Tag Archives: moisture resistance

Comparing Rockwool and Fiberglass Insulation

Comparing Rockwool and Fiberglass Insulation  

Fiberglass insulation has long been a popular option for slowing heat transmission through building walls and ceilings. While it may have an added benefit of creating a fire-resistant layer between interior and exterior walls, fiberglass still may not measure up to Rockwool’s natural abilities. Like fiberglass, Rockwool is an insulation material regularly used in residential, commercial, and industrial buildings.

Rockwool can be differentiated from fiberglass by comparing heat retention, fire resistance, moisture resistance, and soundproofing capabilities. 

Rockwool insulation’s manufacturing processes helps to explain true fire-resistant potential of this product. It’s composed primarily of basalt rock and a recycled steel-making byproduct known as slag. These components are superheated, allowing them to liquefy and mix together into a lava-like liquid. In order to melt these substances, temperatures must exceed 2,900 degrees Fahrenheit.

This mixture is then blown into a large spinning chamber designed to stretch superheated liquid into fibers. These fibers are then gathered together and compressed into a mat, then cut into slabs of Rockwool insulation.

By creating Rockwool through this process, all organic matter is eliminated, greatly increasing finished product’s mold- and mildew-resistance.

Confusion about recycled material amounts used to make Rockwool insulation can mostly be attributed to statistics about mineral wool insulation in general. Rockwool is a brand-specific type of mineral wool insulation so popular its name became synonymous with mineral wool. Brand-specific Rockwool insulation is typically between 16 to 40 percent recycled materials. Department of Energy has stated mineral wool insulation contains an average 75% recycled materials. Department of Energy’s estimate is hard to back up as they make a distinction between standard ‘rock wool’ insulation and ‘slag wool’ insulation, but don’t note difference in recycled material amount for each product. Also, this is a perfect example where ‘Rockwool’ brand name being used in place of generic material name, blurring lines between products.

In general, it can be derived recycled material amount in Rockwool insulation is not precise because it ultimately depends on specific product. Rockwool insulation may only have between 16 to 40 percent recycled material, while slag Rockwool insulation can be made with up to 75 percent recycled material.

Both fiberglass and Rockwool are effective at keeping buildings cool in summer and warm in winter, but specific thermal efficiency of these materials favors Rockwool. While fiberglass insulation is capable of offering an R-value of about 3 to 3.2 per inch of insulation, Rockwool has an R-value between 4 to 4.2 per inch of insulation.

Fiberglass insulation also may lose some thermal efficiency over several years if it begins to degrade. Due to construction methods and materials used to make Rockwool insulation, its thermal performance remains stable over a building’s lifetime. Rockwool does cost more per square foot than fiberglass insulation.

As noted previously, Rockwool insulation is formed from literal rocks and steel slag and must be heated beyond 2,900 degrees Fahrenheit in order to mix component materials and create this highly effective insulation. With this in mind, it makes sense mineral wool products in general can resist fire, flames, and heat up to 1,400 degrees Fahrenheit, while some Rockwool products are capable of resisting temperatures up to 2,150 degrees Fahrenheit without melting, smoking, or catching on fire.

This impressive heat-resistance is ideal for buildings because it forms a fire-resistant barrier between building interior and exterior, between rooms, and even between floors, slowing fire spread. It should be noted fiberglass insulation is also highly heat-resistant, though it begins to melt at about 1,100 degrees Fahrenheit.

Rockwool’s durability is difficult to dispute, given it is capable of retaining its thermal efficiency over a building’s entire lifetime without degradation in its R-value. This is due to materials used to make Rockwool insulation, including rock and steel slag, both known for having a high level of durability and natural resistance to decay and corrosion.

However, Rockwool’s durability isn’t limited to material’s heat retention quality. Impressive water-resistance, mold-resistance, mildew-resistance, and fire-resistance also contribute to Rockwool insulation’s durability and capabilities. 

Some people may not appreciate Rockwool insulation’s heftiness because it does tend to be thicker than fiberglass insulation, but this helps to slow heat transmission and it has an added effect of slowing sound waves. As sound waves attempt to move through Rockwool material, they are slowed and sometimes completely blocked, creating built-in soundproofing.

