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Timberjack
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# Posted: 2 Jan 2011 10:43am
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The earlier thread concerning metal roofing and condensation made me wonder about our own roof. Heres the setup: exposed rafters (i guess you would call that a cathedral ceiling), exposed 1X12 pine decking, 30lb roofing felt, 1" polyiso foam board with foil on both sides between 2x4 lathing, 26ga galvalume panels. Due to the roof/celing construction there is no ventilation like soffits or ridge vents. The exterior walls of the cabin are wrapped with tyvek and then i went over that with 15lb felt. I intend to insulate the floor and walls with R-13 roll insulation. It shouldn't be drafty but that might be a negative in this situation. I know there will be condensation on the underside of the metal but it should contact the foil and then either migrate out through the 1/2" airspace or find its way down to the felt. Am I overthinking this or will it be a problem?
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MtnDon
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# Posted: 2 Jan 2011 12:44pm - Edited by: MtnDon
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What's your climate?
Full time or part time use?
1" of foam doesn't seem like much to me, but you are on the right track with placing insulation on the roof deck. Climate affects what the best practice is.
BTW, you didn't really need housewrap and felt on the walls. Felt would have been sufficient. In some cases it is superior to housewrap.
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Anonymous
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# Posted: 2 Jan 2011 01:54pm
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Timberjack, I am in the same boat as you on teh condensation issue with metal roofs. Im trying to figure a way to install my metal roof over the purlins from the inside out of the building so Im trying to avoid doing the traditional underlayment method and so far all my ideas are coming up short. how about when you figure it out, let me know! haha
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Timberjack
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# Posted: 2 Jan 2011 02:23pm - Edited by: Timberjack
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Don, thanks for your input. I've read several of your posts and was hoping you would reply. We are in Mississippi and this cabin will be for part time use. It will probably see more usage in the winter and spring during the hunting seasons. Our summers are blazing hot with high humidity and our winter temps usually range from the 20s-50s. I knew the felt over the wrap was somewhat redundant but I had extra and wanted to use it somewhere (I love that black paper!). The 1" polyiso foam has an R value of 6.5 without an airspace but I wanted to leave the 1/2" space to try and address the moisture issue. Also, I haven't decided on what form of heating to install. I would like to use wood but LP gas is also an option but again I worry about the moisture. Maybe a vented LP heater? What do you think about the condensation on this roofing scenario?
More photos here:
http://www.small-cabin.com/forum/6_732_0.html
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MtnDon
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# Posted: 2 Jan 2011 02:42pm - Edited by: MtnDon
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I made some extra comments in my first post.
More later... I'm in the middle of a workshop/garage clean out. Too much crap laying around so it's organization time!
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MtnDon
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# Posted: 2 Jan 2011 05:27pm
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Outside: Moisture will likely condense in that 1/2" air space on the underside of the metal roofing. If there's a lot it may puddle along the horizontal 2x4's? Where is it going to go then? My concern is that it might seep down the joint where the foam meets the 2x4. If the sun is warm enough any water there may evaporate and vent to the outside.
Sideways air venting is not as effective as vertical venting though.
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MtnDon
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# Posted: 2 Jan 2011 05:37pm
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Inside:
I believe that a non vented propane heater is a bad idea everywhere, but more so in a climate that is humid. A direct vent propane heater has many advantages over the unvented variety. The unvented type add all the products of combustion to the air and it's my personal belief that we should not breath them. That covers my feelings on kerosene heaters and lamps as well as candles FWIW.
The inch of foam plus the thickness of the pine decking might be enough to prevent condensation inside on the ceiling/roof. I can't say for sure. It may be that some condensation might occur sometimes when the interior is cold, but may dissipate when warmed. Hard to say.
The inside surface has to be kept above the dew point to prevent condensation. That is easier when the inside is heated and the outside temperatures do not go below freezing.
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MtnDon
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# Posted: 2 Jan 2011 06:32pm
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A note on metal roof panel installation: This is for roofing made from Galvalume steel which I believe is what most of the roofing materials are made from. It is still possible to get older zinc only galvanized corrugated roofing but it has lower corrosion resistance than the aluminum-zinc ally Galvalume. I believe most colored or painted panels use Galvalume steel.
The manufacturer of my ribbed metal roofing recommended installing the metal panels directly on the roof deck. That is, the roof framing is first covered with structural OSB or plywood panels and #30 roofing felt, or the equivalent, is installed over that. Then the metal panels.
If metal roofing is applied directly against a flat surface, such as tar paper over OSB or plywood sheathing, that moves the cold plane where condensation can take place. If the air can not get in there between the metal and the deck, neither can water vapor. The possibility of some moisture condensing in a raised rib is still there but that's much reduced. Any moisture that would condense in the rib will run down the rib or the roofing paper.
In re-reading the printed information from the Galvalume steel manufacturer I note that they make two types of metal roofing panels. One is called structural and the other architectural. Structural is applied directly to framing without decking and is non painted. It is used mainly on low slope commercial roofs. Architectural is usually painted and is frequently not as thick as the structural panels. I'm thinking that most colored metal roofing is not rated as a structural panel, especially those stocked by places like HD, Lowe's, etc.
FYI, there is also a note that Pressure Treated wood should not be used in direct cintact with Galvalume steel. The chemicals in the PT wood can cause corrosion. Ditto on contact with concrete. Copper, lead, graphite and unprotected steel should also not be used against the Galvalume panel although aluminum is okay. Also some spray on foams may contain fire inhibitors that are corossive; check with foam manufacturer.
Don't use silicone sealants that smell like vinegar.
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MtnDon
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# Posted: 2 Jan 2011 06:48pm
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I found the printed information I had online HERE. This is a Galvalume Steel manufacturer not the maker of roofing panels.
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elkdiebymybow
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# Posted: 17 Jan 2011 06:01pm
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My metal roof was installed on a tar paper covered OSB deck on 2 x 12's with vents installed in each bay. I was told I could have gone every-other bay but didn't think the additional venting would hurt. I haven't noticed any condensation at all. Just a water leak around my tripple wall stove pipe where the rubber boot gasket has lifted a bit off the metal.
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toyota_mdt_tech
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# Posted: 17 Jan 2011 06:09pm
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Quoting: MtnDon Outside: Moisture will likely condense in that 1/2" air space on the underside of the metal roofing. If there's a lot it may puddle along the horizontal 2x4's? Where is it going to go then? My concern is that it might seep down the joint where the foam meets the 2x4. If the sun is warm enough any water there may evaporate and vent to the outside.
I'm with Mt Don. The set up you have now, you should remove the metal roof and add a layer of 15# felt over the top of the foam and horizontal 2X4's, layer the felt to shed water downhill. I have an enclosed metal cargo trailer (7X16) trailer it has a heater inside of it. When its heated inside and cool outside, I get big beads of sweat build up on the metal screw heads going into the metal studs. I know the metal skin on the inside of the wall is full of condensation, but its designed to roll off and own and drain out the bottom of the wall. In your case, it will sweat bad, but looks like it will have no place to go. .
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