# need advice concerning vapor barrier



## 163481 (Jul 8, 2015)

I'm going to be doing some renovation to a frame and stucco house with a sprayed foam roof. The reno will include removing the drywall from two rooms, installing rigid foam (polystyrene) insulation, sealing the edges and voids with spray foam and re-drywalling the rooms.

The question is: Where does the vapor barrier go? I've heard that it is to be installed on the warm side of the wall while my neighbor, who's a contractor, says on the cold side.

Who's right? And why? Could either of us be confusing the Tyvek house wrap with a vapor barrier?


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## Stick486 (Jan 4, 2013)

the polystyrene is the vapor barrier...
it's in the right place for vapor barrier...


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## DaninVan (Jan 1, 2012)

Yes. The Vapour barrier goes on the _inside_; the Tyvek on the outside. Tyvek is NOT a vapour barrier!
In fact, just the opposite. It breathes water vapour through it, but not water itself. In other words a molecule of H2O in vapour form can pass through Tyvek but a water droplet can't.
You probably should check with your Building Dept. re your local Code requirements regarding 'rainscreen'.
Here (Canada) it's mandatory and very closely inspected. Condo rot focused a _lot_ of attention on this issue.
The pic below shows a system for commercial or highrise construction, but the principles the same for residential...


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## 163481 (Jul 8, 2015)

Thanks, guys, that's very helpful.


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## DaninVan (Jan 1, 2012)

Any more vapour questions, Chuck, we're here 24/7...


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## Stick486 (Jan 4, 2013)

@sfchuck...
Dan meant to say vapor


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## DaninVan (Jan 1, 2012)

If Dan had meant to say "Vapor" he'd have driven down to Blaine Washington. 
https://www.google.ca/search?q=vapour&ie=utf-8&oe=utf-8&gws_rd=cr&ei=8cOyVef3AsL1-AHr4I2oAQ


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## Stick486 (Jan 4, 2013)

DaninVan said:


> If Dan had meant to say "Vapor" he'd have driven down to Blaine Washington.
> https://www.google.ca/search?q=vapour&ie=utf-8&oe=utf-8&gws_rd=cr&ei=8cOyVef3AsL1-AHr4I2oAQ


may be you should have...

*British vapour. a visible exhalation or noxious gas...
*


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## Cherryville Chuck (Sep 28, 2010)

One of the escape routes for vapour is through light outlets and plug in receptacle. I spray foamed some wiring when I installed a range hood quite a few years ago. I recently changed the hood and found that the spray foam had dissolved the outer layer of insulation on the wire loom. The insulation on the individual wires was still fine, different plastic I would guess. You might want to use a latex based foam for around the wiring. 

The Canada building code also classes rigid insulation as vapour barrier especially if you finish sealing it the way you intend. The reason for the vapour barrier going on the inside of the wall is to keep the vapour inside the house from getting into fibreglass, cellulose, or any other insulation that is capable of holding vapour. Once those types of insulation get wet the R value goes waaay down. Also, if it's cold, the dew point at which the vapour will condense into water is somewhere between the outside of the wall and the inside. This leads to mold and mildew which are serious health hazards as well as eating up the structure of the house over time.

Of course if you are in the States then you are dealing with water _vapor_ which, according to Stick, is something completely different.:wink:


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## Stick486 (Jan 4, 2013)

sfchuck said:


> I'm going to be doing some renovation to a frame and stucco house with a sprayed foam roof. The reno will include removing the drywall from two rooms, installing rigid foam (polystyrene) insulation, sealing the edges and voids with spray foam and re-drywalling the rooms.


PL 300 is your friend here..
glue your panels up to the studs and the insulation board seams together for a complete VB......
use the stuff as caulk if the need arises..

Foamboard Adhesive PL 300 VOC from Loctite Adhesives


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## 163481 (Jul 8, 2015)

I suppose putting up some 6 mil poly just behind the drywall would be overkill but the San Francisco area is very humid and damp during the cold months.


