Living in 52722, Eastern Iowa, I am wondering if I need to install poly on the interior of the house? I am having ~1 inch of closed cell foam followed by R-19 batts placed within the 2 x 6 walls. Could I just use faced batts?

The closed cell foam will act as a vapor barrier - or at least a vapor retarder. I think if you installed a second vapor barrier you might be asking for trouble. I say go with un-faced batts.

BTW - what is the installed price per sq ft for your 1" thick closed cell foam?

this type of installation is generally called "flash-n-batt". It has been suggested in many other posts that the 1" flash is a waste of money.
1 1/2 works, but 2" does work best, so you may want to review your application.

Thanks for the replies so far. First, I thought that based on previous posts, 1.5"-2.0" of closed cell foam is required for it to become a vapor barrier. Second, in 52722, where I initially overpaid for closed cell foam ($1.20/bd. ft.), you can get closed cell foam for $0.90 a bd. ft.  Open cell foam seems to be available in the range of $0.32-0.37 a bd. ft. $.032 was the absolute best I found for a true .5lb foam. There are "hybrid" foams around, like the Sealection Agribalance (R-4.45) that is $0.37 a bd. ft. I also got a quote for a "hybrid" foam that is R-5.0 and acts as a vapor barrier at 2" (similar to a true 2lb foam). That is available at $0.70 a bd. ft.

I think there is a lot to be desired from the way the installers bid out jobs. They seem to take the square footage (including windows, doors, garage doors, etc.) of a particular room/house and use that to bid out the job. I have offered to pay for material (for example 3 sets for 45,000 bd. ft. or 4 sets for 60,000 bd. ft.) and direct where and how much foam to be sprayed by the installer. They would charge me based on the labor they normally would charge for application. Unfortunately, this did not go over well with the people I tried dealing with. Undoubtedly, they were marking up the material as well. There are readily available "sets" of open and closed cell foam to be purchased for $2000-2400/set. At 15,000 bd. ft. per set, that is a material cost of $.13-0.16 per bd. ft.... and that is at a NON dealer/installer price.

Finally, I didn't think that 1" was a waste of money. You get the air sealing benefit from the foam. You get R-6.5 for that inch instead of R-3.5 in any other non-closed cell foam product, and some (albeit minimal) added structural support.

I still am up in the air regarding using faced or unfaced batts in the "flash and batt" application. Any thoughts?

Evidently the answer is complicated. From the net, this article covers it (not online):

Feburary 2008 issue of SprayFoam magazine

Would it be possible to apply the foam to the backside of the drywall (ie, insulate from the outside)?

I've seen research that indicates that polyethylene rips slightly around staple holes when there is a high wind.

At 1" the closed cell foam is only a class-III vapor retarder- semi-permeable, so kraft-faced batts with the facers or poly on the interior side would be fine, the assembly would still dry toward the exterior.

At 2" or thicker it becomes a class-II vapor retarder (semi-IMpermeable) in which case retardent facers on the fiber would make it a moisture trap, so unfaced batts are required, and interior side vapor retarders need to be avoided. When you do this, if the R-value of the batts is too high, interior air can condense in the cavity causing mold issues. You have to either model it or at a minimum do the simple math so that the average wintertime temp within the fiber layer stays well above the dew point of interior air. The dew point of 68F air at 30% RH is about 35F the average January temperature in Bettendorf IA is about 20F. (30% is the minimum recommended RH for human healh & comfort.) The point where it stay 35F or cooler must occur toward the exterior of the batts.

