Help identify exterior wall underlayment please.

[SHOOTALL]

Plastic is used here all the time. Placement is important.

[mannyrock]

    Okay guys, though interesting, the information about non-vented attics isn't much help.  Mine wasn't built that way, so its irrelevant.


    I agree that using plastic sheeting would not work well here.


    The sole purpose of me putting up an underlayment here, would be to prevent condensation on the inside of the T-111, due to rapid temperature changes on the exterior of the house.   (The sun shines on it blazing hot for 4 hours, and then as soon as the sun sets, the temperature here in the mountains falls and it can get really cool really fast.)


    So, as an underlayment here, I think I should got with the Celotex, half inch(?) insulation board, with an aluminum facing.


    Should the aluminum face outward (i.e, into the back of the T-111 siding) or inward, into the attic space?


Thanks, Mannyrock


   

[FPH]

I would say inside.....Celotex has an expansive web site with applications.

[SHOOTALL]

I can't think of one time in 40 years of construction where the foil side was faced in . BTW the foil works like any other vapor barrier even plastic.

[Old Syko]

I can't think of one time in 40 years of construction where the foil side was faced in . BTW the foil works like any other vapor barrier even plastic.

Now wait a minute.  In all your years as a plumber you have never walked into an unfinished basement and found yourself surrounded by foil extending below the frost line, which by the way puts the insulation against the concrete? 

[FPH]

I can't think of one time in 40 years of construction where the foil side was faced in . BTW the foil works like any other vapor barrier even plastic.

For the small amount of material and his climate I think inside would be fine,  I was a framer-corice man for the early part of my 40 years of construction and I have seen and installed the foil backing placed to the inside in the E. Texas climate.......it also provides better protection for the fragile foil backing.  JMHO.  Check with the Celotex rep.  he will advise you as to the best installation method.  Now I was wrong once, but that was when I was right and I thought I was wrong!!  Don't I wish.

[SHOOTALL]

I can't think of one time in 40 years of construction where the foil side was faced in . BTW the foil works like any other vapor barrier even plastic.

Now wait a minute.  In all your years as a plumber you have never walked into an unfinished basement and found yourself surrounded by foil extending below the frost line, which by the way puts the insulation against the concrete?

 
Many many times and it was wrap insulation with foil out and fiberglass wool in not foamed backed foil. I have also seen the foil paper insulation also. In most cases you put the vapor barrier to protect the wood from rot. So you place it between the surface that sweats and the wood. I hot areas it is between the stud and sheetrock for a conditioned space. Most times it was over the insulation backing. Long time ago 970's  - 80's ) the VB was on the out side but mold would come thru. on the sheetrock after a few years . It is important that the area breath. Foam and foil can sweat on either surface.

[FPH]

My understanding is that the attic is vented and unfinished on the inside.  A cheap layer of
poly against the studs and then a layer of Celotex and then the T 1-11 should be fine.  I assume this is off the ground and not subject to the earth.

[mannyrock]

  Thanks for all info guys.


   Attic space is one and a half stories up, well ventilated above and below, unheated, unfinished, in climate where it gets really hot and humid, and where the sun will beat directly against the siding 6 hours a day, till the sun suddenly sets.


   My local supply store only has Johns Mansville, Foil faced, foam insulation sheathing, half-inch, four ft by eight ft.  I'm going to go with this, with the foil facing outward, so it is directly against the back of the T-111 sheathing.  I will nail it to the studs first, and then the T-111.


  Happily, this is only a small area of siding, about 3 ft by 14 ft, so I don't have to make a decision for the whole house!


  Thanks again,


  Mannyrock

[FPH]

You are correct.  If you are in Florida, no vapor barrier is recommended.  Also, the foil would be installed on the out side of a vented attic space to reflect heat outward,

[SHOOTALL]

Kind of a moot point now with the thermal barriers aval. that reflect heat or trap it as the need maybe.