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## DaninVan (Jan 1, 2012)

That's not overkill, Chuck, that's mandatory. It should be labelled "Vapour barrier" or words to that effect. Not all 6mil poly is created equal.
The whole point, as *Other Chuck* explained is to prevent any interior moisture from migrating into the wall cavities. Moisture is a building wrecker.
https://www.google.ca/search?q=fung...oTCMTw19qM9cYCFYg5PgodIccI0Q&biw=1120&bih=524


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## Stick486 (Jan 4, 2013)

sfchuck said:


> I suppose putting up some 6 mil poly just behind the drywall would be overkill but the San Francisco area is very humid and damp during the cold months.


it's not over kill but not required in this locale rigid polystyrene board is used and all seams and penetrations are sealed...


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## Ghidrah (Oct 21, 2008)

In the 80s when super insulated homes were becoming the rage the poly barrier was applied to the ext side of the studwall before sheathing then once the wall was standing and insul'd the barrier was applied again to provide a "DEAD" airspace. When Tyvek came out it pretty much killed poly barrier use. You want moisture to expand out but not suck in. 

Something to consider, if you're applying the barrier because of insul fuming or fear of it, I understand but if you apply it and the exterior wall is compromised in some way it will likely hold moisture in the wall longer than would be wise. Seen in the past renovations and damage control repairs, in areas of high moisture, (coastal, surrounded by trees and shrubbery and points south) mildew build up, moisture damage and all the critters that go along with moist wood has been an issue regarding int. poly barrier applications. The int. barrier prevents one from seeing and or smelling the results of moisture damage from inside the house. 

If your ext wall is already covered in Tyvek the house is protected.


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## 163481 (Jul 8, 2015)

Just for clarity, the exterior walls are stucco and the house is about 65 years old. I don't know how...if at all...that figures into the equation.


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## Cherryville Chuck (Sep 28, 2010)

Stucco is breathable like Tyvek is. You can't seal both sides of a wall or you get dry rot or worse. A building has to be able to breathe. I bought a book from Lee Valley a few years ago that is an excellent resource on this subject. Mine is "Builder's Guide to Cold Climates" but he also wrote "Builder's Guide to Mixed Climates" for locations where the temperature rarely falls below 45F. The author is Joseph Lstiburek and he is a forensic engineer who is internationally recognized as an authority on moisture related building problems, including rain penetration, air barriers, and vapour barriers. The books are pricey so you probably want to try and find it at the library.


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## Gaffboat (Mar 11, 2012)

sfchuck said:


> I suppose putting up some 6 mil poly just behind the drywall would be overkill but the San Francisco area is very humid and damp during the cold months.


San Francisco cold MONTHS? Reminds me of the Mark Twain quote, "The coldest winter I ever spent was one summer in San Francisco." :laugh:


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## DaninVan (Jan 1, 2012)

*I Like Succo*



Cherryville Chuck said:


> Stucco is breathable like Tyvek is. You can't seal both sides of a wall or you get dry rot or worse. A building has to be able to breathe. I bought a book from Lee Valley a few years ago that is an excellent resource on this subject. Mine is "Builder's Guide to Cold Climates" but he also wrote "Builder's Guide to Mixed Climates" for locations where the temperature rarely falls below 45F. The author is Joseph Lstiburek and he is a forensic engineer who is internationally recognized as an authority on moisture related building problems, including rain penetration, air barriers, and vapour barriers. The books are pricey so you probably want to try and find it at the library.


Further to Chuck's info above, Stucco is breathable, but the multiple coats of oil based paint applied over the decades isn't.
That's why you'll often see paint blisters on stucco...well, not just stucco. That's moisture trying to get out that causes them.
It was finally recognized that if you could stop the moisture migrating from the interior, the problem went away.

Having said that, the rule is, the vapour barrier goes on the warm side, as Chuck said, to prevent it from condensing went it gets to the cold zone.
Well what do you do in Florida, where it's hot and very humid outside and you've got A/C inside? I'm guessing you reverse the procedure? :nerd:


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## 163481 (Jul 8, 2015)

Twain was absolutely right. When the rest of the Bay Area is experiencing 70-80 degree weather (and Sacramento is in the 80s or 90s), the city can be in the low 60s. Sweat shirt vendors on the streets do a land office business among the tourists and vacationers who show up in summer attire.