With flash & batt, & interior facers, you'll be fine as long as you take care to air seal the facer around any penetrations and at the floor/ceiling. If you go with unfaced batts you MUST install an interior side vapor retarder and seal it well. In a 5.5" cavity you've put in ~R6 of foam, shrinking the depth of the cavity to 4.5". An R19 batt compressed into a 4.5" cavity yields about R16, for a total center-cavity R of R22. When it's 20F outside, and 68F inside the place where the 35F point occurs is (35-20)/(68-20)= 0.31 of the R value in from the outside, or (0.31x R22=) R6.8 in from the sheathing, which is a bit into the fiberglass. If interior air is allowed to reach that point, it'll condense and the moisture will pretty much stay there for weeks since it has scant drying time at low vapor pressure. If the wall is highly permeable, the cavity air moisture will track the interior air moisture, same problem. But vapor and air sealed at the interior the cavity won't be accumulating interior-air moisture, and the cavity air will track the average exterior air humidity (which will be much lower when it's cold outside.)

Do the same math on 2" of foam + 3.5" of fiber and you'll see you have some margin. While peak cold may see condensing temps in the fiber layer those events are short-lived, and moisture won't continue to accumulate for weeks.

I plan to encapsulate a mobile home in Central Florida with 2.5 lb closed cell foam, 2 inches on the roof and 1 inch on the exterior.
Should I have any vapor concerns? Do I need to spray the bottom also?

Posted By buck3647 on 15 Mar 2010 12:31 PM
I plan to encapsulate a mobile home in Central Florida with 2.5 lb closed cell foam, 2 inches on the roof and 1 inch on the exterior.
Should I have any vapor concerns? Do I need to spray the bottom also?

Florida is the flip-side of the coin- where the air-conditioned interior is often below the dew point of the exterior air, and keeping exterior air/vapor from getting to structural materials is key to mold-control. 

A 2" lift on the roof is fine on the exterior- not so fine on the interior if it's a membrane roof (which wouldn't allow the roof deck to dry easily), but OK if applied to the underside of the roof deck if it's a roofing felt & shingle or roll-roof type system above.  I have no idea what's standard for trailer roofs.  In general it's safest to put it above the roof and add a ventilated nailer deck for standard roofing above the insulating layer.  It may be cheaper to put 3-4" of tapered EPS on the outside of the structural roof held in place with furring/purlins, and build yourself a rain-roof above that.

On the exterior walls, putting your flash-coat on the exterior sheathing is preferred.  It's not as vapor-tight as a 2" lift, but vapor drives from walls are typically lower than from roofs in hot-humid climates (the exception being masonry or stucco walls, which can hold a lot of water after a rain, and will experience HUGE vapor drives when the sun hits it, but that's not the siding on most trailers, eh?)

If you're air conditioning the place you BET you'll want to spray the bottom too, or the sub floor will rot from the bottom up from outdoor air moisture condensing on the mechanically cooled subfloor. When  it's 90F outside with only 70% relative humidity  the dew point is ~78-80F.  If you're keeping it 76F inside, there will be ample periods where the subfloor will be absorbing moisture from the outdoors. But even 1/2" of cc insulation will be enough to keep the wood above condensing temps, and it'll dry toward the interior if it doesn't have highly retardent floor coverings (like vinyl), but toward the exterior otherwise, since it won't be enough foam to make a strong vapor retarder.

Everything I find on the issue is along these lines:

"To be safe, thicker foam is better -- one inch of closed-cell foam in Indiana or Massachusetts, and two inches in Montana, Minnesota, or Vermont. Once you're up to two inches, though, you aren't saving much money compared to a full-foam job."

Posted By jonr on 15 Mar 2010 04:14 PM
Everything I find on the issue is along these lines:

"To be safe, thicker foam is better -- one inch of closed-cell foam in Indiana or Massachusetts, and two inches in Montana, Minnesota, or Vermont. Once you're up to two inches, though, you aren't saving much money compared to a full-foam job."

That's highly dependent on what means "...a full-foam job."  Another 3.5" thickness to fill out a 5.5" thick cavity with 2lb foam is about $4/square foot in my neighborhood.

Filling the same cavity-remainder with sprayed cellulose or batts results in a center-cavity R-value of ~R26-27 instead of ~R30- roughly a 10% performance hit, but it's a quarter the installed price (or less!) for that 3.5" remainder-fill.

