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tusk
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# Posted: 13 Dec 2011 06:47pm
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Parameters for Flat Roof:
1. Building Dim: 16'x48'
2. Roof Dim: 20'x51'
3. Slope: 1:16
4. Metal: 3'x20'x29 ga. Gavlalume Panels
5. Underlayment: 15# roofing felt (2 layers)
6. Sheathing: ½"x4x8 OSB
7. Rafters: 2x8x16 (16" or 24" o.c.)
8. Insulation: R-13 or R-15 (2 layers @ 3 ½" each)
As you can see, with a rafter depth of 7 ¼", there is only ¼" of "dead space".
My question is this: If the void between the ceiling and roof sheathing is completely full of insulation, could I have condensation issues?
It is my thinking that any condensation on the underside of the metal will roll down the felt and into the gutters.
I plan on heating the cabin with LP and cooling with ceiling fans and 115v AC window units.
Average low temperatures in my area are 34F during the winter. Humidity is higher than most places.
Thanks for your time,
TUSK
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Just
Member
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# Posted: 13 Dec 2011 07:04pm
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I am sure you need a vented space between the insulation and the steel . I have a home with 2 x16 prefab rafters.. The plans called for 12 in. of insulation and a 4 in. air space with a steel roof .
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tusk
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# Posted: 13 Dec 2011 07:47pm
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is there sheathing and felt between the steel and the rafter? how cold does it get there?
additionally, the steel has ribs in it....
hell, i don't know what i'm talking about....
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MtnDon
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# Posted: 13 Dec 2011 08:45pm
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The usual ribbed metal roofing panels where the first rib on the second panel overlaps the last rib of the previous panel is normally rated for a minimum pitch of 3/12. Using those on a pitch as low as 1/16 may result in water getting under the lap joints. There are some metal roofing products made for low pitches. Standing seam can go as low as a half in 12. Check what the roofing manufacturer states about the panels you want to use. Consider using a membrane roofing material like EPDM, which is designed for flat roofs.
In a normal pitched roof the minimum recommended air space between the roof sheathing and the insulation in the rafter bays is 1 inch. Some engineers recommend a minimum of a two inch air space. With an extremely low, basically flat roof that 1/4" air space is probably woefully inadequate. R13/15 does not seem like much insulation at all for a roof/ceiling.
I would suggest much deeper rafters even though they may not be needed for the proposed span. Personally I would not want less than R30 in a roof/ceiling.
Another choice would be to install foam sheet insulation on top of the OSB sheathing. Two layers of 2 inch EPS or XPS foam with staggered seams. Another layer of OSB screwed through the foam to the rafters below. Then felt, then the final roofing. This would also eliminate the possibility of condensation on the underside of the roof sheathing. Then the rafter bays could be fully filled with batt insulation and the space would not require ventilation.
Moisture will not condense on the underside of metal roofing material when the metal is flat against the roof sheathing or roofing felt. There are foam rubber pieces made for ribbed roofing at are fitted under the metal at the ends. These seal the rib areas and prevent air circulation in the rib area, also eliminating condensation.
I would thoroughly;y investigate any choice of roofing for its compatibility with such a low pitch roof.
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Martian
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# Posted: 14 Dec 2011 07:12am - Edited by: Martian
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We built an 18'X66', with a 2/12 pitch, add-on to a mobile home. Decking was 5/8" OSB covered with 29ga Strongbarn panels. For underlayment, the metal supplier recommended a rubberized snow and ice shield type material which stuck to the OSB. The lap seams were sealed with a bead of gutter caulking. Its been two years, now. So far, so go.
I would strongly suggest a styrofoam layer between the fiberglass and the decking. Better yet would be spray foam insulation. Have your metal supplier bend the drip edge with a 12" flashing up the roof. With a roof this flat, I'd suggest looking at rubber roofing which glues down. That way there are no fasteners to leak.
Closure strips do prevent air circulation, but they won't stop condensation. In fact, cutting down the circulation will exacerbate the problem since condensation will occur in the ribs and will run down onto the decking. The closure strips will slow the evaporation of any moisture that does form.
I'm currently rebuilding a structure where the foam strips were placed between the metal and the bottom wood perling (no drip edge trim). Condensation had formed and run down the panel, saturating the foam. The wet foam has rotted the wood.
Out of curiousity, if you are using 16' rafters and 20' metal, how are you supporting the 2' overhang on each end?
Tom
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tusk
Member
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# Posted: 14 Dec 2011 09:48am
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Martian,
The rafters will extend 18-24" on either side of the structure. I wanted additional shade and shelter from the elements.
It sounds like i should consider "foam board" underneath the metal to avoid potential condensation issues...
thanks to all!
TUSK
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tusk
Member
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# Posted: 14 Dec 2011 09:54am
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MTNDON,
RE: ceiling insulation: I planned to go with 2 layers of R13 or R15 in the rafter bays. that would give me R26-R30. if i added foam sheathing it would boost the R value to 28 or 32 which is more that ample for the Central SouthEast....
TUSK
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MtnDon
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# Posted: 14 Dec 2011 10:45am - Edited by: MtnDon
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Sorry tusk, I misread about the R values you were aiming for.
Martian, I see the word purlin in the section discussing rubber dams. If a roof is built on purlins there is really no way to stop condensation on the underside short of spray foaming the underside. In order to have condensation occur moisture laden air has to be able to get to the cold underside of the metal.
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Martian
Member
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# Posted: 14 Dec 2011 11:21am
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MtnDon, after getting this job, if I had to build my cabin again, I would deck the roof and use the stickon rubber shield. I'm going to spray the bottom with foam next Summer. At least my closure strips are sitting on the metal drip edge and not the wood.
You're right that moisture has to get to the metal to cause condensation, and that is definitely easier with purlings. But humidity is so pervasive that any space will allow condensation; even sealed metal tanks condensate on the inside. The advantage of purlings is that the condensation can dry.
Tom
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