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mike202025
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# Posted: 26 Apr 2015 11:57am
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In Quebec City area, cabin has 2x4 walls with fibreglass batting. Not enough R in walls. Was told at a Lowes that 1.5 inch SiveRboard on outside on top of existing OSB would get me to correct R (board has R7.5) and that with decent batting would put the condensation point in the board. Was told Silverboard has decent permeability and when taped properly provides the air barrier. Assuming this is correct, the question becomes whether to keep the inside poly vapour barrier. For the bathroom the Lowes guy says keep the poly, the Silverboeard has enough permeability. Looking for comments, confirmation and / or other alternative. Re. alternatives, I like what the Lowes guy says because I can keep the 2x4s, batting and poly and just work from the outside.
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creeky
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# Posted: 26 Apr 2015 02:53pm
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No. No poly vapour.
The silverboard has a tinfoil cover and is, according to the mfct, a vapour barrier.
but you're kind of between a rock and a hard place. a bathroom is pretty humid.
is this a summer / seasonal cabin?
if you have the vapour barrier installed, I would look at the roxul exterior board instead of the silverboard. it will breathe.
roxul is all round one of the best insulation choices.
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rockies
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# Posted: 26 Apr 2015 06:08pm
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You can try looking at the "Greenbuilding.com" advisor" site.
http://www.greenbuildingadvisor.com/search/node/silverboard
If these don't answer your questions, maybe other articles on rigid foam applications will.
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MtnDon
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# Posted: 26 Apr 2015 06:38pm - Edited by: MtnDon
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The specs on that product claim the material has a high water vapor permeance. The laminated surfaces are micro perforated much like the roll house wraps (Tyvek, etc.) The info states.... "SilveRboard XS - Exterior Radiant Wall Sheathing has been engineered with a ventilated surface film to meet not only the demands of the job site toughness, but new 2012-/13 codes both for R value - High Water Vapor Permeance and Low Air Permeance"
So the Lowe's guy could be correct in his claim. I did not try to verify the fact that the 1.5" thickness would place the dew point in the foam, but that is the goal when there is wall cavity insulation. I willtry to find the info I have used before to verify the required R-value needed for the exterior foam.
SilveRboard web site = "The reflective laminated surfaces of SilveRboard XS are micro perforated in order that the SilveRboard XS exterior wall sheathing insulation materials can "breath". "
Poly is not always recommended as an interior wall vapor barrier. It might be best to eliminate it. Theories vary. With no inside poly the wall can dry to the interior where it is usually drier and warmer. I would do some research on the link provided by rockies above as well as looking on buildingscience.com
Manufacturer link There's also an installation manual link on that page.
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MtnDon
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# Posted: 26 Apr 2015 07:03pm - Edited by: MtnDon
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Here it is.... another greenbuilding.... link
Depending on the climate zone the 7.5 may be a tad low. I think Canada uses a different zone system than the USA.. Here is USA info ....
a google map
IECC info
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MtnDon
Member
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# Posted: 26 Apr 2015 07:54pm
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here's what we need; a map that includes Canada and the USA
QC is a 6... give the Lowes person a thumbs up.... 
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creeky
Member
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# Posted: 26 Apr 2015 09:10pm - Edited by: creeky
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I'm editing this. i wrote it after coming in from working in the fields. my wife says I thought everything was funny. but rereading this ... I'm not so sure.
this part stays the same: and I'll only add that builders and building sites seem to conclude in the majority of cases:
amvic says "a layer of reflective lamination on both sides. This highly effective combination of materials performs better than traditional flat-sheet insulation by providing built-in vapour and air barriers"
and this too remains the same:
green builder (2014) says about silverboard ... "Even better would be to add rigid rock wool panels (and no rainscreen mesh), which are inherently highly vapor permeable." and not use silverboard.
finally I leave:
roxul is very cost competitive in Canada. and for our climate with hot muggy summers and cold winters, is in all probability a better solution.
 sometimes the edit button is your friend.
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mike202025
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# Posted: 30 Apr 2015 04:46pm
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Thanks for all the replies. I will check the links, the roxul exterior board/rigid rock wool without rain screen mesh. Once I have digested all the additional info I will probably be back with some final questions. Consensus seems to be no to inside poly vapour barrier for walls.
This was seasonable but we are spending more time out there in fall/winter so have to upgrade. Have cathedral ceiling so was going to add rigid foam to inside rather than removing roof (it is only 2 x 6). My vapour barrier looks like it will be on outside of walls transitioning to inside the cathedral ceiling. For transition was going to use rigid foam between rafters, over top of the wall and sealed to the Silverboard/roxul wall layer. The floor will be wood for permeability with batting in between the joists, foil layer down (got this from building science.com). May be doing more work on the roof and crawl space in a few years, so hopefully the above is good to start.
Again, thanks a lot, you guys are life savers.
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