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Scott G
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# Posted: 27 Feb 2015 06:19pm - Edited by: Scott G
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Hi,
Live in east coast Canada - it gets cold and stays that way for a good long time each year.
So, building my cabin using reclaimed material, namely the rafters. They are 2x4. I am insulating only to R15 by placing 3" foam between the rafters. I added a small leanto to the building and used full dimension 2x6 for that rafter.
I've attached a pic showing the roof and the layers from T&G, Vapor Barrier, Foam, Strapping, Rhino Roof stuff, Metal roofing. (Also Soffit and Ridge vent is in place).
As I was building all the local builder guys said my plan would be fine in my small 14x20 cabin, (main part is 14x20, leanto is 6x20 - cathedral ceiling 16' to top) that I'd have no problem with the lower r value and keeping the place warm - maybe just add another log to the fire - fair enough.
Today I mentioned what I was doing to another local contractor and he was very concerned that with only r15 the warm air hitting the cold will lead to tons of condensation, ice buildup on the roof, all kinds of nasty problems with water...
As a result of that conversation I'm VERY, VERY stressed out over my roof - and it's built already, just putting in the 3" foam this week. I could build up the rafter and add 3" of roxul inside before adding the vapor barrier. I'd loose that much head room in the loft but I can only stand up dead center as it is.
I'm rambling, sorry. Any and all opinions are really appreciated. Thank you very much in advance.
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felineman
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# Posted: 27 Feb 2015 07:15pm
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You would be better using an A/B spray foam not only doe it stop condensation but provides a vapor barrier too. And is alot more then R15. If you want it to last build it right.
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MtnDon
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# Posted: 27 Feb 2015 07:43pm - Edited by: MtnDon
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You could have some issues. How much will it be used in winter? If it is not used in cold weather there won't be a hot - cold differential to cause problems. The more it is used the greater the likelihood of problems.
Use XPS foam to fill in the rafter bays and seal them well using foam in a can. Do not bother building up the rafters. Do not bother with the air space, push the XPS foam against the roof sheathing and seal with the foam in a can. The idea being to seal that cold roof underside so air and moisture can not condense there. The sealing has to be done well enough to stop air movement. You do not need the exterior roof vents if the space between the rafters is fully filled with foam. and you do the next step as well.
Or skip all that messing around and fill the bays with spray in foam. Closed cell foam will get you close to R7 per inch. Open cell will be less than XPS at about R4.
Then install sheets of XPS foam against the inside surface of the rafters. That also provides a thermal break at the rafters and makes the insulation more effective. There are 1.75" diameter nylon washers made for this purpose; used with screws long enough to anchor in the rafters. OR use 1x furring strips and screws through to the rafters to hold the XPS in place. Do that and you don't need the vapor barrier sheets; the XPS is a barrier when installed tightly. Best results in 2 layers with the panel seams staggered.
Yes you lose some interior space / headroom, but it is a loft and most lofts don't have full headroom anyhow. It's just a sleeping space, or not?
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Scott G
Member
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# Posted: 27 Feb 2015 08:15pm - Edited by: Scott G
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Thanks.
We will be fulltime in this. Weekends for the next 2 years then fulltime. At that time the cookstove will be fired 24/7 pretty much from late october until may.
Even if I push the foam as tight as i can I'm only against the strapping and rhino roof wrap, not tight against metal. Foam on leanto is in and sprayfoam sealed...3" gap above that. Argh.
I have 3" roofmate foam board. If I add 2 1" layers I'd achieve R28-29.
That with the airgap? Thoughts?
What about adding boards over the additional 2" of foam?
Yes, just a sleeping loft. Its a no-jumping-jack zone.
Thank you both for commenting.
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creeky
Member
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# Posted: 28 Feb 2015 09:33am - Edited by: creeky
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The problem you'll have with putting the xps up against the roof now is that any moisture that gets in there between the vapour barrier and your xps insulation has nowhere to go. This could be a problem for your strapping and your rafters.
So I would leave the 1/2" gap as you have it. Make sure this can vent somewhere (I suspect you've thought of this already, watching for insect infiltration etc).
Then I would put up my xps between the rafters as you have it. I would spray foam to seal the edges. Being aware that you now have a second vapour barrier.
This brings your insulation flush to the outside edge of your rafters. Now you could add another layer. Maybe poly iso? Right across the rafters. Even one inch would bring your well sealed r15 up to r22 (give or take). Two inches and you're at r28.
Now you have a breathing area for your rhino/strapping/rafters. And you have a high insulation roof/ceiling complete with vapour barrier. You're all set.
It'll work great. Congrats. Nothing wrong with this design.
And you'll need some longer nails/screws for your interior finish layer.
(Ideally the insulation goes on the outside of the roof on top of the vapour/rhino barrier.)
