# Rigid Foam Board under sheathing



## milfordtom (May 10, 2009)

i'm new here, great site. I just had a quick question. Framing in an addition. i keep reading that when using rigid foam board, its general practice to place the foam board on the outside of the sheathing. Is it acceptable practice to place foam board (in my case 1/2 inch foil faced ISO panel) under the wall sheathing? In my case, i would have cedar clapboards, tyvek, OSB, foam board, 2x4 stud, unfaced fiberglass bats, then drywall.


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## greg24k (May 19, 2007)

First introduce yourself. Second, put your location in. Third why would you want to put foam before the sheeting and not over it... what code will allow you to do that? :no: In addition, why are you using unfaced insulation? Are you sure that your field is construction industry?



milfordtom said:


> i'm new here, great site. I just had a quick question. Framing in an addition. i keep reading that when using rigid foam board, its general practice to place the foam board on the outside of the sheathing. Is it acceptable practice to place foam board (in my case 1/2 inch foil faced ISO panel) under the wall sheathing? In my case, i would have cedar clapboards, tyvek, OSB, foam board, 2x4 stud, unfaced fiberglass bats, then drywall.


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## milfordtom (May 10, 2009)

wow. well, i'm matching what was originally on the home. i do not want to use faced batt insulation, that would give me two moisture barriers within the wall. As for the osb before the foam, again that is what is existing. Essentially, isn't that what a sip panel is. The homeowner wants her 3 season room to become regular living space. I posed this question for some insight and im fairly certain im in the construction industry.


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## SLSTech (Sep 13, 2008)

A SIP panel is basically a sandwich - 2 slices of bread, a piece of bologna, and mustard spread on both slices of bread evenly & equally - not fancy lines or artistic renderings

For an Interior finished wall you have your drywall, glue, foam, glue, osb which creates a solid structure.

Sorry, but you are new & this question reeks of a HO acting as GC & you really should introduce yourself properly in the appropriate area


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## greg24k (May 19, 2007)

You said you framing an addition, so you faced with the new construction and you handle it just as that, *new construction*. When installing faced insulation you install faced side to the interior of the house, which acts to reduce water vapor penetration into the wall cavity from inside of the house. In hot climate areas, the code will often call for placing the vapor barrier toward the exterior of the house, so check your local codes because I have no idea where you from.



milfordtom said:


> wow. well, i'm matching what was originally on the home. i do not want to use faced batt insulation, that would give me two moisture barriers within the wall. As for the osb before the foam, again that is what is existing. Essentially, isn't that what a sip panel is. The homeowner wants her 3 season room to become regular living space. I posed this question for some insight and im fairly certain im in the construction industry.


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## milfordtom (May 10, 2009)

sorry about not fully introducing myself. my name is tom and i'm from milford michigan. im not a homeowner. i got started in the business 10 years ago after i graduated from college, started working with my buddy for his father as a laborer. after that i started working with a good builder doing spec homes, basements, baths, kitchens. i still work for him, but times are slow in michigan and when i get a job on my own i take it. i'm young in the business, but i feel like i have a solid background of working with people who did things the right way. i'm licensed and insured. 

my question basically had to deal with possible problems of using Foam Board under the OSB. i do not know of any. It seems to me to be acceptable, but if it is not, i just wanted an explanation as to why. code allows foam on the outside of a home, why not under the OSB? it will be easier to nail my cedar siding on and i won't have to have furring strips to keep the siding off the foam


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## greg24k (May 19, 2007)

It will weaken the structure by allowing structural deflection. To ensure adequate structural stiffness when the structure is subjected to bending under various code predicted loads, i.e. lateral wind pressure, or negative suction forces,etc. you must apply sheeting directly to the studs.:thumbsup:


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## SLSTech (Sep 13, 2008)

http://www2.dupont.com/Tyvek_Construction/en_US/uses_apps/suppliers/FAQ_suppliers.html #3, 5 & 25

You will still need furring strips or the cedar must be fully sealed prior to installation

To further what Greg stated --- You basically have 2 options, strap & anchor the he11 out of the framing & install foam sheathing over it, or install OSB to take care of the wind & shear issues & foam over that


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## Roofus (May 10, 2009)

You will want to make sure that you have enough R-value in that rigid insulation so that you will not be creating a condensation point on the inside of the wall. In the winter months, during the heating season, air will move through the wall assembly from inside to out. If that warm "wet" air hits the backside of a cold surface it has the potential to condensate. If your rigid insulation is thick enough you should be alright...if not you could end up with a potential risk of condensation. 

