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Thread: Roof Vent Advice Needed

  1. #1

    Default Roof Vent Advice Needed

    I am putting up a new cabin (28x24) with a 5/12 roof (metal). The roof is done with rafters and is vaulted inside (no attic). What is recommended for venting the roof. The rafters are 2x10's and the insulation in the roof will fill the gap. I will put up a vapor barrier on the inside and then plywood on top of that and at some time in the future I will put a paneling over the top of the plywood.

    Thanks for your advice and time

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    Member thewhop2000's Avatar
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    You need airflow so make sure your insulation does not smother that. A Ridge vent will work since you are doing a metal roof anyway and vent your soffet. Just don't forget to screen off your blocking on the soffet to keep the bugs and bats out.
    If a dipnetter dips a fish and there is no one around to see/hear it, Did he really dip?

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    Member dkwarthog's Avatar
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    You have a couple of options, imo. You are planning, based on your description, to build an UNVENTED roof, AKA a HOT ROOF. I wouldnt do that, I think you will have trouble with that eventually ice damming,etc.

    If you cant vent under the roof deck, then you should vent it above the roof deck, IE, build a second sheathed surface on 2x lumber on top of the first layer of sheathing. Either that or provide an air gap above your insulation and vent the eave AND peak.

    I opted for a variation of that on my home and ran 2x4s across my "vaulted" rafters at the 8' height to create a small attic. I then spray foamed my roof so I could get enough insulation in to still allow for an air gap. Air comes in at the eaves goes into the "attic" and out the gable end vents. So I have a cold roof.


    If you live somewhere where it gets cold and snows, you should have a cold roof. Good luck...

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    Quote Originally Posted by gpearston View Post
    I am putting up a new cabin (28x24) with a 5/12 roof (metal). The roof is done with rafters and is vaulted inside (no attic). What is recommended for venting the roof. The rafters are 2x10's and the insulation in the roof will fill the gap. I will put up a vapor barrier on the inside and then plywood on top of that and at some time in the future I will put a paneling over the top of the plywood.

    Thanks for your advice and time
    I did the same thing 20+ years ago. Used 8" fiberglass batts and vented the eaves and ridge. The air gap between the top of the insulation and bottom of the plywood has been adequate and I've never had any trouble keeping it warm. I've had opportunity to peek inside the roof a time or two with additions/projects. It looks like the day I did it.

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    Although this project has several differences from yours, I thought the clear pictures might help you visualize what the other 3 posters have already explained: http://www.homeconstructionimproveme...th-foam-board/

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    Have you built it already? This is the no. 1 reason I always recommend using 2x12's in a cabin roof. You can put 10" of R38 high performance cathedral ceiling fiberglass insulation. It is stock in some SBS stores and Galco. Standard R38 is 12". It's the best case scenario besides going with 14 or 16" BCI's or trusses.

    Be sure to have a substantial ridge beam. In a vaulted ceiling you MUST have a properly sized ridge beam so that the load doesn't flex the ridge and push the exterior walls out. Especially with only a 5/12 (which won't reliably shed snow).
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    Default cold roof vs. hot roof, and mold is BAD

    Quote Originally Posted by dkwarthog View Post
    You have a couple of options, imo. You are planning, based on your description, to build an UNVENTED roof, AKA a HOT ROOF. I wouldnt do that, I think you will have trouble with that eventually ice damming,etc.... So I have a cold roof. ...

    If you live somewhere where it gets cold and snows, you should have a cold roof. Good luck...
    I've only built structures that have a cold roof, so I can't give you good advice about how to do yours - except I can tell you to not screw up on this issue or you will have mold.

    And once you get mold in there there is no process yet invented to remove it other than to remove your entire roof structure (stupidly expensive). The present state of the art for corrective processes after accumulating mold up there is to apply a sealer to the mold, and hope it (all) stays sealed up (but its still there, and its still mold). Yech.

