Our insulation choice, Rockwool Comfortbatt, does double duty – thermal resistance for temperature as well as sound absorption. Rockwool was very easy to work with – it’s lightweight and cuts with a handsaw. Technically it can make you itchy if it touches skin, so when we started we put on gloves, long sleeves, and a dust mask. After a couple sessions with it we ditched that gear and there was no unreasonable skin irritation.
The insulation is made of stone wool, described as: “a natural material formed from volcanic diabase rock. The rocks are graded and crushed to a suitable size, mixed with coke and slags, then melted in a furnace at a temperature of 1500 °C. The melt is directed onto a series of rotating wheels where it is spun into wool. From the spinning machine the wool enters a chamber where small quantities of resin binder and mineral oil are added to lock the strands together and make them water repellent. ”
Rockwool has a sound transmission class (STC) of 45 in a standard residential. At that rating “loud speech is not audible” across rooms. That’s good enough for us! Per the Rockwool website, “Living close to an airport, the noise you hear inside your house is reduced by 40 percent if your roof is insulated with ROCKWOOL stone wool.”
Rockwool Comfortbatt has an R-Value of 4.3 per inch of thickness. We purchased the 3.5 inch thick product, hence a total R-Value of 15. This insulation is not sold at Home Depot, only Lowe’s.
Rockwool also makes a similar but different product, branded Rockwool Safe N Sound, which is available at Home Depot. Thickness is the difference between the two products, as the Safe N Sound is 3″ thick, but has the same amount of material and is therefore denser. This improves the sound absorption but decreases the amount of airgap and therefore Rockwool has stated Safe N Sound has a lower R-Value of 3.9. We prioritize thermal insulation over sound absorption, so we traveled to Lowe’s and bought Rockwool Comfortbatt.