Rockwool-Not Your 1950’s Mineral Wool Insulation

This post first appeared on the Rockwool R-Class blog.

If you’ve been in the construction industry long enough, chances are you’ve had to remove an old mineral wool insulation product during a renovation.  I know I have.  It’s itchy, easily falls apart, and it often doesn’t completely fill a cavity bay.  I’ve had many conversations with other builders who will not consider using a mineral wool product because of their past experiences with the older mineral wool insulations.  I can tell you; the old stuff is nothing like modern stone wool.

Itchy, fragile, and does not fill an entire 2×4 cavity bay. This R-7 batt of mineral wool insulation from the 1950’s is much different than the modern stone wool equivalent.

I’m currently working on a room renovation in my own home where I removed all the existing wall and ceiling finishing materials and insulation.  The existing 1950’s era kraft faced mineral wool had a listed R-value of R-7 for the original 2” x 4” wall assembly.  This same product was used for the sloped roof of my Cape Cod design.  Ice dams and comfort issues were my main complaints.  My goals for the project is to increase both the wall and ceiling R-values from the interior, (no exterior wall or roof insulation is included) and improve the air tightness of the space.  (And get rid of the popcorn ceiling!)

During the demo process, what I found was someone had installed a second layer of drywall over the original drywall, which had some funky 1950’s wallpaper.  Water staining was present over a large portion of the original drywall and wallpaper.  The water damage was caused by air leaks.  Warm, moist air inside the home finding a cold, condensing surface during the heating season.  Frost would accumulate on the interior side of the roof and wall sheathing until spring when temperatures would warm.  The frost would then melt, water would run onto the interior surfaces of the exterior walls.  Fortunately, there was no structural damage or rot, and only a very small amount of mold, mostly just water staining.

My funky 1950’s era wallpaper with water staining.
The ceiling insulation was supposed to act as the air, vapor, and thermal control layers. It failed at all.

Removal of the existing insulation was the worst part of the entire job.  The process required eye protection, a respirator, and long sleaved clothing with gloves, and I still ended up itchy and coughing.  The 70-year-old insulation required removal from the home in small loads, if I tried taking too much outside at a time, the product would fall apart leaving a trail of shredded insulation.  Once the old insulation was removed and everything cleaned up, the rest of the process was just like working on a new home.

The new cavity insulation for both the walls and ceiling is ROCKWOOL R-15 Comfortbatt®.  The Comfortbatt® I was using is designed to fit inside a 16-inch-on-center wall cavity.  The batts are also sized for a standard 8 foot tall, each batt is 47-inches long, two batts fit perfectly in the cavity.  At 100 ½ inches, my wall height is a little taller than standard which required a small additional piece of insulation be added.  I found it easier to install the small piece between the two full batts, rather than either at the floor or ceiling.  Something else I did to simplify installation, I ran all my electrical cabling through the walls at 47 inches above the bottom plate, eliminating the need to split a batt to fit around the electrical wire.

By installing the electrical cabling at the height of a single Rockwool Comfortbatt, I was able to save installation time by eliminating the need to split the batt to fit around the cable.

Maximum performance of all fibrous insulations happens when the assemblies are airtight.  Air sealing can happen on either the exterior or interior sides of the walls and ceiling.  In the case of this project, I will be using both.  As you can tell from some of the previous photos, my home has board sheeting, individual boards nailed to the studs on the exterior at a diagonal to add structural support (a common practice before plywood or OSB).  Though board sheeting offers resiliency with its ability to take on and release moisture, it is poor at providing any type of air sealing.  I chose to use a water resistive barrier (WRB) on the exterior that also functions as an air barrier.  The product is a self-adhered WRB (basically a giant piece of tape) called Henry Blue Skin.  This product is airtight but will allow some moisture in the form of vapor to move through when drying is needed.

The interior air (and vapor) control is provided by a product called a responsive membrane, also known as a smart vapor retarder.  I chose to use Siga’s Majrex.  When installed correctly, Majrex is very effective at stopping air, but similar to the Henry Blue Skin WRB, it also has the capability to allow water vapor to move out of a wall cavity when needed.

I’ve carefully selected the materials I’m using in my own home based on the attributes associated with each of the products.  ROCKWOOL’s Comfortbatt® has the density to completely fill a cavity and can more easily provide a high-grade installation than other insulation products I’ve used in the past, thus maximizing the R-value.  Both the WRB and interior air/vapor control products provide high levels of air tightness but still allow a good drying potential of the assembly if needed.

Renovation projects often have limitations to the amount of performance gains based on the work scope and budget.  Choosing the right products for the project can help a home last, I’m hoping my home, originally constructed in the 1950’s, will still be around in another 70 years.

2 Replies to “Rockwool-Not Your 1950’s Mineral Wool Insulation”

    1. Hi Ron,
      I did do a blower door test on my home, I was around 10 air changes per hour at the test pressure of 50 Pascals, very leaky for my climate. I have all my own gear for testing, so I’ll probably pull another blower door test later this fall, after temps cool a little, thermal imaging works better during the cooler times of the year. My home is a multi-year renovation project, I’ve got a few more years before I will be finishing up.
      Thanks for the question.

Leave a Reply

Your email address will not be published. Required fields are marked *