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Natural air sealing material for stick built cabin

 
steward
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I've been asked for advice about insulating and sealing a tiny cabin on piers.  Currently it has board and batten on the outside, stud walls and pine board interior siding.  It is very drafty.  The interior walls and ceiling can be removed for renovations and the floor can be accessed from below.  The wall boards are kinda natural and a bit wavy so you can see through the gaps.  Even when removed and reinstalled you'll probably still be able to see through the wall to the innards.

For a slightly better than conventional build, I'd say to use rockwool insulation and then cover the inside with black poly and cover it with the interior siding.  I'm hoping the black poly won't really be identifiable through the wood siding cracks.  Question:  If there isn't housewrap outside the insulation, will that be a problem or is it minimal?  The climate is cold and generally dry (Montana).  We can't remove the exterior siding so I'm not sure if there's a way to accomodate.

If the black plastic is a cosmetic issue, would covering it with brown kraft paper be a reasonable option?

For the floor, should they try to run the poly around the floor joists and up into the joist bays and then put in the insulation and then add something to hold up the insulation?  I'm assuming we'd want the poly on the "conditioned" side of the insulation.  But wind blowing under the building would wash the heat out of the insulation if it's exposed.  Or would it be better to just install rockwool and cover under it with housewrap (not poly).  But would that block enough air leakage?  I doubt it.

If they wanted to do something a bit more natural, and yet proven, easy and reliable, what might that be?

Thanks!
 
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Hi Mike,

House wrap, for me, is a fairly new invention .... if you are speaking of the wrapping that goes under the siding.  Its main purpose, from what I understand, is to stop drafts.  Some people feel it does too good of a job of doing this.  So, much will depend upon how tight the house is and the threshold of the occupants.

When I built my cabin in MN, I did not use housewrap.  We did not have a problem with drafts.  We did have 6.5 inches of insulation in the walls ....1 inch wood siding ....and 1 inch paneling inside.  
 
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Interesting project! A bucket of random thoughts:

Interior:
- I agree, rockwool and heavy-duty black poly on the interior facing side would make a huge difference (I would avoid fibreglass; it breaks down, and rodents love it, and it's miserable to work with)
- Perhaps house wrap could be tacked to the outer siding from the inside, before the insulation is installed
- With paper, I worry about fire hazard; as well as the risk of inducing rot if it gets wet (on the exterior side)

Exterior siding:
- Somehow, the spaces between boards need to be closed with some sort of caulking or chinking material (to shut out the wind and slow heat loss, to keep moisture from saturating the insulation, and to discourage tiny creatures from colonizing the spaces)
- Commercial latex caulk would be fast and effective, but the plastic waste may be a concern
- The only natural chinking material I can think of is heavy jute or sisal swine, tamped into the spaces; maybe soaking in raw linseed would help it stick in place?

Underneath:
- Some sort of skirting aroung the base to stop the wind is just about essential; it creates a bonus dry space for storing yard tools etc. so it's multi-purpose
- The insulating method has to protect against rodents and insect colonizers; that usually means a tightly sealed and caulked insulation sandwich; some people use heavy roofing tar paper, or a full covering with plywood; a fine weave hardware cloth would work too, but it's spendy; I think house wrap would work very well, but I don't know its rodent resistance
- Foam sheets are easier to work with, but certainly not natural, and not rodent resistant
- Even creating a dead air space with house wrap would make a difference, I think, but if going to the work it's better to insulate at the same time
 
pollinator
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Is it lived in year round? Seasonal? Weekend retreat? How good are the windows and doors? Any ceiling/roof insulation?

I am really not a fan of vapor barriers added to older homes and cabins.  They cause MAJOR moisture issues if the rest of the wall is still leaky.

The easy answer is closed cell foam, but even the greenest foams aren't very green.

If the owners are OK with the price, I would use rockwool in the walls and floor and then use either rigid rockwool or plywood as a breathable barrier.  Tyvek or another "breathable" barrier would be the next choice, something that has at least a chance to dry through.

Floors are a real challenge depending on the ground clearance and moisture levels.

Don't over insulate the walls if the ceiling and windows are still leaky.  Insulation is only as good as the weakest link.
 
Mike Haasl
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Thanks Douglas and R!  The building is occupied as a weekend retreat style place.  The windows are shitty and need to also be replaced.

I'm thinking that preventing air infiltration is more important than insulation so I won't suggest they do anything more than R13 or so.  The attic is about 8" tall and there is fiberglass insulation there currently that goes above the ceiling joists.  Not sure if it's abutted nicely or gappy.  It might be good enough as is if they can get some visqueen up there.  I'm pretty sure the attic isn't vented so there probably isn't much wind passing through it.

