Replies

1. 
   northeastvt | Nov 15, 2007 06:07pm | #1

   Newpoint,

    The foam with aluminum facing (Thermex is one brand) would not need Tyvek in my opinion. Use aluminum tape to cover all joints etc. There was a thread about using foam in cold regions recently. Try searching for that. You would be putting the vapor barrier on the outside of the building envelope. Not suggested in a cold area. But, I see it done a lot on older houses in this area. Curios to see if there are any problems from it in the future. I am assuming you are in a cold area, since your profile is not filled out:)

   northeastvt

1. 
   Newpoint | Nov 15, 2007 07:48pm | #3

   Winters somewhat cold, hence trying to put insulation but if it will cause a problem...

   Thanks for your help.

   Edited 11/15/2007 11:56 am ET by Newpoint

2. 
   RichBeckman | Nov 15, 2007 07:16pm | #2

   What insulation is in the walls now?? How much thicker are you willing to make your walls?? You're adding ply and foamboard, how much thicker can you go (remembering what happens to the doors and windows receding into the walls....)?

   Since you are concerned about insulation, it seems reasonable that you are up north.

   The problem is the vapor barrier.

   I believe almost all rigid foam constitutes a vapor barrier.

   When the weather is cold enough, the inside surface of the foam can get cool enough for water vapor that is in the wall to condense on that inside surface. When that happens, you have liquid water in your wall, not a good thing.

   There is always water vapor in your house. If you already have a vapor barrier on the inside of your wall you might think that there will not be a problem with condensation on the exterior barrier, but you now have the problem that if water gets into the wall, it can't get out (and somehow, water always seems to find its way in...)

   Housewrap is designed to stop water (rainscreen) but allow water vapor to pass through. Felt does the same.

   If the housewrap is detailed with enough perfection, it can help prevent air movement. I'm not sure how often that level of detail is attained.

   I have seem fan fold insulation that is perforated so it is not a vapor barrier. But is wasn't foil covered and taping foam board (to increase its effectiveness as a rainscreen) is often an exercise in futility since the foam surface separates from itself so easily. And fanfold is so thin, it hardly constitutes "insulation".

   I was about to think out loud about putting a drainage plane behind the foam board so condensate could simply drain out, but that would simply allow cold air in behind the foam board and defeat its purpose.

   As northeastvt indicated, what you are suggesting is done, but I will be surprised if you can get anyone on this forum to endorse the idea.

   I also agree with northeastvt that if you do use the foil covered foam board, tape it and skip the housewrap. Not because I endorse what you are doing, but the taped foam board should be adequate as a rainscreen.

   Be sure to flash all the windows/doors/other protrusions in such a way that water is kept in front of the rainscreen.

   Rich Beckman

   
   

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   Edited 11/15/2007 11:19 am ET by RichBeckman

1. 
   Newpoint | Nov 15, 2007 07:54pm | #4

   Only half of the front wall of the house is insulated. Fiberglass batts. Thickness of insulation 1/2". Only window depth would be a concern and had planned for it. Northeast climate. Winters somewhat cold. Thanks for your help.

1. 
   RichBeckman | Nov 16, 2007 04:46am | #5

   If your walls do not currently have insulation in them (half the front and all of the rest??), and you are putting on new siding, then you really should be dense packing cellulose into the walls before you side.You will be very glad you did. It will insulate better than the foam board and doesn't bring up the possible negative issues.

   Rich Beckman