While insulation thickness helps to block noise, it is Rockwool insulation’s density providing soundproofing. Fiberglass insulation has a density of about 0.5 to 1.0 pounds per cubic foot, allowing it to reduce sound by 4 to 10 decibels. Rockwool insulation has a density of around 1.7 pounds per cubic foot, capable of consistently dampening sound by 10 to 15 decibels.

Rockwool’s construction and composition make it ideal for rooms prone to high levels of humidity, like bathrooms or kitchens. Rockwool insulation’s moisture-resistant and vapor-permeable qualities mean any liquid water will drain away from insulation instead of soaking into it, while gaseous water vapor will pass through without dampening material.

Additionally, Rockwool insulation is inorganic, so it makes a poor medium for mold and mildew to grow as there is nothing for them to use for energy. In fact, Rockwool products are also tested and certified as resistant to fungal growth, reducing chances users will open up a wall and find a dangerous biological problem waiting for them.

Meeting IRC Slab Edge Thermal Breaks With Post Frame

Meeting IRC Slab Edge Thermal Breaks With Post Frame

Reader CHUCK in MUNCIE writes:

“Morning sir, I read your link in your post about post frame buildings for barn houses… one thing I am wondering, is how does the building pass energy code for residential construction, plus the IRC talks about a building being used for residential occupancy needs a thermal break at the foundation wall…. in a conventional post frame building the posts are on footings, and a slab on grade is poured, so how do you provide the thermal break to meet the building code?”

Mike the Pole Barn Guru says:


Post frame construction for residences has no appearance of going away at any time according to my crystal ball. And why should it? Post frame is more economical than stick frame, very DIY user friendly and can be readily super insulated. Here, I previously expounded upon post frame’s residential virtues: https://www.hansenpolebuildings.com/2022/01/why-your-new-barndominium-should-be-post-frame/

Slab edge thermal breaks (slab perimeter insulation) is only required in Climate Zones 3 and greater. You can look up your Climate Zone at codes.iccsafe.org/content/IECC2021P2/chapter-3-re-general-requirements When required, it must be a minimum of R-10 and down two feet (Climate Zones 4 & higher adds a horizontal R-10 component or becomes down four feet).

A common question with rigid foam insulations is how well it resists water. A number of studies show EPS retains less moisture than XPS. A case in point is a side-by-side analysis of these two rigid foam types installed on a commercial building foundation in St. Paul, MN. When extracted and tested after 15 years in service, EPS had 4.8% moisture content by volume, compared to 18.9% for XPS (a four-fold difference). A testing lab also found  XPS holds water longer than EPS. After 30 days of drying time, XPS still had elevated moisture of 15.7%, while EPS had dried to 0.7%.

For installations where insulation will be exposed to large amounts of water or frequent wetting, rigid foam insulation is available with water-resistant facers or pre-cut drainage grooves. Insulation with polymeric laminate facers keep water from entering insulation and also provide an added barrier to water wicking or diffusing through.

Moisture resistance is also important for below grade and under-slab insulation, since wet products provide much lower thermal resistance. Side-by-side insulation comparison found EPS retained 94% of its specified R-value, while XPS lost nearly half of its insulating capability over 15 years.

In addition to higher moisture resistance, EPS also is not subject to thermal drift. This means its R-value stays same over time. By comparison, XPS’s manufacturing process uses blowing agents diffusing from foam’s cellular structure over product life, thereby reducing its thermal performance. EPS manufacturers typically warrant 100% of published R-value for 20 years or more, while common XPS warranties cover just 90% of published R-value.

Whether selecting EPS or XPS insulation, to ensure performance, confirm product was manufactured to meet requirements of ASTM C578, Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation. This standard provides a key quality check on rigid insulation.

As insulation becomes increasingly common at slab edges, understanding performance and cost factors of these different materials is important. EPS offers a number of advantages over more commonly installed XPS, including having highest R-value per dollar among rigid insulations, making it a cost effective choice for many jobs.