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Attic venting question – No ridge vent

Richard_Perrin | Posted in General Questions on

Hello, I’m working on improving my attic space as I have condensation issues going on. I am working on air sealing my ceiling. I know thats the main win. In parallel I was going to work on improving venting of my roof seathing. My roof actually does NOT have a ridge vent, but rather soffit vents and then 1 motor attic vent fan.

Could it make sense to do site built baffles (rigid foam) all the way up my rafter bay from the soffit to the roof line so the rafter cavity is enclosed (with 2 in gap for air flow of course) and then run duct from each rafter bay to the attic vent fan to keep air flowing, could that work ?

Again my tricky situation is I don’t have a ridge vent to get the continuous air flow, but rather 1 attic fan near the middle of my attic space. So I’m thinking I can enclose each rafter bay fully, then duct each encapsulated bay to the vent attic fan. The fan would “pull the air” from soffit up through rafter bay and then out via the attic fan.  This ensures the air flow / movement from soffit up against the sheathing and then out the attic.

I’ve attached a crude drawing. Hope it makes sense. (Again, this is in parallel to air sealing the ceiling).

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  1. Expert Member

    User ...111,

    If you have an attic, you want the ventilation to remove moist air from the whole area, not just flow from the soffit to ridge. My suggestion is to air-seal the ceiling as you are doing, remove the motorized fan, which is probably making things worse by de-pressurizing the attic and drawing air from the house, and install several roof vents near the ridge. ideally you want to add about 1/3rd of the vent area you have at the soffits in these new higher vents.

    Roof vent:

  2. Richard_Perrin | | #2

    Thanks malcolm. If the ceiling seal is good, shouldnt the moisture problem be minimized ? Then the attic fan wouldn't be pulling air from the whole attic causing depressure, but rather it'd be just pulling from the bays. The de-pressure would be sucking outside air via the soffits.

    Just thinking out loud. Appreciate your thoughts / pointers.

  3. GBA Editor
    Martin Holladay | | #3

    First of all, can you tell us your name? (I'm Martin.)

    Two points:

    1. You don't want to have a powered attic ventilator -- this type of fan is an energy hog and is potentially dangerous. Read more here: "Fans in the Attic: Do They Help or Do They Hurt?"

    2. Installing a ridge vent is easy. If you don't want to do it yourself, hire a roofer to install one.

  4. Richard_Perrin | | #4

    Hi Martin, I'm Richard Perrin. (Just signed up here and didn't realize it creates a funky username).

    Was trying to find / see if there is a "creative" way to make my situation better with what I have my given constraints (no ridge vent and vent fan) versus an expensive roofer project.

    Appreciate the follow up and thoughts.

  5. Peter Yost | | #5

    Hi Richard -

    There are three ways to get air to move:

    1. stack effect
    2. wind
    3. mechanical fans

    One of the problems with "clugeing" mechanical approaches to ventilation is that the other two are "cheaper" and may even "compete" with what your fan(s) is/are trying to achieve.

    I am not sure we completely understand how high/low or soffit-to-ridge works with each the two forces (wind and stack effect) and I am actually working on some "wingnut" testing of roof ventilation for three educational sessions I am doing at the Better Buildings Better Business (B4) conference in The Dells, Wisconsin next week. Will do a GBA blog on this work in the next month or two--stay tuned.


    1. Jon_R | | #7

      On the other hand, of the three, fans work when there is no wind or stack effect and can be controlled intelligently based on humidity, temperature or pressure (to control moisture or ice dams).

      Be aware that almost all of what has been written about attic fans is referring to unbalanced (and usually over-sized) exhaust fans - while these are common, there are other options. For example, in Richards's case, one intake gable fan and one exhaust gable fan would remove moisture while maintaining the optimal (in Winter) slightly positive attic pressure. Fans use some electricity, but the cost may be insignificant compared to the cost of other solutions.

      1. Richard_Perrin | | #14

        Thanks Jon.

    2. Richard_Perrin | | #9

      Thanks Peter, will look out for the blog.

  6. tommay | | #6

    You're really turning this into a fluid dynamics problem. Is the attic used for anything or will it ever be finished off? Adding foam baffles, creating multiple ducts, brings more friction and restriction into the equation and each bay may not vent evenly depending on fan location.Use these of course, and install correctly, if you plan on finishing off the area. Are there any windows in the attic space? Do you have gable ends where you can install vents?
    Replacing the centrally located fan with a passive vent as Malcolm mentioned, and a couple of properly sized gable vents may balance out the exhaust, for even intake along the soffit. Of course, if you can add a window you can always stick a fan in that if needed at times, and free light is always a good thing.

    1. Richard_Perrin | | #10

      Hi Tom, don't plan on turning it into finished space. Just solving the attic condensation / frost going on is main goal. Nope, no windows in the space or gable vents, just a attic fan motorized in center of attic.

      Thanks for the thoughts here.

  7. seabornman | | #8

    I have air-sealed and insulated two houses that did not fit the standard soffit/ridge venting system. I found that very good air sealing, a good insulation job, keeping insulation away from the roof deck and some amount of ventilation provided a house free of icicles and ice dams. Both houses had gable vents only and they were not anywhere close to 1/300 area of attic. These were both my own houses so I was able to get into attic space occasionally to verify they were free of problems.
    My current house project does not have a ridge vent. It's a metal roof and I don't like the details I've seen for a ridge vent in metal. I have code minimum ventilation at eaves and gable and it's performing well this winter (zone 5). I paid a lot of attention to air sealing.
    So I think you can accomplish what you want without elaborate ventilation schemes.

  8. Richard_Perrin | | #11

    Thanks Joel, appreciate it.

    Just out of curiosity, how did you get air flow ( ventilation ) from your soffits to your gable vents? Curious as you mentioned you indeed kept insulation off the roof decking so there must be some channel or cavity from soffits to gable vents. And then you insulated around that channel ?

    Thanks again Joel.

    1. GBA Editor
      Martin Holladay | | #12

      For attic ventilation, the main driving force in any case is almost always wind.

      The second most important driving force is the stack effect -- but that's way down the ladder of importance compared to wind.

      Wind pressurizes one side of a house and depressurizes the other side. As a result of wind, air moves through attic vents -- sometimes as expected, and sometimes in surprising ways.

      1. Richard_Perrin | | #16

        Thanks Martin. Got it, makes sense.

    2. tommay | | #17

      The channels or cavities are the rafter bays and the attic itself. No baffles are needed. Insulation should stop at the bottom of the rafters leaving the rafter bay open. You should be able to see light through the soffit vent holes while looking down the rafter bay. Air comes in there and goes out somewhere else.

  9. seabornman | | #13

    Richard, I installed field-fabricated baffles at soffit to provide full 2" air space in the house with trusses and thin plywood on the rafters on the other house (rafters were only 4"). Gable vents were at both ends which I'm sure encourages some airflow. Current house has one large gable vent at one end. It's facing into the wind so I've had to create a large box baffle to intercept blowing snow.

    1. Richard_Perrin | | #15

      Thanks Joel. So you didn't insulate all the way up the rafter cavity to the roof line, but maybe half way up your baffle or so if i'm following.

  10. seabornman | | #18

    I installed the baffles so they were comfortably above the insulation.

    1. Richard_Perrin | | #19

      Thanks Joel. Best,

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