b'T&R Update Cold Weather, Insulation, and R-Value ReneDupuis, PbD PE M. T is the time of the year when sharp build Even under still air conditions in cold weather, his ing owners and operators mentally takewe would expect to find beads of condensation stock of their heating system. Many believe theon the underside of the single ply along these is thermal envelope on their roofsatisfactory be joints. cause they installed aR-value recommendedAir Gaps roofingThe recommendation could be system.Steel roof deck construction is the most from a roofing contractor or professional de problematic because air feeds up into today\'s signer. R-value can be away to specmisleadingroof insulation systems. Deck flutes are usually ify insulation if tapered insulation is used. Theone inch or more clear of the wall at the biggest issue is how the insulation boards are in deck ends. Interior roof drains and roof curb stalled; open joints may exist from top to bot openings allow for building airmove up tom even in the deep tapered systems shown into a\'sliterature.under the roof membrane at deck cutouts. manufacturer olderPolyisocyan Side lap air feeds from the steel roof deck can urate is most often the insulation material used.also be aon how they were In contrast to the older BUR roofs that hadproblem depending successive layers of insulation hot moppedconstructed at the deck/wall interface. together, our single plys usually have aDry Down single layer insulation with gaps occurring, eitherNumerous white roof condensation issues have due to original installation, insulation boardarisen in the past due to the cool operatingFigure 1 shrinkage, or both. Gaps in rooftemperatures of the membrane in summer\'s insulation over metal decks lead to the passage ofheat.Asan industrydid not fully understandR-Value Considerations warmerwe (and sometimes more humid) building airand appreciate how the older BUR assembliesThe 2011 NRCA Roofing Manual: Membrane right up to the single ply membrane. Ifunderwent amoisture gain in coldRoofingrecommends an in-service itnaturalSystems mechanically attacheddry-vis amembrane, theweather and underwent adown in summerR alue design of 5.0 for heating conditions membrane moves up and down under windheat. Of course the BUR membranes sawusing polyisocyanurate insulation. This is depending load, pulling in and exhaling interior airtemperatures well above 140 Fonan 11% reduction from the previous design through the gaps in insulation.gravel color or coating used. Smooth blockrecommendation of 5.6 R-value for that In the northern states we observe thin icemembranes run well above 160 Ecoolproduct. The NRCA manual recommends that Our youdry lenses building up under the single ply Ifwhite membranes cannotthemselves downpolyisocyanurate be designed for long-term walk the rooffreezing weather, youwillas the typical temperatures range in summer\'sin-service summer time cooling (hot inhearweather) the ice crunch beneath your feet. Ifheat is in the 120 FThat is why youR-value of 5.6. These new recommendations you usedrange. white membrane,June ayou can expect the "cool"can do test cuts inon some mechanicallyare based on evaluating the R-values at roof to stayMore condensation ensues andfastened systems with single layer insulationoperating temperatures reflecting its actual cool. the ice lenses keep growing. Butinover steel deck and find wet insulation facersuse. Prior testingdone at 75 F, now 40 eventuallywas the warmer sun, the ice lens comes back downwith biologic action taking place. Cyclical dryF110 Fconditions are used to more ortest in the form ofinto the interior. Thisdown with white "cool"is veryaccurately predictconditions. drippagemembranesactual generally begins to occur in late morning andlimited at best in the northern states.In summary, ifover a constructedmetal typically stops in early afternoon. For this reason,deck,roof systems are prone to air AirNapor Barrier: ASolutionlightweight asingle ply is highly desirable and logicalPossibleexchanges with the building. It is a blackWill aair/vapor barrierfunction touse in the northern climates. Also, the NRCAtightly constructedof design,,interior work?Itif tightly sealed at all walk andconstruction andoperating Roofing Manuals have recommended two layersmay,conditions. Wind pressure on the building ofroof penetrations. However, we cannot shoot itenvelope can further drive air passage. insulation minimum.full of holes with mechanical fasteners withoutBead Air gaps and potential interior air feeds aresome diminishment. The cut off to building airadhesives are good for anchoring roofing best illustrated in Figure 1, which shows howhas notably curtailed the lifting and bellowingmaterials but physical air gaps abound. We one manufacturer\'s literature (circa 1990)offastened single ply membrane,need to keep in mind that the R-value of perpetuates this problem. Note the three-to mechanicallypolyisocyanurate insulation diminishes with four-layer taper does not stagger or break joints,especially the wide sheet assemblies. Oncetemperatures below 75 F, so stopping intra roof and the stacked jointover aagain, an adhered assembly is the best long air flow is important. fallsdeck flute.term performer but at a higher cost due to the This will allow an air exchange to take place.construction. 8www.mrca.org- Midwest Roofer'