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scotia1
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# Posted: 5 Sep 2017 09:36am
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Hey there,
I have a 16x20 cabin with and loft and a 3/12 single pitch roof that I'm in the midst of putting up. Initially I was planning on not insulating it, but have changed my mind. In any case, I'm now wondering about suggestions/options for insulating and venting the roof as this is my first roof build.
Rafters are 2x10, roof is 5/8 plywood with novaseal underlay and steel roofing (see pics). I'm planning on installing T&G soffiting. I live in eastern Canada, and will mostly be using the cabin in the summer/fall, but also now and then in the winter. It will be wood stove heated. Obviously since I'm already pretty far into the build my options are somewhat limited, but would be interested in what approaches people would use.
Thanks!
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Mike 870
Member
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# Posted: 5 Sep 2017 10:14am
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If it were me, I'd install 8 inch batts, drill holes at the top of the blocking in the middle and the blocking on each end of the shed roof to allow airflow. I'd be willing to accept less than code for such a small cabin that I'm using only several times per winter. Since you are venting it, you will not run into moisture problems. I think this is a pretty easy solution. Also I really like your design, looks cool. Looking forward to seeing your progress.
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DaveBell
Moderator
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# Posted: 5 Sep 2017 04:55pm
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About the same as the R value of those giant windows using Closed cell spray foam.
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Steve_S
Member
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# Posted: 5 Sep 2017 05:02pm
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IMO from deep NE Ontario...
As mike mentioned, venting from soffit to top ridge using baffles all the way. Drill in 1.5" holes to allow for airflow AND staple Galvanised Steel Wire Mesh (big screen) over the holes !!! Provide no easy access for critters... SOFFITS will NOT keep critters out.
REF:
https://www.homedepot.ca/en/home/p.raft-r-mate-attic-rafter-vents---rigid-extruded-po lystyrene.1000503435.html
Next, use Roxul ! *NOT* Fibreglass !! 16oc or 24oc depending on how you built. You can use "galvanised retainer wire" between rafters to keep them up snug (not needed 90% of time but good to do anyways). You could probably do 6" batt R22 & 4" batt R14 for a total of R36. Remember offset your joints between layers.
REF:
https://www.homedepot.ca/en/home/p.comfortbatt-r22-for-2x6-studs-24-in-on-centre.1000 123047.html?autoSuggest=pip
Lastly: Good sense & building Code means Vapour Barrier, 6mil Poly. IF you have seams when you roll it out, ensure 4-6" overlap, use Accouti-Seal on the laps & joints AND tape edge with Red or Blue Tuck Tape. Then cover with whatever ceiling finish you want. NB leave no openings in the Poly and provide good "flaps" for when insulating & barrier-ing (cute word eh) the lower walls.
REF: Accousti-Seal
https://www.homedepot.ca/en/home/p.pl-acousti-seal-vapour-barrier--sound-reduction-ad hesive-825ml.1000409510.html?autoSuggest=pip
FYI: Accousti-Seal is an amazing product BUT get it on your hands, clothes, hair... you will resort to very extreme drunken sailor talk !!! Worse than any Newfie could manage, even with a few gulps of Screech !!! and it will not wash out in the laundry (don't even try) so if your a messy person, wear crappies that are ready to chuck.
Hope that answers your question and is of help. Enjoy the Cabin and may you have many good years filled with good times & memories there.
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ICC
Member
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# Posted: 5 Sep 2017 05:26pm
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Quoting: Steve_S You can use "galvanised retainer wire" between rafters to keep them up snug
What do you mean by that...?
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rockies
Member
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# Posted: 5 Sep 2017 06:10pm
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This is where you run into a mess of options (which can easily turn into a mess). Once you start adding insulation, vapor barriers, venting, baffles, vent holes you have to get them in the right places and in the right order or you risk trapping moisture within the roof assembly.
The EASIEST (though perhaps not the cheapest) option is to spray foam the underside of the roof sheathing, put up a sealed poly vapor barrier and then your ceiling finish. No vents, baffles or access holes.
I would order a Tiger Foam kit and spray it yourself. When you're done, send the empty tanks back for recycling. The spray foam is insulation and vapor barrier all in one and your roof needs no venting. Spray it and forget about it.
http://tigerfoam.ca/
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Steve_S
Member
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# Posted: 5 Sep 2017 08:09pm - Edited by: Steve_S
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@ICC a thicker galvanised wire about the size of a coat hanger & 1/2 longer than the space between rafters so it bows slightly up... keeps the insulation from sagging or shifting. Used to be common practice (in some areas) long time ago for other batt insulation.
Example from Lowes:
https://www.lowes.com/pd/100-Pack-Insulation-Support-Common-23-5-in-Actual-23-5-in/10 07637
@Rockies, you are correct on the spray foam points but the cost...
The "Fast Rise 600" kit = $850
This kit produces approximately 600 square feet at 1†or 50 cu. ft. yield
As for R-Values... looked & looked and can't find it... Somehow I think it should be pretty obvious that detail should be quick to find / see.
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ICC
Member
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# Posted: 5 Sep 2017 10:17pm
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I know what they are. I have used them to hold batt insulation up against a sub floor when placing insulation under a floor. That was not my query.
What I don't understand is your recommendation for using them in a roof, a ceiling. We don't want to hold the insulation UP against the roof sheathing. We want the air space above the insulation, don't we?
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Steve_S
Member
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# Posted: 5 Sep 2017 10:37pm
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Quoting: Steve_S venting from soffit to top ridge using baffles all the way
Would allow for the airflow and not "compressing" the insulation up, just keeping it from shifting down. Shallow roofs not so much of an issue but steep roofs could lead to issues. Roxul is a pretty tight fit as it expands into the cavity and rarely shifts at all.
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DaveBell
Moderator
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# Posted: 5 Sep 2017 10:56pm - Edited by: DaveBell
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Rockies, I don't think they get it. (not an attic)
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ICC
Member
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# Posted: 5 Sep 2017 11:13pm
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Quoting: Steve_S just keeping it from shifting down.
It seems to me you are overthinking this. Roxul used in walls does not have any problem; does not shift down in the stud bays. Why would a roof?
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scotia1
Member
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# Posted: 6 Sep 2017 11:15am
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Thanks for all the info, much appreciated. Going to sit down with a beer and ponder my options regarding cost/work/effectiveness
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KelVarnsen
Member
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# Posted: 6 Sep 2017 02:02pm
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[img=null]null[/img]Quoting: rockies This is where you run into a mess of options (which can easily turn into a mess)
Everytime the phrase "vapour barrier" comes up you can't count on Rockies recommending against it. I think he must have been assaulted by a vapour barrier as a child and hasn't gotten over it.
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snobdds
Member
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# Posted: 6 Sep 2017 02:39pm
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Spray foam is the best option; not in price, but in time, function and results.
Never stuff a roof joist rafter to the underside of the roof with insulation.
Understand the difference between a cold roof and a warm roof.
Depending on whether you go cold roof or warm roof, dictates how to vent the roof.
Depending on your material for insulating, dictates if a vapor barrier is needed.
Too many variables to consider for a vague(ish) question.
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DaveBell
Moderator
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# Posted: 6 Sep 2017 03:35pm - Edited by: DaveBell
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Those two big windows form part of the roof as far as heat containment goes. Two inches (~$370) of Green 602 spray foam would provide about R14 and seal against moisture. Then vapor barrier and finish ceiling would trap air also providing more R value. Anymore insulation in the ceiling might be just a waste considering the R value of the windows. I don't know why a cathedral ceiling would be vented.
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rockies
Member
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# Posted: 6 Sep 2017 06:34pm
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6 ml poly was considered the classic interior vapor barrier for decades. It's the equivalent of wrapping the interior of your home in a plastic bag. Unfortunately, unless all the other details (flashing, air sealing, siding and trim, drywall install, etc) are exactly right you can trap moisture in the walls.
people always think of moisture getting into the walls from the exterior (only when it rains) but it can be driven through the wall assembly by sunlight and also get into the wall cavity from the interior (through electrical outlets, ceiling lights, etc).
Fortunately the building scientists at Green Building Advisor go out and build sample walls and test them for moisture damage. As far as they are concerned the only place for 6 ml poly in a building is under a concrete slab on grade.
http://www.greenbuildingadvisor.com/blogs/dept/musings/vapor-retarders-and-vapor-barr iers
http://www.greenbuildingadvisor.com/blogs/dept/musings/do-i-need-vapor-retarder
There are much better choices than 6 ml poly.
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ICC
Member
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# Posted: 6 Sep 2017 06:38pm
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Quoting: DaveBell I don't know why a cathedral ceiling would be vented.
A non-vented, so called hot roof, where the insulation is placed against the underside of the roof decking can NOT be done with ANY batt type insulation and still meet any building code. Do a hot roof with spray on foam under the decking OR with sheet foam panels on top of the decking.
some resources.............
http://www.jlconline.com/how-to/insulation/insulating-cathedral-ceilings_o
http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral- ceiling
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