Here is a calculation you can use to figure how much rigid you need. I took this from another site:


Building Science Corp.

Interface temp. = Indoor temp. - [(Indoor temp. - Outdoor temp.) X (Cavity R / Total R)] 

Lstiburek's organization, Building Science Corp., has spent years running detailed computer simulations to predict moisture conditions within walls and experimenting with different wall assemblies to verify the calculations. Eventually, the group settled on a simple way to specify exterior foam thickness: "You take the average temperature of the three coldest months of the year in your location," says Lstiburek. "Take the average temperature for December, the average temperature for January, and the average temperature for February — and you average those, and use that average as your design temperature for outside. You set your interior design condition as 70°F and 35% relative humidity. Then you do a simple calculation to make sure that the condensing surface doesn't drop below the dew point. As long as you don't see 100% relative humidity at the interface between the foam and the cavity insulation, you won't have condensation on the back side of the sheathing."


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## Leo G (May 12, 2005)

greg24k said:


> It will weaken the structure by allowing structural deflection. To ensure adequate structural stiffness when the structure is subjected to bending under various code predicted loads, i.e. lateral wind pressure, or negative suction forces,etc. you must apply sheeting directly to the studs.:thumbsup:


Right there is why this should never be done.:notworthy


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## milfordtom (May 10, 2009)

thank you very much. very informative


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## B.K (Dec 15, 2008)

Over East in Canada, in New Brunswick, we use foam board over OSB or Plywood on the outside of the walls. 

Reason Being:
- Extruded Polystyrene, is a great moisture retarder/air barrier, it doesn't adsorb any moisture that will get behind your siding, and when it does get behind the side it will just slide down the board and drip out at the bottom. 
*Always strap your siding when you get the chance, you want that barrier between your siding and exterior wall.*

- It does give an increased R value to the house as well, don't quote me on this one at all but if i remember correctly .5 R per/sq in

If you did end up putting the board behind your OSB, it wouldn't do a darn thing for you, it could possibly create a larger problem with moisture.


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## JamesKB2 (May 13, 2009)

greg24k said:


> It will weaken the structure by allowing structural deflection. To ensure adequate structural stiffness when the structure is subjected to bending under various code predicted loads, i.e. lateral wind pressure, or negative suction forces,etc. you must apply sheeting directly to the studs.:thumbsup:


That is a turn key answer and 100% correct. Also, I might add, use proper fasteners and spacing.


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## sprayit (Sep 9, 2012)

*board foam under sheathing, yay or nay?*

Hello im John from moncton New brunswick and im a general contractor, I thought i would throw this cenario at you guys and see what you think. I will be building a timberframe home for myself the wall construction will be this, sealed(varathane) timbers left exposed to the inside, 2 inch rigid board foam, plywood, tar paper, board and batton. in between timbers 2lb closed cell foam covered with plaster. timberframes dont use sheathing as structural support, 2lb spray foam gives you 30% structural stength and is a vapour barrier. no air pockets, the sprayfoam will connect to the rigid giving a 100% thermal break behind all of the timbers. no air pockets to allow condensating. what do you think fellas?


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## Brutus (May 29, 2007)

foam goes on top of the OSB. Always. greg24K hit the nail right on the head with his comments. 


Also, two guys in here from New Brunswick!? Hello fellow maritimers! :thumbup1:


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## sprayit (Sep 9, 2012)

if no structural support is needed then why on the outside of the sheathing?


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## WilsonRMDL (Sep 4, 2007)

I'm in MI as well, inspectors always want the seams taped as well. I doubt it would even pass code being on the inside of th osb, but I guess I've never even thought about using it that way


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## Tom Struble (Mar 2, 2007)

Brutus said:


> foam goes on top of the OSB. Always. greg24K hit the nail right on the head with his comments.
> 
> 
> Also, two guys in here from New Brunswick!? Hello fellow maritimers! :thumbup1:


no..not always,structural sheathing is not always needed and when it's not it's probably best to get rid of the wood all together


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## 91782 (Sep 6, 2012)

greg24k said:


> It will weaken the structure by allowing structural deflection. To ensure adequate structural stiffness when the structure is subjected to bending under various code predicted loads, i.e. lateral wind pressure, or negative suction forces,etc. you must apply sheeting directly to the studs.:thumbsup:


Huh?

