Replies

1. 
   MartinHolladay | Oct 16, 2007 04:12pm | #1

   Talking Dog,

   "Does condensation ever form under any circumstances inside the glass fiber layer?"  No.  The idea that condensation can occur inside a fiberglass batt is a common misconception;  building scientist William Rose demolished the fallacy in his recent book, Water In Buildings.  (Condensation can occur on cold hard surfaces, including sheathing, but not inside a fiberglass batt.  If a fiberglass batt is soaking wet, it is usually an indication of a problem caused by wind-driven rain or a roof leak.)

   "It seems to me that ... you wouldn't get any condensation anywhere inside the wall during the cooling season."  True.  During the cooling season, the exterior is more humid than the interior;  the exterior polyiso in your wall assembly prevents inward vapor drive, keeping your wall assembly dry during the summer.

1. 
   talkingdog | Oct 16, 2007 05:11pm | #2

   So, what is it with these buildings I see, which, when demo'ed, the FG walls are full of black mold? This is all leakage?

1. 
   MartinHolladay | Oct 16, 2007 05:18pm | #3

   Talking Dog,

   Without inspecting the house, it's impossible to tell where the moisture is coming from.  Most moisture problems in walls are caused the the entry of wind-driven rain due to flashing and WRB (water-resistant barrier) errors.  Some wet-wall problems are caused by condensation of moisture that piggy-backs on exfiltrating air.  If long-term condensation problems cause the wall sheathing to remain wet for many weeks or months, mold can grow, and the mold can be harbored on the damp fiberglass batts.  But in such cases, the condensation occured on the wall sheathing, not within the fiberglass batts.  Once the stud cavity is damp, the entire cavity gets damp;  so in some sense, the location of the original condensation is somewhat irrelevant.

1. 
   talkingdog | Oct 16, 2007 05:46pm | #4

   So, condensation may form inside the sheathing, but not if the wall is covered with foam--is that right?Now, a somewhat related question, since I know you have been following this story for a long time. Polyiso doesn't absorb water, but what do you know about phenol foam? I'm looking into using phenol foam instead of polyiso, as it has slightly better cost performance and is besides rather fire resistant. But I understand it soaks up water. Do you know anything about this?

1. 
   MartinHolladay | Oct 16, 2007 06:02pm | #5

   Talking Dog,

   Just to be sure you understand -- your original question concerned summer condensation, not winter condensation.  Condensation on the interior surface of wall sheathing is a winter phenomenon, not a summer phenomenon.  If your exterior sheathing consists of foil-faced polyiso, there is a very low likelihood that your wall will ever experience either winter condensation or summer condensation.

   "Phenol foam" is an obsolete term usually used to refer to foam insulation products that are no longer sold.  There are three main types of rigid foam insulation:  expanded polystyrene, extruded polystyrene, and polyisocyanurate.  There are two main categories of sprayed-in-place foam:  closed-cell foam (also called 2-pound foam, because its density is usually around 2 pounds per cubic foot) and open-cell foam (also called 1/2-pound foam).

   Some types of expanded polystyrene can absorb water;  others don't.  Open-cell spray polyurethane foam (like Icynene) readily absorbs water.

2. 
   talkingdog | Oct 16, 2007 06:43pm | #6

   That's reassuring. Summer condensation is the problem here.I know that phenol foam is no longer sold in North America, but there's a new, improved formulation of the stuff on the market here that outperforms everything else, in addition to satisfying the fire regulations, so it's a best seller. Only drawback is apparently that it gradually soaks up water. Same problem as aerated concrete siding.When phenol foam went off the market in the US, back in the 1980's (?) was it due to this water absorption, or was there some other factor?

3. 
   MartinHolladay | Oct 16, 2007 06:59pm | #7

   Talking Dog,

   Why are you being so mysterious?  What's the brand name of this foam?

4. 
   BigBill | Oct 16, 2007 08:48pm | #8

   I think that RetroFoam might be the name of the foam he is referring to.

    

   http://www.retrofoam.com/

   Edited 10/16/2007 1:50 pm ET by BigBill

5. 
   talkingdog | Oct 17, 2007 02:00am | #9

   It's called Neoma Foam, produced by Japanese chemical giant Asahi Kasei. It's certified for a one hour fire rating in walls. Here's the thermal performance comparison (heat loss in W/m x K): Neoma 0.0017, Urethane 0.0024 EPS 0.0040. I guess that would make it R-8 or so. Cost is the same as polyiso.