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At what point is extra additional attic intake air detrimental? Please judge my attic venting strategy

rocket190 | Posted in Energy Efficiency and Durability on

Here’s the scenario:

48′ x 104′ building in climate zone 5. 48′ x 64′ of that area will be unheated garage space. I’m still debating the merits of insulating this area. The other park of the building, 40′ x 48′ will be heated only (no a/c). The area above that is a room in attic area that will be conditioned year round. The room in attic area is 20′ x 40′, with two 10’x11′ spur areas that were allowed by implementing a gable dormer on the main dormer roof line.

My proposed attic ventilation is as follows:

Main roof line consists of continuous Rollex vented soffit, with a free area of 9.72 sq inches per linear foot, which will provide 2021 square inches of net free area (9.72 x 104′ x 2 sides). The ridge vent provided by the builder and preferred by the roofer is Lo-Omni, with specified net free area of 11 square inches per foot, which will provide 1,872 square inches of net free area. I have two questions. Is this enough total exhaust ventliation? If so, am I better off with a more balance overall ventilation, or is the extra intake okay?

My second concern is venting the gable “eyebrows”. At the gable I have three ganged trusses on each side of this opening to give me the ten foot opening. The gable eye brow was then stick framed from there, and has rafters for the roof system. The building was sheeted up to the 10′ opening, and the widest part of the gable is 24′, which means the rafter tails extend beyond the knee walls onto the previously sheathed roof. The net result is that I have a 10’x11′ conventional attic above this room that will be insulated with blown cellulose, and I have the triangular attic areas created by the extended rafter tails. I apologize if there is a proper name for this type of construction, but I don’t know what it is. My ventilation strategy for this area was to install continuous ridge vent in conjunction with gable end vents. With two gable end vents I can easily match of exceed the capacity of the ridge vent. I will mount the gable end vents as low as possible to maximize air flow, but I would like to keep them a minimum of 20″ above the roof deck to keep snow from coming in. The roof to wall junction is also wrapped with ice&water to the bottom of the window rough opening.

Comments or concerns?

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  1. rocket190 | | #1

    On the attached diagram, the red lines are an approximation of framing members. The interior knee walls were insulated with 3" of xps while the exterior gable wall will be insulated with 2" of interior eps. My plans are to install continous 2" eps on the ceiling, strapping, and the balance of attic insulation to be blown cellulose to appox r-60 level.

  2. rocket190 | | #2

    Also, baffles will be installed at all top plate/rafter junctions.

  3. GBA Editor
    Martin Holladay | | #3

    First of all, when you have kneewalls, it almost always makes more sense to have the insulation layer follow the sloping roofline, and to leave the kneewalls uninsulated. For an explanation of why this is so, see Two Ways to Insulate Attic Kneewalls.

    Keep these principles in mind: the most important thing you need to do is to establish a consistent air barrier without any interruptions, and to pay attention to airtighness during construction.

    The second principle is to install a thick layer of insulation in contact with your continuous, uninterrupted air barrier.

    If these principles are followed, attic venting details aren't very important at all. Too much venting or two little venting hardly matters, as long as you follow the two principles I listed.

    For more information, see All About Attic Venting.

  4. rocket190 | | #4


    As usual, thanks for the advice. In my case, the knee walls are full height 8'9" and will be fairly easy to install a continuous air barrier on the backside. I just can't swing the costs at this time to do an insulated roof plane. The roof plane is so large 7200 square feet compared to the bonus room area (2200 sf) that I can't justify spending $35,000 up front for an insulated roof plane. Also, I have to admit that installing thick rigid insulation or spray foam from underneath both seem risky from an installation standpoint. If errors were made, it could take years to materialize and entail rotten roof sheathing and or damage trusses.

    Since I can't do an insulated sloping roof plane, I will try to do the next best thing and do a rock solid air barrier and high r attic insulation. Even at the eaves, my trusses allow for 15" of insulation. As I move from the eaves, my trusses' dead load allows for approximately 20" of cellulose. My plan for the air barrier at the knee wall is two 2" layers of type one EPS. The EPS will be spray foamed right to the insulation baffles. I will offset the seams and tape one of the layers. Inside the stud walls will be 2.5lb density spider fiberglass.

    I would like your opinion on how to treat the intersection of the bottom of the knee wall. I intend to insulate the entire bottom chord for the width of the building. The building width is 48' and the bonus room is 20' wide, and the bottom chord is 2x12" syp. Do you still recommend blocking directly below the knee wall, which would effectively split the bottom chord into three areas (20', 14', 14')? The middle area (sealed on all sides) could be dense packed, with the two outside areas loose filled and mounded up against the knee walls. If I didn't add the blocking, I could essentially do the same thing, but I wouldn't have the blocking to help dense pack the middle area.

    In addition to the fluffy insulation, I intend to install 2" of type 1 eps on the underside of the bottom chord to prevent thermal bridging and add additional r-value to the assembly.

  5. GBA Editor
    Martin Holladay | | #5

    I'm not sure I understand your question, but here's my best stab at an answer. Whenever you decide to insulate a kneewall, you are making a jog in your air barrier. That means that you need solid blocking between each floor joist under the kneewall's bottom plate, and you also need solid blocking between each rafter above the kneewall's top plate.

    Installing these pieces of blocking is fussy work. If you go this route, pay close attention to air sealing on every single piece of blocking.

  6. rocket190 | | #6

    It's always difficult to describe these scenarios, but maybe I can find a picture that would help.

    Essentially, I'm going to end up with a 48' wide building that has bonus room trusses allowing a 20' wide room. The bottom of the trusses will be covered with a ribbed steel "liner panel" for the full 48', which based on my research is a pretty decent bottom side air barrier. The top of the bottom chord will be sheathed with 3/4" tongue in groove OSB, which will be the topside air barrier. The floor trusses will be completely filled with cellulose.

    Behind each knee wall I was hoping to install enough loose fill insulation on the attic floor that I wouldn't need to air seal (install blocking underneath knee wall). Note: The attic floor behind the knee wall will not have a topside air barrier. I thought that if I installed sealed foam for the full height of the knee wall, I'd essentially be able to bury the bottom junction in enough insulation that I would be okay. My knee wall is created by the webbing of the trusses, so I don't have a bottom plate unless I cut and install nailers between the trusses where my floor sheathing meets the knee wall.

    If that doesn't seem like an effective strategy, I'll have more work to do installing small pieces of foam between the joists, but it's certainly something I can handle if the ends justify the means.

    Regarding blower door testing, is it best to do after air sealing (but prior to insulation) or is it best to do air both air sealing and insulation have been installed?

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