impermeable layer 12 is fabricated from a material which is impervious to the passage of water vapor with sufficient impermeability to limit the water vapor transmission to 0 025 grams per 24 hours per square meter tested in accordance with astm e 96 66 procedure bw material such as butyl rubber or other comparable elastomeric materials may be utilized to fabricate layer 12 to meet the above impermeability standard also a built up bituminous membrane may be utilized to form layer 12 if special accessories well known to the roofing trade such as vapor tight reglets or metal transition plates are provided to make a vapor tight connection with layer 17 overlaying water vapor impermeable layer 12 i provide a non permeable insulation layer 13 which in addition to serving as insulation for the building also serves as a bridge to hold overlaying permeable but lower cost low adsorption insulation 15 15a up out of any vapor condensate water the arrangement of the insulation 13 provides channels 14 through which any water vapor can pass to the atmosphere and condensate can pass to the roof drainage system generally designated as 16 as shown in fig1 and 4 insulation layer 13 is comprised of rectangular or square shaped sections disposed in spaced apart relationship thereby defining channels 14 between edges 52 thereof as shown in fig1 channel 14 extend from the top surface of insulation layer 13 to the bottom surface thereof preferably for reasons related to the insulation properties of layer 13 the spacing between sections should be limited to a maximum of approximately one half of an inch alternatively other configurations or other materials could be utilized to fabricate layer 13.