diff --git a/examples/minimal_torus.rs b/examples/minimal_torus.rs
index ae0b7d7..82670ad 100644
--- a/examples/minimal_torus.rs
+++ b/examples/minimal_torus.rs
@@ -1,5 +1,5 @@
 use coremem::{Driver, Flt, mat, meas};
-use coremem::geom::{Cube, Index, InvertedRegion, Meters, Torus};
+use coremem::geom::{Cube, Index, InvertedRegion, Meters, Torus, Union};
 use coremem::stim::{CurlStimulus, Sinusoid1, TimeVarying1 as _};
 
 fn main() {
@@ -34,16 +34,23 @@ fn main() {
     let drive_region = Torus::new_xz(Meters((half_width - ferro_major, half_width, half_depth).into()), wire_major, wire_minor);
     let sense_region = Torus::new_xz(Meters((half_width + ferro_major, half_width, half_depth).into()), wire_major, wire_minor);
 
-    driver.fill_region(&ferro_region, mat::db::minimal_square_ferrite().into());
-    driver.fill_region(&drive_region, mat::db::conductor(conductivity).into());
-    driver.fill_region(&sense_region, mat::db::conductor(conductivity).into());
-
     let boundary_xy = half_width - ferro_major - ferro_minor - buffer;
     let boundary_z = half_depth - wire_major - wire_minor - buffer;
     let boundary_lower = Meters((boundary_xy, boundary_xy, boundary_z).into());
     let boundary_upper = Meters((width - boundary_xy, width - boundary_xy, depth - boundary_z).into());
     println!("boundary: {}um; {}um", m_to_um(boundary_xy), m_to_um(boundary_z));
     let boundary_region = InvertedRegion::new(Cube::new(boundary_lower, boundary_upper));
+
+    let vacuum_region = InvertedRegion::new(Union::new()
+        .with(ferro_region.clone())
+        .with(drive_region.clone())
+        .with(sense_region.clone())
+        .with(boundary_region.clone())
+    );
+
+    driver.fill_region(&ferro_region, mat::db::minimal_square_ferrite().into());
+    driver.fill_region(&drive_region, mat::db::conductor(conductivity).into());
+    driver.fill_region(&sense_region, mat::db::conductor(conductivity).into());
     driver.add_upml_boundary(Meters((boundary_xy, boundary_xy, boundary_z).into()));
 
     // J=\sigma E
@@ -86,6 +93,7 @@ fn main() {
     driver.add_measurement(meas::MagneticFlux::new("mem", ferro_region.clone()));
     driver.add_measurement(meas::Power::new("mem", ferro_region.clone()));
     driver.add_measurement(meas::Power::new("boundary", boundary_region.clone()));
+    driver.add_measurement(meas::Power::new("vacuum", vacuum_region.clone()));
 
     let prefix = format!("out/{}/{}-flt{}-{}-feat{}um-{}mA-{}ps--radii{}um-{}um-{}um-{}um",
         base,
diff --git a/src/geom/mod.rs b/src/geom/mod.rs
index dcef303..bdbb000 100644
--- a/src/geom/mod.rs
+++ b/src/geom/mod.rs
@@ -3,7 +3,7 @@ mod units;
 mod vec;
 mod vecu;
 
-pub use region::{Cube, CylinderZ, InvertedRegion, Region, Sphere, Torus, WorldRegion};
+pub use region::{Cube, CylinderZ, InvertedRegion, Region, Sphere, Torus, Union, WorldRegion};
 pub use units::{Coord, Meters, Index};
 pub use vec::{Vec2, Vec3};
 pub use vecu::Vec3u;
diff --git a/src/geom/region.rs b/src/geom/region.rs
index 0cec8c8..8cf650f 100644
--- a/src/geom/region.rs
+++ b/src/geom/region.rs
@@ -162,7 +162,6 @@ impl Region for WorldRegion {
     }
 }
 
-/// Region describing the entire simulation space
 #[derive(Clone, Serialize, Deserialize)]
 pub struct InvertedRegion(Box<dyn Region>);
 
@@ -179,6 +178,29 @@ impl Region for InvertedRegion {
     }
 }
 
+#[derive(Clone, Serialize, Deserialize)]
+pub struct Union(Vec<Box<dyn Region>>);
+
+impl Union {
+    pub fn new() -> Self {
+        Union(Vec::new())
+    }
+    pub fn new_with<R: Region + 'static>(r: R) -> Self {
+        Self::new().with(r)
+    }
+    pub fn with<R: Region + 'static>(mut self, r: R) -> Self {
+        self.0.push(Box::new(r));
+        self
+    }
+}
+
+#[typetag::serde]
+impl Region for Union {
+    fn contains(&self, p: Meters) -> bool {
+        self.0.iter().any(|r| r.contains(p))
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;