Have your Rockwool bass traps/Superchunks caused damp probs?

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Have your Rockwool bass traps/Superchunks caused damp probs?

Postby Max Dread » Sat Jun 05, 2010 12:42 am

Hi

I'm in the process of moving house... from quite an old house with single solid walls to a house built in 1993 with brick cavity walls.

In the old house, I had a alcove which I filled with rockwool to make a bass trap. I pulled the trap out today and found that the wall (and the cotton the RW was wrapped in) was quite black with damp. To be fair, even with nothing in that corner of the room the wall will go a little black after time - but nothing like this!

I plan to build superchunk floor to ceiling traps in the new house and am a little worried that the same thing might happen. Has anyone else experienced similar problems? Am I likely to have an issue seeing as the house is newer, seems to have no issue with damp, has better heating, etc.

Would appreciate any advice before I go ahead and start with the build.

Cheers

Max
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Re: Have your Rockwool bass traps/Superchunks caused damp probs?

Postby Eric.Desart » Sat Jun 05, 2010 11:42 am

You are confronted with hygrothermal problems.

An interesting link:
http://www.buildingscience.com/doctypes/digest
A lot of these pages are very well supported by sketches and pictures explaining the principles.
On many pages you find a link "expand article" (right-top side). Notice this and use it to see the complete article.
Via the left menu you can find even more documents. This site swarms with good solid information and downloadable documents.

The basic idea is: Condense forms at the coldest side of insulation. Therefore one must have a damp/vapor barrier at the hottest side of the insulation.
Hence in fact acoustic absorption without a damp barrier on top by definition is a risky application, because it is hygrothermally a wrong application.
You have NO problems if the temperature of that wall against which you applied this absorption is about, and on an average, equal to your inside temperature.

If you apply this however against an outside wall (in cold climates of course) you ask for trouble.

The way to solve this is either having some ventilation behind this absorption (which can be a shortcut as well to the inside of the room, or the outside when you intent the absorption to act as thermal insulation as well), or you can cover the inside of the absorption (room side) with a damp screen (whichever) which will slowdown the transport of damp through the absorption. The latter of course will influence the acoustic properties of your absorption (which does not mean it will become necessarily worse, can be positive as well).

The basic idea of thermal insulation is that the vapor resistance on the hot side is larger than the vapor resistance on the cold side. That way vapor has the opportunity to escape rather than condensing into water once the RH (Relative Humidity) equals 100%. Note that vapor is a gas, not water (a liquid), and has a smaller molecular structure. Therefore materials can be watertight, without being vaportight (in a varying degree).

Wrong hygrothermal conditions can cause disasters. For instance: You can put rockwool outside in all weather conditions, rain, whatever, for years. The board will start looking ugly but it still remains rockwool.
I've seen the same kind of rockwool used in a ship against the outer steel-skin, without ventilation whatsoever (hygrothermal complete wrong application). I couldn't use a knife anymore to take a sample but had to use a spoon to collect some of this stinking material having something of a gel. I've seen rotting cellular concrete falling apart, cotton fabrics which you normally can wash a million times felling apart just trying to pick it up.
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Re: Have your Rockwool bass traps/Superchunks caused damp probs?

Postby Max Dread » Sun Jun 06, 2010 6:35 pm

Many thanks for the reply.

My gut feeling is that things will be better in a new build property with no existing damp issues. The old house already gets damp in the corners even when nothing is in the corners. So I imagine not letting the corner breathe and dry out made it a lot worse. As the new house has a cavity wall construction with insulation in the cavity, and has an effective damp course, I'm hoping the inside wall will be closer to the room temperature and so less problems should occur. I'm going to try to set up the superchunks in a way that will allow me to check behind them from time to time.l....

I wonder also whether the fact the RW was covered in cotton made things any worse? In the new house the RW forming the superchunks won't be covered in cotton so no cotton will be up against the wall...

Cheers for the help

Max
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Re: Have your Rockwool bass traps/Superchunks caused damp probs?

Postby Eric.Desart » Sun Jun 06, 2010 9:39 pm

Max Dread wrote:I wonder also whether the fact the RW was covered in cotton made things any worse? In the new house the RW forming the superchunks won't be covered in cotton so no cotton will be up against the wall...

Unless some accidental situations occurred, this has nothing or little to do with it. The temperature difference between the inside of the room and the temperature of the wall is the defining factor (and in old houses maybe this moist can crawl up via other paths as well).
If you want to understand read about RH (relative humidity) versus temperature via the link I gave.
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Re: Have your Rockwool bass traps/Superchunks caused damp probs?

Postby Scott R. Foster » Mon Jun 07, 2010 2:02 am

sounds like you need help with properly insulating the building with a vapor barrier - get this taken care of and moldy mineral wool corner traps probably won't be a problem any more
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Re: Have your Rockwool bass traps/Superchunks caused damp probs?

Postby Eric.Desart » Mon Jun 07, 2010 2:29 am

Scott,

If these traps are mounted against a wall (double brick with insulation in cavity) maintains about the inner temperature then the problem probably becomes less risky.
This can be e.g. when there isn't 100 % coverage. The heat collected by the naked wall will also spread out behind corner traps or absorbers loosely distributed against these walls.

However if there should be some risk, then 2 choices
1) Either using a foil on top (room side) of the absorber (e.g. some plastic foil),
2) either ventilating the absorbers by leaving a space between them and the wall, and mounting them as such that there is free space on top and bottom, hence that natural convection behind the absorbers will take care of the ventilation. For wall panels that's no problem or limitation to leave such a cavity since that only improves low frequent absorption. For super-chunks just introducing some fantasy in the mounting design.
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