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Thread: roof problems

  1. #1
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    Default roof problems

    I need some advice. Here is my situation; I have a 24x24 cabin in the mountains, had it for about 20 years. The roof is metal with a 12x12 pitch. The problem I am having is it condensates terribly, at least that is what I think is going on. As I am no builder, I relied on the advice of a buddy that told me we could run 1x4 perlins across the rafters and then screw the metal to that. It is a dark roof so I am thinking that the roof heats up during the day, winter time, and condensates dropping the moisture into the insulation and then it freezes. When I show up and start a fire it melts and drips so badly it will get the walls wet. Do I have to rip the metal off and sheet the roof with plywood to stop this. I have asked SBS and they don't seem to know what to do. Any advice is appreciated.

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    The only way excessive moisture can condense on the underside of the metal like you're describing is if your ceiling vapor barrier is leaking. Especially if your roof venting isn't good. I'd work to vapor seal the warm side and improve the ventilation on the cold side.

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    Sounds like an inadequate (nonexistent?) vapor retarder.

    Describe the building envelope (walls and ceiling, how they are constructed inside to out) in more detail.

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    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by Avid outdoorsmen View Post
    I need some advice. Here is my situation; I have a 24x24 cabin in the mountains, had it for about 20 years. The roof is metal with a 12x12 pitch. The problem I am having is it condensates terribly, at least that is what I think is going on. As I am no builder, I relied on the advice of a buddy that told me we could run 1x4 perlins across the rafters and then screw the metal to that. It is a dark roof so I am thinking that the roof heats up during the day, winter time, and condensates dropping the moisture into the insulation and then it freezes. When I show up and start a fire it melts and drips so badly it will get the walls wet. Do I have to rip the metal off and sheet the roof with plywood to stop this. I have asked SBS and they don't seem to know what to do. Any advice is appreciated.
    Think of this as analogous to a boat with a leak; in that situation your solution is not to attempt to empty the water from the boat more effectively, rather the solution would be to stop the leak and prevent water from getting into the boat in the first place.... This is no different and you have to look at how the moisture is getting into your insulation layer in the first place. You need to install a good sealed vapor barrier between your living space (where all that moisture is being generated) and your insulation layer. No amount of ventilation above your insulation will solve your problem as long as you're pumping moisture into your insulation from below.
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    Avid,

    While you're at it....

    I built the majority of my cabin with a plywood sheathed roof under metal. The last addition I did the 1x4 slats across the rafters and screwed the metal to that. While the slats make a very nice ladder and installation was easy the screw retention of the slats isn't good. I have to re-set every screw that's set in the 1x4s every summer. The hot/cold cycle apparently backs the screws out. That's NEVER happened in the much older plywood sheathed areas. If your screws are loose your insulation may be getting wet in the autumn rains and then freezing with the cold weather of winter. One more thing to check, especially if you think your current vapor barrier is good.

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    If you use propane to heat or even cook it will make it worse.

    Be sure to remove all molded insulation whule wearing proper gear. Now would also be the time to fur out the ceiling to move to R34-38 insulation also if it doesn't have the depth you need. Be sure to tape the vapor barrier overlaps and any staple holes or the insulation will mold behind them.

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    I appreciate all of the valuable input. What I currently have is the insulation with the backing that is stapled to the rafter, not sure of the R-factor. Then visqueen over that. Started to put tongue and grove over it but decided to wait until I figure this out. Some years are worse than others. What is odd to me is that I can go in the cabin at zero degrees and fire up the wood stove as the place heats up you can see the moisture coming thru the insulation backing and running down the visqueen to the walls. After about 6 hours it seems to cook it all out. And then its fine for the rest of the weekend. I am sure that I could do a better job taping the vapor barrier together. Is it possible that the vapor barrier could be creating that much moisture? How does it create moisture when there is no heat in the cabin?

