Wall insulation in old house

[El_Viajero]

I'm applying some soundproofing treatments to the spare bedroom in a 50s ranch I just bought. There isn't any insulation in the walls (no surprise), but I need to insulate one exterior wall as part of my project. The old drywall has been torn out (actually, it is "new" old drywall because it appears to have been installed during an 2007 renovation) and I'm happily packing mineral wool batts into the cavities.

My question has to do with potential water/moisture issues. My understanding is that old houses often can't accommodate wall insulation because they were built to "breathe" and the framing might not ever dry out if it gets wet and insulation holds the moisture in. Should I be concerned about this with the one room I'm working on?

It's a brick house and there's some kind of fiberboard sheathing behind the brick. I'm adding the insulation batts against this fiberboard-type material. Hoping someone here has some experience with this...

[BudgetSlasher]

I have no personal with doing exterior walls in an older house, but I recall seeing a youtube video that deals with it. I think they mentioned making an air gap b/t the sheathing and insulation to allow air to continue circulate and dry "if it can't dry it will die" (as opposed to has the breathe). In that case they used a dimple mat like those usually found outside for poured foundations.

Edit: I found the video I mentioned above.https://www.youtube.com/watch?v=FnfnLAhtTvk

[Paper Chaser]

What's your location? Humidity and temperature difference are the two big ingredients for moisture. So, where you live really impacts stuff like this.

If you live in a dry climate, someplace that's pretty temperate, or both (southern CA for example) then moisture control probably isn't a huge concern. If you live someplace with high humidity levels and/or the outside air temps differ a lot from your indoor air temps then it's more of an issue.

Condensation forms on the warm side of a surface when there's enough moisture in the environment and a significant temp difference between the two sides of that surface so the General rule of thumb in the past has been to put an impermeable vapor barrier (not a semi permeable material like tyvek) on the side of the wall that is typically warmest. That keeps the condensation from getting inside the wall cavity (There are of course some exceptions depending on the type of insulation, etc but for rockwool I'd stick with the rule of thumb). So if you have cold winters and heat the room, you'd want a vapor barrier between the insulation and the drywall. If you live in a very hot climate, it might be better to install the vapor barrier before the insulation so that it would be between the sheathing and the insulation.

[bacchi]

As always, Building Science has an article about it but it's almost entirely about new construction.

https://www.buildingscience.com/documents/digests/bsd-controlling-cold-weather-condensation-using-insulation

Condensation within walls during cold weather is a common performance problem. Most such condensation is due to air leakage, not diffusion.

Unfortunately, the best way to solve it is rigid insulation on the outside of the sheathing. The warmer the area, the less rigid foam.

For retrofits, seal air leaks (plugs and switches and baseboards) and put an air barrier where appropriate, as Paper Chaser mentioned.

[Papa bear]

Vapor barrier on the warm side as mentioned above.  In cold climates, that means on the inside.  Any moisture should then be able to dry to the outside. 


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[El_Viajero]

Thanks, all. I actually live in NC, so it's a warm, humid climate for most of the year.

I forgot to mention: I'm also planning to put up a layer of plastic material known as mass loaded vinyl (MLV) over the studs after insulating the walls. MLV is standard for many soundproofing projects. I'm not sure if the MLV qualifies as a vapor barrier but it would almost certainly help prevent moisture intrusion into the home, right?

So basically, the layers would be:

Exterior bricks --> Black fiberboard material from the 50s --> Mineral wool batts --> Heavy plastic (MLV) --> Drywall

Does the fiberboard itself function as a porous air gap of sorts? I'm just wondering what about this setup might cause a problem and what the problem might be. There's no siding on this house that's going to rot, so that's good. Am I supposed to be worried about the framing rotting away? Mold growing on everything? What, exactly?

[lthenderson]

Thanks, all. I actually live in NC, so it's a warm, humid climate for most of the year.

Exterior bricks --> Black fiberboard material from the 50s --> Mineral wool batts --> Heavy plastic (MLV) --> Drywall

Does the fiberboard itself function as a porous air gap of sorts? I'm just wondering what about this setup might cause a problem and what the problem might be. There's no siding on this house that's going to rot, so that's good. Am I supposed to be worried about the framing rotting away? Mold growing on everything? What, exactly?

I'm not from a warm climate so don't have a working knowledge but I would guess most vapor barriers in a warm humid climate are between the sheathing and the exterior bricks. If you install it up against the drywall, you risk having the humid air condense up next to the mineral wool batts turning them into giant sponges. Since I have no working knowledge, I don't know what the solution is but I would guess it would be better to have the vapor barrier up against the black fiberboard material before the mineral wool batts and drywall are installed.