Replies

1. 
   DanH | Dec 11, 2013 07:45am | #1

   Define "reflective rigid foam

   Define "reflective rigid foam board".  How thick?  And how cold has it been there in the past two weeks?

1. 
   pdunnigan | Dec 11, 2013 08:36am | #2

   Rigid Foam

   1" Polyisocyanurate

   20's-50"s

1. 
   DanH | Dec 11, 2013 08:27pm | #6

   First off, it's not an issue of a "double" vapor barrier, but rather of having insulation on the warm side of a vapor barrier.  When you do that then the temperature at the vapor barrier surface can drop below the dewpoint there.  Having a "double" barrier actually helps a bit by reducing the amount of humidity that can get into the assembly.

   In your case you have a vapor barrier, R19 of fiberglass, another vapor barrier, and about R6.

   If we assume an inside temp of 70F and an inside humidity of 35%, and an outside temp of 30F, the temperature at the inside surface of the foam will be about ((70 - 30) / (19 + 6)) * 6 + 30 = 39.6.  Unfortunately, the dewpoint of 70-degree 35% air is 41F, meaning that the foam surface is apt to be below the dewpoint.  You would probably need to about double the thickness of the foam to be reasonably confident about condensation.

   (There used to be a nice online wall assembly calculator that could do these computations much more elegantly, but I lost the link when my computer went casters-up a couple of years back, and I have't been able to find it again.)

2. 
   sapwood | Dec 11, 2013 12:10pm | #3

   Although faced fiberglass isn't a really good vapor barrier, it is one. So, yes, adding the poly iso would be a double vb. Why not simply add some more fiberglass instead?

1. 
   pdunnigan | Dec 11, 2013 03:52pm | #4

   Video

   I'm lookin at the video on this site "air sealing a knee wall" and he is using the faced foam board on a insulated knee wall. I was hoping to do the same. 

1. 
   calvin | Dec 11, 2013 05:53pm | #5

   pd

   To further confuse the issue-or not.

   Here's a readable short blurb from Martin Holliday-one of the Green Building Advisor brain trusts.  He has some good experience and a pretty reliable resume re. air sealing, vapor/moisture barriers etc........

   https://www.finehomebuilding.com/how-to/departments/energy-smart-details/two-ways-to-insulate-attic-kneewalls.aspx?nterms=62150

   I looked for the video you mention and couldn't find it-could you post a link so we can take a look at it.

   thanks.

1. 
   pdunnigan | Dec 12, 2013 06:15am | #7

   Video

   Here is the link to the video

   https://www.finehomebuilding.com/videos/index.aspx?id=621819021001&c=9

1. 
   calvin | Dec 12, 2013 07:07am | #8

   pd

   Too bad, membership required to view-many will miss out.

   I think if you take Mike Guertin's video and Hollidays print article together with perhaps a visit HERE for some more insight-you might come up with a plan.

   Note that sometimes this info is climate specific-adjust to yours if necessary.

2. 
   pdunnigan | Dec 12, 2013 07:43am | #9

   Video

   thanks for the replys

   Pat

3. 
   sapwood | Dec 13, 2013 11:58am | #10

   The simplistic bottome line is that the wall has to dry to either the inside or the outside. This assumes that moisture will get in despite all ones efforts to the contrary. So, a double vb will inhibit that drying effort. That's why its not a good idea. 

   If one believed everything seen on videos, magicians would rule the land and monsters would be stomping us in the streets.

4. 
   DanH | Dec 13, 2013 06:30pm | #11

   If you took away the inside vapor barrier and trusted it to "dry to the inside" you'd very possibly have a mess, as condensation drained off of the foamboard and got into the ceiling below.

5. 
   pdunnigan | Dec 13, 2013 06:54pm | #12

   Leave as is

   I think I will just leave it as is

6. 
   calvin | Dec 13, 2013 09:15pm | #13

   pd

   Remember, open to aire-fibreglass is merely an air filter.

7. 
   pdunnigan | Dec 14, 2013 07:24am | #14

   Leave open

   what should I covers the attic side wall with?

8. 
   MarkH | Dec 25, 2013 07:53pm | #15

   "If one believed everything

   "If one believed everything ..., magicians would rule the land and monsters would be stomping us in the streets."

   Maybe it's just me, but I really think that is happening right now. 

3. 
   albion10 | Dec 27, 2013 12:51pm | #16

   Howdy, is your atic vented? R19 knee wall is not much of an area  for heat loss compaired to the rooms ceiling. How much insullation in it?

    Instead of foam consider installing Typar house wrap it has a good perm rating. Caulking it to the wood framing boards too reduce air infiltration into/ threw your exisitng  fiberglass . The typar allows vapor moisture movement but greatly retards air movement. Attics , vented ones, are drafty- i know i should use a more technical term- an fiberglass allows air movement - air movement threw the fiberglass is not what you want.