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Venting a hip roof

C L | Posted in Energy Efficiency and Durability on

What is the recommended solution on a hip roof, with an existing ridge vent that is not long enough to provide sufficient exhaust?  The recommended roof venting is 1 sf per 300 sf of attic floor space, split evenly between ridge and soffit.  There is plenty of soffit intake, but due to the shortened length of the ridge resulting from the hip roof design, the ridge vent provides only 65% of the recommended exhaust venting.  The roofers I have consulted seem all over the place on this and don’t seem to have sufficient knowledge of building science.  
In researching this, it seems that simply adding box vents could cause short circuiting between the box vents and the ridge vent.
http://www.roofingmagazine.com/fact-or-fiction-mixing-exhaust-vent-types-is-problematic/

– What is the downside risk of leaving the existing setup with insufficient ridge venting?
– Are there solutions other than removing the ridge vent and replacing it with sufficient quantity of box vents?
– Is there a ridge vent that offers up to 65% more venting than the standard 18 square inches/foot?
– Do specialized hip vents (which vent the top 3′ or so of the hip slope) also cause the short circuiting issue?
– There is a second floor over part of the structure, so some portions of the roof end at the second floor walls rather than a ridge.  Would installing box vents near the top of the slopes with walls prevent the short circuiting, since those areas are away from the ridge?

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Replies

  1. Jon R | | #1

    That article contains this quote, which is so wrong that I'd ignore everything in the article.

    "...air will always follow the path of least resistance. Air will always look for the easiest, closest path to take."

    Nope, air follows all available paths according to the resistance and pressure differentials. Closest is irrelevant.

    If you want to worry about short-circuiting (I wouldn't), consider that ridge vents can short-circuit themselves.

  2. Jason S. | | #2

    The downsides of underventilation depend entirely on the climate, the airtightness of the ceiling air barrier and whether we're taking about an unconditioned truss-framed attic or insulated rafters over a conditioned attic. With the former, any functional ridge ventilation would make "ridge rot" unlikely even if undersized, but ice dams are definitely possible.

    I'm unaware of any codes that prohibit using both box vents and ridge vents, as that article claims.

    You could frame up however large of ridge vent you like furred out on the trusses and sheath/shingle over it. IRC requires means of mitigating weather and pest intrusion.

    Having more soffit than ridge vent area is good. The alternative is a net stack effect pressure imbalance that can pull conditioned air out of holes in the ceiling.

    1. Jon R | | #3

      > pressure imbalance that can pull conditioned air out of holes in the ceiling.

      Or, in the Summer, such pressure might stop the reverse stack effect flow that would normally pull hot attic air into the house. Would be nice to see the pressure data behind all these attic venting claims. Say a years worth of both sides of intake vents, exhaust vents and attic floor.

      1. Jason S. | | #8

        My cold climate bias is showing :)

        On this I think we have consensus: airtight ceiling and good insulation is the best defense.

        https://www.greenbuildingadvisor.com/article/lstibureks-rules-for-venting-roofs

  3. Expert Member
    Malcolm Taylor | | #4

    CL,

    The rule of thumb currently suggested for the proportion of venting at the soffit to that at the ridge is 60% to 40%. You are fine.

    1. C L | | #5

      Thanks all.
      The current proportion of total area of venting is 78% soffit and 22% ridge.

      Using the 1 sf of venting per 300 sf of floor area rule, the roof needs a total of 3.08 sf of venting or 443 si total min. If the ridge needs 40% of the min total, it would need 177.2 si (1.23 sf)

      Currently the roof has 3.5 sf (505 si) of venting in the soffit (more than the total required of both soffit and ridge) and 1 sf (144 si) at the ridge vent.

      Should things remain as is, or the ridge augmented with a turtle/box vent, or with hip vents, or should the ridge vent be removed in favor of a field constructed ridge vent with sufficient venting or in favor of turtle vents?

  4. Jon R | | #6

    I'd add 1.66 sf of additional up high vents - which meets both the code 150:1 option and provides an allegedly good high/low ratio.

    Always air seal well to keep house moisture in the house.

  5. Expert Member
    Akos | | #7

    Before going nuts on venting, is there an actual problem you are trying to solve? If the roof is fine and no sign of moisture issues, leave everything alone. You have plenty of venting there already.

    Generally for roofs to work well you need some venting, but what that exact amount of "some venting" is very dependent on climate and how well your ceiling is sealed. There are plenty of 100 year old houses around me (zone 5) with minimal/nonexistent roof venting and generally hold up well until somebody shotguns the ceiling with pot lights.

    If you want to improve things, air seal your ceiling. This will not only reduce the risk of roof issues but will reduce your energy usage and improve comfort.

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