I live in Central Iowa and I have 2x6 walls with 1/2" OSB sheathing + 1" polyiso R-board insulation sheathing (not taped at the seams) on the exterior, with Tyvek paper over that and fiber cement siding topping it all off.

The plan was to spray 2" closed cell spray foam inside the 2x6 cavity and fill the rest with R-13 batts or cellulose. The plan is to live in the basement (which is ICF), and initially only spray the 2" foam so that we can work rough ins upstairs, yet have a bit of insulation in the walls. Well, due to budget, the spray foam can't happen until next summer at the earliest.

After inquiring the local spray foam contractors, they have all raised concerns about moisture buildup in the wall, and two of them have said I need to use either faced batts over the 2" foam or unfaced and poly before drywall. I was under the impression that poly in my situation would not be a good idea. Two others have told me flash and batt is asking for trouble with moisture, and I need to fill the entire cavity with spray foam or fiberglass, not both. So now I'm concerned and I'd like your opinions and advice.

Thanks!

What is your concern with just using drywall, a poly barrier, 2x6s walls filled with cellulose , 1" foam, Tyvek then cement board? In any case, not using poly or equivalent at the inside would be a bad idea and is unlikely to satisfy code.

Posted By jonr on 13 May 2010 08:55 PM
What is your concern with just using drywall, a poly barrier, 2x6s walls filled with cellulose , 1" foam, Tyvek then cement board? In any case, not using poly or equivalent at the inside would be a bad idea and is unlikely to satisfy code.

Well if that 1-inch of exterior polyiso is the foil-faced kind, that'll make for a nice double vapor barrier, no?

I would hope that's not the thought, but to be more descriptive: 1" of unfaced XPS foam. And reinforcement for the racking issue.  And vapor retarder paint if you want something more permeable than poly (ie, you think some inward drying is more important that blocking the moisture in the first place).

The R-board that I have on the house does not have foil on it. It's this stuff right here: http://www.atlasroofing.com/tabbed.php?section_url=21

Jonr - What do you mean by racking issue?
This whole thing started because we wanted ICF, couldn't afford it, wanted SIP's couldn't afford it, so went with 2x6 walls. We added the extra 1" of insulation on the outside to help a little bit with thermal bridging in the stud and plates. As I understood, air infiltration is a biggie, and a way to do a good job of sealing that is spray foam. (Sure we can hand caulk and foam everything, but who has the time?).
Also, we wanted to maximize the r-value. I'm not certain but I think a 2x6 wall of cellulose is about R20 in the cavity, with the R-board we'd have an R-26 wall insulation.
With the 1" rboard, 2" cc foam, and R-15 fiberglass batts that brings us to about R-33, which the Hvac guy said makes a difference between a 4 ton or 3 ton heat pump.

Also, when I looked at wet spraying cellulose, the contractors I contacted only do cellulose in the attic, and if they knew, they didn't tell me of anyone local that does wet spray cellulose in walls.
So you are saying that even in my assembly as proposed I still need a sheet of poly before the drywall? Wouldn't that be trapping moisture between the cc foam and the poly? Would condensation be an issue with warm inside and cold winter outside? I've tried to follow other posts about where the condensing plane ends up, but I am a bit sleep deprived and can't make sense of the math, even though it looks simple enough.

Thanks.

If you skip the OSB, then the building frame needs some reinforcing to make it rigid. A builder should be able to tell you more about the options. You may want to look more at costs and ROI of the different options - you might be headed towards a wall that costs more than ICF.

You can get lots of different opinions about interior vapor barriers/retarders with most building codes, big organizations and simulations supporting low perm values.  If code requires it, then there isn't much point in further discussion. The condensation plane concept is overly simplistic - don't use it. 1" of XPS foam is somewhat permeable (allows outward drying).   Adding OSB will reduce this.

Air in/exfiltration and control of interior humidity are critical - but spray foam isn't the only way to limit the former.

Posted By jonr on 14 May 2010 09:22 AM
If you skip the OSB, then the building frame needs some reinforcing to make it rigid. A builder should be able to tell you more about the options. You may want to look more at costs and ROI of the different options - you might be headed towards a wall that costs more than ICF.

You can get lots of different opinions about interior vapor barriers/retarders with most building codes, big organizations and simulations supporting low perm values.  If code requires it, then there isn't much point in further discussion. The condensation plane concept is overly simplistic - don't use it. 1" of XPS foam is fairly permeable.   OSB and plywood are poor (ie, act as exterior vapor retarders).