As for the "To be safe, thicker is better..." argument, that's highly dependent on the stackup and climate.  At 2"  on the exterior it can be insufficient R to prevent moisture buildup from interior permeation in many climates, yet too vapor retardent to allow interior-side vapor retarders, forcing you to go thicker.  Thicker is better only if applied to the cold side of the assembly-  but it doesn't take hard math to calculate the minimum thickness that provides some margin.

But applied on the warm side of the assembly in a heating-dominated climate you can go as thick or thin as you like without issues. If the climate calls for higher vapor retardency on the interior that vapor retarder can be a lot cheaper stuff than  2-3" of closed cell foam.  Getting flash & batt right with the batts on the cold side of the assembly takes no arithmetic skills whatsoever.

I had a contractor install 0.5lb open cell foam. THey did a horrible job and in fact defrauded me out of the job that they were paid for. Some areas of y home only has 1inch to 3" of spray foam. This material was sprayed on the roofline. I cant afford to have another spray foam contractor to come back and spray additional material. I have had the worst case of ice damming and the whole perimeter of my home has 3-5 fot icecycles hanging from the gutters.

Would it be okay to apply the new atticat pink insulation product to the floor of my attic? Obviously this is not what I had intended but I need to add more insulation. Also I was given many sheets of 4 inch roofing insulation panels that I was going to lie on the floor and then add atticat insulation on top. Does anyone forsee any problems with this? Thanks

I'd put down a barrier and then lots of cellulose.


I downloaded ORNL WUFI which can simulate moisture in walls. Some observations (using Detroit, MI data):

1) control of indoor humidity level is critical.
2) definitely add the vapor barrier behind the drywall (I suspect that polyethylene leaves fewer gaps, but don't use staples directly).
3) consider open cell foam instead
4) 2" does better moisture wise than 1"
5) consider 1" of XPS + 1" of spray vs 2" of spray

Posted By jmbeam59 on 15 Mar 2010 07:57 PM
I had a contractor install 0.5lb open cell foam. THey did a horrible job and in fact defrauded me out of the job that they were paid for. Some areas of y home only has 1inch to 3" of spray foam. This material was sprayed on the roofline. I cant afford to have another spray foam contractor to come back and spray additional material. I have had the worst case of ice damming and the whole perimeter of my home has 3-5 fot icecycles hanging from the gutters.

Would it be okay to apply the new atticat pink insulation product to the floor of my attic? Obviously this is not what I had intended but I need to add more insulation. Also I was given many sheets of 4 inch roofing insulation panels that I was going to lie on the floor and then add atticat insulation on top. Does anyone forsee any problems with this? Thanks

To safely split the R-value between attic floor and roof deck you have to do the same climate-temp averages & R-value math as if it were a single wall or roof assembly to ensure that moisture from conditioned space air doesn't condense and accumulate in the attic causing mold issues.

Air leaks into the attic space are more likely, and likely to be larger than in typicall wall assemblies.  To further mitigate risk and improve performance, air-seal the attic floor/conditioned space interface to the best of your abilies, before laying down the floor insulation, including weather stripping the access doors/hatches.    If there are lots of gaps & penetrations the air-sealing can be time consuming. If you have much to do, use purpose-built guns and the large (20+ oz) cans of 1-part foam rather than the cheezy li'l 12-16-oz cans with breakable cloggable plastic nozzles (eg. Dow Great Stuff, etc.)  Grainger sells the stuff as does EFI.  If you have recessed lighting cans sticking up there, box over them with sufficient clearance and seal the edges with foam/caulk/mastic.

If it's a single simple attic space you can also control RH in the with a dehumidifer after placing the floor insulation.  It needs to stay below 60% to keep mold from getting started- at 70% & up the mold hazard explodes off the charts.  If it's reasonably air-tight the duty cycle of the dehumidifier will be quite low. Monitor the RH- there's no need to over-dry the place "just to be sure", since the hazard periods are temporary & seasonal.

