b'Technical ArticleMind the Gap Thermal bypass occurs through gaps between insulation boards in low-slope roof assembliesBy Sudhakar Molleti, PH.D.Reprinted with the permission ofDuring roof system design, theGaps in insulation joints areexpandorcontractunderThe GapsProfessional Roofing magazine. selectionofinsulationdependsan unwanted but often unavoid- temperaturevariationsde-A s you know, roofs representon several factors, such as R-valueable fact of life in low-slope roof- pending on a coefficient ofRegardlessofwhethergapsare large portions of low-slopeperinch,dimensionalstability,ing. Causes of gaps are listed inexpansion. This effect of lin- formedbecauseofimproperin-building envelopes. And incompressivestrength,agingofFigure 1 and can be attributed tostallation or panel defects, they are ear expansion is not perma- present in a majority of low-slope light of declining energy resourc- insulation, flexural strength, facerthe following: nent as the boards will revertroof systems. Figure 2 shows in-situ es,escalatingenergycostsandcharacteristics and moisture per-Gapsbetweentheinsulationtotheirinitialdimensions increasing concerns regarding cli- formance.Attheroofassemblyboards occur as a result of instal- examples of the extent of gap for-mate change, roof systems effectslevel,equallyimportantarethelation procedures. During con- when returned to the initialmation.on energy consumption and envi- compatibilityofinsulationwithstruction, it is common to seeconditions. The effects of gaps on the overall ronmental control are becomingotherroofsystemcomponents1/16- up to 1/4-of-an-inch gaps effective R-values of roof assembliesGapformationalsoresults increasingly important. and attachment within the assem- betweenboardseventhoughfrom the dimensional stabil- depend on a gaps size, shape and Heat flow through roof assem- bly(loose-laid,securedwithad- all attempts are made to placeity of insulation boards. Thislocation. There also may be lateral blies is impeded by exterior andhesives or mechanically fastened).boardscarefully.Deficienciesis the degree to which a mate- heatflowwithinaroofassembly interior air films, roof decks, ther- Selectingappropriateinsulationinmanufacturedboardscanrial maintains its original di- (between the deck and insulation) mal insulation and cover boards.is a roof system designers respon- compound the issue. mensions when subjected tothatcomplicatestheoverallheat Aroofsystemsthermalperfor- sibility. assembliesaresub- changesintemperatureandflow pattern. Even if a roof system Roof mance is evaluated based on itsInsulations thermal resistancejectedtotemperaturevaria- humidity. Unlike the coeffi- designerbelievesadesignhasad-steady-statethermalresistance,referredtoasratedR-valueistions, both diurnal and sea- cient of linear expansion, thisdressedthethermalperformance which is the ratio of the surface- reportedbymanufacturersatasonal. Insulation boards willis permanent deformation. requirement, most likely it has not.to-surface temperature differencemean temperature of 75 F. This across the roof to the heat flowvalue is obtained from laboratory through the roof. Within a rooftestsconductedinaccordance assembly,thesinglecomponentwith ASTM C518, Standard Test thatprovidesthegreatestresis- Method for Steady-State Thermal tance to heat flow is thermal in- Transmission Properties by Means sulation. Therefore,thecurrentof the Heat Flow Meter Appara-and most effective way to reducetus. From this rated R-value, the building energy demand throughamount of insulation required to a roof system is via the effectivecomplywithprescriptivecode use of insulating materials. values can be estimated. The ac-curacy of such estimates depends Background on whether the insulations ther-The North American roof insu- mal performance is fully realized.lation market has evolved duringIn low-slope roof systems, in-the previous decades, developingsulationmaterialsusuallytake insulationwithR-valuesrangingthe form of four- by four-foot or fromR-1perinchtoR-60perfour-byeight-footrigidboards. inch. Apart from providing ther- As the boards are laid, gaps occur mal resistance, roof insulation hasbetweentheboards.Thesegaps expanded to fire protection, soundhavelowerthermalresistance reduction,wind-upliftresistance,than the insulation itself. If insula-drainage and as a surface for in- tion boards were continuous and stalling waterproofing membranes. heat flow one-dimensional, a roof assemblysthermalperformance easily could be determined from theone-dimensionalU-factor calculationapproach.Butwith the presence of gaps in insulation joints, the heat flow analysis be-comes more complicated.Theheattransferthatby-passes the conductive heat trans-fer between two regions, such as the gaps between the insulation boards,canbereferredtoasa thermalbypass.Inotherwords, thermalbypassdescribesheat loss through intentional or unin-tentional openings in the roof as-sembly via air, heat and moistureFigure 2: Typical examples of gap formations in low-slope roof assemblies.Figure 1: Causes of gap formation. movement. Courtesy of 2001 Company, Waterbury, Conn., and Canadian Roofing Contractors Association.12ONTARIO ROOFING NEWSISSUE 1 2021 THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ORN'