But to the specifics: During the winter months (approximately November through April), the prevailing winds blow across the Pacific before making landfall. They pick up a lot of moisture and the relative humidity is always high. There is a heavy dew virtually every morning and it often lasts well into the day. A lot of the older houses are not well insulated and that's the problem. With the inside air heated and the exterior walls cold, there is always moisture on the surfaces. With that, of course, comes all the problems you might expect...dampness, mildew, etc. That's what I'm trying to mitigate.

I do appreciate the responses to my questions. They confirm what I thought I knew and I'll proceed. My plan is to install a double thickness (3") of polystyrene in each wall cavity and seal the edges and voids with sprayed foam to inhibit air infiltration. The polystyrene is available from the BORG down the street at what seems a reasonable price and it's work I can do myself. I'll then staple up the 6 mil polyethylene and hang the drywall.

Any other advice?


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## DaninVan (Jan 1, 2012)

Yup. Find a couple of Italian boardmen to hang your drywall. 
a) they get a better price from the drywall wholesaler.
b) let somebody else deal with muscling 12' sheets up the stairs.


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## 163481 (Jul 8, 2015)

DaninVan said:


> b) let somebody else deal with muscling 12' sheets up the stairs.


That's REALLY good advice. :wink:


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## Cherryville Chuck (Sep 28, 2010)

You guessed right Dan. In hot humid climates Lstiburek says that you should dehumidify the cooled air inside the house and run it at positive pressure to prevent moisture laden air from infiltrating from the outside. A mixed climate like San Francisco is more complicated since moist air could be on either side of the assembly. Lstiburek's best suggestion is to insulate with a moisture impenetrable insulation on the outside of the wall. This keeps the dew point on the outside of the house where it can't hurt anything. (I was watching an episode of Mike Holmes and they were interviewing the insulation contractor and he said that what his preference is too.) That won't be possible here because the outside is already finished but it looks like the next best alternative is to use the rigid (moisture impenetrable) insulation up against the outside wall. The dew point will be inside the insulation layer where it can't do much harm.


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## Stick486 (Jan 4, 2013)

sfchuck said:


> That's REALLY good advice. :wink:


I second that advice...


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## Ghidrah (Oct 21, 2008)

sfchuck,

Something else to consider, this is from my personal experience in the framing and remodeling trade. I've worked on many older houses, a few of them built from 1900 to 1940, off the top of my head at least 6 built from 1875 to 1900. One built in 1778, (scratched into mortar at the back of the fireplace) and one was the Thornton W. Burgess house/museum, in Sandwich, not sure how old the house was when he was born in it in 1874, but when I helped reroof it in the late 1970s we found readable print newspaper covering the barn board roof sheathing from the late 1890s. 

One common thread ran through all of them, they all leaked air like no tomorrow. Many of the earlier roofs and walls weren't sheathed, they were strapped and red cedar shakes were applied to the lath, when the shakes and shingles were dry you could see outside. Minutes after rain began they closed up quicker than a miser's purse.

Later, 1880-1930 barn board became very popular in residential structures as prototype sheathing because they improved the lateral structural integrity over inlaid wall corner braces. The walls and roofs still breathed like a champ racehorse. 

My point in all this is those structures rarely ever showed signs of wood rot or insect damage. Most back then were framed in old growth fir so that was a plus. The only places I ever found rot or insect damage was in damp crawl spaces under the basement less houses. Moisture and insect damage began a gradual increase in the mid 50-60s when wall cavities were being stuffed with newspaper, horsehair, whatever one could think of to increase the thickness of the wall and slow air movement. By the 80s wood rot and insect damage changed gears as insulation improved and dead airspaces provided by poly barriers made the scene. Even worse when AC and yr round closed windows caught on. 

Do I have any scientific proof to back this up? Nope, only 34 yrs building, digging into and tearing down structures on Cape cod.

For what its worth.