Air in/exfiltration and control of interior humidity are critical - but spray foam isn't the only way to limit the former.

What Jon is describing is basically what we did.  We were actually in a very similar boat as you - didn't really consider ICF, but wanted to do SIPs (REALLY wanted to do SIPs).  In our area, the prices seemed higher across the board than what I've read here about other areas, so we had to eliminate that option.  We ended up doing 2x6 construction w/1" of foam board on the outside.  Our local code allows non-structural sheathing in most wall areas, with structural sheathing required on corners and some other areas.  We did the 1" of foam with no OSB on a good portion of the house, and on the rest was sheathed with 1/2" osb & then 1/2" foam board.  In the cavity we did 1" of spray foam for the same reasons you mentioned - air sealing, mostly (the insulators also caulked in the seams between studs/headers/etc).  After that we did 5.5" batt.  It's actually compressed to about 4.5" so we lose a little r-value but not a ton.  Basically we ended up with R22 in the cavity (R6 for the inch of spray foam + about R16 for the fiberglass) and R6 on the exterior - with really good air sealing and a thermal break at the studs.   Oh, and our foam board was foil-faced in the 1"..

A couple things to consider, depending on how far along you are in your project -

There is a structural insulated sheathing product available that I believe gives you about R6.  You can use this instead of OSB and then foam board.  It was fairly expensive compared to OSB (something like $30/sheet compared to $8) but I never got as far as to compare the price of that material to the price of OSB+foam board+labor to put up 2 layers..

Last point - we looked at doing 2" of foam & 3.5" batts instead but for the added cost we didn't get a ton more performance out of it so it wasn't really worth it.  Spray foam is R6/inch, fiberglass batts are 3.6/inch.

2" @ R6 + 3.5@ R3.6 = R24.6
1" @ R6 + 4.5@ R3.5 (reduced for compression) =R21-R22.  An extra inch of spray foam for the entire house is going to add significant expense, for R2-3.  We didn't think it was worth it.

A really good solution, although a bit more expensive than the others mentioned, is spray foam & sprayed fiberglass - the blown blanket system.  That fiberglass is R4/inch & there are no compression issues - so an inch of spray foam + 4.5 BIB gives you roughly R24 and should be cheaper than doing the 2" of spray foam (another benefit is that homes with the BIB insulation are supposedly very quiet)

Posted By Boontucky-girl on 13 May 2010 04:29 PM
I live in Central Iowa and I have 2x6 walls with 1/2" OSB sheathing + 1" polyiso R-board insulation sheathing (not taped at the seams) on the exterior, with Tyvek paper over that and fiber cement siding topping it all off.

The plan was to spray 2" closed cell spray foam inside the 2x6 cavity and fill the rest with R-13 batts or cellulose. The plan is to live in the basement (which is ICF), and initially only spray the 2" foam so that we can work rough ins upstairs, yet have a bit of insulation in the walls. Well, due to budget, the spray foam can't happen until next summer at the earliest.

After inquiring the local spray foam contractors, they have all raised concerns about moisture buildup in the wall, and two of them have said I need to use either faced batts over the 2" foam or unfaced and poly before drywall. I was under the impression that poly in my situation would not be a good idea. Two others have told me flash and batt is asking for trouble with moisture, and I need to fill the entire cavity with spray foam or fiberglass, not both. So now I'm concerned and I'd like your opinions and advice.

Thanks!

From the outside >> inside you  have:

R6 of permeable-iso

R0.5 of sheathing

R12 of semi-impermeable  ccSPF foam
^--^--^--^--^--^--^--^--^--^--^--^<(vapor retarder layer & first condensing surface)

R13 of  air & vapor permeable cellulose or fiberglass

This puts ~R18.5 on the outside of the first condensing surface (the interior of the foam), and ~R13 or so inside the vapor retarder I think you're good.  The OSB & section of the stud outside the SPF dries to the exterior, and the section of studs inside the foam will (almost) never drop below the dew point of 30% RH 70F air in central IA.  You have almost 60% of the R value to the outside of the vapor retarder (the SPF foam) that's more than enough.  Use at least semi-permeable interior wall finishes and you're good- no interior vapor retarders necessary (or desirable!). 

Air tight, but not vapor methods on the interior wall are still a good idea.  You can get anoter R1 or so in the fiber layer going with best-in class spray fiberglass stuff than with batts or cellulose (sprayed or blown.)  I'd go for sprayed-goods for fewer gaps, lower internal convection, but it's far less critical to performance in this stackup than if the bulk of the R was the fiber layer, since the delta-T across the fiber layer will be lower resulting in lower convective drive.

