Foam board insulating sheating?

[Nate R]

Hypothetical wall from inside to out:
1/2" drywall
vapor barrier
insulation/stud
7/16" OSB sheathing
1.5" Foam board insulating sheathing
Tyvek

So, if I install a new construction vinyl window, it gets secured to the foam board!? If I secure it to the OSB, the integral J channel in the window would get buried by the foam board.

Seems not very secure to me. Also makes for larger jamb extensions.

Obviously foam sheathing and vinyl windows go together all the time in new construction, so how is it done? Anyone know?

[frenchie]

1-1/2" of foam and a vapor barrier? Trouble. How's your wall assembly going to dry out? Might want to spend some time over at Lstiburek's site:

http://www.buildingscience.com/bsc/

The typical detail for windows over foam is to block out, with wood, flush to the foam - then attach your window to that. Requires careful detailing of the drainage plane to prevent water intrusion at the seam between them. Either housewrap before, or over, the foam.

a few relevant links:

http://www.eere.energy.gov/buildings...ndows_foam.pdf
http://www.coastalcontractor.net/cgi...icle.pl?id=123
http://www.umass.edu/bmatwt/publicat...e_outside.html
http://www.ibiblio.org/london/renewa.../msg00741.html


These other links, are from a pro-only forum, be careful about posting there if you're not in the trades; the forum regulars can be very mean. The building science sub-forum is open to anyone, as is the electrical. The rest of the forum... only pro's (as you'll see from the 2nd link, from the exterior details sub-forum, where the moderator closed the thread):

http://forums.jlconline.com/forums/s...ad.php?t=39586
http://forums.jlconline.com/forums/s...ad.php?t=38479
http://forums.jlconline.com/forums/s...ad.php?t=33080
http://forums.jlconline.com/forums/s...ead.php?t=8047
http://forums.jlconline.com/forums/s...ad.php?t=11328

[BigLou]

Nate,

Why do you feel that you need a vapor barrier ? you live in a cool enough climate where there is nothing wrong with using one. However its just not necessary. Most moisture enters a wall cavity on air currents very little enters through vapor dispersion. A wall that leaks a lot of air leaks a lot of heat and money. Drywall is a good air barrier especially with some vapor barrier paint.

What do you plan on using for insulation ? Dense pack cellulose gives you the most bang for your buck. It also does a decent job of preventing air movement in the structure. Fiberglass is the hardest to install and get the rated R value altough its not impossible.

Many products are available that render the sheating un necessary. Simpson makes several shear panels that accomplish the same thing. Nothing bad will probably come of installing the sheathing its just not needed. I would just install the foam directly to the studs and tape all the seams. If you wanted you could block out around the windows with a 2x4 flat. The details can be fairly complex. but doable JLC has some good articles and books on the subject

TYVEK: just do a Google search on Tyvek and Dick Seibert and see what comes up.
Jamb extentions are done everyday. most window places will add them for you if you are confident in the final all thickness

hope some of this helps

Lou

[Nate R]

1-1/2" of foam and a vapor barrier? Trouble. How's your wall assembly going to dry out?

Hmm, didn't realize/think about that.

Does that mean when someone installs vinyl siding w/ the fanfold underneath that they should not have an interior vapor barrier? Since the fanfold is the same material as insulating foam sheathing, no?

What do you plan on using for insulation ? Dense pack cellulose gives you the most bang for your buck. It also does a decent job of preventing air movement in the structure. Fiberglass is the hardest to install and get the rated R value altough its not impossible.

I had just defaulted to fiberglass in my mind. Never really thought about anything else. I was thinking sprayed urethane would be too expensive, so glass was next in my mind. The dense pack cellulose: Is that the stuff with the adhesive in it so it can be installed when the walls are still open?

[Nate R]

Oh, and thanks for the links! I'll check out those other forums.

[construct30]

A vapor barrier is required on the inside of the insulation, you can not do it without it.

[BigLou]

A vapor barrier is required on the inside of the insulation, you can not do it without it.

says who ?

[interalian]

A vapor barrier is required on the inside of the insulation, you can not do it without it.

It's a code requirement here (6mil), and Alberta is extremely dry ambient. Strangely though, contiguous vapour barrier around exterior wall electrical boxes is required during renovations, but not on new construction. My 3 year old house leaks through all the wall plugs. Exterior is vinyl over tar paper over OSB over 2x6 with R20 fiberglass.

[geniescience]

i'd listen to the guy from Alberta. His climate is closest to yours.

Where to put the vapor barrier, (inside or out) or whether or not to put one at all, -- it all depends on your winter temperatures and dew point. Local climate.

david

[frenchie]

Does that mean when someone installs vinyl siding w/ the fanfold underneath that they should not have an interior vapor barrier? Since the fanfold is the same material as insulating foam sheathing, no?

No, 1/2" is fine; but 1-1/2", you've got a vapor retarder.

Permeability decreases as thickness increases: 1-1/2" blocks moisture transport 3 times more than 1/2".

A vapor barrier is required on the inside of the insulation, you can not do it without it.