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## DaninVan (Jan 1, 2012)

Parallel lives, Ron! 
Dont'cha just love finding those rodent droppings (or better yet, the dead mummified rodents themselves) ?


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## Cherryville Chuck (Sep 28, 2010)

There is no arguing with the fact that if the walls and ceiling can breathe they won't rot Ron. The problem is they weren't insulated and on a cold day you could see your breath inside the house as well as outside. Back in the 50s and 60s when I was still fairly young, energy was cheap so it didn't matter that the heater rarely shut off. Those facts have changed and we have to make our dwellings more energy efficient these days. The insulation and keeping the insulation dry and efficient is what has led to all the other problems and I don't really think we have it down to a finite science yet but we are on a path we can't go backwards on.


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## Ghidrah (Oct 21, 2008)

Charles,

I completely understand, my point is the trade, lead by people that didn't quite understand the tactics of moisture and how it works created guidelines that did more to trap moisture than prevent or actively repel incursion. 

Now to beat the proverbial dead horse.

How much sense does it make to go to 2X6, shell 2X4 or even 2X8 stud walls, super insulate them then increase the window footprints to let more passive light in when the R factor of the best of the windows are still about the same as 2X4 walls with sagging fiber fill insulation? Building code reqs walls and ceilings since the 80s with R25 -30, larger gas filled void winds at R5.

Now, open those winds for that breeze AC just can't replicate let all that moisture in and trap it there. It's wonderful that technology can discover advances in materials that can make a home warm with minimum heating, a conservation salute. The Inuit have done it low tech and doing it well enough to warm their houses and igloos with heat developed by their own bodies. We've had techniques that will cool them just as well for decades but they aren't attractive to the average/common psyche, maybe after the apocalypse. 

Charles even "way back then" houses were built different, Take a look around, count all the houses with fireplaces, wood stoves or gas fireplaces. Now count how many of them are on the gable end and how many rise up from the center of the house, not furnace stacks but honest to goodness central fireplaces. 

Back "then" the cold in the house was at the exterior walls, (and yes because no insul and the fireplace drew fresh cold air from the leaky walls, windows and doors). The closer one got to the center of the house the hotter it got and because of the material and technique used to construct the fireplaces they radiated heat throughout the houses maintaining temp for days without firing it up again. The radiant heat from cooking hearth/fireplaces was integral to all areas of a home. 

Cold walls became less of an issue once we got the idea of intentionally drawing cold fresh air from outside, although it in some instances, not all, created other problems like negative draw issues due to pressure imbalances between the int and ext of the house. Some passive techniques have been used to solve that by allowing air draw to be from multiple sides of a home, (windward side) to force feed the unit. Others use electric fans to pressurize.

When my house was built I had 15lb felt, 1/2" ply 1" high density corning, R15 Fiberglas in 2X4 walls with a poly barrier under the shtrk. In 1980 we were told electric heat was the way to go, in another 10 yrs oil prices would rise to equal and or pass elect., heat.


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## Ghidrah (Oct 21, 2008)

Dan, 

I added a dormer to a cape back in the late 90s; the owners had already packed up and moved back to the winter home. We found 5 desiccated squirrels in the voids behind the kneewalls and between the rafts. Over the yrs, they must have squeezed in through the vinyl louvers above the ceiling and starved because they couldn't get back out. 

In the late 80s when plotting the renovations to the Falmouth Playhouse which had been closed about 5-6 yrs we found a mountain of racoon poop about 8' high on the stage. The ***** blew town when we sent people into the building. We didn't get the contract, then later in the 90s it mysteriously burned down.


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## DaninVan (Jan 1, 2012)

"...then later in the 90s it mysteriously burned down."

Did anyone question the raccoons?


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## Stick486 (Jan 4, 2013)

DaninVan said:


> "...then later in the 90s it mysteriously burned down."
> 
> Did anyone question the raccoons?


vengeful/vindictive critters...


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## DaninVan (Jan 1, 2012)

I know,_ I know!_
Feisty too!


