Newbie here. Only my second thread. We are doing a 20x40 Country Plans cabin in Southeast Alaska.  The climate is temperate; cool most of the year, with high humidity. I've been reading some of the archive posts on this subject but am still perplexed. Common thinking seems to be to vapor barrier the heated side. I planned to do this but on a five hour ferry ride I chanced to meet a builder from Juneau and he told me not to provide vapor barrier. He says he has had to do mold repair in not-that-old structures that did vapor barrier. He said structures done 40 years ago, though not as energy efficient, don't suffer mold problems. I also know a man who had to put new plywood on his entire roof up in SE Alaska because condensation could not evaporate. He had not provided a vent space between plywood and insulation, a situation he remedied second time around. He showed me some of the old black decomposing roof plywood and was so adamant about not providing wall vapor barrier that he removed a section of wallboard and let me take a look behind insulation to show me the condition of wall studs. He did o wall vapor barrier and the studs looked good. He too advised it would be better to let the walls be able to breathe somewhat. Perhaps roof and walls are two different questions.  I know I am going to provide air vent gap between roof plywood and the insulation, and vent roof apex and eave. Providing vapor barrier is a hot topic among builders up there. I was given the impression many seem to have tried it and are now leaning the other direction. The thinking seems to be that the small amount of vapor allowed through doesn't make much difference in energy efficiency. Firemen seem to not like all the plastics in construction too. I'm leaning toward craft faced insulation in the walls.

Should I plastic vapor barrier my interior walls and roof rafters up in SE AK?
If so should I allow vent space with a furring strip between the exterior wall covering and the insulation?  Or what other precaution?
Has vapor barrier technology stood the test of time in a cold damp climate?

Mel

I would recommend reading the following "House for very Cold Climate" from buildingscience.com
This page has a download link for the PDF document
http://www.buildingscience.com/documents/houseplans/hp-very-cold-recommendations/view?searchterm=cold%20climate
Basically on this design they've not used an interior vapor barrier. They have sealed the interior drywall to the framing with caulk. Then they use rigid foam over the exterior housewrap and leave the wall cavities empty. The roof is also rigid foam with a healthy 3 1/2 inch vented space above the foam board and under the shingles. Foam floor too.


The "Cold Climate" version could also be informative
http://www.buildingscience.com/documents/houseplans/hp-cold-recommendations/view?searchterm=cold%20climate

Here's an image from the Very Cold Climate design



The article has more images that illustrate the drainage plane, vapor management and air barrier

THanks Don. I edited my post slightly before seeing your reply.  I'll try to digest your suggest reading. I appreciate the help.

Quote from: MelFol on January 03, 2011, 09:36:56 PM
If so should I allow vent space with a furring strip between the exterior wall covering and the insulation?  Or what other precaution?

Mel

One thing I've noticed while reading the build science web articles and Joseph Lstiburek in particular is that he likes/prefers 2 inch air spaces in vented roofs. IRC and IBC codes specify 1 inch, but he likes more as indicated by the 3 1/2 inch space in the Very Cold design. He uses a 1x furring strip under the siding on the Very Cold design

Quoteknow a man who had to put new plywood on his entire roof up in SE Alaska because condensation could not evaporate. He had not provided a vent space between plywood and insulation,

Was the roof insulated with fiberglass batts?  Little doubt that would create a big problem if not vented. The moisture in the air inside the house could make it through the fiberglass and condense on the cold plane of the underside of the plywood and be held there by the fiberglass. That can be prevented by installing some rigid foam sheet on top of the roof plywood. Then another layer of plywood on top and finally the shingles/metal. Then the roof could be built unvented.

Or vent it

Here is a link to a PDF done by the university of fairbanks on insulation and vapor barriers for 1 1/2 story structures.http://www.uaf.edu/ces/publications-db/catalog/eeh/EEM-04550.pdf.  Hope it helps.

