Rigid Foam Board under sheathing

[milfordtom]

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.

[greg24k]

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? In addition, why are you using unfaced insulation? Are you sure that your field is construction industry?

Originally Posted by milfordtom

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.

[milfordtom]

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.

[SLSTech]

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

[greg24k]

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.

Originally Posted by milfordtom

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.

[milfordtom]

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

[greg24k]

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.

[SLSTech]

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

[Roofus]

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."

[Leo G]

Originally Posted by greg24k

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.

Right there is why this should never be done.

[milfordtom]

thank you very much. very informative

[B.K]

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.

[JamesKB2]

Originally Posted by greg24k

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.

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