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## Ghidrah (Oct 21, 2008)

Dan,

If I recall right the playhouse was his 2nd property to burn down.

Further back in the thread 7/25, 11:31, you mention blistering. Mostly seen here, (Cape Cod) in contemporary latex, (more stretchable?) huge blisters and some on many multiple layered, (enough paint to hold the shingle on the house long after the nails rot away) older oil based paints. 

The old rule of thumb in our area was to prime and paint the exterior of trim only rationalizing that whatever moisture was trapped in at painting or made it in after the fact could off gas via the unpainted hidden side, in particular soffit, fascia, and frieze, not so much C boards and rakes. Seems to work fine as seen when old trim was removed and replaced really on the old houses, 50s and earlier. I see/saw far more blistering on primed and painted both sides, not sure why if wood was thoroughly dry at painting. I speculate it may occur because of nail erosion in today's anodized, mech and hot dipped galvy. SST doesn't erode and the really old cut nails didn't. I've pulled cut nails out of 100 + yrs old houses and been astounded at their condition.


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## DaninVan (Jan 1, 2012)

Terry 'The Torch'...


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## Ghidrah (Oct 21, 2008)

That bad boy looks to be having an evil plotting moment


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## Stick486 (Jan 4, 2013)

Ghidrah said:


> That bad boy looks to be having an evil plotting moment


only thing missing is the singed hair...


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## Ghidrah (Oct 21, 2008)

***** are too smart to leave trace.


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## schnewj (Nov 18, 2013)

Ghidrah said:


> ***** are too smart to leave trace.


Ron, 

Not always! Some are just not smart, or agile, enough. Sometimes they just blow the entry and exit strategy.

Case in point, while mowing the lawn Thursday I found one of the pesky critters lying next to the house behind my garbage can. He apparently fell off of the roof and had broken his back. 

I agonized over the course of action to take. I would normally, humanely dispatch the suffering animal. However, I called the city Animal Control department to come remove the poor, suffering thing. It was better then having the neighbor report me for animal cruelty or discharge of a firearm in a residential neighborhood. 

So, two hours later they show up and remove the raccoon. Probably only to eventually put the animal down. 

Moral, sometimes the Darwin Awards are not just for humans.

Bill


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## Ghidrah (Oct 21, 2008)

schnewj,

I framed in a couple development a town over, the MO was to let the found. crews in to set up to 10 foundations then clear out for the framers. The bad thing about that was the many animals that would end up broken but still alive in the holes. We usually drew straws to see who would do the job. The worst were skunks, I still get messed up thinking about the ones I drew short on.


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## 163481 (Jul 8, 2015)

Ghidrah said:


> ...skunks, I still get messed up thinking about the ones I drew short on.


They tell me canned tomato juice works wonders.


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## DaninVan (Jan 1, 2012)

With vodka. Lots of it! _Then_ you do the cleanup...


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## Stick486 (Jan 4, 2013)

you run out of beer ???...


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## DaninVan (Jan 1, 2012)

You're joking, right?


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## Stick486 (Jan 4, 2013)

not me...


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## DaninVan (Jan 1, 2012)

I'll run out of air before _that_ happens.


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## Stick486 (Jan 4, 2013)

good to know we'll be right there...


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## DaninVan (Jan 1, 2012)

Uh oh...


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## Stick486 (Jan 4, 2013)

DaninVan said:


> Uh oh...


that pillow some kind of sign language of Canada???...


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## DaninVan (Jan 1, 2012)

It's the 'Running out of Air' icon...


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## Stick486 (Jan 4, 2013)

oh....


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## Stick486 (Jan 4, 2013)

was wondering what nap time had to do w/ a beer raid...


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## Stick486 (Jan 4, 2013)

better icons???

.


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## DaninVan (Jan 1, 2012)

Time out? OK, that makes sense if your tank's empty.


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## schnewj (Nov 18, 2013)

DaninVan said:


> Time out? OK, that makes sense if your tank's empty.


Not what I was taught!

Thank God I never ran dry, I did have a buddy that blew his regulator out on a 110' wreck dive in the Florida Keys. Luckily I was running Genesis Steel tanks and had plenty of air for us both to get through a decompression stop.