This stackup has excellent drying capacity in both directions, and will not condense interior moisture within the wall. Ya done good, and your foam contractors are all wet!  (It seems few foam installers really understand the moisture dynamics of wall structure.  They all seem to understand that cc foam blocks water vapor and oc foam doesn't, but little about setting up a good stackup to avoid condensation or moisture traps.)  Buy putting the vapor retarder in the middle of the R you don't have to go extra-low-permeance, which is a good thing, since the assembly has much better overall drying capacity than with an interior poly or kraft vapor retarder.

I'd be slightly concerned about the OSB if you'd used foil or poly faced goods on the exterior, but if it's the fiber-faced with a permeance over 1.0 you're good.  (It's probably over 10.0 perms, but check the spec sheets.)  If it's not too late, foaming, taping or caulking the seams of the iso will make it a better air-barrier, and limit a slight convective heat loss at those seams.

WUFI (simulation software) shows the OSB between layers of foam staying wet and molding if it ever gets wet. Not really a good design.

Posted By jonr on 14 May 2010 11:45 AM
WUFI (simulation software) shows the OSB between layers of foam staying wet and molding if it ever gets wet. Not really a good design.

It's iso roofing insulation which is ~10x as permeable as XPS.

Adjust the permeance numbers for the 1" exterior foam in the WUFI model to at least 15-20 perms, (not 2 which is where 1" XPS would be.) Dry enough yet?  How low can the permeance go before the model indicates a problem for the OSB? 

I'm guesstimating even 5 perms would be fine in central IA, but I confess I'm a bit surprised that 2 perms would  be an issue- how is that different from putting 2-perm XPS on the exterior of a building with a 1-perm interior-side vapor retarder?  If that's truly a problem, painting the interior surface of the SPF with vapor retardent latex would lower the permeability of the SPF to about that of a kraft vapor retarder or less.

Yes, I used polyiso foam for the outside 1" which WUFI says is perm=2.5. But R-board claims < 1.0 (probably has a film to help maintain R value), so that makes things worse.

It says XPS is .75.   Sprayed polyiso 1.45.

The walls are already built. I have the studs, OSB on the whole wall not just corners, 1" r-board, tyvek, and siding already installed. Bare stud cavities inside.
So if I'm understanding this correctly, according to this WUFI, whatever it is, my OSB is going to rot if I go with 2" cc foam? Or is it going to rot no matter what I do?
So what do I do now then? The website does say that R-boar is <1 perm.

And price wise, even with 2" cc foam and R/15 batts, OSB, R-board and framing labor this wall assembly is cheaper than ICF or SIP for my area. We built the shell in 2008, and lumber prices were really cheap for my area, so SIP and ICF couldn't even compete.

Posted By jonr on 14 May 2010 01:29 PM
Yes, I used polyiso foam for the outside 1" which WUFI says is perm=2.5. But R-board claims < 1.0 (probably has a film to help maintain R value), so that makes things worse.

It says XPS is .75.   Sprayed polyiso 1.45.

Huh????

I thought she'd said "no facers"!?!  If it's under 1 perm, you can't go with cc foam on the interior-  it's a moisture trap for the OSB for sure! 

I'm surprised that the generic WUFI spec for 1" of XPS  is modeled at 0.75 perms(?)  (That sounds more like a spec for 1.5" or 2" XPS.)

The spray iso number sounds about right though- it's usually sprayed as a higher density open-cell, whereas the the sheet goods are much lighter and very permeable until vapor retardent facers are applied. Roofing iso sheet goods I've used in the past has all been fiber-faced and highly permeable.  It looks like Atlas has gone all low-perm, even on their fiber facers except their uncoated glass-facer (3-7.5 perms according to their generic web info, which is a lot lower than I'd anticipated.)

Good thing you looked up the spec on the R-Board- it changes everything!  Putting a Class-II vapor retarder like R-Board on the exterior is just plain wrong for a central IA climate.  (It's important to read the specs before committing to it, eh?) 

Something like 1.5" unfaced EPS would have a higher permeance and still deliver ~R6, and still let the OSB dry with 2lb foam on the interior. (What perm rating does WUFI use for R6 EPS, jonr?  I'm guessing 5-ish.)

