to expand on Dan's answer (code was roo long for comment) here is method I use to calculate perimeter for building. You could adjust this for multiple possible boundary conditions and also for an individual space or surface.

As is it has a single counter for all edges common to an exterior wall and a floor with ground boundary condition.

    # currently takes in model and checks for edges shared by a ground exposed floor and exterior exposed wall. Later could be updated for a specific story independent of floor boundary condition.
def OsLib_Geometry.calculate_perimeter(model)    
  perimeter = 0
  model.getSpaces.each do |space|
    # counter to use later
    edge_hash = {}
    edge_counter = 0
    space.surfaces.each do |surface|
      # get vertices
      vertex_hash = {}
      vertex_counter = 0
      surface.vertices.each do |vertex|
        vertex_counter += 1
        vertex_hash[vertex_counter] = [vertex.x,vertex.y,vertex.z]
      end
      # make edges
      counter = 0
      vertex_hash.each do |k,v|
        edge_counter += 1
        counter += 1
        if vertex_hash.size != counter
          edge_hash[edge_counter] = [v,vertex_hash[counter+1],surface,surface.outsideBoundaryCondition,surface.surfaceType]
        else # different code for wrap around vertex
          edge_hash[edge_counter] = [v,vertex_hash[1],surface,surface.outsideBoundaryCondition,surface.surfaceType]
        end
      end
    end

    # check edges for matches (need opposite vertices and proper boundary conditions)
    edge_hash.each do |k1,v1|
      next if v1[3] != "Ground" # skip if not ground exposed floor
      next if v1[4] != "Floor"
      edge_hash.each do |k2,v2|
        next if v2[3] != "Outdoors" # skip if not exterior exposed wall (todo - update to handle basement)
        next if v2[4] != "Wall"
        # see if edges have same geometry
        next if not v1[0] == v2[1] # next if not same geometry reversed
        next if not v1[1] == v2[0]
        point_one = OpenStudio::Point3d.new(v1[0][0],v1[0][1],v1[0][2])
        point_two = OpenStudio::Point3d.new(v1[1][0],v1[1][1],v1[1][2])
        length = OpenStudio::Vector3d.new(point_one - point_two).length
        perimeter += length
      end
    end
  end

  return perimeter
end