Laying batts under existing rock wool


Old 10-04-15, 12:17 AM
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Join Date: Feb 2015
Posts: 52
Laying batts under existing rock wool

Hi. I was about to start my fall project Saturday, which is to scoop up the existing rock wool on my attic floor in order to lay down R-19 batts that have a vapor barrier. I would then be putting the rock wool back on top of the fiberglass batts, creating a nice high R. This was going to be a fairly straightforward project.

I then realized that maybe it's not. There's bound to be some dust under the rock wool. Maybe a little, and maybe a lot. It's been there a long time. I'm now thinking that the attic floor must be free of ALL dust before I lay the vapor retardant batts down over it. This, of course, would make this job more tedious.

Any advice would be appreciated. Thanks so much.
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Old 10-04-15, 04:37 AM
Join Date: Jan 2014
Location: usa
Posts: 466
The effectiveness of a "vapor barrier" is not related to dust in proximity to it.

There are other issues you may want to address though in your quest for insulation upgrades.

Rock wool is heavier than an r-19 fiberglass batt, therefore if you have several inches piled on top of fiberglass you will compress it and lose R-value.

Removing the rock wool and doing a good job of air sealing any gaps that may be present in the ceiling plane would be a wise move as an initial approach. You can then add additional insulation. You can purchase rock wool batts at the home centers and install those prior to re-installing the loose material that was there originally.

As far as a vapor retarder is concerned, you may not need to be overly concerned about it. Allowing indoor moisture to pass through the insulation is ok as long as some well defined path of ventilation is present. I would review the venting issues in the process of doing your upgrade. As others will tell you, more moisture is transported by leakage paths than by diffusion through building materials, thus the need to address the gaps and cracks is of greater concern than the vapor retarder.

Of course, being cognizant of how moisture is generated within the house is always a wise move. Deal with spot generated moisture, ( kitchen,bath) with appropriate mechanical fans. Being at a level around 50% is regarded as safe! being at 60% is on a path to developing serious issues.
Old 10-04-15, 08:47 AM
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Join Date: Feb 2015
Posts: 52
Thanks so much for such an informative post, calvert!

I'm glad that I won't have to vacuum up any rock wool dust before applying the batts. That saves a lot of work. I'm also glad that the vapor barrier is not something to be overly concerned about. I was thinking the same thing, but I also felt that it wouldn't hurt to purchase batts with a vapor barrier.

I'm looking to stick with the fiberglass batts, while attempting to avoid compression other than what would happen naturally over time through gravity. The reason I prefer to stick with the fiberglass batts is that they are already purchased, they weren't purchased recently, and I can't return them because the retailer doesn't even sell the brand anymore. So as long as there is no tremendous harm in doing so, I would still stick with the plan and focus on the other issues that you made me aware to be cognizant of.

I have never seen signs of moisture in the attic, which is a good thing. The attic is spaceous for a small house. It has 2 gable vents (with 1 attic fan), and 2 roof vents. So I believe that is considered good venting? There are no soffit vents, and I considered installing them, but it seems that doing so in my situation would just be overkill. I was thinking I should just be sure that my insulation doesn't block air flow where ceiling meets floor. Thoughts?

In terms of spot-generated moisture, the bathroom would be the focus. It's a small bathroom, and a hot shower will steam it up. We do keep a dehumidifier stationed outside the bathroom in summer, and the vapor shouldn't be harmful in the winter as the house tends to be a little too dry in winter. But in terms of addressing the bathroom, there is a 1-foot square cutout in the ceiling with a clear plastic panel sealing the cutout. I can only assume that the previous owner had intentions of installing a ceiling fan of some sort. I've considered a fan/heat lamp. How would you deal with this?

I have seen it written that, indeed, gaps and cracks in the ceiling plane are of utmost importance to address. I suppose these have a tendency to develop over time. I'll look for articles on what to look for and how to correct gaps and cracks.

Thank you for helping me...

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