Replies

1. 
   DanH | Apr 20, 2014 10:36am | #1

   Heat rises, water vapor (if anything) sinks.

   What he's proposing is a "hot roof", which is a bit controversial, but no one has ever come up with compelling reasons to not use it.

   (If he does it, it will no doubt cancel whatever warranty he has on his shingles, but that's only because shingle companies will use any excuse to cancel warranties.  And most shingle warranties aren't worth sith anyway.)

1. 
   damunk | Apr 21, 2014 06:32pm | #2

   Roof shingles are slate.  Water vapor rises, it's lighter than dry air.  So, you are going to have water vapor rising coming in contact to a very cold surfaces, (3 windows) and i'm imagining a serious condensation problem.  Just wondering if anyone out there had experience with this set of circumstances.

1. 
   DanH | Apr 21, 2014 09:33pm | #4

   Why would he have a problem upstairs if he doesn't have a problem downstairs??

2. 
   Mongo | Apr 21, 2014 08:09pm | #3

   ??

   It's conditioned space. So moisture should not be a significant issue.

   I finished off my own attic years ago, I'm also in CT. Sealed the attic with foam. I have RFH in the house on the first two floors, but no heat in the attic. It's tight enough where it doesn't need it and the attic stays prefectly cofortable throughout the winter and summer. The beauty of foam.

   With RFH throughout the house, there is no "forced air" circulation from the attic to the other floors of the house. I have zero moisture issues in the attic. I do get occasional winter condensation on some of the second floor windows. Windows are Marvin double hung with insulated glass.

   If he has no existing condensation issues, I doubt if a well-sealed and well-insulated attic will suffer.

1. 
   damunk | Apr 22, 2014 09:01pm | #5

   Good information...

   I guess where my head was going with this is that because water vapor rises, it will be most prevalent in the attic and with winter in the northeast, this area would be the most suspect as far as "wet windows". I guess no big deal.  The attic windows are single pane because prior to this it was an unconditioned space so probably worst case he has some sashes to replace.

1. 
   DanH | Apr 22, 2014 10:52pm | #6

   Once again, water vapor doesn't rise.  It mixes with air and will achieve nearly equal "partial pressure"  throughout the enclosed space, regardless of elevation.  That's physics.

1. 
   AndyEngel | Apr 23, 2014 02:46pm | #8

   Are you sure of the physics?

   I read Allison Bailes blog, Does Open-Cell Spray Foam Rot Roofs? over at GBA today. He contends that water vapor does indeed rise, because the molecular weight of water is lower than that of most components of air. If we ever covered this in my chem classes, it was more than 30 years ago, so I'm out of my depth.  

1. 
   DanH | Apr 24, 2014 12:26am | #9

   If gasses rose or fell based on their molecular weight then nitrogen (molecular weight 28) would rise above oxygen (molecular weight 32) while CO2 (molecular weight 44) would sink to the bottom.  You'd suffocate because the layer of CO2 near the earth's surface would displace all the oxygen.  In reality they all mix together in the atomsphere and maintain relatively constant distribution throughout.

   Yes, CO2 will "sink" when released from a CO2 tank, but that's only because it hasn't (yet) mixed with air.  Once mixed, each gas maintains the same "partial pressure" throughout the mix.

   Water vapor (molecular weight 18) behaves like any other gas in this regard.

   (If someone blogs that water vapor rises, you should question whatever else they put in their blog as well.)

2. 
   AndyEngel | Apr 24, 2014 10:40am | #10

   Excellent points. I do wonder if there's a time factor involved, though? The mixing doesn't happen instantaneously, as evidenced by the steam that collects at the ceiling in my shower. Is there any chance that water vapor does rise until it mixes with the air? Again, 30 years since my last chem class, and I'm just trying to understand the process.

3. 
   DanH | Apr 24, 2014 11:34am | #11

   The steam from your shower (mixed with air) is going to rise mostly because it's warm.

2. 
   damunk | Apr 24, 2014 09:40pm | #12

   hygric buoyancy-

   moisture laden air is lighter and less dense than dry air. I believe it (water vapor) rises.  My friend says he's going to create a little barrier at the single pane glass at attic windows with plastic to prevent condensation.

2. 
   Mongo | Apr 23, 2014 09:35am | #7

   Understood...

   ...any installation can be buggered up.

   Whenever it comes to retrofitting an older house with foam, if the lid (attic) is going to be sealed, it makes sense to address moisture in the lower portions of the house.

   The basement. Address moisture generation in the basement if it's an issue. Minimize moisture vapor getting from the basement into the living space above by foaming any electrical/plumbing/HVAC penetrations in the basement ceiling. Detail work.

   Same inside the living space. Address moisture generation at the source; cooktops, bathroms, etc. Might already be done, and it might already be effective. It's just a matter of having open eyes.

   One thing to consider. Okay, one of several things to consider. Right now if the attic is leaky there might be air movement within the house. From the basement, into the living space, and onward up into the attic, then out the currently vented attic.

   When the attic is sealed off, that can eliminate some of that vertical stacking through the envelope of the house. So it's good to at least attempt to take a holistic aproach. You mess with the lid of the house, at least take a look at the rest of the house.

   With this attic beign made conditioned space, it's not like it'll be an isolated area. If it's properly conditioned, it'll become one with the rest of the house.

   It's good you're thinking things through.

3. 
   F_Timmons | Jul 21, 2014 10:31am | #13

   In my honest opinion, you want to avoid using spray foam. I only say this because I know it has proven faulty for many people and it always gets bad reviews. I think you want to go with foam board insultation such as the the blue boards. They are more durable and last longer then the spray foam. univfoam.com might have what your looking for, but I definetly would tell your friend to do his research first!   

1. 
   damunk | Jul 21, 2014 11:44am | #14

   House is huge

   To cut and install rigid would be brutal.  I think the close cell spray is the way to go. Get all those nooks and crannies, ya know?  But who knows? I don't know when they 1st started using it in attics so the frame of reference may not be "complete". Maybe in 30 yrs the stuff turns to dust. (like I will be in 30)