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I am in the process of finishing my unfinished basement and had a question about stacking insulation.

When framing the basement we realized we have alot of areas which have about 6~ inches of extra space we could utilize for more insulation (see image). I figured this would be a great opportunity to add some mineral wool insulation for the added r-value and its sound proofing benefits.

If I were to add mineral wool insulation on top of my existing fiberglass insulation (on the exterior wall) what do I do with the vapor barrier (i.e: The paper back on the fiberglass)?

  • Is it okay if the paperback on the fiberglass is now behind the mineral wool?
  • Can I remove the vapor barrier on the fiberglass and have the mineral wool act as a barrier?

Any advice would be great - I am concerned about trapping moisture.

Additional Notes:

  • I live in Virgina
  • The home is split level so the exterior walls are barely "underground"

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Vapor barriers are installed on the warm side in winter on walls.

If you leave the vapor barrier in the middle of the insulation envelope, it will trap the moisture in the wall causing mold or dryrot or both.

Vapor travels from a warm environment to a cool environment. When it reaches its dew point it changes from vapor to water. If this occurs in the middle of your wall and can’t dry out, it’ll create mold, dryrot, etc.

If you add insulation, I’d remove the paper vapor barrier (or slice it into a zillion cuts) so the vapor or moisture can travel through the wall space.

This may help:

https://www.energyvanguard.com/blog/75130/What-Happens-When-You-Put-a-Plastic-Vapor-Barrier-in-Your-Wall

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There is a sophisticated site where all questions concerning dew points, moisture and drying time, U value, phase shifting and more are calculated and displayed after inserting layer for layer of the wall. Each layer can be easily edited and switched on/off to instantly see any change if an (insulation) layer is added or removed. But it seems to be mainly based on European materials and standards. It is free for private use.  www.ubakus.de

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