That is a good article. Thanks

In the roof design detail, there is a long screw used to go through (12") foam insulation and roof sheathing ... into the rafters to hold the 4" wood strips in place.  Anyone have any tricks for accomplishing this?  ... for driving the screw blindly through the insulation and yet finding the rafter?  I've only gone through 4" of insulation, and my method of finding the rafter is a bit clumsy.... would like to find a better scheme.

Mt Don: That is exactly what happened to him.  I plan to do the roof as you suggested. The foam sheets I've heard of up there are called Proper Vent. Probably others available too but I haven't come across them.  Then R-30 under that in the 2x12 rafter.  Does that sound workable?

Haven't read the very cold climate pdf yet but am trying to digest a lot of info. I'll get too it.

Chuck:  Thanks for the link.  They probably have a good feel for AK.

JRR:  I have the same question. Layout should be close, but I need to hit studs solidly. I also haven't come across long screws with truss head that don't cost an arm and a leg. I have 800+ sq ft of 3" EPF foam to wrap 3 sides of the structure. Brought the windows and doors out to accommodate 3.5 inches.  I'm figuring 1/2 inch air gap between EPF and siding (per one of the drawings I found on countryplans) and desire to put prefinished hardi plank on the exterior for longevity and fire protection. HB weighs 3lb per sq ft. I don't want it coming loose down the road. My EPF is 2' wide. My studs are 16 OC. I hate to cut every piece of EPF so hoped to use steel studs mounted vertical for furring 2' OC.  I'm still thinking it through. I may have to cut the foam to 16".  Hopefully the article will enlighten me.

I have helped out on a number of cabins up here and some of the people have used T&G (2"x6" boards) as their roof deck followed by foam, then osb or plywood.  We used screws from this website http://www.fastenmaster.com/lok-line.html.  The heavy duty flathead one.  They are a bit pricey but work well.

I have used Headlok a few years ago

http://www.mcfeelys.com/search.aspx?q=headlok&src=M8GLSCW&s_kwcid=TC|4266|headlok%20screws||S|p|4712748385&gclid=CMnc686moaYCFQTNKgodDCfOog

I had considerable success on a 12/12 pitch roof with 2x12 rafters by filling the entire roof cavity with 11 inches of closed cell foam----expensive but eliminates venting requirements and gives me about R60+ in the roof.  No condensation problems and no separate vapor barrier required. Cost me about $3600 for 816 sq ft of roof area. You can actually achieve the same vapor barrier with about 2 inches of closed cell foam if you don't need all that R factor-----and you can add FG batts over the foam too.

In the above design for the "very cold climate" ... I wonder what negative impact it would have if loose or blown in cellulose were added between the studs and rafters.  Would it move the condensation plane out of the exterior foam?  I wouldn't think so ... should still be vapor friendly, and would add a few "R's" to the wall and roof.  ??

Its kinda curious that the wall framing and the roof framing are "inside and warm", and the floor framing is "outside and cold".  Would it be that difficult to add the foam up against the flooring system underside?

JRR. 
I wondered the same thing about the empty walls. On the "cold" house they have insulation in the walls and 1" rigid wrap around the outside.
I also wondered why they have an air gap between the siding and rigid insulation on the Very Cold house but not on the Cold house recommendations?  I wish they had explained both of those issues. These contradictions can be confusing.

I calculated the heating degree days for our location (rough calculation w/o accurate charts), and it comes out in the high end of the "Cold" range. I will probably go with my original preparations; 3" rigid foam outside the house wrap (I already have the foam) with 1/2 air gap between siding and rigid foam. Original plan was fiberglass insulation in the 2x6 walls. (cellulose machine would be needed for longer than they want to let it go. Couldn't get it back soon.) I could go either R19 or R11 if I see a compelling reason for the dead air space between insulation and OSB.  Due to this reading I'll not put up a plastic vapor barrier on the walls.  Instead I'll install the drywall to be the vapor barrier.  But I'm still reading and looking. 

I am very interested in this subject. I have been trying to do some research on vapor barriers and have also read the information at buildingscience.com.

My location, however, is the central area of western Arkansas.
I am building a simple one story wood frame structure on a pier/post foundation with a trussed roof.
The crawlspace will remain open, no perimeter skirting.

