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rockies
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# Posted: 22 Oct 2018 20:35
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Everyone worries about moisture getting into their cabin walls from the interior and causing mold and rot but few people know that sunshine can drive moisture into the wall cavities from the exterior and your 6 ml poly vapor barrier could be making things worse.
https://www.finehomebuilding.com/2017/05/10/sunshine-drives-moisture-walls
Check the link to get the article as a PDF
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fiftyfifty
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# Posted: 23 Oct 2018 08:31
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This happened at my cabin. It's an off-grid cabin that is not heated when we are not there. We put up the poly over the insulation on one trip and left it like that. When we returned a few weeks later, it was a warm, humid and sunny day. But the inside of the cabin was still very cool. We noticed that there was lots of condensation on the inside of the poly, to the point that it was making some tiny rivulets that were dripping down, especially on the south side. We knew the moisture hadn't come from inside the building (from showering, cooking, breathing etc) because we hadn't been there. We eventually took down the poly. So now our building is just stained T1-11 on the outside, fiberglass in the stud bays, and then quarter inch plywood on the inside as the walls. No vapor barriers of any type. Not ideal, I suppose, but better than having a dripping mess.
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Nate R
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# Posted: 23 Oct 2018 11:18
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I'm thinking of using fiber cement panels for siding, but the resivoir effect of it can be a concern.
To alleviate problems like Rockies linked to, I'm planning on using the B.O. Hydrogap WRB on the exterior of my cabin. Kind of a mini-rainscreen. I'll also have full 24" overhangs on all sides, minimizing rain/snow buildup and splash on the siding. On the interior walls, I'm thinking of using asphalt building paper or kraft paper if I can find it vs poly. (I'm intending to insulate with mineral wool, which only seems to come unfaced.)
Hoping this all together will minimize the risk of issues like those cited in the PDF. And, off-grid with no A/C should also decrease risk.
Thanks for sharing, Rockies. I had already chosen these materials for other reasons, but solar driven moisture was something I hadn't thought about much.
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NorthRick
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# Posted: 23 Oct 2018 14:11
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Few homes up here in Alaska have air conditioning. And, our winters are long, meaning there is a lot more time where warm moist interior air is trying to get out through the walls. So, for us, interior vapor barriers is still the way to go.
If I were building a house in the deep south, I'd put a vapor barrier on the exterior since much more of the year you are using the AC. For locations in between, I'm not sure what I would do.
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ICC
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# Posted: 23 Oct 2018 14:46 - Edited by: ICC
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Quoting: fiftyfifty No vapor barriers of any type. Not ideal, I suppose,
In some climates no vapor barrier is quite good. Whether or not a vapor barrier is needed or can actually create a problem is variable depending on the climate zone you are in. That has been known for a couple of decades now.
https://buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
The idea for using a vapor barrier began in the northern areas before residential A/C was commonplace.
NorthRick has the right idea re north and south. But I am not sure if A/C in summer complicates things in climates like ND, Manitoba, etc where winters are definitely cold and summers can be warm enough that many homes there have A/C as well.
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rockies
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# Posted: 23 Oct 2018 19:45 - Edited by: rockies
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A good option to a poly vapor barrier is a product like Membrain.
https://www.certainteed.com/building-insulation/products/membrain/
This is a "smart" air and vapor barrier which remains "tight" in winter but becomes more vapor permeable in summer to allow moisture to escape.
Whatever you do to a wall any moisture in the wall cavity (either from the interior or driven in from the exterior) must be allowed to escape ( usually through the opposite side). The worst thing to do is put a non-permeable layer on both sides, thus trapping any moisture within the wall.
Nate R: As long as there is a drainage plane behind the fiber cement siding you're fine. It can be 30 lb felt paper on the sheathing and vertical spacing strips 16" OC too. Mineral wool insulation is the best for inside the wall cavities. You might consider adding Comfortboard mineral wool panels to the exterior sheathing too.
Northrick: For extremely cold climates research shows that all the insulation should be on the exterior of the sheathing and NO insulation in the wall cavities.
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NorthRick
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# Posted: 24 Oct 2018 12:20
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Quoting: rockies Northrick: For extremely cold climates research shows that all the insulation should be on the exterior of the sheathing and NO insulation in the wall cavities.
Maybe. My house is almost 50 years old and has 2x4 stud walls with fiberglass batts in between and 6-mil poly just behind the drywall. It's still quite habitable.
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95XL883
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# Posted: 24 Oct 2018 15:54
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Hmmm? As I think ahead to building our cabin, I'm wondering if I'm really missing something.
My thought was to avoid a separate vapor barrier entirely. I have the impression that condensation occurs when moist warm air meets a cool surface. From what I have read, use of board/sheet type insulation, (poly-iso, Roxul comfort board, etc) of sufficient thickness and properly taped would basically act as a vapor barrier while keeping the warm moist air away from a cool surface that the moisture could condense on.