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Scott G
Member
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# Posted: 28 Feb 2015 04:19pm - Edited by: Scott G
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Quoting: creeky So I would leave the 1/2" gap as you have it. Make sure this can vent somewhere (I suspect you've thought of this already, watching for insect infiltration etc).
Check. Yes, I have tons of ventilation in the soffit and ridge vent.
I should add that the rhino roof layer is only tuck taped on one side, forgot the other. Not sure how much that matters. Its well overlapped. Saw it more as a failsafe if the old tin roof leaked than anythong else.
Quoting: creeky Then I would put up my xps between the rafters as you have it. I would spray foam to seal the edges. Being aware that you now have a second vapour barrier.
Yup, got 12 cans of foam. 
*So you're saying that the rafter foam/spray foam is itself a vapor barrier? Then no need to add a layer of plastic, black glue edges, over this? (like I will be doing on the walls)
Quoting: creeky Now you could add another layer. Maybe poly iso? Right across the rafters. Even one inch would bring your well sealed r15 up to r22 (give or take). Two inches and you're at r28.
Looking into locally available options....searching homedepot.ca (if you know of a suitable product feel free to link - thank you.)
So...R22+ will be ok? That's a real load off. Bigtime.
Huge thank you.
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creeky
Member
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# Posted: 28 Feb 2015 06:50pm
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ridge vents rule.
do you have a lowes out there? they have tin foiled polyiso.
and yup. you can skip the extra layer of plastic if you foam over the rafters. can you pop down to the US? ridiculous that foam sheets made in canada are cheaper there. but...
as for R value. i mean. you can always throw on another log. it's just how long do you want to wait before you have to put another one on. and another. and ...
I told the story of John in winter tales. he has to put a log on every hour. man I got tired of getting up at 3 a.m. to put wood in the wood stove after one winter. and He's been doing it for decades. each to their own.
insulation costs now but pays back for a long time. I watch kijiji for deals. they come up from time to time.
and everybody. if you're doing foam. look into getting a gun and the foam canisters. the cans last wayyyyyy longer. plus you can close the gun and keep the foam for months.
this Fomo Products Inc. F61050 HT - 300 7" Polymer Dispensing Unit is excellent. It came recommended to me after I broke my first gun (which cost 50 bucks). These cost 22 bucks now.
I've had three guns over 5 years. the first one lasted two months. all plastic. wand snapped. the second one lasted almost 3 years. forgot to close the gun and left it for some months. dried solid. this third one has been going for over a year.
the spray foam I'm most fond of is TYTAN PROFESSIONAL Extreme Climate PRO. available online and at Lowes.ca
just watch the vapour. you don't smell it. but whew. when you find yourself suddenly very clumsy ...
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Steve_S
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# Posted: 28 Feb 2015 07:16pm
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I have to agree whole heartedly with Creeky... Kijiji rules when looking for deals, just be ready to act and have cash on hand... I got almost all of my foam at seriously deep discount through Kijiji.
Also ++ on the Tytan foam, nothing beats that stuff just make sure your wearing gloves and even one of those "toss away" type painters coveralls come in handy when working in close quarters & tight spots, you'd be amazed how quickly that stuff glues to you !
If your in New Brunswick, I think the recommended R values are R20 for walls & floor & R40 for roof.
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saskboy
Member
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# Posted: 28 Feb 2015 07:33pm
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I think your worrying too much, I finished insulating my small 12x16 cabin with r 14. My rafters were 2x6 and I put up the plastic rafter vents from lowes, so I only had room for roxol r14. Let me tell you, it got cold in saskatchewan at -30 to -40c outside when I did the vapor barrier. I had to open windows in the cabin with a small fire going, my small 2000w wall heater I installed cycles off at those temps even. I am going to have to chop small pieces of wood for my stove or Ill be cooked out of there. So im sure your insulation will be plenty warm......
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Scott G
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# Posted: 28 Feb 2015 08:51pm
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Thanks a million for the comments guys. Very appreciated. Honestly im not worried about heat, im sure ill have to crack open a window. It was the potential moisture problem that had me frazzled.
Im no longer frazzled.
We have a wood cook stove so ill have to use smaller logs and fill oftern anyways. Man i sure appreciate the advice. Major thank you all.
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Scott G
Member
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# Posted: 28 Feb 2015 09:19pm
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One last question; any thoughts about using double bubble foil poly? Claims to be made for cathedral ceilings and adds r10.
Hoax? Just marketing? Dont bother my r15 well sealed will be fine?
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Don_P
Member
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# Posted: 28 Feb 2015 10:47pm
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Google "FTC and radiant barriers".
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Scott G
Member
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# Posted: 28 Feb 2015 11:02pm - Edited by: Scott G
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Ha! Thanks for saving me $200+. Wild.
Came across this as well: http://www.small-cabin.com/forum/2_4454_0.html#msg65910
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