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Theory on solar chimney / spire / cupola vented roof

Burninate | Posted in General Questions on

The safest bet for roofing in a humid snowy climate is to run a steep 12:12 (or above) roof with soffit vents and a ridge vent, to ensure that the stack effect is strong enough to maintain the underside of the roof sheathing/cladding at a low temperature difference to the ambient outdoor temperature.  A vented roof works in nearly every climate, about as effectively for cold temperatures as for warm temperatures.

Steep roofs aren’t trivial to build at scale.  You get a lot of what people perceive as wasted space (and then they immediately want to destroy your building envelope & moisture management strategy by building in the attic, adding dormers and stairs),  the trusses develop transportation problems, and regulations often base home size restrictions on average roof height.

As you get progressively shallower roofs, which tend to be cheaper and easier to build from a zoning, code, covenant, and restriction perspective, the ridge & soffit venting loses effectiveness, because the stack effect that drives it ceases to be very strong.  The force of the stack effect that a vented roof captures to move air should be proportional to the height that the heated air traverses, just as the force on a hydroelectric dam’s turbine is proportional to the height that the water drops.

Could you supplement the stack effect by using soffit vents and inexpensive, tall center-roof vents?  A 2:12 roof with a half dozen 12″ wide, 12ft tall smokestacks coming out of the ridge rather than a ridge vent?  Or with chimney-lookalike shafts that don’t extend down into the attic?

Similarly: In a shallow-pitch hip roof, where ridge venting is difficult, could you vent with a tall architectural parapet for added stack effect?

https://en.wikipedia.org/wiki/Solar_chimney

(This principle weaponized to provide electricity)  https://en.wikipedia.org/wiki/Solar_updraft_tower

If it does work in theory: Why isn’t it popular in practice?  Even very modest height changes (eg making a 3:12 roof behave like a 5:12 roof) can have a significant impact on ice damming & all-weather leak issues, and this strategy is inexpensive to retrofit relative to re-doing a roof.

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Replies

  1. Expert Member
    Peter Yost | | #1

    I have been doing some cathedral roof vent testing (see GBA Wingnut Ridge Vent testing blogs) and agree pitch can make a big difference in stack effect-driven ventilation effectiveness. I bet there is a relationship between roof pitch and "needed" solar chimney height for stack effect-driven roof ventilation but you might have to do your own wingnut testing on this...

    Also note, though, that the other real driver of roof ventilation is wind and in fact, it is probably a much stronger driver than stack effect, and probably also much less dependent on roof pitch. I am testing the impact of stack effect versus wind, but have not done enough yet to say more.

    I note that you are looking at soffit-to-ridge attic ventilation, which is quite different from cathedral roof assembly venting.

    Peter

  2. Jon R | | #2

    > The force of the stack effect that a vented roof captures to move air should be proportional to the height

    And roughly proportional to the delta-T between the attic and outside. This isn't fixed so you get diminishing returns as you increase height. Compare costs (and aesthetics) of your plan to simply adding more vent area. Also consider something (no idea what) to maximize the effect of the more significant wind (very light wind is when this would help).

    I completely agree that one should step back and consider the whole issue, including costs and non-conventional solutions. In other areas of building design, passive techniques have been mostly eliminated (too unreliable, expensive to build, etc). New ECM attic fans are super efficient. About $10/year to get about the same airflow increase (edit: ~1000 CFM, limited hours) as going from 3:12 to 12:12 or adding stacks. When they aren't installed backwards (ie, as exhaust fans), they have a bonus effect of decreasing air leakage from the interior. Compare to greater height (12:12 or tall vents) which increases leakage from the interior.

    Consider avoiding top chord overhangs (one can still have eaves) and "eave coolers" (aka soffit vents) - cooler eaves encourages ice dam formation and while common, soffit vents are not needed (there are lower vent alternatives).

    Also consider the danger of falling ice/snow (a serious issue) with a high slope roof - 12:12 isn't the safe bet.

  3. Burninate | | #3

    Jon R,

    > Also consider something (no idea what) to maximize the effect of the more significant wind.

    Turbine vents would do this. I just don't understand why you need it, in any weather conditions in which you have significant wind (which will tend to cool hot shingles in summer), and it seems like you wouldn't *desire* more airflow in conditions with extreme wind, where there may be wind-driven rain present.

    I think active attic fans are not a bad idea, but I don't know what kind of cfm airflow we're talking about here, I'm not sure reliability is what you would want it to be, and ~10 watts probably represents an insufficiently powerful fan.

    > Consider avoiding top chord overhangs and "eave coolers" (aka soffit vents) - cooler eaves encourages ice dam formation and while common, soffit vents are not needed.

    This part flies in the face of everything I read. Without overhangs the walls are vulnerable to moisture. You put generous overhangs on in order to permit the walls to survive while well-insulated and well-airsealed.

    Soffit vents don't cool the eaves, they cool the whole roof and the attic. They dry water that does get into the roof sheathing, and they keep the roof cool enough that ice dams do not rapidly build up. At night in humid places, they warm the underside of the roof up enough that the sheathing doesn't get lots of condensation. Without soffit vents, there's not a lot of point to ridge venting or an exhaust fan (unless you're building your house in a very mild climate where outdoor air is comfortable enough that you don't seal the house up well from the attic).

    Maybe I'm not understanding what you mean by 'warm eave truss'?

  4. Expert Member
    Dana Dorsett | | #4

    >"I think active attic fans are not a bad idea..."

    Active attic-exhaust fans and turbine vents usually ARE a BAD idea, since depressurizing the attic that amplifies air leak defects between the conditioned space and attic, wasting energy, and in colder climates drawing in more moisture from conditioned space air than it purges.

    Attic ventilation is more about purging moisture in the attic than cooling the attic or roof deck. On steep pitched roofs the convective cooling on the open exterior has a much higher effect on attic temps than active or convective ventilation of the attic. On low pitch roofs the SRI (solar reflective index) of the roofing material is more important than ventilation, but potentially risky for the roof deck in cold/very cold climates.

    >"> Consider avoiding top chord overhangs and "eave coolers" (aka soffit vents) - cooler eaves encourages ice dam formation and while common, soffit vents are not needed.

    This part flies in the face of everything I read. Without overhangs the walls are vulnerable to moisture. You put generous overhangs on in order to permit the walls to survive while well-insulated and well-airsealed"
    ------------
    Keep reading.

    Deep overhangs need to be insulated in high snow load areas to avoid ice dam formation due to melt-out from the rising column of warm air at sun exposed walls creating a warm stripe of roof just exterior to the wall, that re-freezes further out toward the edge of the roof. I've personally seen this happen locally with overhangs as shallow as 2 feet, though it's more common to see it with 3-4' overhangs. This movie is shows near me often enough- we get more than 10' of annual snowfall some years, though the long term average is more like 4-5'. (And it's mostly 10%+ H20 maritime sludge, not 3-6% Wasatch Eiderdown.) For long overhangs outside the eave vents it takes ~2" of rigid foam to fully control it near me. YMMV.

  5. Jon R | | #5

    +1 on Dana's last paragraph. See here for "We want to insulate big overhangs on the underside and push the air inlet to the outside edge." Understand what's going on and it's clear that using one of the soffit vent alternatives can work even better at avoiding the "cooler outer edge" issue. Review "heat of solidification" and think about how venting away this heat is counterproductive.

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