# foil-faced polyisocyanurate foam board



## rmcfall (Dec 14, 2009)

I've read some about people using extruded polystyrene in walls IN BETWEEN the joists (not layered against the joists on top of existing insulation).  A lot of work cutting the foam board and sealing it up, but probably worth it when the high R-value is considered (at least in my opinion).  From what I understand, the extruded polystyrene doesn't cause any problems in terms of moisture, since it allows moisture to pass.  What about using foil-faced polyisocyanurate instead of extruded polystyrene?  A couple questions...

1. What about using foil-faced polyisocyanurate foam board for an even higher R-value?  That is, cut, placed, and sealed BETWEEN the wood joists of a wall?  Would this create any problems with moisture in the walls?  I can't see how, if the entire cavity were filled with ONLY layers of the polyisocyanurate, but maybe I am missing something?  

2. Can the polyisocyanurate be used against concrete block walls that are ABOVE grade?  I know that extruded polystyrene is OK, but again, would using polyisocyanurate with the foil facing cause any problems by trapping moisture?   In this scenario, the polyiso would be layered against concrete block walls, then a new 2x4 wall or furring strips would be installed to serve as framing for drywall.  

I appreciate the feedback.


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## velvetfoot (Dec 15, 2009)

I don't know, but that stuff might not need a fire barrier (thermax?).  XPS you have to cover I know.


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## dave11 (Dec 15, 2009)

rmcfall said:
			
		

> I've read some about people using extruded polystyrene in walls IN BETWEEN the joists (not layered against the joists on top of existing insulation).  A lot of work cutting the foam board and sealing it up, but probably worth it when the high R-value is considered (at least in my opinion).  From what I understand, the extruded polystyrene doesn't cause any problems in terms of moisture, since it allows moisture to pass.  What about using foil-faced polyisocyanurate instead of extruded polystyrene?  A couple questions...
> 
> 1. What about using foil-faced polyisocyanurate foam board for an even higher R-value?  That is, cut, placed, and sealed BETWEEN the wood joists of a wall?  Would this create any problems with moisture in the walls?  I can't see how, if the entire cavity were filled with ONLY layers of the polyisocyanurate, but maybe I am missing something?
> 
> ...



1. I assume you mean putting the cut panels between the wall studs, not joists, correct? The issue about moisture depends on what is on each side of the wall. If either side maintains a higher moisture level at any time of the year, then it will need to be considered at the time the wall is insulated. What sort of wall are you asking about? 

2. The answer here also depends on what is on each side of the wall. Moisture will still move through concrete block walls above ground. As far as I know, all foil-sided panels are relative vapor barriers, which could be a problem in certain situations.


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## rmcfall (Dec 15, 2009)

I should have been more clear.  

1.  Yes, I mean cutting the panels to fit between the wooden wall studs.  Some of the walls consist of wood sheathing and then brick on the outside, whereas others consist of solid surface sheets of exterior siding.  These are 4x8 sheets of siding.  

2.  Nothing else is on the other side of the concrete block walls other than paint.  I did actually see something put out by Building Sciences that shows polyiso used in this type of situation, so I imagine it must be fine.  

thanks for your help!


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## dave11 (Dec 15, 2009)

rmcfall said:
			
		

> I should have been more clear.
> 
> 1.  Yes, I mean cutting the panels to fit between the wooden wall studs.  Some of the walls consist of wood sheathing and then brick on the outside, whereas others consist of solid surface sheets of exterior siding.  These are 4x8 sheets of siding.
> 
> ...



But is there soil on the outside of the walls, or another room, or outside air?

Are you talking about basement walls?


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