Image courtesy foo.ca

Phase-change materials add thermal mass to lightweight building materials without the need for all that pesky mass.  Historically, if you wanted to use passive cooling techniques in your buildings you built them in the desert with two-foot thick walls.  Since it would take all day for the heat to make its way through the massive walls, the rooms would stay cool.  At night when the temperature dropped precipitously (as it is wont to do in the desert), the heat that had finally managed to come through the walls would take all night to escape out again, and the rooms would be warm.  But most of us aren’t priviledged to live in the desert, and thick walls can be expensive to put up. 

Since the 1950s, several companies have tried and failed to develop passive cooling systems that take advantage of phase-change materials because incorporating them into common construction materials has been less than easy (Bourzac).  In consequence, we continue to see widespread use of lightweight construction materials that don’t store heat, leading to wild fluctuations in temperature that we use HVAC systems to overcome.

In light of all this I think you’ll be glad to know that “chemical giant” BASF has developed wax capsules that can be incorporated into lightweight construction materials such as gypsum drywall, ceiling panels, or aerated concrete blocks.  The capsules contain “paraffin waxes that melt at around room temperature, enabling them to keep the temperature of a room constant throughout the day. The waxes work best in climates that cool down at night, allowing the materials inside the capsules to solidify and release the heat they’ve stored during the day” (Bourzac).  The desert wins the “best passive cooling building site award” yet again!   

Image credit Peter Schossig

 The process for making these capsules is complicated but it’s predicated on the fact that wax and water find each other repulsive.  When BASF researchers beat melted wax into hot water, they wind up with small droplets of wax similar to what you might see an ill-mixed salad dressing (not that I recommend putting wax in salad dressing).  Acrylic precursors coat the droplets and then a catalyst is added to form an acrylic polymer shell around the wax.  At this point you can add the mix to gypsum to make drywall or you can dry it and add it to concrete or plaster (Bourzac).