A Deep Dive Into Polyiso and Mineral Wool

Based on the white paper, Mineral Wool and Polyisocyanurate Insulation: A Comparative Study of Water Absorption, Drying and Rewetting by M. Steven Doggett, Ph.D, Principal Scientist, Built Environments, Inc. View infographic

Mineral Wool Absorption & Drying*

WHEN TESTED FOR WATER ABSORPTION (ASTM C209) IT WAS FOUND THAT:

  1. Mineral wool absorbs 18% to 78% of its weight in water.
  2. Mineral wool takes 3 to 7 days to dry.

ASTM C209 is the standard sorption test for closed pore foam insulation. This test requires insulation samples be immersed in one inch of standing water for a period of two hours.

FINDINGS NOW DEFY COMMON CLAIMS REGARDING WETTING AND DRYING BEHAVIORS OF MINERAL WOOL.

Mineral wool absorbs 18–78% of its weight in water when wetted with liquid water because it is an open pore, fibrous material. Sorption potentials for mineral wool have, in the past, been evaluated utilizing water vapor via ASTM C1104 (95 ±3% RH). That makes the product appear to perform better than it really does because it doesn’t measure bulk water. Sorption potentials for mineral wool have therefore been underestimated.

For example, it has always been assumed that water freely drains from the void structure and that water retained by mineral wool dries quickly due to its inherently high vapor permeance. Based on this study, neither is true. Instead, water is suspended by capillary forces and the network of fibers.

POLYISO HARDLY ABSORBS ANY WATER, AND IT DOESN’T TAKE DAYS TO DRY.

SEE THE WHITE PAPER FOR MORE INFO.*

*Based on the white paper, Mineral Wool and Polyisocyanurate Insulation: A Comparative Study of Water Absorption, Drying and Rewetting by M. Steven Doggett, Ph.D, Principal Scientist, Built Environments, Inc.

TERMS & DEFINITIONS

Sorption: A general term in physical chemistry used to describe the combined processes of absorption and adsorption.

Absorption: Uptake of matter in bulk by other matter, for example, the penetration of substances into the bulk of another solid or liquid.

Adsorption: Surface retention or adhesion of an extremely thin layer of molecules to the surfaces of solids or liquids with which they are in contact.

Holding Capacity: The ability of a pore structure to physically hold water against the force of gravity.

Drastic Performance Variations in Mineral Wool Products*

When tested for water absorption (ASTM C209):

It was found that two different brands of mineral wool product, with similar declarations for densities and binder type, had large differences in water absorption rates.

This indicates other factors that can’t be identified visually. Factors may include the raw materials used to make the fibers, chemicals used to bind the mineral fibers together, additives to increase water repellency, the direction of the fibers and how much the fibers are compressed during manufacturing.

Even within mineral wool product bundles from the same company, the range of sorption values varied significantly between individual pieces.

A LACK OF SORPTION CONSISTENCY WITHIN MINERAL WOOL PRODUCTS ERODES CONFIDENCE IN THE THERMAL PERFORMANCE OF A BUILDING

A lack of sorption consistency in mineral wool products creates a wider variation in drying times, which is why tests showed a range of 3–7 days to achieve the same endpoint of 0.5% water content.

MINERAL WOOL IS NOT CONSISTENT IN WATER SORPTION.

SEE THE WHITE PAPER FOR MORE INFO.*

*Based on the white paper, Mineral Wool and Polyisocyanurate Insulation: A Comparative Study of Water Absorption, Drying and Rewetting by M. Steven Doggett, Ph.D, Principal Scientist, Built Environments, Inc.

TERMS & DEFINITIONS

Sorption: A general term in physical chemistry used to describe the combined processes of absorption and adsorption.

Absorption: Uptake of matter in bulk by other matter, for example, the penetration of substances into the bulk of another solid or liquid.

Adsorption: Surface retention or adhesion of an extremely thin layer of molecules to the surfaces of solids or liquids with which they are in contact.

Drying: Loss of water due to drainage, evaporation or desorption.

Rewetting: Mineral Wool’s Absorption & Holding Capacities*

When mineral wool was tested in accordance with ASTM C209 and then rewetted, it was found that:

  1. Rewetting mineral wool increased water absorption by 130–190% and extended dry times by up to 4 days.

  2. Repeated wetting of mineral wool revealed moisture holding capacities that vary due to changing pore and fiber structure.

Rewetting tests conducted on mineral wool specimens revealed a substantial increase in water absorption. This is significant, as results from standard sorption testing don’t account for prior exposure to water. Even a single previous exposure to liquid water may result in increased sorption.

When rewet over seven cycles, separation of fiber layers and notable physical voids became obvious. These changes caused the product to absorb and drain differently than it had during the previous cycle, implying physical product changes are the main cause of greater water absorption upon rewetting.

*Based on the white paper, Mineral Wool and Polyisocyanurate Insulation: A Comparative Study of Water Absorption, Drying and Rewetting by M. Steven Doggett, Ph.D, Principal Scientist, Built Environments, Inc.

