sentences:  In March 2008 The US city of Portland, Oregon, approved an appeal to waive the requirement for a vapor barrier/retarder when using cellulose insulation.The appeal can be viewed in the Portland Bureau of Development Services search form by searching for appeal ID 4996.Fundamental to any appeal is mentioning that recent studies show air movement is the primary problem for vapor, that cellulose is an effective barrier to air movement, and that cellulose acts to diffuse vapor.