Great point about the skirting!  Critter proofing the floor insulation is also a great idea.  I've done that with 1/4" plywood and tight seams and it kept mice out of a cabin very well.

 
Douglas Alpenstock
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For the attic, sometimes a thin layer of roughed-up cellulose is enough to fill the gaps and augment the existing batts. It's not hard to fluff up compressed cellulose bales with standard garden tools, at least enough for this job.  Bonus: mice don't care for it.
 
pollinator
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Sealing anything with black poly may cause moisture entrapment issues. If you can get breathable wrap that would be ok, even for an old building.
Can you give us a photo, its hard to create an image of what you describe.
 
Douglas Alpenstock
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It's normal in cold locations in North America to have a vapour barrier on the interior side of the insulation (in fact, it's Code). This keeps normal house moisture vapour from migrating through the insulation and forming frost on the exterior side, which both wastes the entrained heat and reduces the insulation's effectiveness. Also, in a dry climate (like mine) it's hard to keep enough humidity in the house to keep it comfortable in winter.
 
John C Daley
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I live in a dry climate. I have never been aware of the humidity requirements.
What are they please?
 
Mike Haasl
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When it's cold and the humidity is down under 20% you can get static electricity build up so whenever you touch a doorknob you get an annoying shock.  Plus, if you're from a normally humid area (central north America for instance) when it's dry in winter you get cracks on your hands and other issues.  I'm guessing there are other "humidity issues" but those are the ones that come to mind.

For buildings here, the risk is definitely moisture getting into the insulation in winter.  If you have a humid bathroom or kitchen, the humid air could get into the insulation and if it reaches the dew point before it leaves the building, it condenses inside the wall.  If it's really cold out, it likely freezes at that point.  Then in spring, all the ice in your walls will thaw and be a problem.  I'm not a building science expert so hopefully I said that somewhat correctly...
 
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Its not a dry vs humid climate issue, its a cold climate issue.  Warm, moist air on the inside space wanders through the insulation to a spot where its cold enough to create condensation ... and now you've got a mini waterfall either in the insulation itself or at the interface between the insulation and the exterior sheathing.  Mold and rot result.  So an impermeable barrier on the inside keeps the moisture out of trouble.  There is generally enough air leakage around windows, doors, kitchen vents, etc that the interior doesn't become as wet as a swamp!  If the structure is REALLY tight (like in a PassivHaus) then you need a special air exchange unit.

In non-cabin builds with sheetrock, impermeable paint can be applied.  Wavy pine board need more help, and a layer of plastic is an option, and provides better protection than faced batts (and I've never seen faced Rockwool anyway).  Another option is to just use thin sheets of plywood - minimize the number of seams, tape them (with fancy tape*), and paint the plywood to seal it (may not be necessary).  The plywood path seems more difficult, but I prefer it because: 1) its a lot more durable than plastic, and a picture hanging screw or nail is less of a compromise if it manages to penetrate 2) you can add sealant behind the plywood to ensure the top and bottom sills are throughly sealed - with plastic you're counting on the interior wall to compress the plastic against the sill (or you're using tape), and I've never found the plastic to work with glue 3)I _think_ the plywood has fewer nasty bits that the plastic 4) its much more rodent resistant 5) the plywood adds tremendous additional strength.

I wouldn't worry about housewrap - that is largely to keep rain from getting into areas with low air movement.  If the exterior cladding is in good shape against rain, has some overhangs, etc then moisture in that dry climate shouldn't be a concern.

On the inside ... unless $ or interior volume are really precious, more insulation is always better.  If its just a 2x4 cavity, then you can get r-15 rockwool in there.  There is also a rockwool "board" (https://www.rockwool.com/products/comfortboard-80/) that is mostly used for adding insulation on the outside, but can be used as an additional layer on the inside.  Its nice too because instead of insulation in the cavity, and the studs as something of a thermal bridge, there is a continuous layer of insulation.  Yeah, you'd have to rework the window and door jambs to accommodate.









 
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The building science info would scream DO NOT!! use black poly there as it will cause moisture troubles and wall rot.  The outer barrier in that climate must be a vapor permeable air barrier so the wall can dry.


Suggest watching videos on perfect wall.(especially Matt Risinger videos)  Given your constraints you can not do it but you can probably rob the principles and still make it work.  Another one to watch is the building principles videos by Joseph Lstibuerk

The outer layer in perfect wall is just designed keep most of the water from reaching the next layer.  Cracks and gaps are actually a good thing.  The next layer in is an air gap.  Then comes a vapor permeable air barrier.  Then the insulation.  In perfect wall both the membrane and insulation should continuous.  Then you have the sheathing and finally have the standard stud wall.   You can't do that so you will have fudge seriously.

