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Liner system roof assembly (right) shows single piece vapor retarder encapsulating purlins vs. filled cavity assembly (left) shows vapor retarder sealed atop framing, leaving purlins exposed. |
New energy codes have increased energy efficiency stringency
(lowers the assembly U-factor), thus featuring different prescriptive R-value assemblies and thermal barrier specifications for insulating metal building roof and walls. The importance of the thermal barrier/vapor retarder location is indirectly referenced in the newer energy codes, whereas the 2012 International Energy Conservation Code
(IECC) and ANSI/ASHRAE/IES Standard 90.1-2013 both feature a Liner system (LS) roof assembly and is heavily listed in each of the prescriptive tables. Standard 90.1-2013 lists liner systems in climate zones 4 through 8 while the 2012 IECC lists liner systems in all eight climate zones throughout the country. The newly published 2015 IECC lists liner systems in all eight climate zones.
Liner system’s high performance can be contributed to two primary factors: 1) uncompressed insulation in the purlin cavities and 2) the vapor retarder is properly placed entirely below the purlins which isolates them from the inside conditioned space. The vapor retarder specification for a liner system is typically continuous and is essentially a large, single piece tarp that is made to fit the width and length of a bay without erector applied field seams. The vapor retarder is typically held in place with steel straps and fasteners into the bottom side purlin flange, creating the depth space for the unfaced, uncompressed fiberglass insulation to be installed from the topside of the building in new construction. The vapor retarder is sealed around perimeter atop rafters and below side wall eave struts.
The other newly listed prescriptive method is in Filled Cavity
(FC) assembly, also known as a long tab banded system. This method is prescriptively listed in only climate zones 1 through 3 in Standard 90.1-2013. The IECC voted not to include this assembly in their prescriptive tables in their 2012 IECC or in the recently published 2015 IECC.
Similar to the liner systems, the filled cavity method includes steel strap bottom side installation to the lower purlin flange, so ideally the uncompressed insulation can rest upon it. However, there are crucial differences in the length, width, and most importantly, location of the vapor retarder. Unlike the liner systems, filled cavity method uses insulation in which the vapor retarder is glued/laminated onto the fiberglass. Thus, each roll of fiberglass has its own vapor retarder that must be field seamed during the installation process. Installed high performance claims are based upon the two adjoining vapor retarder facings sealed continuously on the top of each purlin flange, no gaps along the purlin webs and without insulation compression of purlin bracing/stiffeners within the cavities.
Long vapor retarder tabs with sufficient width on each side of the fiberglass roll are needed to allow the full thickness of insulation in the cavity. For example: 8-inch purlin depth with purlins spaced 60 inches on-center would require each long tab extending beyond the insulation width to be at least 11 inches to allow the vapor retarder to travel up the vertical purlin web (8 inches) and be sealed horizontally over the top purlin flange (3 inches). Considering this must be done on both sides, each 60-inch purlin spacing would require at least 82 inches of vapor retarder
(11 inches + 60 inches + 11 inches). This type of sealing is required to be done every linear foot atop each purlin flange, essentially twice considering the overlap field seam. The long tabs of the filled cavity method establish the depth within the cavity and the objective is that it’s long enough to rest on the support straps.
Industry instructions state not to pull the tabs so tight that it pulls away from the purlins. This disclaimer or warning is stated to help prevent a larger gap along each vertical purlin web where the inside conditioned air can easily circulate within the cavity and hit the exposed roof (or standing seam clip) fastener tip. This is a caution area where condensation may be likely to first occur with the slightest percent of humidity inside the building.
Considering the new energy codes, the demand for high installed R-values and the overall finished interior appearance, specifying a proper thermal barrier/vapor retarder may be dependent on where it is specified in metal building roof and walls to achieve the thermal performance and aesthetics desired.
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Brad Rowe is the national marketing manger at Thermal Design Inc., manufacturer of the Simple Saver System, based in Stoughton, Wis., and Madison, Neb. To learn more, visit www.thermaldesign.com.