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## Ghidrah (Oct 21, 2008)

I dove some in the service setting cable and S&R but never went below 120ft and had a WO2 master diver in charge every tank dive.


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## Stick486 (Jan 4, 2013)

S2 recon diver...


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## Ghidrah (Oct 21, 2008)

Small world, the only thing I would have had to fight off would have been belly up carp, curious Musky or floaters


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## Stick486 (Jan 4, 2013)

not a primary job...
used only if required and that was rarely...


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## pgwisn (Oct 31, 2014)

I know this is an older thread, sorry to interrupt your fun, but back to the topic:

I am finally likely to get my Building Permits approved today for an addition to the back of the garage for a wood shop (24' x 30' /w 10 ft walls and cathedral ceiling, and an addition of a new kitchen and a great room on the back of the house (also with cathedral ceiling). Interfacing a new construction with a house built in 1959 will give me access to the whole of the rear of the house, and the ability to upgrade it's thermal performance, with a certain unease as to managing water vapor, especially with the cathedral ceilings/roofs. I've attached a sketch of the wood shop attached to the end of the garage (with cold frames along the south side for winter veggies  ).

Earlier you were mentioning that the vapor barrier 'was' the polystyrene panels, sealed with the foam.

I found a chart in Ching's "Building Construction Illustrated" stating building materials have rated values assigned for Permeance (or perms*)
some of interest:

*Polystyrene (molded) 2.000*
*Polyethylene (6mil) 0.060*
2 coats paint 0.900
3/8" (10mm) gypsum board 50.000 
Asphalt paper (tar paper) 0.200
Polystyrene (extruded) 1.200
Polyethylene (4mil) 0.080
Aluminum foil (1mil) 0.000

*(a perm is a unit of water vapor transmission, expressed in grains of vapor per one square foot per hour per inch of mercury pressure difference. For those of you north of the border that is ng/Pa.s.m2)

What is your experience with cathedral ceilings and vapor barriers? Vented eaves and ridge vent above a sealed barrier over the insulation? Or totally filled bays with insulation,sealed with 2" (50mm) polystyrene or isocyanurate over the sheathing and then another layer of sheathing and the roof shingles to move the dew point outside the roof cavity?


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## PriscillaCNewman (Aug 18, 2016)

Vodkaa...


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## Stick486 (Jan 4, 2013)

pgwisn said:


> I know this is an older thread, sorry to interrupt your fun, but back to the topic:
> 
> What is your experience with cathedral ceilings and vapor barriers? Vented eaves and ridge vent above a sealed barrier over the insulation? Or totally filled bays with insulation,sealed with 2" (50mm) polystyrene or isocyanurate over the sheathing and then another layer of sheathing and the roof shingles to move the dew point outside the roof cavity?


welcome back Pat...

no interruption... just keeping the thread alive....
we were also warding off boredom...

here in the mountains of Colorado the most common way is filled bays, 2' of extruded Polystyrene and 5/'' fire rock (Type X)...

NOTES...
3/8 rock is a no no.. 
Bay filers..
Closed-cell polyurethane foam, closed-cell, hard foam, two-pound foam....
Wet spray cellulose, spray cellulose, dense pack cellulose...
High density fiberglass...

EPS and XPS foam boards are typically used because they are semi-permeable, which means the allow moisture to pass through and allow the wall or floor to dry instead of rot.


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## Stick486 (Jan 4, 2013)

@pgwsin...

Pat, the Mooney wall and variations there of are very popular here too...
in lieu of the 2x2's Z bars are substituted and the interior walls covered w/ closed cell..

install unfaced 2x8' panels of blue or pink polystyrene and NOT the white Styrofoam. horizontally...
we do this in 2 or 2.5" because of sustained very low negative double digit temperatures.. depending on climate, 1.5" is all you may need...
start the lay up w/ a "J" or a "U" channel at the floor.. put them in horizontally (channel and styrene)... spot glue the panels and add a "Z" metal at each 2' seam as a clip to hold the styrene and to function as a furring strip...
continue up the wall...
fasten the Z bars to the framing w/ ¾~1'' truss-head screws...

the order is "U" channel (at the floor), styrene, "Z" channel, (continue sequence to the ceiling) and finish with a "U" channel at the ceiling.... vertically "U" channel the corners and also around the windows and doors...