When it's time to re-roof, an inch or three of rigid foam board above the roof deck is a reliable cure for ice damming in much of the lower 48 under 2000' of altitude.  Had you gotten 5-6" of half pound foam under the roof deck that probably would have been enough from an ice-dam potential point of view, but thin spots of 1" half-pound foam isn't much insulation to speak of. 2-6" of iso with pre-applied nailer deck applied above the roof deck is available from multiple vendors (Atlas, Hunter, et al.) and would put another R13-R36 between the snow and attic.  In central MA (7000HDD climate) many just use 1" of XPS under the roofing felt and long shingle nails (but that's usually over an insulated attic).  Even R5 above the roof deck can even out the temperature differences enough to make an ice-damming difference here.

My home is only a couple of years old and I used the best material available. I did this because I did not want to have to re-roof this home in my lifetime. I have alot of 4" insulation board that I was going to cut and place between the 24" floor boards and then place Atticat blown in insulation.I was told not to put a vapor barrier since most of the attic roof has 6" of 0.5lb foam on it .The attic is pretty tight.

I live in Calgary Canada and the city has recently adopted the position that 2lb foam cannot be applied without a vapor barrier under any circumstances...  which, seems problematic to me...  Our climate will frequently yield a 80-90 degree temperature differential and to put a layer of poly as a secondary vapor barrier, seems to me like they are asking for trouble.  They are also insisting that there be continuous venting if 2lb foam is used in the roof, and by extension insisting that you cannot spray directly to the underside of the roof sheathing... Does anyone else see a problem with this?  Does anyone know where there is any good information - independent scientific research that supports the claim that 2lb foam is a viable vapor barrier???

I'd call the city and ask them why they adopted such a policy. I can tell you that using the WUFI simulation software, moisture content is much worse without a PE barrier (the insulation/foam barrier is soaked at 100% humidity much of the time without the PE). I'd worry less about trapping moisture and more about keeping it out.

Posted By EnvyEcoEnviro on 27 Apr 2010 01:04 AM

I live in Calgary Canada and the city has recently adopted the position that 2lb foam cannot be applied without a vapor barrier under any circumstances...  which, seems problematic to me...  Our climate will frequently yield a 80-90 degree temperature differential and to put a layer of poly as a secondary vapor barrier, seems to me like they are asking for trouble.  They are also insisting that there be continuous venting if 2lb foam is used in the roof, and by extension insisting that you cannot spray directly to the underside of the roof sheathing... Does anyone else see a problem with this?  Does anyone know where there is any good information - independent scientific research that supports the claim that 2lb foam is a viable vapor barrier???

The permeance of 2" of 2lb foam is ~ 1 perm.  The permeance of 6mil ply is ~ 0.05 perms.  It would take  over 15" of 2lb foam to achieve the same vapor retardency of 6-mil poly.

With more than 2-3" of  2lb foam on the underside of the roof deck you run the risk end of saturating the roof deck if it isn't vented, since it could go for months without the capacity to dry in either direction. Below 1 perm drying toward the interior through the foam would take forever.  And since 2" of 2lb foam isn't anywhere NEAR enough insulation for the climate, and would otherwise form an exterior vapor retarder, I can kind of see that aspect.

Interior-side 6-mil poly doesn't seem to make much sense in walls with more than 2" of foam on the foam though.  If you're filling a 2x4 or 2x6" cavity the studs can't dry quickly, but they also can't take on diffused moisture quickly.  With poly on the interior it could make things worse, since leak moisture could take longer than a summer-season to dry to the exterior through several inches of foam.

jonr - I spoke to the city and unfortunately, they don't have an answer to the question. They are basing their decision on the insulating practices covered under the 1995 Canadian National Building Code which was basically based entirely on fibreglass insulation... I am not really clear on what you are suggesting from your note... Based on the WUFI Simulation, it seems that the poly vapour barrier is a good idea? Our relative humidity here tends to be quite low (although I wish I could speak on a more educated level about that.)