That depends on his local climate, and codes.

It's a code requirement here (6mil), and Alberta is extremely dry ambient.

You mean, because Alberta is extremely dry ambient. Vapor pressure is related to the difference in humidity between inside and outside. So for the same indoor humidity, a drier outside means there's more vapor pressure.

i'd listen to the guy from Alberta. His climate is closest to yours.

Not sure I agree. Alberta's drier, and colder, than Wisconsin; I think they're on different sides of the line for plastic. But I'd still check local codes.


Anyways, the point is that you can't do both, 1-1/2" of foam on the outside, and plastic vb on the inside, in any climate, and not cause problems.


Nate -

Another thing to look out for - you'll pick it up in those links - is that if you want foam on the outside, you can only put so much insulation inside the wall. The inside of the foam has to be kept warm enough to not get condensation. If you over-insulate inside, you move the dewpoint* further in, and the framing rots.

*"first condensing surface", if you want to get technical.


Is this new construction, or retrofit? There are other ways to get high-R-value walls:

- spray foam
- dense-pack cellullose
-Thicker walls, either by using bigger studs, or (if you want to avoid thermal bridging) strapping cold spots
- Insulated Structural Panel construction
- rigid foam on the inside, would work pretty well in your climate; use the foil-faced, and it's a vapor barrier on the correct side...

...Lstiburek's climate-zone-specific builder's guides, from the buildingscience.com site, are the bible for this sort of thing, and well worth the price. Written for contactors, rather than academics: clear explations, detailed recommended specifications and drawings... makes it all very easy to understand how everything plays out in your specific climate.


Last note: bear in mind that nominal R-value is just one piece of the equation. The average house loses a lot more heat through drafts, than through not having enough insulation. I remember Calgary as pretty freakin windy in the winter...

[construct30]

says who ?

MOLD

Moisture inside the wall equals mold. Vapor barrior outside the insulation bad, ask the people who made the stucco that would not let moisture out.
Dryvit and others.

Hot or cold inside or out, when they meet they condense, climate doesn't matter unless you live someplace that does not require insulation due to heat or cold, very rare climate.

Don't accept bad advice.

[BigLou]

Construct30,

are you saying that you need an interior vapor barrier no matter what the climate?

I agree that excess moisture inside a wall cavity that can't dry out may lead to mold. However you obiously don't understand that the majority or moisture moves on air currents not through vapor dispersion. You would need a steam bath or something to create enough vapor drive through well painted drywall. If you wall is leaking that much air its leaking too much money and you need to build tighter walls.

Going farther with your mold theory, you need a colder surface for the moisture laden air that should not be there in first place to condnse on. The foam sheathing will go a long way to preventing that. On all but the coldest days it won't be cold enough to allow condensation to take place. If said condensation does happen it can just dry back the way it came in

Lou

[jadnashua]

In places where a/c is the primary use, a vapor barrier on the outside is sometimes used since the a/c is removing moisture and the dew point is manageable. You don't ever want two vapor barriers that sandwich moisture in a space.

[construct30]

BigLou, you are saying insulation is 100% effective, FALSE. Do you know what an R value is? It would be S for stopage or TB for totally blocked. An interior vapor barrier stops the vapor from cooking, Showering and even people from getting inside the wall. Not having a vapor barrier outside allows the little bit from condensation out.

Do a search for EIFS, it is what they call the system that fixes the new stucco's that won't allow vapor out. They make a place for water to drip out the bottom. Look at the holes at the bottom of sidings. They call them weep holes. If you put a vapor barrior on the outside of the wall system you will get mold and rot. Aluminum siding when it first came out had no weep holes and the houses grew mold and rotted then the stucco.

Most outside sheathing is designed to allow vapor out and stop water and air infiltration, tyvek and other house wraps do the same.

How many contractors will not learn the lesson from water proof sidings and have 5 to 10 year old houses rottening down around the poor homeowners?

[frenchie]

You want a vapor barrier on the inside in cold (heating) climates, because vapor pressure is from inside to outside - condensation happens on the inside of the vapor barrier in those conditions.

But in hot/humid climates (AC), you want a vapor barrier on the outside - because the vapor pressure is reversed, outside to inside - and condensation will happen on the outside of the vapor barrier.

A vapor barrier on the inside, in a cooling climate, would cause the same problems as a vapor barrier on outside in a heating climate.


In 80% of the continental US, you want the wall to be able to dry somewhat in either direction - avoid plastic sheeting like the plague... you want vapor retarders, not vapor barriers. Kraft-faced FG batts, or carefully detailed "airtight" sheetrock & a few coats of vapor-blocking primer.


The big problem with EIFS systems was that they didn't allow the wall to dry in either direction. Most of the damage wasn't related to condensation, but to water intrusion (poor/typical details on window flashing, etc.).


You should check out some of those links I posted... maybe drop by JLC's building science forum. This stuff isn't as simple as "always put a vapor barrier on the inside".


Hey, BigLou - are you my old friend BigLou from JLC, or somebody else with a similar handle?