Replies

1. 
   MikeHennessy | Sep 30, 2009 09:36pm | #1

   Not foil -- it'll put the vapor barrier on the wrong side. As for in the cavities, consider poly vapor barrier and blown-in cells. Cheaper and just about as effective.

   Mike Hennessy
   Pittsburgh, PA
   Everything fits, until you put glue on it.

1. 
   AzjaST | Sep 30, 2009 09:56pm | #2

   By blown-in cells you mean open cell foam ?

1. 
   MikeHennessy | Oct 01, 2009 02:22pm | #11

   "By blown-in cells you mean open cell foam ?"

   Nope. Celulose. About 1/3 the cost. The big advantage of foam over blown-in is that it acts as its own infiltration barrier. But since you're going to be re-doing the sheathing, you can get a good infiltration barrier on the outside and not have to rely on the wall cavity insulation to do that job.Mike HennessyPittsburgh, PAEverything fits, until you put glue on it.

2. FHB Editor
   JFink | Sep 30, 2009 10:43pm | #3

   Not foil? It seems that foil-faced polyiso on the outside would be a fine vapor barrier situation, Mike. Or are you referring to something else in his question?For my money, I'd do polyiso on the outside because if you've decided on an inch, polyiso will give you the best performance (tape all the seams with housewrap tape). If the outside is detailed correctly then you should have a good air-seal, and fiberglass or cellulose (aka: cells) would be an effective yet economical insulator, if installed neatly to avoid settling (for cells) or major gaps and voids that can lead to convective loops(with fiberglass).Justin Fink - FHB Editorial

1. 
   atrident | Sep 30, 2009 11:01pm | #4

      Foil on the outside will trap moisture in the cavity. Lets be thinkin foil on the inside  as a vapor barrier. Whatever you use the vapor

    

    

    

    

    

    

    

    

    

    

    

    

    

    

    

    

    

    

    

    

1. FHB Editor
   JFink | Sep 30, 2009 11:51pm | #5

   Foil on the outside will only trap moisture if there is also a class 1 vapor retarder on the inside as well. Otherwise the house is designed to dry to the inside, and is perfectly acceptable.Justin Fink - FHB Editorial

1. 
   Piffin | Oct 01, 2009 01:24am | #6

   I think this is six of one and half a dozen of the other, Justin. We want the VB on the inside and foam same side when it is a heating climate of 6-7000 degree days or more, and on the exterior side when it is a cooling cliumate, but I am thinking his location is borderline either way.So it would come down to how to do the detailing around windows etc.I would probably do foil faced on the exterior, with a rainscreen detail to help the foil do its thing. The Polyisoanurate is close to R7 vs the EPS at R5.Same amt of labour would give R 10 with 1-1/2" foil polyiso.Then I would fill the stud cavies with denspak cellulose or BIBs with a n air barrier/retarder on the inside.As you know, I am in a borderline climate too, and I sometimes do the foil polyiso on the inside of the walls, with strapping, then the sheetrock. It all depends on space available, detailing for trim, etc.With an existing house like he has, He would probably be giving upp too much to put the foil polyiso on the interior face 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

2. 
   Piffin | Oct 01, 2009 01:27am | #7

   Now then, is there anybody working on some changes to the forum?http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=124713.1I addressed a Q to sysop and no reply and no fix. 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

3. 
   MikeHennessy | Oct 01, 2009 02:30pm | #12

   Well, I'm no insulation expert (I'm not even sure there is such a thing), but vapor barriers on the outside always give me pause. Sure the walls will dry to the inside given the proper temps, but what stops condensation on the sheathing when vapor is allowed to get there due to no inside VB?  

   I guess I'm just used to the vapor barrier being on the inside in a heating climate.Mike HennessyPittsburgh, PAEverything fits, until you put glue on it.

4. 
   Clewless1 | Oct 01, 2009 03:27pm | #13

   Right, but I've seen the XPS and the polyiso done on the exterior w/out a problem. I think that given an interior vapor retarder (e.g. paint or poly), and proper sealing AND proper ventilation (get those exhaust fans in right!!), you can do the insulation on the exterior. Both XPS and polyiso are 'vapor retarders' ... polyiso more so as foil has no permeablility, I think. Also the rigid on the outside often assures that the dewpoint isn't reached w/in the wall cavity. Someone menttioned 1 inch. A colleague of mine substituted OSB sheathing with 1/2" polyiso (and diagonal bracing). That seemed to be successful. He built high quality houses and was well versed in the energy industry.

5. 
   Clewless1 | Oct 01, 2009 03:29pm | #14

   polyiso or XPS ... pay your money, take your choice. If you want calc the cost per R-value. If you need more R per inch, go polyiso. As far as the moisture issue, they are both similar barriers to moisture movement.

6. 
   atrident | Oct 01, 2009 06:19pm | #15

      You are right JF. I was assuming a vaporbarrier on the inside. The poster is now asking if he should use faced or unfaced.

7. 
   NRTRob | Oct 01, 2009 08:03pm | #16

   why not use EPS?polyiso and XPS will both de-rate to the same ballpark EPS (no more than 1 more R per inch, and you're unlikely to do much more than 1" in this situation) is in to begin with, and EPS is much cheaper than either other product.-------------------------------------
   -=Northeast Radiant Technology=-
   Radiant Design, Consultation, Parts Supply
   http://www.NRTradiant.com

2. 
   AzjaST | Oct 01, 2009 03:18am | #8

   Thank you guys for taking time to answer my questions. I am not surprised to find contradicting solutions as it is exactly what I was finding checking forums, buildingscience.com and various articles by Joe Lstiburek and Paul Fissette. To further clarify, it is borderline climate, mostly heating but in the summer we got few days with humidity reaching 100% at 95 F. So it looks like I am getting closer to final solution. From inside latex painted drywall 1/2 ", inside cavity either cellulose or batts( faced or unfaced ?), plywood 1/2" and 1 or 1 1/5 " rigid foam (still not sure xps or polyiso as even you guys can not agree on this), housewrap ?, rainscreen, cedar.

1. 
   RedfordHenry | Oct 01, 2009 04:40am | #9

   Your plan with cells (or unfaced batts) in the stud cavities and 1.5" foil-faced iso on the exterior.  Housewrap helps to detail the flashing over windows and such (which is extremely important if you are adding foam to the exterior).

   By rainscreen, do you mean vertical battens to which you can nail the siding?

1. 
   AzjaST | Oct 01, 2009 01:50pm | #10

   Rainscreen from Benjamin Obdyke, Home Slicker.