Replies

1. 
   rdesigns | Jun 25, 2010 03:55pm | #1

   The house wrap is intended wrap inside the rough window openings so that it serves as a drainage plane and drains any water that gets past the joint between windows and trim/siding. This way, it is under the window flanges and any stick-on flashing.

   If you put it on last, any water that gets past the joint of the window where it meets the siding or trim will, by capillary attraction, be able to get behind the wrap and be absorbed by the osb or whatever sheathing you use.

2. 
   Matt | Jun 25, 2010 06:34pm | #2

   In addition to what RD said....

   The flashing is supposed to direct the water outside of the secondary drainage plane - the house wrap.  The way you are proposing I'm not seeing that your flashing is doing much of anything, and further I'm not seeing what is keeping water from getting behind the house wrap at your penetrations - ie windows, etc,  Don't try and reinvent the wheel - square ones never worked...  There has been thousands of people who have thought this thing through.  Further, I'd recommend against double house wrap.  The beauty of house wrap is the perm rating.  Double house wrap = 1/2 the perm rating - unless you live in Appalachia and the first house wrap was in place for 3 years - exposed :-)

3. 
   DanH | Jun 25, 2010 11:44pm | #3

   They won't tell you the real reason, which is that it's simply a heck of a lot easier to install the wrap before the windows.  You can put it on like stretch wrap, without having to stop at every window and make sure it's laying flat.  And you can leave the wrap covering a window opening to keep the house "dried in" until the windows are installed.

   I've retrofit wrap on a house, and its a minor PITA to work around each window and be sure that the taping around the windows is good.  Not a big issue, but enough of one that I'd go with wrap first if I had the choice.

   With regard to over vs under & window drainage, over is better on top and worse on the bottom.  But given that the bottom is the more likely area of leakage, under has a slight edge.  However, with proper taping (and proper tape) neither arrangement should have a significant advantage.

   Your two layers of wrap is no big deal.  Even if you're unlucky enough to have moisture trapped between the two layers, due to the unlucky alignment of a leak in the outer layer, it's certainly no worse than having the water just go on through.

1. 
   Hokuto | Jun 26, 2010 05:29am | #4

   Thanks for the thoughtful responses. I am trying to "think like a raindrop" and do the flashing right, but overall I want to agree with Dan in thinking that an extra layer of Tyvek is probably not a negative. One thing I will say: after 16 years, the original Tyvek looks pretty good in most places, although it's easy to tell where moisture has gotten in--the staples they used are rusted in those spots. I'm trying to patch all substantial tears and holes (anything larger than a single nail hole--I can't get to all the holes left by the previous siding, but if it's at all ripped or enlarged, I patch it), and I'm adding Tyvek tape to all the seams in the new layer (they omitted the tape the first time around).

1. 
   DaveRicheson | Jun 26, 2010 06:03am | #5

   Go over to the JLC site and look for thier book on moisture control (can't remember the exact title). It is an excellent resource book. It gives detailed step by step methods for flashing on new installations and retrofits, repairs, etc. Dupont also has good ltterature available for using Tyvek, Tyvek straight flashing tape, Tyvek flex fashing tape and seam tape.

   You don't have to "think like rain drop". It has already been done for you and field tested.

   If more "builders" read and followed the instructions on the products they use, there would be a major decrease in the repair side of the building trades.