Polyiso Absorption and Drying Results*

When tested in accordance with ASTM C209, it was found that:

  1. Foil Faced and Coated Glass Faced Polyiso absorbed less than 4% water by weight and less than 0.13% by volume.

  2. Both polyiso products were effectively dry within 24 hours.

Specimens of foil faced and coated-glass faced polyiso were immersed in one inch of standing water for two hours. Specimens were drained in accordance with standard protocols.

Fast drying times for the two polyisocyanurate samples were attributed to low water absorption coupled with the ability to rapidly release water vapor.

REWETTING POLYISO RESULTED IN NEGLIGIBLE CHANGES TO ABSORPTION AND NO CHANGES IN DRYING TIMES.

When rewet, the polyiso specimens showed no appreciable increase in water absorption – and no significant changes in drying time. Polyiso sorption behaviors and dry times remained constant because the structure of polyiso is unaltered by wetting and rewetting.

Rewetting resulted in no changes to the drying time for polyiso.

IN CONTRAST TO POLYISO—

Mineral wool absorbs up to 178% of its wEight
in water, and takes at least 7 days to dry.

*Based on the white paper, Mineral Wool and Polyisocyanurate Insulation: A Comparative Study of Water Absorption, Drying and Rewetting by M. Steven Doggett, Ph.D, Principal Scientist, Built Environments, Inc.

TERMS & DEFINITIONS

Sorption: A general term in physical chemistry used to describe the combined processes of absorption and desorption.

Absorption: Uptake of matter in bulk by other matter, for example, the penetration of substances into the bulk of another solid or liquid.

Desorption: The release of an adsorbed substance from another matter.

Wetting: Displacement of a solid-air interface with a solid-liquid interface.

Drying: Loss of water due to drainage, evaporation or desorption.

Mineral Wool Absorbs More Water Than Polyiso*

When tested in accordance with ASTM C209, it was found that:

  1. Mineral wool absorbs 8–38 times more water than foil faced polyiso.

  2. Drying requires 2–6 days longer for mineral wool than polyiso.

These test findings show that water absorption is directly linked to pore structure. Open, fibrous materials like mineral wool absorb and retain significantly more water than closed-cell foam. The testing even allows for specimens to drain for 10 minutes prior to initial weighing, but mineral wool still absorbs a minimum of 8–38 times more water than foil faced polyiso.

SORPTION VALUES FOR MINERAL WOOL ARE GREATER THAN THOSE DERIVED FROM STANDARD ASTM C1104 TESTING, WHICH ONLY SUBJECTS THE SPECIMENS TO HIGH HUMIDITY.

Previously, it was assumed that water drains freely and retained water dries quickly due to the structure and high vapor permeance of mineral wool. Testing in accordance with ASTM C209 shows neither is true of mineral wool. It takes 2–6 days longer to dry than polyiso.

These findings also indicate corresponding effects on insulation. Reductions in claimed R-values are expected, as even partial wetting negates thermal performance.

mineral wool absorbs even more water when rewetted.

See Issue #3 of Mineral Wool vs. Polyiso

*Based on the white paper, Mineral Wool and Polyisocyanurate Insulation: A Comparative Study of Water Absorption, Drying and Rewetting by M. Steven Doggett, Ph.D, Principal Scientist, Built Environments, Inc.

TERMS & DEFINITIONS

Sorption: A general term in physical chemistry used to describe the combined processes of absorption and adsorption.

Absorption: Uptake of matter in bulk by other matter, for example, the penetration of substances into the bulk of another solid or liquid.

Drying: Loss of water due to drainage, evaporation or desorption.

Testing Standards*

To equally compare mineral wool and polyiso, use the same test.

Mineral wool and polyiso have used vastly different test methods in the past, despite being considered for the same function within the building envelope.

When compared using the same test standards (ASTM C209), the water resistive characteristics of polyiso and mineral wool can be more accurately compared.

Standard testing for mineral wool (ASTM C1104) exposes tests specimens to water vapor. When both polyiso and mineral wool are tested using liquid water, significant differences can be observed. Mineral wool’s test methodologies have been based on the need to demonstrate low water absorption to achieve the perception of quality performance.

Sorption potentials for mineral wool have been underestimated, as shown by recent studies involving both partial and full immersion. Even partial wetting negates the insulating performance of mineral wool.

Polyiso, on the other hand, has low absorption potential and high drying potential.

DUE TO DIFFERING TEST METHODOLOGIES, DESIGN PROFESSIONALS LACK A CLEAR CONCEPTUAL UNDERSTANDING OF POTENTIAL RISKS WHEN CHOOSING MATERIALS.

*Based on the white paper, Mineral Wool and Polyisocyanurate Insulation: A Comparative Study of Water Absorption, Drying and Rewetting by M. Steven Doggett, Ph.D, Principal Scientist, Built Environments, Inc.

TERMS & DEFINITIONS

Sorption: A general term in physical chemistry used to describe the combined processes of absorption and adsorption.

Absorption: Uptake of matter in bulk by other matter, for example, the penetration of substances into the bulk of another solid or liquid.

Drying: Loss of water due to drainage, evaporation or desorption.