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    Supporting Member iofthetaiga's Avatar
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    The vapor barrier is not creating moisture. Moisture comes from all your human activities in the cabin. It might look like you eventually dry the insulation out after a few hours of cranking up the heat, but you're not really. You're only drying out the inner portion. Likely, somewhere toward the outer portion of your insulation you are still maintaining a layer of ice. If your vapor barrier is not completely sealed at the edges, and taped at every seam and hole, what happens is this: You come into the cabin, fire up the stove and dry out all your sweaty wet clothing and boots, you cook, breath, perspire, perhaps shower, etc. All that moisture has to go somewhere. A tiny portion of it escapes whenever you open the door or perhaps a window, but only a very tiny percentage. Throw a door open in the winter time and stand there for a few minutes. See all that water vapor condensing into clouds of steam as that warm moist air rolls out the door? Winter air outside your cabin is relatively cold and dry, air inside your cabin is relatively warm and wet. The sole source of moisture affecting your insulation is the warm wet air from inside your living space. Normal human activity inside a building, especially in winter in a small cabin when your drying wet clothing items etc., generates large amounts of moisture. You're constantly generating moisture inside your living space, and that moisture is constantly being pumped into your insulation through every tiny hole in your vapor barrier. At some point within your insulation layer the temperature will equal the dew point and all that moisture will condense and freeze, forming a layer of ice in the insulation. If your insulation layer is very thin (low R-value), and your inside temp is very warm, you may drive enough heat through your insulation layer such that the temp within the full width of the insulation is above the dew point, but still, all that moisture is going to condense on the first cold surface it encounters, such as the underside of your roof. In that event, your insulation might start out dry, but that huge amount of frost accumulating under your roof will periodically fall onto your insulation, saturate it, thus further reducing your R-value.... No amount of under roof ventilation will prevent this. The only solution is to prevent that moisture from escaping into your insulation layer in the first place. The only way to prevent this is by installing a bombproof vapor barrier. This is applicable to ceiling, walls, and floor.
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    Mr. PId is right, as usual. The vapor barrier MUST go on the warm side of your insulation, and SUFFICIENT cross ventilation must be provided above that insulation and its vapor arrier, and beneath the roof diaphragm, leaving enough space for the cross ventilation to be sufficienently effective. Gable end vents would certainly help dramatically in your conditon of a 12/12 pitch roof, too. Easy to install, and probably the very most effective answer to your ventilation problem, which it certainly is. The metal roof has nothing to do with your problem. Your insulation may a bit on the lean side, but you will need sufficient cross ventilation above that insulaltion, and below the roof sheathing, to carry away the warm and moist air you create below the insulation and its vapor barrier. In short, your insulation shouldnt' be at the roof, but at the ceiling. It would be different if you had an exterior, rigid insulation OUTSIDE the roof sheathing. Not really the answer with a 12/12 pitch roof.

    There must be NO HOLES OR SPACES in your vapor barrier. It isn't a "water" barrier, it is an airborn "vapor" barrier, moisture that you cannot see until it becomes a veritable fog. Or else manifests itself in "leaks" through your ceiling and down the walls.

    Almost no heat will be lost through the walls of your cabin. Infra-red photography has verified that. As for a heat source, you'll find that wood fires will see less moisture created than by oil or gas fires. A Coleman (pressure appliance) stove, for instance, or propane (gas) heat, will increase the moisture content of the air within the cabink considerably.

    You noted that your visqueen is stapled OVER the insulation. That puts it on the cold side, which is not the proper side. You now have TWO vapor barriers, and that's a BIG problem Moisure is guaranteed to form below that visqueen. Any vapor barrier must - - - MUST - - - be placed on the warm side of insulation. Your insulation surely has a vapor barrier on one side at any rate. The visqueen has no place in your installation at all. That is best saved for use beneath a slab-on-grade, which you probably don't have anyway.

    If your rafters are nominal 6" rafters (i.e, 2x6 materials), the insulation should be full depth of the rafters. Otherwise the insulation isn't thick enough to give you the R-rating ( U-factor) you need for Alaska weather. The insulaton's backing, surely a vapor barrier type backing, should always be installed on the warm side. Ideally, that insulaton should be down in the ceilong joists, or between the bottom chord of roof trusses, whichever you used. ADEQUATE cross ventilation is REQUIRED above that insulation !!!

    Corrective measures for you may take a little extra work, but they will solve your problems forever. They will be well worth it. If you try shortcuts, you'll forever be fighting wet ceilings and walls. Period. Drop the insulation to the ceiling joists (if any) and remove the visqueen. That's the most important factor. Get rid of it!!! You don't have to be neat about that, just rip it out.

    The base problem you have constructed is with the visqueen vapor barrier. It's holding moisture inside the insulation, and there is nowhere for it to go except back down into your cabin through every crack and pinhole in that insulation. In the end, the vapor barrier must go on the warm side of the insulation, and that visqueen is on the cold side. Won't ever work for you.

    iofthetaiga and I are on the same page, but we do get a little separated somewhere along the line. Cross ventillation is definitely required, and the quickest answer to guarantee that you now can get enough is the installation of gable end vents. IN EACH END OF THE CABIN !!!

