Replies

1. 
   Perry525 | Aug 16, 2012 04:08pm | #1

   Water vapor is created in the home by people.

   It is programed by nature to always move from warm to

   cold.

   The problem is, that water vapor moves from the warm

   comfort zone of your home into the walls and roof trying to

   get to the cold outside.

   If it cannot easily escape, it condenses inside the walls and

   roof making things wet, possibly freezing and possibly

   leading to mold and wood rot.

   Water vapor can easily pass through drywall, fibreglass

   and rockwool, so providing it can escape to the outside no

   problem.

   You don't mention which type of foam you had sprayed on

   your roof and walls.

   Closed cell insulation, is both airtight and waterproof. An

   inch should save 90% of your heat from escaping.

   However, it may only be sprayed onto the inside of the

   outside walls, and roof,  leaving the timber frame in contact

   with the sideing and shingles on the outside and the drywal

   on the inside.

   There is likely to be a solid path for heat to travel through

   by conduction meaning that 20% or thereabouts of your

   walls and ceiling are in direct contact with the outside cold

   and inside warm wet air, this will lead to condensation on

   the frame. Not good.

   Placing rockwool inside the frame, will stop the circulation

   of warm air, that would raise the temperature and possibly

   avoid condensation. On the other hand it will help to keep

   your heating costs down, until it becomes full of water and

   then becomes a liability as water is 4000 times better at

   conducting heat (outwards) than dry air.

   The key thing is the indoor temperature and humidity and

   whether the inside of the frame is at "dew point."

   It is very difficault to create what amounts to a plastic water

   vapor proof room as the water vapor molecules are very

   tiny and can make their way through the smallest crack.

   Installing a water vapor plastic sheet requires care and

   time to do the job correctly.

   Turning a room into a water vapor proof box does have

   benefits, it does away with drafts and thereby keeps the

   room warmer at lower cost.

1. 
   DanH | Aug 16, 2012 06:55pm | #2

   Actually, water vapor does not move from warm to cold, but from areas of high partial pressure to areas of low partial pressure.  "Partial pressure" is what one might call "absolute humidity" and is essentially tied to "dew point", the temperature at which condensation will occur.  Condensation, then, does in fact occur if, at any point in the wall, the dew point is lower than the temperature.

   If water vapor works it way through a structure slowly enough, moving from a warm, high partial pressure area to a cold, low partial pressure area, the partial pressure never gets high enough to reach the dew point at that point in the structure, so condensation does not occur.  This depends on the presence of both vapor retarding materials and insulation. 

   In general, having the vapor retarding materials concentrated near the (warm/moist) inside surface and having the insulation materials concentrated near the (cold/dry) outside surface increases the "spread" between dew point and temperature in the wall and reduces the chance of condensation.

   On the other hand, if there is a strong vapor retarding layer near the outside surface then the dewpoint inside of that layer can get too high, and condensation is apt to occur.  The "trick", then, is to have more vapor retarding near the inside surface than elsewhere in the wall (and also have adequate insulation, especially towards the outside of the wall).

   (I've not figured out why, after about 50 years of obscurity, rock wool has suddenly become popular again.  50 years ago it was the cruder, less efficient, and more unpleasant cousin of fiberglass, but for some reason it's been "rehabilitated".)

1. 
   Tim | Aug 24, 2012 02:20pm | #4

   Canadian Based DIY TV

   shows.

   You said that "I've not figured out why, after about 50 years of obscurity, rock wool has suddenly become popular again."

   Thank Mike Holmes and Scott McGivelroy.

   Something I cannot figure out is why the Canadians have tubs spounts required in all shower (only) stalls.

   It is a uniquely qualified product to add both sound deadening and fire resistence to a floor/ceiling separtiing apratments. Spray foam is a smoke production nightmare and fiberglass is not dense enough to help much sound isolation and it melts/degrades in the presence of flame, for the most part.

2. 
   workman67 | Aug 17, 2012 09:45pm | #3

   I used closed cell spray foam. Thanks for the input. I pretty much cannot believe that paper face on fiberglass would make a 100 percent vapor barrier therefore after your input, I would say that rock wool could be just as productive a material as fiberglass. I have yet to see any contractor in my area ever tape plastic up on walls like you see them do on TV shows. 

    My reason would be time and money at this point and i want to offer to people the most efficient job at a cost that is possible to handle. I felt it was easier to install the rock wool in odd size cavities which i seem to come across all of the time with the old homes that i work in.

2. 
   davidmeiland | Aug 30, 2012 10:45am | #5

   What climate zone

   are you in?