The floor:
Attached to the underside of the joists will be hardware cloth for rodent protection.
Between the joists I will probably use Dow's Safetouch unfaced polyester batts.
Over the batts will be 3/4" OSB subfloor followed by 30# felt, then snap together bamboo flooring.

(I thought about using 7/16" OSB (cheap) in place of the hardware cloth as an air barrier since the crawlspace is open, but wondered if it along with the subfloor OSB might act somewhat like a sort of double vapor barrier, trapping moisture)

The walls:
OSB for exterior sheathing. Over the OSB will be vinyl siding. Again, probably Dow's unfaced Polyester batts for the wall cavity insulation. Latex painted drywall over that.

Ceiling:
Latex painted drywall on the bottom of the trusses for the ceiling. Cellulose blown in on top of the drywall.

As far as recommendations from the insulation companies, Dow will not recommend any information concerning vapor barriers with their Safetouch product. The cellulose company, Concoon, actually recommends not to use a vapor barrier with their product.

Unless I can see a definitive answer to this vapor barrier question, my current thoughts are to not use a vapor barrier anywhere for this climate.

I don't want to make a big mistake, however, so I would welcome all comments on this vapor barrier subject for this Arkansas climate, using the above mentioned materials.

Thanks,
Arky

A/C or no A/C?

If A/C then no vapor barrier on interior walls.   

If no A/C, not 100% sure but I think still no vapor barrier, in your climate. 

Quote from: MountainDon on January 09, 2011, 02:58:58 PM
A/C or no A/C?

If A/C then no vapor barrier on interior walls.   

If no A/C, not 100% sure but I think still no vapor barrier, in your climate. 

No A/C, but hope to eventually cool the house with a system using underground cooling tubes.

Would no vapor barrier in this climate also apply to floor and ceiling ?

In a Hot-humid or mixed-humid climate my understanding is no vapor barrier on the interior anywhere; walls, ceiling/roof and floor when there is interior cooling. When the interior is cooled if there is a vapor barrier or even vinyl wallpaper, the moisture will come from the outside humid air through the wall and condense on the vapor barrier in the wall.

That's all gleaned from buildingscience.com

Quote from: MountainDon on January 09, 2011, 04:22:23 PM
In a Hot-humid or mixed-humid climate my understanding is no vapor barrier on the interior anywhere; walls, ceiling/roof and floor when there is interior cooling. When the interior is cooled if there is a vapor barrier or even vinyl wallpaper, the moisture will come from the outside humid air through the wall and condense on the vapor barrier in the wall.

That's all gleaned from buildingscience.com

That's about what I figure also.

But with osb for wall sheathing, and no interior vapor barrier, should one not add a layer of foam board on the exterior of the osb to keep it warm enough in the cold months to prevent condensation on it's interior surface. I've heard estimates of using foam board with a R-value of 1/4 to 1/2 of the R-value of the wall cavity batts to prevent this.

Also, in the floor, I would rather use plywood or preferably osb (cheaper) instead of hardware cloth on the bottom of the joists, but again wonder if it's inner surface would be a condensation point without putting foam board over it. (Just thinking that with the hardware cloth, there might be quite a bit of air infiltration due to the open skirted crawlspace.)

Arky

Arky, you should go have a search on buildingscience.com for info on insulation in your climate. They have a pretty good search function. There's also a section named designs that work. Loads of info.

Arky:  I have tried to digest all the recommended reading mentioned here because I too don't want to make a mistake.  Here's how I summarize what I've gleaned from that reading.  Heat always moves toward cold, and moisture vapor hitchhikes with the heat. If there is enough moisture vapor in with the heat it can condense when it reaches dew point due to the cooler surface.  Where temps go back and forth from warmer outside than inside, to colder outside than inside, a vapor barrier on either side can cause moisture problems. If VB is on the inside moisture can condense in the wall in summer when warm humid air from outside migrates inward and reaches cooler interior conditioned space. It doesn't necessarily matter how you cool. The thermodynamics remain the same. And if the VB is on the outside then condensation can take place inside of that wall in winter when warm interior humidity migrates toward that cool outside surface.   (I hope that doesn't sound too wordy).