1) at the bottom of the web page for your link to Rboard, it calls it a vapor barrier.
2) in the properties area it says Perm < 1 (this means it is called vapor semi-impermeable)
3) BuildingScience.com is an excellent resource. Try http://www.buildingscience.com/documents/information-sheets/irc-faqs/IRC-FAQ-insulating-sheathing-vapor-retarder-requirements/?searchterm=perm
4) Your assembly MUST be able to dry to one side.
5) Your building codes (if like mine in MN) probably require an interior vapor barrier
6) i've seen two good videos, one on public TV, one on the net called "A House is a System" - I just looked for the net based one and didn't see it. Read all you can at BuildingScience.com

1.76 for EPS. When you get into high perm numbers, say over 15, then wet OSB starts to dry out in a few months. But if OSB is not needed, why take the risk and the expense?

Does MN code still mandate two vapor barriers on basement walls?  I understand it worked fine as long as the block was dry (ie, no real basement water issues).

Most importantly, what should BoonTucky do with the outside foam and OSB already in?    You can get away with some crazy things* if the indoor humidity stays low, you don't allow any positive pressures in Winter, you air seal well and don't get water in the wall.  Initial thought -  I'd stick with the spray in foam plan - it is a good dry wall as long as the OSB doesn't get wet from some external source.  I'd lean towards cellulose (more absorbent but let it dry well before sealing up the wall) and do what ever you have to for code vapor barrier wise (WUFI shows any of the interior VB options working fine - I prefer poly since it is another air barrier and another chance to stop in/ex filtration, but others prefer the maximum permeability that code allows).

* - there are documented cases of the worse possible walls (cold climate, external only vapor retarder, no foam) not having problems.

Dana, the unfaced I was refering to was the fiberglass batts without kraft paper inside that an insulator recommended I use with poly sheets just before drywall. And I did read the R-board spec when we first purchased it but, I can't say I knew what having low perm meant. I thought that was good, like the tyvek, lets things breathe but keeps moisture out. It's not until now that I've been at greenbuilding talk that I've learned to be concerned with things I didn't care about before. We went with R-board because that's what the local lumberyard recommended. They've been supplying it to framers around here for a decade or so and several houses have been built with it on the exterior of the OSB. Now I'd be curious to know if those houses are experiencing rotting OSB. But most likely they only have fibeglass batts in the walls.
We wanted spray foam to minimize air infiltration leaks and increase R-value capacity of the wall.

Jonr - I live out in the country and I don't have any inspections or code, so technically I'm not required to do it any certain way. My concern was that with spray foam then fill the rest of the cavity with fiberglass or cellulose, the thought of putting that poly sheet before the drywall makes me think of moisture trap inside the stud cavity. I had thought the OSB was going to be fine since it isn't really going to be getting wet from rain infiltration, and I didn't consider the r-board to be sealing anything. The stuff looks porous enough, and the joints aren't taped.

Not sure what else I can do to keep the OSB dry. But it'll probably just wick in moisture from humidity in the summer time, won't it?

I guess what I'm interested in now is figuring out what to do. We will have an HRV but not sure if that does anything for moisture control inside the house. How do you prevent positive pressure in the winter? And the air sealing I thought the foam would take care of that. We were going to spray 2" cc foam on the ceiling sheetrock as well and pile the insulation on top that, and the rim joist as well.

I really appreciate your comments and help. Now I need to figure out what would be best in my case now that I'm this far along.

Spray foam.... IMO should be used or sheet foam... in a wall assembly, not both... and definitely not with any wood sandwiched between any foam.

I like slow breathing walls... which to me is densepack Cellulose. If you have to go foam... then no sandwiches. I still have concerns though with no actual evidence.. with spray foam with any combo of other materials... the more layers... the more chance of making lobster traps. And... I will never use poly again most likely... the new poly that auto changes perm is a great idea if someone has to put poly up.

The one guy who I thought was smarter than Joe L... was getting a doctorate in building science... to do with making home enclosures equivalent to our own human skin... like our skin... no air passes through like an open door...but all kinds of molecules can cross through it... IT BREATHES. Permeable... yaa know.

That's also why I am a open cell foam guy... with great results on two residences... but now going greener and am into cellulose dense packed into thick thick wall assemblies.

Greener to me right now is R-60 assemblies with local native wood use...and ground up newspapers. Foam is not economical at R-60, and not enviromental to produce right now and as to worker health... watch what the spray crew has to do to spray and read the spray foam industry magazine about barrel and left over product disposal... and OSHA and on and on... the whole mag.. is devoted to the dangers.. risks... lawsuit prevention... OSHA fine prevention... and on and on.

