IE 9 and below doesn't support more than 4095 individual selectors. SCSS can blow through this pretty easily. To get around it. We can implement a simple class in our config.rb file and implement a save hook to write the additional styles. These should be included with an IE conditional.

config.rb

# Requre a specific version in this file:
# gem 'zurb-foundation', '=4.3.1'
require 'zurb-foundation'
# Require any additional compass plugins here.

# IE 9 and below only support up to 4095 CSS selectors
# This class will split all selectors after that point
class CssSplitter
 
  def self.split(infile, outdir = File.dirname(infile), max_selectors = 4095)
 
    raise "infile could not be found" unless File.exists? infile
 
    rules = IO.readlines(infile, "}")
    return if rules.first.nil?
 
    charset_statement, rules[0] = rules.first.partition(/^\@charset[^;]+;/)[1,2]
    return if rules.nil?
 
    # The infile remains the first file
    file_id = 1
    selectors_count = 0
    output = nil
 
    rules.each do |rule|
      rule_selectors_count = count_selectors_of_rule rule
      selectors_count += rule_selectors_count
 
      # Nothing happens until the selectors limit is reached for the first time
      if selectors_count > max_selectors
        # Close current file if there is already one
        output.close if output
 
        # Prepare next file
        file_id += 1
        filename = File.join(outdir, File.basename(infile, File.extname(infile)) + "_#{file_id.to_s}" +    File.extname(infile))
        output = File.new(filename, "w")
        output.write charset_statement
 
        # Reset count with current rule count
        selectors_count = rule_selectors_count
      end
 
      output.write rule if output
    end
  end
 
  def self.count_selectors(css_file)
    raise "file could not be found" unless File.exists? css_file
 
    rules = IO.readlines(css_file, '}')
    return if rules.first.nil?
 
    charset_statement, rules[0] = rules.first.partition(/^\@charset[^;]+;/)[1,2]
    return if rules.first.nil?
 
    rules.inject(0) {|count, rule| count + count_selectors_of_rule(rule)}.tap do |result|
      puts File.basename(css_file) + " contains #{result} selectors."
    end
  end
 
  def self.count_selectors_of_rule(rule)
    rule.partition(/\{/).first.scan(/,/).count.to_i + 1
  end
  
end
 

# Set this to the root of your project when deployed:
http_path = "../"
css_dir = "css"
sass_dir = "scss"
images_dir = "images"
javascripts_dir = "js"
fonts_dir = "fonts"

# You can select your preferred output style here (can be overridden via the command line):
# output_style = :expanded or :nested or :compact or :compressed
output_style = :compact

# To enable relative paths to assets via compass helper functions. Uncomment:
# relative_assets = true

# To disable debugging comments that display the original location of your selectors. Uncomment:
# line_comments = false
line_comments = true

# If you prefer the indented syntax, you might want to regenerate this
# project again passing --syntax sass, or you can uncomment this:
# preferred_syntax = :sass
# and then run:
# sass-convert -R --from scss --to sass sass scss && rm -rf sass && mv scss sass

# On save, write a new stylesheet
on_stylesheet_saved do |path|
  CssSplitter.split(path) unless path[/\d+$/]
end