3rd layer – rock wool insulation

I shared our experience with the 1st and 2nd layer of insulation in recent posts.

We originally intended to use spray foam for the entire wall section, but decided after some more research to add another material, or a third layer: a rock wool batt product.


What is rock wool? It was first discovered as a byproduct of volcanic activity, where lava came into contact with air and cooled into fibers. Modern manufacturing processes spin molten rock into thin fibers. The process is said to look like cotton candy production.

After adding a binder to the fibers and letting it cure, the batts get cut into the required dimensions and are ready for packaging.


The rock wool option subtly slipped into the picture for a number of reasons:

  • It has the thermal resistance we are looking for.
  • Unlike spray polyurethane foam (SPF), rock wool is not a petroleum based product.
  • It has a high recycled content.
  • Because rock wool is made out of stone and slag, it won’t burn.
  • It is a very economic insulation material (typically around $0.16 per board foot), more economic than SPF or recycled cotton batts.
  • Rock wool is very easy to handle and install.

I found two rock wool manufacturers (Roxul and Thermafiber) that distribute their products in Chicago. The Roxul Comfort Batt was sold for a price that fit our budget at the Chicago Green Depot, where I placed my order and picked up the material.


The batts are very light and dimensioned to fit between 16 inches on center wall framing or floor joists. Due to their somewhat soft nature, we could squeeze the batt, slide it between the framing and let go. Right away it expands back to its original size and firmly sits between the studs or floor joists.

We appreciated the ease with which rock wool can be cut. A long, serrated bread knife was the perfect tool to trim the batts to the required length or fit them around outlets and light switches.

The heat has been on for a few weeks and with all three insulation layers in the walls (an R-value of 25 to 27), we should be able to keep that heat where we need it.

About Marcus de la fleur

Marcus is a Registered Landscape Architect with a horticultural degree from the School of Horticulture at the Royal Botanic Gardens, Kew, and a Masters in Landscape Architecture from the University of Sheffield, UK. He developed a landscape based sustainable pilot project at 168 Elm Ave. in 2002, and has expanded his skill set to building science. Starting in 2009, Marcus applied the newly acquired expertise to the deep energy retrofit of his 100+ year old home in Chicago.

2 thoughts on “3rd layer – rock wool insulation

  1. Small bathroom,gutted.Roxul Mineral wool Or denim insulation on outside wall? The denim batts at Home depot are only R-6.7.That doesn’t seem enough for an outside wall?

    Why is this Roxul insulation banned as carcinogenic in California?

  2. Briefly searched on Google and didn’t find any references on the California ban on Roxul. Not sure where that information is coming from.

    Regarding how much and what insulation material to use: That depends on a number of factors.
    – The climate zone of the project
    – The kind of wall assembly construction the insulation goes into
    – Moisture management across the wall assembly

Leave a Reply

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

This site uses Akismet to reduce spam. Learn how your comment data is processed.