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    Assuming you install a sound vapor barrier any issues related to a hot roof will be outside the building envelope. The issue with hot roofs is internal condensation, compromised insulation, and rot. While it's hard to fault Doug's preferred insulation value we need to consider that most of our cabins are for occasional use. We heat them for a few days and then go away. Also, most of us have heating capacity well above what we need to maintain even heat in a comparable city home that's heated every day of the year. I think the total insulation value is less important and the vapor barrier is every bit as important as in a city home. You'll learn quickly that the insulation is not very important during initial cabin heat-up. You're heating the internal mass within the cabin. That takes time no matter how much insulation you have.

    Heating and insulation strategies are different for occasional use cabins and full time homes. But construction is construction. Vent the roof. Its important.

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    Quote Originally Posted by gpearston View Post
    I am putting up a new cabin (28x24) with a 5/12 roof (metal). The roof is done with rafters and is vaulted inside (no attic). What is recommended for venting the roof. The rafters are 2x10's and the insulation in the roof will fill the gap. I will put up a vapor barrier on the inside and then plywood on top of that and at some time in the future I will put a paneling over the top of the plywood.

    Thanks for your advice and time
    The corrugations in the metal roof will provide for some air flow from the eaves to the ridge, most metal ridge caps provide some air flow, but they do make "vented ridge caps" and they are typically a little bigger than the standard issue ridge cap.

    The eaves need to be open or screened so air can flow from eave to ridge. 2 X 10 is a minimum to get much insulation in the cavity, but I dont know where you are building. Cabins built in the interior have diffrent considerations than something built in SE AK.

    I dont know if you plan to install any plywood sheathing over the 2X10's? Many do not and just use 1 X 4 or 2 X 4 purlins to provide a place to screw the metal roof to. This is a poor option IMO, metal roofing will expand and contract and will work out the screws unless your cabin is in the shade all the time. This can be alleviated somewhat by using big screws and pre-driling the holes through the metal. Better yet is to sheath the roof and put on some ice & water shield or at least some 30# felt paper.

    Any roof penetrations, vents, chimmneys, etc should exit near the peak to prevent damage when the snow slides off, snow-stops are are another option to prevent damage, but be aware they will keep leaves & branches from sliding off also. Hope this helps..........!
    “Nothing worth doing is easy”
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    My cabin has 12/12 and 8/12 pitches. Both hold snow pretty well with typical exposed fastener metal roofing. Especially after a few years of birch weep accumulation.

    Combs Sheet Metal in Anch makes nice crickets in metal roof colors. I have one stack at the bottom of a long slide. One small cricket in the middle and a big one adjacent to the pipe, no problem. In fact the pipe is supported by the big cricket. Works great.

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    Mr. Pid, I totally agree. It's funny, the lengths people go to super insulate a smaller cabin (even a medium cabin) and then find out that they can't get the woodstove turned down enough to keep from cooking themselves out. However, a super vapor barrier is the key to keeping mold from building in the walls and roof. If your vapor barrier is super tight in the ceiling, even without roof ventilation, you shouldn't see a moisture issue. Ventilation above the insulation is not to remove moisture, it's to remove heat so that it doesn't warm the roof surface and cause ice problems.

    I should have qualified my post with the fact that the 2" of airspace above the insulation and under the plywood is extremely important on lower pitched roofs. I found out, through just a tiny research, that R-30 cathedral ceiling insulation is available in 8 1/4" thick.(You'll probably have to order it in AK) That would provide the needed ventilation. If you use 2x8 rafters you can use 5 1/2" R21 and get the needed ventilation. I have stayed in a 20x20 cabin with only R21 insulation in the walls and roof (12/12 using 2x8's) at -30F and it heated just fine. In fact, the fire held (Blaze King Princess) for 8 hours. This was a well built cabin with a super tight vapor barrier.

    I do not agree that the ribs in roofing metal do a darn thing for ventilating a roof. Roof ventilation is through convection. Cold air enters the soffits, is warmed by the heat escaping the insulation (regardless of R value, some heat is going to escape), the air is warmed and it rises along the underside of the roof surface and exits either a vented ridge or gable end vents (or both).
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    I would say strip the roof using 2x's then putting metal over it, but unless you sealed all the open ends with screen you could have a Bat problem.


