Updated: Dec 8, 2022
Watch our video of two different roof insulation types being immersed in water for 3 days. Roof insulation placed on top of a flat reinforced concrete roof will often end up being immersed in standing water, so it is important to specify a roof insulation that does not get soaked. The standing water comes from rainwater seeping through cracks in the roof screed. This standing water will sit on top of the water proofing membrane and effectively immerse the roof insulation in water; see illustration below:
There are different water absorption standards for insulation materials, so it is important to specify the right one. Otherwise, one might end up using insulation materials that quickly will end up getting soaked with water, and hence, renders the insulation ineffective. In fact, a completely wet mineral wool insulation will easily see a 100-times lower thermal resistance.
In our simple video test, we immersed two different roof insulation materials in water for three days. The two insulation materials were:
Extruded polystyrene foam (Roofmate SL from Dow)
Stone wool insulation ("Hardrock" mineral wool from Rockwool)
The insulation materials responded very differently to being under water for 3 days. While the closed-cell extruded polystyrene foam insulation did not seem to absorb any water, the open-cell mineral wool was completely soaked with water. Therefore, the extruded polystyrene foam or similar closed-cell insulations should be used for top insulation of flat roofs where standing water likely will occur. For this purpose, specify a roof insulation material that complies with the ASTM C272 standard for "Water absorption verification of structured core materials when immersed or in high relative humidity environments" and ask for submisison of a 24-hour water immersion test.
Do not ask for compliance to ASTM C1104/C1104M-13 "Standard Test Method For Determining The Water Vapor Sorption Of Unfaced Mineral Fiber Insulation", as this only applies for exposure to water vapour and not to immersion in water.
Mineral wool insulation is likely to perform well according to the ASTM C1104/C1104M-13 standard, so don't get fooled by this "good result" as the mineral wool will get completely soaked and fail thermally when immersed in water. Hence, ask for compliance to the ASTM C272 standard instead.
On the topic of how moisture affects the thermal conductivity of insulation materials, the effect can be quite substantial, as shown in the graph below. Here, three different mineral wool insulation products were subjected to different degrees of moisture absorption, and a significant increase in the thermal conductivity was measured, see graph:
For a 8% moisture gain, the mineral wool insulation effectiveness was reduced to roughly half. As such, it is recommended to keep the moisture gain of insulation materials to 3% or less (by weight).
Note: The article only focuses on thermal conductivity of insulation immersed in water. Other aspects of insulation, such as fire safety, cost, durability have not been addressed.
"Effects of moisture content on thermal conductivity of thermal insulation materials", A M Gusyachkin et al. 2019. IOP Conf. Ser.: Mater. Sci. Eng. 570 012029
"Effects of Moisture on Thermal Insulation", Steve Badger, Ph.D. Sustaining Our Success 108th Annual Conference & Trade Show. IDEA, June 26-29, 2017.