If I understand the principles correct here is what I would try.  I would start by adding a tapered board sloped to the outside to the top of each section of sill and covering it with a layer of the brush on sill seal that is waterproof/vapor permeable that covers the tapered board and goes just a bit up each stud.  The goal here is simply to try and get any moisture that gets in the insulation and goes down to the sill out of the wall to the outside as fast as possible while still hopefully allowing the all existing wood to dry.  Then I would add a bunch of vertical strips say 1/2" square to the inside of the siding.  Its job is to space the barrier away from the siding.  Then staple the first layer of barrier to the studs.  The barrier might be tar paper or Tyvek or many other answers.  Tar paper would give you your black background but would be really hard to work with.  So guessing you will want some one of the other materials.  Then I would fill the rest of the cavity with rock wool insulation.  Over that I would put another layer of barrier except now unbroken down the inside studs.  Over that I would try and get my layer of continuous insulation.  My best bet from reading would be the rockwool comfort board.  It is a bit more rigid than rockwool plan.  Cover the entire walls in it in as continuous insulation.  Then nail or screw the interior siding to the studs thru the comfort board.  

This in less than optimum form should fulfill most goals of perfect wall.  It won't have quite as much moisture shedding capability but should have decent drying so hopefully there will be enough air flow to dry any moisture that gets in before it rots the wood.  You might want to be adding screened vent holes to the siding top and bottom.  That depends on the amount of cracks you already have.  It does give the rain screen layer, ventilation layer, moisture shedding layer, exterior insulation, close to continuous barrier and an inner nearly continuous insulation layer.  It just stacks them in the wrong order.

For floor joists insulate between, vapor permeable air barrier stapled to the bottom of the joist and then a layer of comfort board insulation to give as close as possible continuous outside without the joists creating thermal bridging.  Just batten boards most likely would be enough to hold it up but more likely I would add a layer of plywood screwed thru to the joists to protect it and hold it up.

Now remember the ceiling is your worst heat loss location as heat rises creating stack affect and sucking cold in everywhere..  So you really do need to insulate it too.  Insulation method would vary greatly depending on type of roof and ceiling.  Watch a bunch of the videos then figure out how to apply the principles in your case.
 
Eliot Mason
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Mike - I was reflecting on Letellier's response and it occurred to me that we are probably missing a detail.  It seems probable to me that the cabin has a layer of tar paper on the outside already.  It has been common practice to do that.  If that's there then the extensive interior work that Letellier suggests may be moot.

Of course, you may not know until you've gained access to that first bay...

Also, I realize that I failed to include two other points in my previous post.

First, any tape you might use to seal the interior needs to be fancy stuff.  No duct tape, packing tape, foil tape, etc. 3M makes some stuff (of course they do...) (https://www.3m.com/3M/en_US/company-us/all-3m-products/~/All-3M-Products/Adhesives-Tapes/Industrial-Adhesives-and-Tapes/Air-Barrier/?N=5002385+8710676+8710815+8711017+8747182+8759538+3294857497&rt=r3).  The other "most popular" brand is Sika (https://usa.sika.com).

These tapes actually bond for a long time.  I can attest that whatever the "standard" tape was in early 1990s Wisconsin had completely failed in a decade.

The other thing ... leaky buildings leak everywhere, and that's one way to avoid problems.  Once you start sealing the place you've go to do the whole thing right, otherwise all that moisture and warm air will be concentrated in one place, a veritable jet blast of moisture into one place and THATS A PROBLEM,  Not long ago here in relatively balmy Portland we had a cold snap and people suddenly found the air leaks... one unfortunate homeowner had a leak in a bathroom vent pipe and it created a giant icicle in the attic that then broke off and crashed through the ceiling!
 
Douglas Alpenstock
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C. Letellier wrote:The outer barrier in that climate must be a vapor permeable air barrier so the wall can dry.


Yes, I think we're all in agreement about that.
 
Douglas Alpenstock
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Eliot Mason wrote:It seems probable to me that the cabin has a layer of tar paper on the outside already.  It has been common practice to do that.


Very true! That was standard procedure for houses with cedar siding as well (including mine).
 
Mike Haasl
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I'll do a bit more digging today to see if I can figure out if there's tar paper under the exterior siding.  I doubt it.  This is a sleeping cabin only, no shower, kitchen or sink so the humidity creation is pretty modest.  

C. Letellier wrote:The building science info would scream DO NOT!! use black poly there as it will cause moisture troubles and wall rot.  The outer barrier in that climate must be a vapor permeable air barrier so the wall can dry.