    I also agree people try to over insulate, in a place that size even R-19 all around is more than enough with a good sealed vapor barrior.

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    You can keep a higher R value with the 2x10's by using foam board to create an air space. Use either 1x2's or ripped foam at 1-1/2" nailed flush (and on the side)with the top of the rafters, then in-fill the rafter bay with a layer of HD foam (1"=5R) against the 1-1/2" spacers, expand foam the edges and fill the remainder of the bay with fiberglass.
    For better thermal bridging you can put a layer of foam on the underside of the rafters prior to your plywood.
    I have used 1x2's and Tyvik the same way...to keep the warmer indoor air from mixing with the outside air.
    Either way a good, positive air space.

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    "I have used 1x2's and Tyvik the same way...to keep the warmer indoor air from mixing with the outside air.
    Either way a good, positive air space."

    If you used Tyvik on the inside as a vapor barrier that is a NO, NO. That product is only for the outside of the walls as it will let water vapor pass through making it no good as a Vapor barrier.

    The one thing I'd add to what has already been said is for the few $$ it costs those cardboard baffels they sell to put in between your rafters is a good idea so as to make sure that the space above the insulation down by the bird screen is open and able to vent well.

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    Quote Originally Posted by power drifter View Post
    If you used Tyvik on the inside as a vapor barrier that is a NO, NO. That product is only for the outside of the walls as it will let water vapor pass through making it no good as a Vapor barrier.
    I didn't know that. Since Tyvek is the defacto official state siding , I just assumed it was good'n waterproof.

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    It's kinda like Gortex. It stops water droplets but allows vapor to pass.
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    Quote Originally Posted by otternorth View Post
    You can keep a higher R value with the 2x10's by using foam board to create an air space. Use either 1x2's or ripped foam at 1-1/2" nailed flush (and on the side)with the top of the rafters, then in-fill the rafter bay with a layer of HD foam (1"=5R) against the 1-1/2" spacers, expand foam the edges and fill the remainder of the bay with fiberglass.
    For better thermal bridging you can put a layer of foam on the underside of the rafters prior to your plywood.
    I have used 1x2's and Tyvik the same way...to keep the warmer indoor air from mixing with the outside air.
    Either way a good, positive air space.
    My roof is built exactly how you describe it otter. I feel a little bit vindicated now as I've never heard of it being used before and was kind of experimenting when we built our home. I didnt use tyvek though and dont understand where it would be needed.

    The only thing to watch for when building this type of roof IMO is you need to run the blueboard past the tops of the wall so that any condensation runs down the blueboard and into the eave soffit area instead of running out on top of the wall...if that makes any sense. ...other than that, it has worked perfectly, it has even helped me located a place where water was leaking in thru a stove jack on the roof (running down the blueboard and dripping into the eave overhang and down the pipe.)

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    Tyvek is an air barrier. It stops the wind from disturbing the dead air in the insulation in vertical walls where there is no vent space. It is not a vapor barrier nor would you want a vapor barrier on the cold side, which is where you always see Tyvek.

    The idea that blue board is beneficial because it channels condensation is flawed. There should be no condensation in the roof. If there is you have a problem that needs to be addressed no matter what type of insulation you used. Personally I think fiberglass is way easier to work with than foam unless you have the ability to spray foam in place. I've tried that using the urethane packs from AIH and never had much success. Friction fit fiberglass is way easier.

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    Condensation within the air space indicates air leakage in the interior envelope. Warm, moist air leaking into the cold dry outside air will find a 'dew point'.
    Seal the ceiling light fixture boxes where the wire comes in and caulk the space between the box and the finished ceiling material.
    Air could too be moving up any holes in the top plates. It is all about the details.
    I sealed my house so tightly introduction of fresh air was required to meet air exchange requirements.

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    Quote Originally Posted by otternorth View Post
    Condensation within the air space indicates air leakage in the interior envelope. Warm, moist air leaking into the cold dry outside air will find a 'dew point'.
    And any water in there where there is heat and dark, will grow mold. And mold can never be eradicated, period; there is no such treatment other than tearing the building materials all down and hauling it away.

    Moisture + heat + dark is not your friend.

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