Thus, my thinking is I would use mineral wool in the wall cavities and probably 2" of Roxul comfort board over the sheathing. For my area, 1" of sheet insulation is considered adequate to prevent condensation. (And experience with my house confirms this.) I would probably use 2" just to be safe.
I am basing my thought on various readings of how to insulate a vaulted ceiling. As I understand it, the idea is to have the comfort board thick enough and air tight enough that it prevents condensation by separating warm moist interior area from cool surfaces. Maybe this is where I'm off. Maybe the no separate vapor barrier approach is for foam insulation. I guess I need to check the permeability of Roxul comfort board. Maybe I will need to add a vapor barrier to the outside of the comfort board?
(I have a situational preference for mineral insulation. Insects and pack rats are a huge problem on our ground. Thus, I'm thinking part of preventing infestations is to use insulation that is uncomfortable for them which means avoiding fiberglass, cellulose and foam products. Please correct me if my thinking is off here.)
I then just have to worry about any moisture buildup in the cabin. During the summer, when in use there would be an air conditioner running with a condensate drain to the outside. And 2" of comfort board would be enough to keep the very humid summer air from condensing due to summer air conditioning.
Is my thinking basically right? Or am I way off base? Thank you for any insight you can give me.
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rockies
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# Posted: 24 Oct 2018 19:29
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95XL883: All these extra concerns over moisture in the walls is due to the trend of "tightening" buildings up by sealing all cracks and stuffing in as much insulation as possible. The trouble occurred in trying to figure out what to do about the vapor barrier.
Conventional wisdom dictated that a 6ml layer of poly be attached to the interior studs to keep moisture vapor fro getting into the walls but that seemed to make things worse.
Your idea of using a board type insulation (and Roxul Comfortbard is probably the best since it doesn't risk trapping moisture between it and the sheathing like a rigid foam insulation board) is a good one since it will keep the exterior sheathing warmer.
A warmer exterior sheathing will help prevent moisture coming from inside the cabin from condensing on a cold sheathing surface (within the wall cavity) but the moisture (if it gets in) must have a way of getting out (hence no poly).
However, even more critical than adding exterior mineral wool boards to the sheathing is the importance of putting an AIR barrier on top of the sheathing. Various house wraps have been designed to do this but they must be installed and detailed perfectly in order to work. A much simpler method is to roll on a liquid air barrier system like Stogard Gold Coat.
https://www.stocorp.com/sto_products/sto-gold-coat-2/
This product is vapor permeable but it stops air from transporting excess moisture into the walls. It gets rolled onto the sheathing like paint and there are special tapes that you use to seal around door and window openings.
So the order of your wall construction is: interior finish (drywall or wood paneling), wall studs, Membrain vapor permeable barrier, mineral wool batts in the wall cavity, exterior sheathing, Stogard Gold Coat, mineral wool boards, vertical nailing strips 16" on center, siding.
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95XL883
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# Posted: 25 Oct 2018 13:44
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Hi Rockies,
Thank you for the detailed answer. Trying to be brief and put it in my own language,
1. my plan of no separate vapor barrier is just not smart
2. actually, I should plan on two separate barriers with one being Stogard Gold Coat and special tapes on the exterior sheathing to minimize outside humidity penetrating the building and the other being Membrain to allow any moisture in the wall to breath out
I do have some specific questions.
First, I'm a little confused on exactly where the Membrain goes. Does it go between the stud wall and the exterior sheathing? Or does it go between the stud wall and the interior finish?
Second, shouldn't I duplicate both the Stogard Top Coat and the Membrain on the floor and roof ceiling? I'm planning on an open vaulted ceiling and due to cost concerns I will probably be building on piers so floor insulation and sealing the floor will also be important.
Third, making the cabin this tight should I be looking at some sort of air exchanger so there is a fresh air supply for the interior?
Finally, my wife really likes and wants a barn look. Thus, I'm thinking any siding nailing strips will be horizontal, not vertical (so my wife can have a vertical "barn board" look siding but then I'd be creating cavities where moist air could accumulate. Do I need to be concerned about these cavities becoming moisture collection areas? If so, I'm thinking that I will either wind up with a second layer of sheathing on top of the mineral wool boards or I'll fill any cavities between nailing strips with more mineral wood board.
Thank you very much for sharing your wisdom.
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rockies
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# Posted: 25 Oct 2018 19:19
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The Membrain takes the place of a typical 6ml poly vapor barrier so it is attached to the inside of the studs under the drywall or interior paneling.
Stogard Gold Coat is only rated for walls, not floors or roofs. Sealing those areas depends on your climate and how you plan on building your floor and roof system. Need more info.
An air exchanger (like a HRV) uses a lot of electricity, which may be too much of a drain if you're using solar PV. There are Lunos fans which cycle fresh air into the building that use a lot less energy and don't require ducts.
https://foursevenfive.com/lunos-e/
If you are planning on having vertical barn board siding then you should use a vent strip that has air channels within the strip like Coravent. This allows air (or rain) to run through the strip which is important if installed horizontally.
http://www.cor-a-vent.com/siding-vent-sv-3.cfm
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