The black poly would be on the interior side of the insulation, not on the exterior.  I'm currently planning on not having a vapor barrier on the outside of the insulation since I can't take the siding off.  If there does happen to be tar paper there, then I'm fine either way.

My current plan is to remove the interior wall boards, install rockwool in the walls and fix any missing spots in the ceiling fiberglass insulation.  Then cover the walls and ceiling with poly and put the interior boards back on.  For the floor, cut poly into strips the width of the floor joist bays (plus 4") and staple them onto the floor from below.  Add insulation and cover it with 1/4" plywood with tight seams.

The door currently has a healthy gap under it for ventilation and windows could be cracked if needed.

Any concerns with buying kraft paper to put over the poly so it looks prettier when folks peek between the boards?  Fiberglass insulation is often kraft faced so I think it would be ok.  For that matter, would kraft paper take the place of the poly?  Not sure if the insulation based kraft paper is waxed or treated in some way so it acts differently from just a roll of kraft paper?
 
Eliot Mason
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Mike:

I've never dealt with kraft paper ... so I'll offer nothing on that point.

As for the floor... I've been wondering if you need to line the floor bays.  It would depend on the permeability of the floor - which is a function of the materials and construction method.  If its a sheet good (vinyl/marmoleum) I think its watertight. Even a wood floor often has a barrier in it to prevent moisture coming from below and warping the floor - and that should stop moisture from getting out.
 
Mike Haasl
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I just went out and did some inspecting in the daylight.  The wall studs are every 22".  Which sucks.  The floor boards appear to just be 1x6s.  They might be tongue and groove but I can't tell.  There is fiberglass underneath but it's not pretty.  There is a max of 14" of clearance to the ground, decreasing to maybe 10" at the tight end.  So working from below will suck.  Luckily it won't be me doing it  The floor joists appear to be every 2' since the insulation is stapled up to them.  The floor boards definitely don't block much air.  But I don't relish the idea of them removing the fiberglass that's in there currently.  

Maybe putting plywood under the joists will be enough to block drafts. Then with poly on the walls and ceiling it will be tight enough.
 
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Perfect Wall links. Once you are there search their website for more bulletins on Perfect Wall.Perfect Wall
 
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Mike, three thoughts.
1.) given the odd framing spacing, maybe blow-in insulation would be easier. Staple up a clear vapor barrier and fill through holes in it, you can see your progress and react accordingly.
2.) Maybe tongue and groove boards for below the joists to retain/protect the insulation? easier than panels in that tight space?
3.) interior wall boards and gaps. The gaps that will stand out will be at eye level (standing, seated, lying in bed). If allowed, maybe run the boards over a jointer to straighten them all (loss could be made up by adding a board, a wider base trim, or cove at ceiling), or selectively plane just the worst high spots down (in the hi-visibility locations).
 
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Mike Haasl wrote:
Great point about the skirting!  Critter proofing the floor insulation is also a great idea.  I've done that with 1/4" plywood and tight seams and it kept mice out of a cabin very well.



I had rodents chew through my cedar duckhouse. In the end, I had to wrap the house in 1/4 inch hardware cloth. That solved the problem.
 
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Our experience in CO with a 10' × 14' room we added, 2×4 frame on a deck set on piers.  STD paperbacked fiberglass insulation in walls, 1,5 inch foam board and 1/2 inch chip board under floor, pad and rug inside.  STD roof insulation. Only air leak e as a light fixture which was not caulked.  Even doing all that, nothing could keep it warm until we closed in the "crawl space".  Used left over galvenized roofing bermed with dirt.  Mainly heat migrates from the main house.  But, a coil of black 4 inch irrigation piping on a south exposure exhausts into that space underneath. We screened the ends.
Seems heating the underside helps the most.

 
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The old timers natural chinking way ...

2 parts clay (or dirt)
1 part sifted wood ashes
1/2 part salt
Water to mix




 
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... if the structure is on piers; would this not be an opportunity to investigate running an ondol/kang/hypocaust system? - with a rmh twist perhaps; Permies is definately the place to brainstorm the idea.... if the floor is heated you may welcome a bit of air infiltration.      
 
Mike Haasl
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I think skirting it won't work due to critters that would love to live under there.  Currently they are too exposed to want to live under there.

What's a ondol/kang/hypocaust system Thomas?  This cabin is already pretty cool since it has a RMH in it already.  The heat just bleeds out of the building too quickly.
 
