As sure as the sun comes up in the East, every BEC or GANA Annual Meeting deals with code developments relating to energy. Tom Culp does a great job of keeping in front of this for GANA. His presentations this year highlighted several developments worth mentioning. After the “Battle for the Wall” with ASHRAE last year about reducing the window-to-wall ratio, ASHRAE’s got some other areas that will require the glazing industry’s attention over the coming months and years. Unlike the wall battle, these changes are more reasonable.
Energy-related issues the glazing industry will need to watch include:
Climate zone map changes
ASHRAE 90.1 is slated for an update in 2016. Climate zone maps are changing, so if you work in areas near the borders between climate zones, you might want to look to see if your area is changing. There’s also a reduction of U-values slated to be in the 10-14-percent range in the colder zones (4-7), which Tom felt were realistic and practical. Also, solar heat gain coefficients (SHGC) in Zones 1-3 will be revised to 0.25, and possibly to 0.22 in Zone 0.
Spandrel glass and building orientation
There’s some discussion about using different technologies, such as highly insulated, opaque, spandrel glass to allow higher percentages in vision glass areas. Also, do different glass types and/or glazing systems need to be installed on the North / South / East / West elevations in response to different thermal constraints (mostly solar that vary by compass direction)? We’re seeing some of that now, but this will be primarily driven by the architectural design, so we’ll have to see if this practice becomes more widespread.
Thermal bridging will get a lot of attention in the coming weeks and years. A couple presentations, most notably that of Stéphane Hoffman of Morrison Hershfield, hit it out of the park.
Let’s preface this by looking at a common condition of condominium projects I’ve seen over the years. Namely, designs in which the building floor slabs extend through the exterior walls, primarily to form balconies. Curtainwalls on these projects aren’t really curtainwall in the true sense because they are discontinuous, with each floor slab interrupting the curtainwall, thus becoming a series of 1-story ribbon windows, extending from the top of one slab to the underside of the floor above.
That floor slab becomes a thermal bridge, with no thermal break between exterior and interior. HVAC systems heat the interior in winter, including that slab, which becomes a conduit for heat transfer to exterior, colder conditions. And, the reverse in summer: the sun heats the slab, which conducts the heat to the interior, where the cooling system has to deal with the load.
A similar type of thermal bridging happens in windows and curtainwalls. When insulation in adjacent walls isn’t aligned AND tied to the thermal breaks in the window or curtainwall system, a thermal break occurs, and energy “leaks” in or out of the building. The question of how much energy isn’t known yet, but since window and curtain wall perimeters tend to be large, the loss numbers might be huge. If they are, then the logic for addressing any heat transfer in these areas will require more attention.
Take for example a typical head, jamb or sill detail and draw a line representing the thermal separation from the glass thru the thermal break in a framing system, and then connect it to the insulation in the adjacent wall. Any time you have to lift your marker because there’s not a material to connect any two of the materials, there’s the thermal break. If the material you’re drawing a line through isn’t thermally isolated from the exterior, a thermal bridge has occurred and that portion of the wall can transfer energy through the wall system. And, for the line to be energy and material efficient, it should be straight, not zig-zag.
Often the glass and building wall insulation don’t align, or the line you just drew zig-zags through the detail. The glass might be at the exterior of the framing system, and generally, it aligns with the thermal break in the framing. Additionally, insulation in the adjacent wall is near the back of that construction. How do you tie the two together when they don’t align? Going back to the condominium example, for the head can receiver, where’s the thermal break in the extrusion in relation to the glass? Does it align with insulation in the adjacent construction? If that adjacent construction is a floor slab, there’s no insulation, hence a bridge has occurred.
The over–simplified answer to the thermal bridging issue would be to extend the building insulation into the glazing cavity and tie it to the glass or to the thermal break in the framing system. That might require zig-zagging the insulation along the perimeter of the window to get it to tie to the adjacent wall insulation.
But there’s a catch. How do you tie the two together when there’s a water barrier to be crossed? Most framing systems currently use the glass pocket as the pressure equalized or rain screen cavity to help drain water from the glazing system, tying that off to the adjacent surrounding substrates with sealant joints, or flexible flashings, etc.
Putting insulation into the glazing cavity might be a start, and sealing it to the glass edge might become standard operating procedure in the years to come. Rigid board products come to mind, since they usually won’t deteriorate in the presence of water. But, incorporating them with a water barrier, and then tying that to the surrounding building insulation will require the system designers to be really creative.
This is one of those issues where the extent of the problem isn’t known, but potentially can be huge. Educating the architects on how window and wall systems have to change their design intent when it comes to perimeter detailing is equally as large.
Finally, as Tom noted when contemplating changes to standards and codes, how long they take to get adopted into the local building code is a 2-10 year window. Changes first have to get placed in the model IBC codes, then local governments have to adopt them, which is a lengthy process.
Stay tuned sports fans: we’re in for a bumpy ride if the thermal bridging issue takes flight.