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Workman67's picture

I have recently used 4" thick rock wool to insulate a room of a cape cod (exterior walls) for the first time. When we went to buy the insulation, Depot had a huge display of it for sale. In my area, it has not really been for sale recently. I have seen it being used all of the time on TV but that is in very cold climates. I felt that it was alot easier to install and cut to fit odd size openings and want to use it again if available. I had sprayed the walls first with an inch of closed cell because i had done the roof with the spray as well. So after all of my research that seems to sometimes confuse me, do i NOT need the paper face of fiberglass in order to have the correct vapor retarder if i use spray foam first? What is the differance of rock wool compared to fiberglass?? I once had to put it on a wall for a commercial oven before i installed some sheet metal for a hood fan to make code, so i know it will not burn but why all of a sudden is this product readily available??

rockwool (post #206642, reply #1 of 5)

Water vapor is created in the home by people.
It is programed by nature to always move from warm to
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

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.

Actually, water vapor does (post #206642, reply #2 of 5)

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".)

Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Canadian Based DIY TV (post #206642, reply #4 of 5)


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.

I used closed cell spray (post #206642, reply #3 of 5)

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.

What climate zone (post #206642, reply #5 of 5)

are you in?