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Community and Q&A

To vent or not to vent

Bascule | Posted in Energy Efficiency and Durability on

I am remodeling a 120 year old folk Victorian. The walls are insulated with open face fiberglass. The exterior was wrapped in 11/2 “of eps foam. Care was taken to avoid gaps. A 3/4” rainscreen and back primed wood siding follows.
I am currently stumped on my attic. The original ceiling height was under 7′. I raised the rafter ties from the top plate, to get the ceiling bright to 8′. The framing is sound, with adequate hardware used. I sheathed the top of the new rafter ties with osb to get a diaphragm. Insulation will go on top. 
My confusion begins at insulating the 30″ of vaulted ceiling. I was going to fabricate baffles, out of foam, to give a 2″ air gap and then spray closed cell to seal completely and add some r value. The baffles would extend to beyond the fiberglass insulation in the flat attic. My worries are that the newly vented attic will promote uplift on the roof, which is a 12/12.
I live in northeast Ohio in a very windy area. I have seen many days of sustained 50 mph winds with gusts over 70 mph. 
Is this fear of uplift warranted? Is there another more simple way to have a non vented attic? Could I pack the 30″ of vaulted ceiling with spray foam up to the cellulose and then just use gable vents?

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  1. ThirtyWest | | #1

    I'm curious to see what others have to say on that. If you did pack the rafter bays with cellulose you could continue that into the small attic above and have a nice continuous layer the whole way around. moisture could evaporate into the attic and dissipate. out the vents. I'm no expert but I'm gonna check back and see if there are any updates on this. good luck

  2. maine_tyler | | #2

    Well Joe from BSC seems to think there is some concern:

    "In high wind regions – particularly in coastal areas, wind driven rain is a problem with vented roof assemblies. Additionally, during high wind events, vented soffit collapse leads to building pressurization and window blowout and roof loss due to increased uplift. Unvented roofs – principally due to the robustness of their soffit construction - outperform vented roofs during hurricanes – they are safer."

    Which is interesting to me; I'm not sure I understand the science. I'm not a fluid mechanics expert, but I'm not getting how venting increases uplift. The negative pressure above the roof would seemingly be further equalized with venting, not worsened. But he mentions soffit collapse, so perhaps with only a ridge vent, there is some issue. In which case, if you have a vented roof above your attic still, I wonder if you would still have some concern (maybe less so with gable vents).

    What is the science (and subsequent concern) as you understand it Bascule?

    At least one company uses vents (with the venturi effect at play) to actually decrease uplift (reduce pressure beneath roof membrane).

    1. this_page_left_blank | | #3

      I don't pretend to understand their claim, but do note that this vent is specifically for a low slope commercial roof, essentially a flat roof. In the animation it also looks like the air gap vent is between the roof deck and the membrane, rather than under the roof. The physics are going to be very different for a residential gable roof.

      1. maine_tyler | | #4

        Yes, not the same. Just pointing out some principles.

        Do you understand how venting (in a residential scenario) increases uplift?
        I'm genuinely asking, because I can't work it out. Very little comes up googling it (besides Joe L's comment).

        The rain issue—a different but related one—I can see.
        My gut reaction (and what little I understand about fluid dynamics) tells me it will only help to equalize pressures. But I'm not saying that's right, only that I can't work out why the opposite might be true...

  3. Bascule | | #5

    I dont fully understand the science behind it. With eave and ridge venting we are promoting air circulation. Air entering the eaves in windy days would promote uplift...I think. As far as the pressure goes, I'm just now being to understand that.

    An additional concern with a vented eave. Is possibility of embers being sucked in. I heat almost exclusively with wood.

  4. maine_tyler | | #6

    Hopefully someone with more confidence on the uplift issue with chime in. From my understanding (and poking around) the dynamic forces are fairly complex. Some places will experience positive pressures while others experience negative (namely positive on windward side, negative on most of the roof and leeward side). Turbulence and eddies might also play somewhat unpredictable roles. The specifics of the venting will also play a role (the type of vent, the placement of them, etc).

    I could see how the windward soffit vents might increase pressures beneath the roof. On the other hand, the ridge vent, and possibly leeward soffit vent, might reduce pressure according to Bernoulli's principle—static air beneath the roof equals higher outward pressure (uplift) than moving air. I don't know how the net forces all work out.

    My hunch is that venting itself won't make or break your uplift issue. There's probably bigger fish to fry on that front, such as if you have large and poorly supported overhangs or inadequate attachment of roofing materials to the rest of the structure (use hurricane clips).

    What does seem to be a concern (and is most mentioned in literature regarding venting and hurricanes) is wind driven rain.

    If you do head the vent-less route, this article should help steer you in the right direction:

  5. the74impala | | #7

    No science, just anecdotal information. For the past year I have been gutting and rebuilding my house. A big part of that has been baffling the roof. I think most of the time people think of convection as the mechanism involved in attic airflow (at least I did). However, after going through some less than mild storms with my head at the level of the plywood baffles i have made, it is incredible how much air flows through them when a heavy wind hits that side of the house. I have wondered about uplift more than once as i hear the rafters creak ever so slightly. It is almost like the roof is a wing and the air underneath is creating lift as it is pressurized against the outside wall and forced up the soffit vents at speeds likely higher than the wind blowing over the top roof surface.

    Please continue with real science....

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