b'Technical Articleeffective R-value was lowered 6.5losses. This is a greater loss in R-per cent. With gap height rang- value than a staggered gap configu-ing from two to 9 1/2 inches, aration of a 10-inch gap height.1/4-inchgapwidthloweredtheFor a roof assembly of similar overallthermalperformanceofthickness with a staggered gap con-theroofassemblyfrom2.4perfiguration,thepercentagelossin cent to 6.9 per cent and a 1/2-incheffective thermal resistance would gap width led to a thermal bypass of 3.5 per cent to 9.3 per cent. be 3.2 per cent and 4.5 per cent for the 1/4-inch and 1/2-inch gap, re-Through vs. Staggered Gap spectively, as determined from gap Athroughgapresultswhenimpact factors shown in Figure 9. a gap in a single-layer insulationThe through gaps result in nearly layout develops as a result of sce- triple the amount of R-value loss narios explained in Figure 1. Theas the staggered gap. Therefore, we gapthatformsbetweeninsula- recommendtwo-layerinsulation tionandconstructiondetails,layouts with staggered arrangement suchasattheexpansionjoint,be standard practice in the roofing parapet(seeFigure2),curbs,industry.drains, etc., also could be consid-ered a through gap. In consulta-tion with consortium members,Valuable DataFigure 8: Through gap versus staggered gap per cent decrease in effective R-value. thethrough-gaptestswerecar- Inlow-slopecommercialroof-ried out on a single-layer insula- ing, the formation of gaps in in-tion layout with 3 1/3-inch-thicksulation joints and at construction polyisocyanurate insulation. details is inevitable. The question Figure9comparesthether- ofhowmuchthermalbypassre-mal performances of a throughsults at these gaps has never been Gap Impact Factor Figure 8 summarizes the gapcreased the overall thermal per- gap versus staggered gap. With a impactfactorsdevelopedforformance of the assembly with1/4-inch-wide through gap in a 3addressed or quantified. The con-Thegapimpactfactoristhetwo gap widths relating the gapincreasing gap height. 1/3-inch-thickinsulationboard,sortium study examined the signifi-outcome of all the assembly testsheight (or insulation thickness)Similarly, with an increase inthere is a drop of nine per cent incanceofgapsbetweeninsulation conductedwitheachinsulationwith the effective R-value. Thegap width, there was a decreasethe effective R-value of the roofboards and produced the thermal typeineachdesigncategoryofpercentage decrease in effectivein the effective R-value. For ex- assembly relative to the opaquebypass data. Gap height and width R-26, R-31 and R-36, maintaining athermalresistancebecauseofample, in a two-layer insulationassemblywithoutanygaps.Ifwere identified to affect a roof as-constant 24-inch offset between in- gaps was found to be linear, andlayout with the top and bottomthe gap width increased to 1/2 ofsemblys thermal performance.sulation board joints. Once again,gap height and gap width werelayersthreeincheseach,thean inch, the effective R-value re- Withincreasinggapheight, it should be noted our results showidentified as having an effect onpresence of a 1/2-inch gap in theduced 13.5 per cent. It should be a simplified analysis applicable tothermal performance. In a two- bottom layer reduced the overallnoted this value is specific to athere is a decrease in a roof assem-the tested case of a four-foot-longlayer insulation layout, the pres- thermal performance of the as- 3 1/3-inch-thick gap height, andblys effective thermal resistance. A single joint within a four- by four- ence of gaps in either the top orsembly 4.2 per cent. If the samewith increasing gap height, therethrough gap is the worst-case sce-foot area. bottom layer or both layers de- gapexistedinbothlayers,theispotentialforhigherthermalnario for insulation gaps, and sin-gle-layer insulation layouts should be avoided in roof system design. Atwo-layerstaggeredinsulation layoutshouldberecommended design practice because it can con-siderablyminimizethermalloss in the event of gap formation be-tween insulation boards.Energycodesarepushingfor higherthermalperformanceval-ues in low-slope roofs, specifically in the form of continuous insula-tion. The question is: does thermal bypass in roofs, either in the form of gaps in the insulation joints or at the construction details, comply with the definition and/or require-mentsofcontinuousinsulation? And if not, should roof system de-signers account for thermal bypass during the design stage to comply withthecodethermalrequire-ments?Sudhakar Molleti, Ph.D., is senior researchofficerattheNationalRe-Figure 9: Gap impact factor. search Council of Canada.16ONTARIO ROOFING NEWSISSUE 1 2021 THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ORN'