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Roof Ventilation

Edward Blackman | Posted in Energy Efficiency and Durability on

I am restoring an 1832 built log cabin. Size is 30×44 feet, 9 ft. ceilings, 2×6 t&g SYP ceiling; thermal barrier on top of t&g, 10/12 pitch roof with a 30′ span. Roof is 2×8 rafters, 16″ O.C., wind rafters 16″ O.C., 5/8 thermal OSB decking, ice and water shield overlayment, Metal roof installed on 2×4 SYP sleepers, 2′ O.C. What type of roof ventilation fan would you reccomend. Am using a Trane VAC system. Help.

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  1. Michael Chandler | | #1

    Where are you located?

    I don't see any reference to insulation in this roof design, assuming that thermal OSB is a reference to foil faced roof sheathing and that wind rafters is a reference to collar ties, and that the "thermal barrier on top of the T&G" is foil faced bubble wrap.

    Lots of questions in my reply but in general we don't recommend any type of roof fan.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Michael. Powered roof ventilation (using a fan) is a mistake.

    Please define the following terms, which are all unclear:
    1. "Thermal barrier"
    2. "Wind rafter"
    3. "Thermal OSB decking"

  3. Edward Blackman | | #3

    We call horizontal 2x8's, nailed to the rafters wind braces; you call them collar ties. Attic will have 18" of blown insulation, 2x6 framed gables will also be insulated. Location is Tracy City, TN, 37387, top of Monteagle Mountian. Thanks. In some areas of Tn., roof fans are a code requirement; Bird's mouth of rafters will sit upon the 2x6 t&g, blocking between each rafter, foam insulated at all four contact points to the rafter. Thank you, Ed

  4. GBA Editor
    Martin Holladay | | #4

    1. My recommendation is the same; whatever you do, don't install a powered attic ventilator or any kind of fan intended to provide roof ventilation. I would be astonished if such fans were required by a local building code. If you know the code reference, please cite it.

    2. Where is the blown insulation? On the attic floor? If so, why are the gable walls insulated? Does that mean that the insulation follows the slope of the rafters?

    3. If the insulation is on the attic floor, do you have any kind of air barrier between the T&G board ceiling and the insulation? I hope so.

  5. edward blackman | | #5

    Blown insulation is on attic floor; Owens-Corning thermal air barrier between floor and insulation; gable walls are insulated because north end of structure is a bedroom with a balcony; south end abuts to a timber frame 16'x24' great room, 10/12 pitch vaulted ceiling; hand hewn rafters with hand hewn "collars", shipp-lapped and pegged. All 6x6 timbers are hand hewn, mortise and tenon joinery, pegged; site made bents, mortise and tennoned, pegged on vertical timbers; FYI; am 69 years young and this is a one man operation; sub-floor is t&g 1 1/8" engineered 4'x8' material , 2x12 SYP floor joists, 16" O.C., bridging continous on both sides of central girder; wrap around porch's using "yellow treated Wood", used 3/16 fender wasrher on 3" deck screws to assemble porch joist system, inside joist is bolted to sub floor joist, ice and water shield on all joists of all decking, ice& water shield at contact joint of joist to sub-floor; two porchs are to be screened in, therefore I used a router to "dado" the edges of the 2x6 decking and made 3/8" x 7/8" splines to fit betrween the decking boards, all of which are countersunk for the deck screws, pegs made of p.t. material and installed using gorilla glue. Vaulted framing, 4x10 rafter and collars , 2x4 framed gables, with 5/8 OSB were assembled on the sub-floor, and lifted into place, one lift, and fit on the first lift. If you want photos, please send me your e-mail address and I will forward them to you. Thank You for the help, Old Man of the Mountains

  6. GBA Editor
    Martin Holladay | | #6

    I'm not sure if you still have any questions. If you do, please tell me what your question is.

    I did a Google search on "Owens-Corning thermal air barrier" and came up blank. I have never heard of that product. Can you describe it?

    It sounds like your attic has an insulated floor. You have a ceiling, then attic floor joists, and evidently an attic floor on top of your joists. If I'm right, then I'm still confused: is the insulation between the joists, or on top of the attic floor?

  7. edward blackman | | #7

    Because this is a log structure, the t&g is the ceiling for the log structure and the floor of the attic space. There are no ceiling /floor joists; just the lonley 2x6 t&g. Ed

  8. Riversong | | #8

    Simple answer is: don't use a fan, as that can create negative pressure and draw warm, moist air into the attic.

    But almost every attic/roof assembly can benefit from passive venting with continuous soffit and ridge vents with wind baffles.

  9. edward blackman | | #9

    Thank you very much. Ed

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