More and more exterior insulation is being used and specced into all types of buildings. There is great benefit to insulating walls from the outside, and this will be a blog topic unto itself. In a nutshell, exterior insulated walls reduce the condensation potential, and handle moisture better due to increasing the sheathing temperature. Mark Gauvin built a test hut in the Vancouver area with a variety of different wall profiles to see the effects of how each would handle moisture. Everyday, water was artificially added to the wall. In every case, walls with exterior insulation performed better than ones without. I would recommend taking a look at Mark Gauvin’s test hut report.
Mineral wool exterior insulation has been the go-to for many architects and builders, however is this based on performance or simply convention and getting on the same bandwagon?
While Roxul claims that their products are hydrophobic and repel water, a quick experiment shows that mineral wool may not be nearly as water repellent as we would like it to be. Dow published a study where they submerged mineral wool (as well as their own foam board) under water for 2 hours. They found that the mineral wool absorbs a significant amount of water, whereas the foam absorbs barely any. Intrinsically this makes sense – there is a lot of open air space in the mineral wool that water can get into. You can take a look at the full report from Dow here.
We tried a similar DIY experiment to see for ourselves. It’s easy – simply take a sample of mineral wool, weigh it, and submerge it under water for only 5 minutes before weighing it again. It will pack on about 35% additional water mass in that short time span. Keep it submerged for an hour, and this increases to 85% additional weight in water. Do the same experiment with any Quik-Therm product and you will see how essentially no water is absorbed. Feel free to read about this test further here.
Moisture getting into insulation is a problem unto itself. Even a small percentage of moisture absorption can degrade thermal performance significantly. In Dow’s study mentioned above, the amount of moisture had caused the Roxul to perform at only R-1, instead of the labeled R-4.2 per inch. Let’s say you have 4″ exterior mineral wool – this would only be performing at R 4 instead of about R 16. This in turn lowers the sheathing temperature and creates much higher potential for condensation in the wall cavity. Not a good situation all around.
Drying mineral wool took a long time in these studies, and would take even longer when trapped behind cladding in a real world application. It would take days – if not weeks – for the product to dry out to its initial state. During this time it is quite likely it will simply absorb more moisture with water infiltration. Once mineral wool gets wet, it can be quite difficult for it to dry and perform at the stated R 4.2 again.
I heard someone say that ‘where there is rain screen, there is going to be rain.’ This makes perfect sense, and in such an event, why would you want to put anything fibrous that can absorb moisture into your exterior wall system? It reminds me of putting fibreglass and poly in an interior concrete (usually residential basement) application, where the chance of condensation and other issues is so high it is coined as a ‘guaranteed failure.’ I have a difficult time believing any type of fibrous product on the exterior can stand up to the elements. Both water absorption and wind washing degrade the thermal performance, and isn’t thermal performance exactly why we use insulation?
Many builders and architects are hesitant to have an exterior vapour barrier. It can be done with relative ease, as long as a few criteria are met, and they end up in many cases performing better and are more air tight. However, some prefer building with products and applications that allow breathability to the exterior. Quik-Therm developed Solar Dry precisely for this purpose.
Solar Dry is an exterior rigid insulation board that has grooves on the sheathing side. This offsets the insulation from the sheathing to allow for drainage and breathability to the exterior. While the product itself is impermeable, the way it is made and the application it is in allows for ventilation.
The product’s impermeability is used as an advantage. The plastic reflective facer will not allow moisture penetration, will not be affected by wind washing, and maintains a stable R-Value. Such cannot be said about fibrous insulation. Solar Dry’s grooves ensure that should any moisture get into the wall cavity, it can be drained, vented, and dispersed to the exterior.
Next time you are looking for ‘outsulation,’ consider a product that won’t absorb moisture. Quik-Therm’s Solar Dry Insulation is the perfect alternative to the pitfalls of mineral wool.