Z-Furring Channel | ClarkDietrich Building Systems

I use 22/24GA channel.. anything less will compromise strength come "hang it on the wall" time..

if you are going to install the channels to masonry install the channels and "Z's" w/ tapcons - often.. 12"OC is highly recommended and forget using a Ramset or Hilti... "blow outs" and poor "sets" will screw w/ the overall mission....
besides, tapcons will hold night and day better over shot in pins...


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## Cherryville Chuck (Sep 28, 2010)

pgwisn said:


> I know this is an older thread, sorry to interrupt your fun, but back to the topic:
> 
> I am finally likely to get my Building Permits approved today for an addition to the back of the garage for a wood shop (24' x 30' /w 10 ft walls and cathedral ceiling, and an addition of a new kitchen and a great room on the back of the house (also with cathedral ceiling). Interfacing a new construction with a house built in 1959 will give me access to the whole of the rear of the house, and the ability to upgrade it's thermal performance, with a certain unease as to managing water vapor, especially with the cathedral ceilings/roofs. I've attached a sketch of the wood shop attached to the end of the garage (with cold frames along the south side for winter veggies  ).
> 
> ...


No need to apologize for resurrecting old threads or especially for getting back to topic. My personal preference for the ceiling would be vented eaves and ridge. My brother just built a steel building in Alabama which has the bays insulated with the standard type insulation for steel buildings and it is cold in the winter and unbearably hot in the summer. He also has about 50% of the building as living quarters with framed walls inside the steel frame with a regular ceiling and 8' walls which are also insulated and that part is comfortable. The insulation between the steel girders has pretty good R value and I don't think the walls are a major part of the problem but I think the unvented ceiling/roof is. In a vented roof the hot air causes a natural draft which displaces the hot air with cooler outside air. In an unvented roof, no matter how well insulated, some of that heat will eventually make it through.

Unless your insulation is impermeable then one side has to remain open to atmosphere so that any moisture in it can eventually evaporate away. In cold climates you want to prevent moisture from entering into the insulation from the inside because in cold weather outside air will be drier than inside air and moisture will attempt to migrate outward if it can so vapour barrier goes on the inside of the framing.

Air plus moisture is heavier than just air so the air at the top of a vaulted ceiling will be the driest air in the house and the most moisture laden air will be at the floor. Having the greenhouse attached can be an issue. Air is an insulator. That is basically how insulation works is that it prevents air movement and the transmission of energy from one molecule to the next. Water is a great transmitter of energy and the more moisture in the air the better it transmits. 

Very dry air will be the warmest but below 30% relative humidity will cause health issues over a long period of time and above 70% can encourage mold growth and reduce the effectiveness of your insulation as well as feeling cold and clammy. With the greenhouse attached you could be fighting with a 70%+ range which means you will either need to dehumidify or exchange inside air with drier outside air.


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## pgwisn (Oct 31, 2014)

Thank you both for your responses.

I have been away from the router forum for a while. We did go with an unvented cathedral ceiling approach, following the current code requirement to move the vapor point outside the structural sheathing.
This first winter saw no evidence of any sorption (what other call condensation). Fingers crossed that it continues that way!

R38 in the ceiling rafter spaces, and two layers of R-Max insulation, staggered, taped seams over the first, structural layer of sheathing, with a second layer of sheathing above (both layers of sheathing seams are taped with Zip Tape), and everything foamed and sealed at the edges too, with fiberboard over the upper sheathing and then an EPDM membrane roof over it all.

No recessed, 'can' lighting, rather sealed J boxes and surface-mounted SLD LED lights, so we minimize and vapor movement into the roof structure.

This article was the basis for our approach:
http://www.greenbuildingadvisor.com/users/user-756436


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