    That's my two cents worth, but it comes from designing buildings in Alaska, including their roofs, for more than 28-years. These included that funny looking building at Lake Hood (the one with the slopy columns and tipped out windows) and the UAA Anchorage chemistery building.


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    Hate to say it, but sounds to me like ya got a leaky roof; tear it off and start from scratch. A small cabin with a good woodstove and a properly operating chimney hardly needs a vapor barrier; in that situation you usually need to ADD moisture, in the winter, anyway.
    " Gas boats are bad enough, autos are an invention of the devil, and airplanes are worse." ~Allen Hasselborg

  11. #11

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    Quote Originally Posted by Avid outdoorsmen View Post
    I appreciate all of the valuable input. What I currently have is the insulation with the backing that is stapled to the rafter, not sure of the R-factor. Then visqueen over that. Started to put tongue and grove over it but decided to wait until I figure this out. Some years are worse than others. What is odd to me is that I can go in the cabin at zero degrees and fire up the wood stove as the place heats up you can see the moisture coming thru the insulation backing and running down the visqueen to the walls. After about 6 hours it seems to cook it all out. And then its fine for the rest of the weekend. I am sure that I could do a better job taping the vapor barrier together. Is it possible that the vapor barrier could be creating that much moisture? How does it create moisture when there is no heat in the cabin?
    First of all what size do you have for rafters ?Do you have any type of ceiling framing or it just your roof rafters ? As others stated if your insulation is not thick enough and also if you lack air space between the insulation and the metal roof you will never solve the problem. If your insulation is touching the metal roofing you could be getting condensation when the roof heats up from the sun. Also as stated nefore you have got to have air flow.

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    To clear up some misconceptions. Warm air carries more total humidity than cold air. That's a scientific fact. Human activities or lack of them aren't terribly important in the vapor barrier discussion. When the inside of a building is warmer than outside the dew point for moisture condensation lies somewhere IN the insulation layer. Dew point is another scientific fact and is easy to calculate. The vapor barrier is there to prevent moisture from the warmer side from transporting into the insulation where it will find the dew point and condense. Ultimately as the roof cavity warms, whether from outside temps rising, an insulating snow layer moving the dew point zone outward, or solar gain, etc., the moisture that's in the insulation will migrate out. And that's when you need ventilation space, so the moisture can exit the roof cavity. Without that potential to exit the moisture is trapped and will build up in the roof. With that insulating value declines and structural members get wet and rot. Sounds like the 70s and 80s around here when hot roof construction was popular. What a mess. We learned about cold weather roof construction by doing it wrong. Now we know how to do it right.

    The vast majority of existing structures in our region have fairly leaky vapor barriers. Vapor retarders if you want to get technical. 70s and 80s vintage homes don't perform well with blower door tests. They weren't built to the tight standards we use now. Yet they're healthy homes and don't have condensation problems. You can attribute that to attic venting. Use a vapor retarder and ventilate the area above the roof insulation. Just an inch or two of space on top will work. Use screen vents at the bottom of the rafters and a vented ridge on top. Easy to do with standard metal roofing and the metal's accessory ridge cap. It's an important detail no matter how much insulation you use.

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    Wow,
    I have certainly learned more here than talking with a few contractors as well as the boys at SBS. Not to say they didn't have good advice but, I am truly amazed at the depth of knowledge here. That said, I apologize for the confusion, as I am no builder I struggle with articulating the proper construction terms. I have 2x10 rafters on 16" centers. Also, the visqueen/vapor barrier, is on the hot side of the insulation. The insulation when newly installed left a good 3 inches between metal and insulation. Thru this discussion I am understanding that should allow the roof to vent. I am alittle confused on the gable end vents? It is a open roof line so where would I install the gable end vents? Perhaps I should go up and pull the rig cap and make sure the insulation at the top is allowing it to vent? I can look up the rafter from standing on a ladder on the outside of the wall and see there is a gap, but not sure I am looking all the way up to the rig cap.

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    Before applying the ridge cap make sure there's a gap in your slats and roofing so you can see your air gap on both sides of the ridge. An inch or two gap is adequate. Apply the ridge cap and leave the ends open at the front and back. Add screen vents at the soffits or rafter blocking and assure air flows freely to your air gap above the insulation. That should be plenty good for an average cabin.

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