Hey Tom, you're in Milford, so it's a 20 minute run to Rose TWP. Find Old Farm Trail, 4th house on left:

1. L-Ranch, built in 1990, pure DTE Energy spec
2. Drywall, 3mil plastic floor to ceiling with wrapover, unfaced bats in wall cav, paint had to meet permeability rating (forget what it was)
3. studs/w metal corner bracing, then 1/2 double face cyano(all seams foil taped), then T-111
4. Scissor trussed great room/kitchen/dining (about 50% of total floor plan) flatbottomed trusses everywhere else on a 9ft full poured wall basement.

All to spec, all engineer & building code approved.

Special notes: First time I ever paid the insulator to foam caulk the outlets, windows & around the register & other penetrations.

No mold, no T-111 curl, no movement - low energy bills.


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## SLSTech (Sep 13, 2008)

SprayIt /John - sorry but that will probably Fail
I don't exactly know what your timber spacing in but you are looking at some warping issues coming up & not enough connectors to hold the plywood & any siding up
Timbers also shrink which will be interesting with the CC foam
Along those lines I am not positive plaster will stick to CC foam & it is not exactly the smoothest product when sprayed on which will require a lot of plaster
How about windows, doors, outlets... sorry but the money you will spend will be pretty similar to SIPS which is what should really be used

Tom - unless high winds are never an issue I would have to disagree on it being best to just leave the wood off & just go with foam


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## Tom Struble (Mar 2, 2007)

damn..wrong again:sad:..just seems like the weak link in the system..moisture wise Sean


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## dom-mas (Nov 26, 2011)

I've worked on a few places that had isocyanate directly on the studs (all seams taped), then 1/4"OSB on top of that with tyvek on top of that. New construction, energy efficient, easily passed inspection. Rather than using the sheathing to stop racking they had cross bracing wherever possible and convenient. 

Sheathing is not the only way to stop racking


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## A&E Exteriors (Aug 14, 2009)

I started framing in Shelby county Tn, framed many multi million dollar homes....6000 sq ft customs, and one acre zero lot style homes where the norm for us...... with asphalt saturated celotex sheeting. Some builders/ townships required or had options for 12' osb corners.

We did have to nail seams every 2-4" and every 4-6" in the field with 1 3/4 roofing nails. Moisture resistance was the reasoning for the celotex, it is very humid near the Mississippi river. We also strapped all exterior headers and any over 4' interior headers. 

Many, many roofs are still sheeted with 1x for the moisture resistance.

I'd also like to add that an August day in Memphis of cutting "blackboard " was a miserable one :laughing:


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## A&E Exteriors (Aug 14, 2009)

I'd also add that high winds and lots of tornadoes are a BIG issue in memphis


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## Morning Wood (Jan 12, 2008)

As others have said. Sheath first then foam board. If you are Applying horizontal or vertical board siding I would house wrap or felt over the foam and strap the walls for a rain screen. If you are shingling, you can always sheath the wall again. I would suggest fir Cdx ply and not osb. Osb doesn't hold fasteners worth a damn


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## WarriorWithWood (Jun 30, 2007)

SLSTech said:


> Tom - unless high winds are never an issue I would have to disagree on it being best to just leave the wood off & just go with foam


Not necessarily, wind braces do the same thing as plywood and will stand up to the same amount of wind. 
http://www.betzwood.com/2011/05/20/shear-bracing-foam-sheathed-walls/


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## SLSTech (Sep 13, 2008)

WarriorWithWood said:


> Not necessarily, wind braces do the same thing as plywood and will stand up to the same amount of wind.
> http://www.betzwood.com/2011/05/20/shear-bracing-foam-sheathed-walls/


:whistling I guess I deserve that though Tom did mention...


Tom Struble said:


> no..not always,structural sheathing is not always needed and when it's not it's probably best to get rid of the wood all together


Yes I know about let in braces, corner sheathing, metal braces etc... which brings me to my real issue with the wind - buildings are designed for 80 mph winds. The problem is once the shell is penetrated you are looking at a lot of damage either from the wind or rain or... Sheathing the whole building is about protecting not only those inside, but the building & your neighbors property (it's not cool when half their roof is laying in your pool)



Tom Struble said:


> damn..wrong again:sad:..just seems like the weak link in the system..moisture wise Sean


No your not wrong as it is acceptable by code, it is just not something I would do primarily for the reasons listed above. Yes that was my pool in Arizona - no tornado's just a nice little storm

The air & moisture control layers should prevent any issues from getting to the sheathing - I know one reverse lap & you are looking at issues but the same applies to foam, anything that gets past it is going to cause issues.

Along those lines, did you ever see the article by Joe Listerbck (sp?) on his house (Lessons Learned?) with the gaps that opened up at the seams?


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## Tom Struble (Mar 2, 2007)

of the foam sheathing? yea i think i did read that


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