Here is my somewhat conclusion for where we are building in cold climate SE Alaska. The thermodynamic logic says to VB the inside to keep moisture from migrating from inside to out. Thus it can't reach the cool outside of the wall where it could condense. But I've talked to folks who say not to VB up there at all because of mold problems they have encountered where a VB was installed. The thermodynamic being unchangeable in how it works I conclude someone must not have provided a proper seal everywhere in the vapor barrier when they built, thus allowing moisture from inside the living space to migrate into the walls. Another possibility is the lumber was not allowed to dry enough before enclosing it. Those are the only logical explanations I can think of for what they had happen. Complete seal would seem to be critical. So would dry lumber.  I thought about VB on both sides, but suspect if there is too much moisture in the lumber when it is sealed up one is asking for trouble too. The problem for me is extruded polystyrene foam wrap tends to create a vapor barrier on the outside. So how do I keep from trapping moisture in the wall in a very humid climate?  The framing has roof and house wrap around the outside of the walls and can breathe while we are gone for the winter. Hopefully that will take care of any excess humidity in the framing before I close up the wall. Plus 3" of R-15  EPF wrapping the outside will hopefully help keep the studs from reaching a dew point.

Then again, I once saw a sign on a Professors door that said his idea of a tragedy was a deduction killed by a fact.   Any other thoughts or insights would be appreciated.

I have two question on this topic as well.  I have read quite a bit on this topic, and I think I have my head around the building sciences for full time residences in my climate, but I am not sure about some other details. 

South central Texas climate, 100 miles from coast, average humidity varies from 40% to 70%, sometimes more.  Temps range from mid 60s average in winter to 110 in summer, of course there are extremes in both directions as the weather changes. 

Building is 90% unused and unconditioned.  1 weekend a month type place with ac or heat as needed.  Exterior siding is a thin plywood based T-111 like product, with fiberglass batts between the studs. 

So, two questions: 

Given the building is not used or even powered except for 3-4 days a month, would I consider the rules about conditioned space as the inside?  If your not conditioning the space, does it even matter? 

Is using felt paper the same as a true vapor barrier in this case?  What would be the effects of 15# felt paper on the inside side of the studs prior to wall sheathing?  The sheathing would likely be wood cedar planks, and the felt is an attempt to 1 reduce draftiness and 2 provide a background neutral color so eventual minor gaps from expanding and contracting planks never show up. 

muldoon, as I understand it felt is not a vapor barrier. Felt can absorb vapor and then transpire it in either direction. It is a liquid water barrier. Used under the siding on the exterior that is one job it does well. At the same time it acts as an air barrier. But not a vapor barrier.


melfol, I believe you are likely right in believing peoples problems stemmed from improperly sealed/installed vapor barrier. The article from Fairbanks states that was a problem they saw with the "old" way of placing insulation in the attic kneewall



another article that may be of interest, from another guy who seems to know his stuff. It is old now but worth a read.

http://bct.eco.umass.edu/publications/by-title/housewraps-felt-paper-and-weather-penetration-barriers/.

I noticed a huge tempurate increase in my cabin once the vapour barrier was installed, it was like night and day.  im in a colder area.   I know it doesnt provide a huge R value, but it seemed to make a huge difference in both keeping cool air out, and the time to heat the cabin dropped significatly.

There are several older threads that have good information on this:

http://countryplans.com/smf/index.php?topic=1629.0

http://countryplans.com/smf/index.php?topic=7371.0

Note that the kraft paper on fiberglass bats is more a vapor retarder than a vapor barrier.  In Arkansas when my family built our home, we sheathed the outside walls with a foam sandwiched with foil backing (except near the corners, where we used waferboard).  This acted as a combination insulation and vapor barrier on the OUTSIDE.  The wall insulation was blown cellulose with a bonding agent.  That house is now almost 20 years old and has never had problems with mold/moisture and the climate is similar to this one.