Douglas Alpenstock
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Mike Barkley wrote:The old timers natural chinking way ...
2 parts clay (or dirt)
1 part sifted wood ashes
1/2 part salt
Water to mix


I would love to understand the chemistry at play there!

I have seen other recipes for natural chinking that involve pure chicken doo as the binder with rinsed-out horse doo as the fibre matrix. I wonder ...
 
John C Daley
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OK, what is CO, STD please?
 
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Mike, as someone else mentioned above, please don't use black plastic, an impermeable barrier, inside the insulation. Using vapor-permeable fabric will stop the wind and allow some moisture out, which is what you want. Someone suggested piecing the vapor barrier into each stud bay, against the exterior siding. That won't work, it should be as continuous as possible and taped at joints.
The floor: the cheapest step you can take is put down a vapor barrier of black poly on the ground. That will greatly reduce the moisture coming up. Instead of plywood attached to the bottom edges of the floor joists, use builder board/Homasote which offers much better insulation, is cheaper, and will make it quieter inside.
I have to agree with everyone who has said avoid kraft paper behind the interior siding. The best recommendation I read is to put up a vapor-permeable house wrap on the inside of the studs and then blow cellulose into the cavity, unless you want to spend lots of money on rockwool or recycled denim or natural wool insulation.
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[url=https://www.builderonline.com/products/building-materials/insulating-homes-with-natural-sheeps-wool_o

https://www.builderonline.com/products/building-materials/insulating-homes-with-natural-sheeps-wool_o

An article about the benefits of using a most natural insulating product.
 
Thomas A. Cahan
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... ondol (Korean), kang (Chinese), and hypocaust (Roman) basically use the underside of the building as a furnace flue; the inlet usually also serves as the cooking area... the flooring is of course stone etc which also retains heat - it would be like living in a chickie brooder.    
.... if the sides/underpinnings need addressed anyway this may kill 'many bird one stone'.... it would also be secure underneath; although any critters inside would be conveniently smoked or bbq'd.      
 
Mike Barkley
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OK, what is CO, STD please?  



I'm not sure but I think Laura meant Colorado & standard.
 
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I just had to remove all of the fiberglass insulation from under my pier and beam Cabin in the Ozarks.  It was held up next to the subfloor with black poly.  It was a huge mistake.  Trapped water/moisture and also provided ample nesting/breeding ground for mice. I replaced it with bubble foil like this guy's video: https://youtu.be/xPTJD9nuWvQ.  So far, so good.  I would highly recommend it and will be using it for other floor construction.
 
Posts: 19
Location: Alaska, South Central
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forest garden fish solar
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Not sure what everybody is looking for but... I've been building in Alaska (cold climate) for close to 40 years. I've watched the building codes change back and forth. but in my opinon (for waht it's worth) from the inside -out.
1) Interior finish, drywall, paneling...
2 vapor barrier - 6 mil visquine - taped and sealed.
3) fiberglass batt - whatever thickness for you studs are
4 ) Sheating.
   a)If it's plywood - maybe just use red vapor barrier tape at the seems.  - unless you intend to use a HVRC system, or,Tyevk
5) siding
6) If you get the house to tight, there are a lot of envrionmental issues that need to be dealt with. You can ( and they Do! ) get indoor air quality issues
 
Doug Steffen
Posts: 19
Location: Alaska, South Central
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forest garden fish solar
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PS: The vapor barrier needs to be on the WARM side of the insulation!!
 
Mike Haasl
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Location: Northern WI (zone 4)
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Thanks Doug and Kacy.  Doug, all of my building experience aligns with what you typed.

Interior finished surface, then poly, then studs and insulation, then exterior sheathing, then housewrap and siding.  

If the poly is on the heated side of the insulation it's good.  If it's on the outside of the insulation it's quite bad (per Kacy's experience).

So my recommendation to the cabin owner (from inside to outside) is pine boards (old, wavy, reused), kraft paper (to show between the gaps in the pine boards), 6 mil poly, rockwool and then the existing board and batten siding.  Ceiling will be to leave the insulation that's up there in place and add more if there are voids.  Then cover with 6 mil poly, kraft paper and reinstall the ceiling boards.  I think the floor will end up with the floorboards untouched, then the fiberglass underneath untouched, then add some mouseproofing hardware cloth, then hold that up and block some of the wind with tightly installed pine boards from their sawmill.  Then they'll get some air exchange through the floor but the "tight" walls and ceiling should vastly cut down the air passing through the building.  
 
Laura Hans
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Off grid so not online much.  Yes. CO = Colorado STD = Standard.   I should also add the interior walls are drywalled with a vapor barrior paint mixed with a heat sink additive.  Little thermal mass beads of a material whose name escapes me right now.
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