Dana1 - My instinct goes with you with regards to the poly on the walls being potentially problematic. I am more concerned I think though about the poly being compromised and moisture from the inside getting into the wall cavity in the winter than I am about leaking from the exterior. Most of our precipitation is snow, so we don't often have significant issues with rain penetration. As for the roof, standard practice here has been to spray 4" of 2lb foam on the underside of the sheathing, and no venting - specifically on flat roofs. We use a minimum 60 mil Firestone roofing membrane, and we are militant about roof penetrations - and avoiding them whenever possible. Sounds like from your experience, you favor venting the flat roof anyhow, just in case of membrane failure? I agree, if the water gets in there, with 4" of foam, it will take forever to get out!

All the feedback is appreciated. I am trying to figure out what to do, because I don't know what the vented flatroof detail should look like... I'm always more concerned about inviting air into a space because air can carry moisture in as well as out...

dw

What's the average annual outdoor temp in Calgary?  I'd think that if it has more drying-toward-interior hours than absorption via diffusion hours in a year the interior side vapor retarder would be a hindrance, but  if it's the other way around it would never dry.  Drying through 2lb foam takes forever, but so does absorption via diffusion.  If the WUFI model was run correctly for your climate, it sounds like it doesn't have sufficient annual drying hours relative to absorption hours, but that will vary by local climate considerably.  It has to be below ~3-4C on exterior for outward drying to be slower than the uptake of moisture from 20C 30% relative humidity interior air.  As long as the exterior is above the dew point of the interior air, it should dry toward the exterior, however slowly... 

Sun on rain or dew wetted siding has a HUGE vapor drive, and without a rainscreen gap the moisture WILL end up in the foam, limiting the ability of the assembly to dry toward the exterior.  IIRC the Canadian Nat'l Building Code (rightly, IMHO) spells out a 10mm rainscreen under siding, which would keep vapor drives from the exterior from entering the foam, and enhancing drying toward the exterior.  Whether that would be sufficient to not need any inward-drying capacity with a foam cavity fill + interior poly is something that can probably be modeled for a particular climate.  I suspect it is in Calgary, but I also strongly suspect that  20C 30% RH air wouldn't saturate the the exterior foam over the course of the winter- it's cold, but not THAT cold there. (cood b rong, offen am)

Fiberglass will dry much more rapidly in either direction when conditions are favorable, but it's internal convection & air-diffusion make interior side vapor retarders an absolute-must in Calgary winter conditions. Any breaches in the interior poly would have a much larger effect with fg due to the many orders of magnitude higher air flow transporting huge amounts of moisture into the cavity.

On a flat roof with only 4" of 2lb foam it's still only semi-impermeable (~0.5 perms- about the same as kraft facers on fiberglass batts), and the roof deck CAN dry toward the interior as long as there isn't any interior-side poly or foil vapor retarder.  It wouldn't keep up at a leak point, but would limit the area of damage.   I'm with you 1000% on the air-transported moisture issues with ventilated structures- as a general rule, air-tight is the right way to go wherever possible.

Bottom line, it's hard to make a high-R assembly that dries easily with 2lb foam, but since it's a perfect air-barrier and semi-impermeable to water vapor, it doesn't take on moisture quickly either.  I've not seen any science or modeling to suggest that interior poly would be a benefit if it's a full cavity-fill (but maybe jonr has.)  If it's only a couple inches rather than 3.5-5.5, maybe, but again, that's not enough insulation for Calgary.

FWIW, the Building Science Corp sample design for Minneapolis (colder than Calgary) recommends only semi-permeable latex for an interior side vapor retarder to enhance summer-months drying of the cavity when semi-permeable (~2.0 perm) XPS  exterior sheathing is used. With a full-foam insulation it should probably be designed to dry in both directions- with a weak interior side vapor retarder, not poly.