills to keep the GBA website alive -- you can sign up for just $14.95 a month. Here is the link: Sign up for GBA Pro.

    To get an idea of the video that you have not yet signed up to see, I'm including a screen shot below.


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