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cabinbiscuits
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# Posted: 6 Sep 2012 08:42pm
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I'm heading up to camp to start setting my roof trusses in place tomorrow evening and now after the discussions in Bobbotron's post "Metal Roof With Cathedral Ceiling", I realize that I have to change my plans and some of my materials prior to leaving for camp.
My building is 16x20 and will have a gambrel style roof with either 1/2 or a full loft. My plan was to place purlins on 24" centers on top of the trusses/rafters, and then place R-19 unfaced fiberglass rolled insulation between the rafters right up against the metal. In reading Bobbotron's post i learned that I failed to account for the metal condensing the moisture that makes it's way through the fiberglas, so now I'm trying to adjust my plans PDQ.
There are no soffits so there is no way to install a soffit to ridge vent type of venting system above the insulation.
Foam insulation is prohibitively expensive, so the most I could afford to spray to the bottom/inside of the metal roofing would only be about 1 inch.
I don't want to use sheets of foam board on top of the roof structure, but under the metal. I know that's how many commercial,industrial roofs are done, but due to the logistics involved in this particular build, I don't want to try to go that way.
I already have the unfaced rolls of R-19 that I got a pretty good price on in a clearance at Home Depot.
Keeping in mind that this building will sit unheated, un-air conditioned for about 90% of the time, my questions are as follows-
1- If I were to do away with the purlins and sheath the roof with 5/8 osb, then tar paper over that, then the metal roof over top of that, would there still tend to be a moisture problem with putting the insulation right up to the bottom/inside of the osb?
2- What about if I switched the roof material to asphalt shingles over osb would that change the moisture problem.
3- I plan to take 4 or 6 mil plastic and staple it to the bottom of the trusses/rafters to make a vapor barrier prior to putting up the inside finish material. I figure the plastic should help to really reduce the vapor transmission into the insulation from below. Does anyone see a problem with doing this, no matter how I proceed above?
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turkeyhunter
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# Posted: 6 Sep 2012 09:12pm - Edited by: turkeyhunter
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this is what i used got it from HD!!!!!
http://store.contractor-pro.com/durovent-70ctn-p-2591.htmli put 2 x 4's on my rafters, no PT as per metal company-- screwed the green metal to the 2 x 4"s........put DURA Vent between the rafter's as a after thought, it has been insulated with rolled insulation and no problems what so ever.......hope this helps
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rockymtngal
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# Posted: 6 Sep 2012 09:47pm - Edited by: rockymtngal
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Did you see this from mojo43?
mojo43
Member # Posted: 31 Aug 2012 09:13
Reply Quote
What about this??
http://www.youtube.com/watch?v=i1guAwzyEOQ
It doesn't seem to hard..
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GomerPile
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# Posted: 7 Sep 2012 09:23am
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Even if you foam the roof you still need to use enough foam to keep the dew point within the foam or you will get condensation on the backside of the foam in cold temps.
The issue in using cut to size foam pieces and can foam is that you need to get the thing air tight or the air leaks will create pockets of dampness as the moist air escapes.
If you don't want to spray foam the roof, your best and safest bet is to vent the roof using the duravent product and add a vapor barrier in front of the insulation.
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MtnDon
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# Posted: 7 Sep 2012 10:28am
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Duravent is great at doing what it is designed for; permitting air to move from the soffit vents up past blown in insulation or batts that is located between ceiling joists near the eves. They allow you to fill the ceiling insulation near the eves and still have air movement. If the thought id to use duravents and pack fiberglass insulation into the rafter spaces there will be much of the space that is really not vented... space where the fiberglass touches the sheathing. Hope that is clear.....
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DaJTCHA
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# Posted: 7 Sep 2012 07:12pm
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Our roof is layered like this: 2X6 rafters, 1/2 osb, owens-corning iceguard G underlayment, metal roof.
I'm still debating on whether to insulate with hard foam and spray foam gap sealer and then overspray the hole thing with DIY spray foam to seal everything tight to the wood structure.
Let me know what you figure out and install, for now...our loft sits uninsulated.
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johng
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# Posted: 7 Sep 2012 08:05pm
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Quoting: DaJTCHA Our roof is layered like this: 2X6 rafters, 1/2 osb, owens-corning iceguard G underlayment, metal roof.
You're good.
Keep in mind your metal is still vented eave to ridge via ribs in metal.
If all else fails seek manufacturers install instructions on web page.
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cabinbiscuits
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# Posted: 16 Sep 2012 07:50pm
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Thanks everyone for the replies. I ended up going with purlins on top of the trusses, although I went back and forth about using the osb and tar paper instead. In fact I went as far as buying
The osb and tar paper and trucking it to the top of the mountain and unloading it before I made my final decision.
Ultimately I decided to go with the purlins due to the pitch of our roof and lack of labor ( just my 72 year old step father helping me).
As far as the insulation issue I guess I will leave our roof unisulated for now and will use spray foam at some point in the future on the inside of the roof.
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toyota_mdt_tech
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# Posted: 16 Sep 2012 09:29pm - Edited by: toyota_mdt_tech
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I just sheeted my roof, laid down 30# felt, the roofers laid the metal right on top of the felt. No leaks, no sweat.
Here is a video on the baffling posted earlier
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DaJTCHA
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# Posted: 17 Sep 2012 10:36am
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No video link...please post!
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