Cross-ventilation by the use of windows works for
underground buildings when the weather permits. But rainy or very cold weather may require something more. Energy- recovery-air-to-air heat exchangers work when power is available but they also have a high up front cost. Recently, I have become aware of an ancient material that is experiencing a revival. Magnesium oxide (MgO) was displaced as the
concrete of choice when Portland Cement became popular. It (MgO) is a much healthier atlternative to Portland Cement as it is hygroscopic (meaning that it allows
water vapor to pass in and out of it, both ways, from inside to outside and outside to inside) yet is resistant to passage of liquid water. It also allows other gases to pass through it, though at a much slower rate than cross-ventilation would. Still, it appears to solve the problem of mold growth that is a continuing problem with buildings that do not "breathe". "Breathing walls" is a metaphor as walls do not actually breathe as we living creatures do, but their vapor permeability do make the term legitimate.
MgO, when cured into a solid material will not degrade even when subjected to prolonged immersion in liquid water, and it does not swell. Currently, MgO is more expensive than Portland Cement, owing to economies of scale but when it's use becomes more common it
should, once again, become the concrete of choice as it consumes much less
energy to produce than Portland Cement. The walls of an underground building that face the outside atmosphere could be constructed of Mg0 board (products such as "Dragonboard"), allowing vapor and gases to pass to the outside atmosphere. These types of products are already very competitive with composit
wood products such as plywood and OSB.
For a short overview see www.geoswan.com There is a link near the top of the page, in bold black lettering for MgO.
John.
