referring to fig1 there is shown an insulated building product 10 in accordance with the present invention . building product 10 comprises a base mat 12 of suitable thermal and acoustic insulation material and semipermeable coating 14 . depending its intended application , building product 10 may further include one or more intermediate layers 16 of hydrophobicity enhancing and / or reinforcing material disposed between base mat 12 and coating 14 . base mat 12 may be chosen from any insulation material appropriate for the building product &# 39 ; s intended application . the base mat may be fabricated from nonfibrous material such as polyimide or melamine foam or the like or from mineral fibers such as glass fibers , refractory fibers or mineral wool fibers . the thickness and density of base mat 12 will be dictated by the levels of acoustic and / or thermal insulation that are desired or necessary for a particular building installation . if , for example , building product 10 is constructed to serve as an hvac duct liner , base mat 12 may have a thickness of about 0 . 5 to about 2 inches and a density of about 1 . 5 to about 3 pcf . it will be understood that wall , floor , ceiling or other installations utilizing building product 10 may require a base mat 12 having greater or lesser thicknesses and densities . when building product 10 is a duct liner , base mat 12 may be made from polyimide or melamine foam or fiber glass . according to a presently preferred duct liner construction , the base mat is resin - bonded rotary or flame attenuated fiber glass . in addition , when constructed as a duct liner , building product 10 preferably includes a woven or nonwoven fibrous layer 16 disposed between base mat 12 and coating 14 . suitable layers 16 include johns manville corp . 8440 non - woven glass mat or 1801fx nonwoven glass mat marketed by lydall manning of fort washington , pa . the 1801fx mat is desirable because it is chemically sized to enhance hydrophobicity and constructed with a modified fiber geometry to decrease the surface opacity ( i . e ., amount of openness ) of the mats . it is believed that the latter property is achieved by combining different glass fiber diameter blends ranging from 6 to 16 microns . various length fiber blends ranging from about 0 . 25 to 1 . 5 inches in length have also been tested in combination with hydrophobic size chemistries . while the above described non - woven glass mats have not been fully evaluated , they do show promise as an alternative method of achieving a semipermeable insulated building product . semipermeable coating 14 is essentially impermeable to liquid water and permeable to water vapor . vapor from water in base mat 12 coated by coating 14 may thus pass through the coating , whereas liquid water is effectively prevented from passing through the coating and entering the insulation . thus , the coating not only prevents dry base mat insulation from becoming wetted by liquid water residing on the surface of the coating opposite the insulation but also promotes drying of moist insulation . semipermeable coating 14 preferably comprises an acrylic latex polymer having fire retardant , antimicrobial or other agents or additives for enhancing the performance of the coating in duct liner or other building product applications . several duct liner prototypes incorporating the features of the present invention were constructed and evaluated by the present inventors for liquid water repellency , liquid water hold - out and water vapor permeability . the composition of the coatings for these prototypes , identified respectively as prototype i and prototype ii in fig2 were as follows . the coating for prototype i was series 8463 acrylic latex polymer , with proprietary hydrophobic additives and an effective amount of defoaming agents , marketed by b f goodrich company of charlotte , n . c . the coating for prototype ii included nw 1845 acrylic latex polymer marketed by rohm & amp ; haas company of philadelphia , pa . under the trade designation roplex ® and an effective amount of defoaming agents . another prototype coating whose repellency performance data is not reflected in fig2 but which nevertheless exhibited excellent water repellency included about 30 to about 80 weight percent , preferably about 45 to about 55 weight percent , rohm & amp ; haas company nw 1845 acrylic latex polymer combined with about 0 . 12 to about 3 . 0 weight percent , preferably about 0 . 2 to about 0 . 5 weight percent , fc - 461 water dispersible fluorocarbon marketed by minnesota mining & amp ; manufacturing co . of minneapolis , minn . when constructing building product 10 as a duct liner , the desired base mat 12 and intermediate layer 16 are selected and brought together and joined with a phenolic resin in the manner known in the art . base mat 12 and intermediate layer 16 are transported by a conveyor to an in - line sprayer that sprays the liquid acrylic latex polymer compound onto the exposed surface of intermediate non - woven glass layer 16 of the moving product . the coated product preferably travels on the conveyor at speeds of approximately 10 - 60 ft / minute and moves through an oven where the dwell time is approximately 15 seconds to one minute and the oven temperature is maintained between about 230 - 450 ° f . once the product exits the oven , the latex compound is fully cured . upon curing , the preferred dry coating weights range from between about 5 to about 15 grams per square foot . referring again to fig1 there is symbolically illustrated the liquid water repellency and water vapor permeability attributes of the building product 10 constructed according to the present invention . the semipermeable surface coatings 14 disclosed herein repel liquid water droplets 18 , resist pooled water 20 , and hold out a ⅛ to 1 inch ( or 200 - 400 grams of water in a one gallon , open bottom can ) liquid water column 22 from penetrating the coating and accumulating in the insulation base mat 12 . as indicated by dashed arrows 24 , the coating 14 and intermediate layer ( s ) 16 , if present , constitute a semipermeable stratum which allows water vapor 26 in the base mat 12 to move through the surface of the coating for release into the atmosphere or into the interior of an hvac duct if building product 10 is a duct liner . not shown on the drawing figures yet demonstrated in the laboratory is the resistance of semipermeable coating 14 to soapy detergent liquids such as those that might be used in cleaning of hvac duct work . this attribute of the coating was discovered in the laboratory as part of the repellency evaluation tests . basically , a diluted soapy solution was poured on the surface of the duct liners constructed as described herein and after a period of one hour the soapy solution still remained on the surface and was easily cleaned off the surface with a paper towel . during research and development associated with the present invention , the present inventors discovered an alternative to the above described processes for producing semipermeable building products , particularly duct liners . more specifically , commercially available toughgard ® duct liner product marketed by certainteed corp . as well as several other duct liner products were sprayed with a product called revivex ®, a water repellent fluoropolymer marketed by w . l . gore & amp ; associates , inc . of newark , del ., and cured in an oven for two hours at 250 ° f . the duct liner products treated with revivex ® exhibited improved water repellency . by virtue of the present invention there is provided an acrylic latex surface coating for rotary or flame attenuated fiber glass duct liner that has the above - described semipermeable properties and hence improves building iaq . the duct liners also have solvent and liquid detergent resistance to enable wet cleaning of the duct the present invention also offers a cost - effective , breathable ( semipermeable ) coating that may be useful in connection with any type of building product , whether insulated or not . the coating allows water vapor to pass through unimpeded , whereby water vapor freely moves into and out of the insulation . by way of example , but not limitation , the coating can be applied to other hvac products such as duct board and the like or other organic or inorganic substrates used in the building material industry where the technology described herein has merit . although the invention has been described in detail for the purpose of illustration , it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims .