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swiarda
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# Posted: 19 Jun 2015 18:53 - Edited by: swiarda
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Ordered our 12'x32' Side Lofted Barn Cabin yesterday from Cumberland Buildings ... it'll be delivered to our Utah mountain property in 3-4 weeks. It's gonna be an adventure, fer shure!
While I have some general construction experience, I've some questions on the nuances, right now mainly insulation. The gambrel roof is raftered with 2"x8" on 24" centers, plywood sheathing under a metal roof.
While foam insulation is the way to go, local contractors want almost as much to travel as what the materials costs, so they were scratched off the "want" list fairly early. I'm left with faced R21 fiberglass batts, so no big issue there (that I see anyway).
My question is this: the building has a solid soffitt. Do I need to drill ventilation holes (like I saw that RifRaf did) then provide an airway to the vent?
Radiant barrier ... yay or nay?
Thanks in advance ...
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rockies
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# Posted: 19 Jun 2015 19:56
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Hmm. 2x8 is borderline for a roof rafter, especially at 24 inches on center. Any idea what the snow load is like for the Utah mountains? I would see if you can have the rafters increased to 2x10, or at least install some extra 2x8's so they are 16 inches on center.
As to spray foam insulation in the roof, you could use this.
http://tigerfoam.com/
Order their kit, do it yourself, then return the kit. Then you wouldn't have to use batt insulation, baffles, eave and ridge vents, etc.
As to radiant barriers, look up info on this site.
http://www.greenbuildingadvisor.com/
just search for whatever term you want, and the articles will come up.
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PA_Bound
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# Posted: 20 Jun 2015 18:43
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You indicated that spray foam may not be an option. That's too bad, because that is IMHO the best solution.
But you are planning to use batt insulation. That will work, but in that case you may need to vent the underside of the plywood, and that likely means drilling holes in the soffit. The problem is what happens to the humidity created in the cabin- particularly during the winter. Humidity comes from many sources, including cooking, bathing, use of propane and other heating sources, and some is just expelled by living bodies. If humid-enough air comes in contact with the cold plywood it will condensate. With the insulation batts, it will likely not evaporate well (at all?), which could lead to rotted wood, mold/mildew, bugs, or some other nasty issue. Faced insulation will help, as would the addition of tight, vapor-impenetrable ceiling materials, but it's safer to assume some humidity will get in that space and the only way to remove it is with air flow using outside air.
RifRaf and I are following a similar cabin plan. Although my cabin is not used in winter (can't get in to it usually), I decided to drill holes in my soffit and install plastic rafter vents that keep the insulation from directly contacting the plywood and provide an air-gap between the two. The holes in the soffit open into those rafter vents and allow outside air to flow between the plywood and insulation. The other piece of this puzzle however, and your initial note does not indicate, do you have a ridge vent? Holes in the soffit allow air to enter the rafter vents, but you also need someplace for the air to exit at the top. A ridge vent is the typical solution for that.
Below is picture that shows what I did (obviously this is a work-in-progress picture- the channels were ultimately completely extended from soffit to ridge, and covered by insulation).
I hope this helps.
Rafter vents and insulation
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Steve_S
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# Posted: 20 Jun 2015 20:45
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Welcome to the forum,
You may want to try and touch base with some local Commercial Roofers. I have gotten a "LOT" of Styro-SM (Blue Foam) 2", 3" & 4" from them. I've also gotten 1", 3" 4" ISO with treated craft paper backing. Last but not least, also 6 lifts of 5-1/4" EPS Foam with bonded fibreboard (both sides) for my walls.
These all come in 2x4, 4x4 (mostly) and some 4x8 sheets.
Easy to cut and handle, Light, Fire Sae, High Insulation value per inch and no rot or water issues with foam.
A lot of commercial roofs require that all layers of insulation is removed when redoing, averaging 60 % reusable... Particularly for folks like us.
Prices... Well I used 4" SM-Blue under my foundation (twice code @ R20) and each sheet cost me $3.00 for 2x4. The ISO for outside my walls & roof (which is getting 8" / R42) are 4x4 sheets @ $2.00 ea.
You can cut it with a Circ Saw, Hand Saw, Olfa Knife, even a Table Saw... all easily and no big hassle (just some time) and stuff them in tight between studs / rafters and seal in with spray can, low expansion foam.
You MUST Vent the roof between the insulation & plywood like PA_Bound shows (those are perfect). You will have to provide air intakes in soffits, often the easiest is the "strip vents", 3" wide that can be pushed into the cut.
Ideally you should have a layer of Foil Faced ISO on top of the sheathing with battens running from bottom to ridge & Tin attached to the battens. That provides the most value for keeping summer heat out and winter heat in and then you do not need the baffles and you can stuff up to 7-1/2" into the rafters. The thermal break and positive venting work in your favour in summer & winter saving you money and the Foil ISO is water proof, reflecting heat in both directions but could still use water proofing layer (depending on your area but Utah I can't say.
Vapour Barriers depend on YOUR Regional Climate and type of insulation used and how tight everything is.
Good Luck & Hope it helps.
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