Kinda like the evening news of late with millions of gallons of oil floating in our beautiful Gulf ocean.. .

If only the fish could talk or take up arms against the greedy human species.  Maybe a future enviromentalist will fund some gene manipulation work to enable non human species to fight back.  It's a big playground... Earth... we can take care of it and play hard at the same time... I know we can.

Go Green... go now.

Green by design, inc.
aj

I live in Minnesota (next door neighbor). Yearly, there's a "Living Green Expo" at the State Fair Grounds.
I missed it this year. (May 1, 2) http://www.livinggreen.org/
They have numerous lectures by consultants who will answer your questions.
It may pay to go to their website and and see if you can find an appropriate consultant or supplier that you can talk with on the phone to iron out.

Above, i recommended BuildingScience.com. Another good website is for the Fine Homebuilding magazine website.
www.finehomebuilding.com

I'm a subscriber to them online, so these links are available to me. I don't know if they will work for you.
regarding walls like you have:
http://www.finehomebuilding.com/item/11050/flashing-windows-in-walls-with-exterior-xps-insulating-sheathing
regarding vapor drive:
http://www.finehomebuilding.com/how-to/departments/how-it-works/vapor-drive-reducing-condensation.aspx?nterms=61678
I'm at work, so couldn't search much. I know they had an article a year+ ago where the lead photo showed someone applying your type of insulation on the house. I may find it one of these nights - or, go to your local library and you'll really enjoy leafing through that magazine - when you're building a house, it's worth the subscription.

If the exterior foam is already up, here's a third vote for going all-cellulose (or only 1" of SPF for air-sealing, then cellulose). Dense-packed dry-blown would be ideal, but 2-hole method would work as well if you've made the sheathing as air-tight as possible. Seal the cellulose- blowing holes with SPF after the fact. Wet-sprayed would take forever to dry at 5.5" depth and it'll need to start out bone dry since you'll have both interior & exterior vapor retarders. The interior vapor retarder probably shouldn't be class-I (like poly), since that would pretty much defeat all inward drying capacity. A class-II vapor-retardent (vapor retardent latex) would be about right. Limiting interior RH to a max 30% during the winter & shoulder seasons should keep it from loading the wall via vapor diffusion.

Fiberglass is not a great option for the cavity fill here (not even the better new-school stuff), since it can't buffer the seasonal humidity or wick moisture away from the wood. Cellulose or cotton (denim batts) offer far better protection to the structural elements, with far lower convective transport of moist air to condensing-temp surfaces, and the capacity to wick & retain/release moisture without losing insulating value.

Building in an exterior rainscreen ventilation gap between the siding & wall is always a good idea, to limit bulk water incursions/moisture wicking at the foam sheathing edges, and manage summertime inward vapor drives.

I'm also partial to the notion of going with most of the R-value being rigid foam on the exterior of the structural stuff, with cavity-fill being the secondary, not the primary insulation. If none of the wood spends much time below the dew point, the risk of rot conditions is near-zero, and you can be much less fussy about interior vapor retarders, and the inevitable eventual air leaks into the stud cavities is also low-risk. That's not likely to be in the budget here, but adding 2" of iso (or 3" of EPS) over what's already there, and dry-blowin cellulose into the cavities (or even unfaced batts, if that's dramatically cheaper), & NO interior vapor retarder would get you there with much higher inward drying capacity overall. The plan for 2" SPF in the cavites wasn't going to be cheap, and more rigid iso on the exterior may be comparable. (Taping lapping the seams of the existing layer with the overlay and taping seams of the exterior layer can make it pretty air-tight.)

My options for doing or improving things on the exterior are NONE, since I will not tear down the siding to get to it. The OSB, the exterior 1" R-board foam is already up, covered with Tyvek and the fibercement siding already installed.

If the wet spray cellulose needs time to dry, that would not be a problem even if it is months before I could drywall.

The main purpose of the foam was for air sealing and max. r-value with the framing I have. I can give up R-6 in the wall and go with 1" if its better in terms of moisture traps, etc.

So, if I can find a contractor to wet spray cellulose in the wall cavity from the interior of the house which would be better? (The foam and OSB are already installed and the house is sided!):

1) Foam + OSB + 1" cc foam + 4.5" cellulose + drywall
or
2) Foam + OSB + 5.5" cellulose (with OSB/stud joints caulked or foamed) + drywall

I don't think I want any sort of poly on the inside regardless of which way I go. Do I?

Thanks