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leebree
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# Posted: 5 Apr 2012 05:35pm
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Hi all,
I'm very pleased to find a forum that believes small is beautiful..!
I have another ceiling insulation question, but I hope it's okay if I start a new thread, even though this subject has been covered recently.
I'm insulating and finishing the interior of a small shed/outhouse that will be used as a bunkhouse. The bunkhouse is around 11ft square.
I'm using Roxul (R-13 I believe) to insulate walls and ceiling - the framing is 2x4 all around. The exterior is just one layer of thin wood siding (large panels). The roof is asphalt shingles over plywood. Although it's a few decades old, all the wood is nice and dry and in excellent condition.
We're in southwestern Quebec. Winters are harsh but we hope to use the bunkhouse (with the help of a small electric heater) in the summer as well as Spring and Fall - perhaps down to zero degrees C. But it will be used intermittently, not more than one or two weekends a month There'll be no kitchen or bathroom inside - it's just for sleeping.
After some indecision and much reading of forums etc I decided not to use a vapour barrier for the walls. But I'm not sure about the ceiling. There are no ridge or soffit vents. The Roxul insulation I have would leave no space for a gap in the roof assembly. I have pine T&G to finish the walls and the ceiling.
So - am i asking for trouble if I just insulate with no venting? Or should I install soffit vents, rafter baffles etc? I'm on a budget, so spray foam etc is out of the question.
Could I go for an unvented roof with the addition of a vapour barrier? Given the fact we're not using it that often..does it really matter?
Any advice would be much appreciated.
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trollbridge
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# Posted: 6 Apr 2012 01:36pm
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Welcome leebree, congratulations on your small cabin!
We have a small bunkhouse that is insulated but we did not use vapor barrier in the walls or the ceiling nor do we have any type of venting in the roof or eaves. Not sure if that was smart or not-our winters are harsh here too in Northern Wi. but we knew that the bunkhouse would not be used a whole lot once our big cabin was built. In that cabin we do have venting and barrier in the walls.
Wish I had an answer for you.......somebody here will be able to give advice I'm sure. Best of luck!
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MtnDon
Member
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# Posted: 6 Apr 2012 06:05pm - Edited by: MtnDon
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IF your use pattern does not change to where the place is occupied more frequently and for longer periods of time, if you use electric heat and not use any fuel burning appliance indoors unless it vents the exhaust directly to the outside, if you don't cook or heat water inside, if you don't do a lot of heavy breathing inside, then maybe you can safely get away with no venting in the roof. Depends. Personally I don't like the idea, wouldn't do it myself, but that's just me, my opinion and my paranoia about things growing in warm, dark and moist places.
Vapor barriers are normally highly recommended in walls and attics or cathedral roofs most places north of 49. But maybe not having one could help the building dry out in between uses. Hard to say.
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tbjohn
Member
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# Posted: 6 Apr 2012 07:08pm
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Don't they call that a "hot roof" these days? I am not so sure I like the idea. Either you have to keep the moisture out or let it out. Some of the old ways are not so bad.
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soundandfurycabin
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# Posted: 7 Apr 2012 05:09am - Edited by: soundandfurycabin
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I would go with a good vapor barrier on ceiling AND walls, sealing the seams and any tears with tape, and taping the ceiling plastic to the wall plastic. Otherwise during freezing conditions, moisture from the air might condense and freeze into a nice layer of frost on the underside of the sheathing. Then when the sun hits the roof, it melts and drips, soaking your insulation and staining your pine ceiling. With the vapor barrier in place and your limited use, I wouldn't lose sleep over the lack of vent space.
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rayyy
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# Posted: 7 Apr 2012 07:00am
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It's true,for the little bit of extra work and cost involved,it will save you a lot of headache down the road.Freezing moister does do some pretty wierd things.
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leebree
Member
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# Posted: 8 Apr 2012 10:32pm
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Thanks for all thoughts. It does sound like it's a risk to avoid vapour barriers entirely, especially below zero degrees C.
As Mtndon suggests, given I have no venting in the roof at all, perhaps a vapour barrier on the ceiling would tend to prevent any drying out. since I'm covering the Roxul with pine, which seems pretty porous, the roof would at least dry to the inside if moisture collects.
There again, since the plywood sheathing is full of nail holes, and the pine T&G has a few holes of its own, perhaps the insulation value of the Roxul is reduced so much anyway that I don't need to worry! (except about being cold..).
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MtnDon
Member
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# Posted: 8 Apr 2012 10:51pm
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What I said was meant as a big maybe. Ideally, being in a cold climate you would have rafters with a deeper depth so you could have both a vented space above the insulation and have more insulation. But because you seem to have finished the structural construction already I don't think there is a best thing to do.
I didn't ask though, if you had already installed shingles or whatever roofing material already? If not then I'd do 4 inches of rigid XPS foam (2 layes with staggered seams) on top of the roof sheathing then a second OSB skin and the shingles. Leave the Roxul out altogether. Four inches of XPS is = R20 and removes the need for an interior water vapor barrier when you leave out the Roxul or other rafter cavity insulation.
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leebree
Member
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# Posted: 9 Apr 2012 09:59pm
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Yes, understood. In fact the structure was already constructed (by a previous owner) but just used as a storage shed. Hence the 2x4 construction all around.
The roof is asphalt shingles on top of plywood - again, already installed.
I like the idea of XPS foam on top, although re-doing the whole roof would be a bit costly. Perhaps in time when we have to replace the roof anyway - at that point we could look at installing the foam and a second skin.
In any case, I think it's unlikely we will be using it in temps below zero, so perhaps we can avoid the condensation-ice problem.
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