thermal aqua

July 16, 2015 § 2 Comments

bsi061_photo_01f_webA tip of the hat to alum and regular correspondent Grant, who pointed me to this enjoyable rant about the problem of thermal bridging in high rise design.

It’s author, Joseph Lstiburek, points out that Jeanne Gang’s Aqua Tower in Chicago very effectively mimics a fin-tube radiator, as each of its curved balconies very effectively radiates indoor air temperature to the exterior.  The thermal image to the left shows, in Lstiburek’s words, “an 82-story heat exchanger” in the middle of Chicago.

This, of course, isn’t just Aqua’s problem, but the tower is certainly a paradigmatic example of thermal bridging.  Concrete’s density makes it a thermally massive material, meaning it not only stores heat well, it also conducts heat well.  Passive solar heating and ventilation, for instance, both rely on well-insulated thermal reservoirs of massive concrete to store heat–or the lack of it–that will dampen down the daily cycle of rapid heating and cooling of lighter building materials around them. problem with high rise construction is that concrete also offers a great work platform from which to assemble and install building cladding.  Full curtain walls are draped outside of the building structure, but as you can see from a closeup of Aqua, a tall concrete frame suggests a much easier way to attach the building cladding–use single-story ‘storefront’ systems that just span between floor and ceiling slab at each story.  This eases the structural issues that wind causes for fully hung curtain walls by limiting spans to a single floor height, and by providing robust connections at each slab.  It also simplifies the labor involved in erecting cladding, since there is always easy access from the interior and since balconies can provide access (intermittently, in this case) from the outside face of the cladding.

The tradeoff, of course, is that if the slab is monolithic, it works as a very effective way to suck heat from one side of the cladding to the other.  If, say, you’re trying to heat your condo during the winter, what you end up doing is heating the air in the room, which then heats the floor and ceiling slabs, which then–because they have so much outdoor surface in addition to their indoor area–try to heat the entire atmosphere.

There are, as Lstiburek points out, details that can reduce or eliminate this.  By making a foam joint in the concrete and dramatically increasing the amount of rebar in the slab, you can essentially make a steel cantilever that’s wrapped in concrete–but concrete that isn’t continuous from inside to out.  This is as expensive as it looks, of course, and it gives structural engineers fits, since they really want concrete to be as monolithic as it can be.

Aqua, it’s often pointed out, was a super-tightly budgeted project–basically a very standard (if very tall) condo tower with the one super-clever balcony detail that enabled its wave-like forms.  There’s no chance that the developer didn’t do a full cost analysis on this balcony detail, and my guess is that this is evidence that we’re still really in a cheap-energy economy.  It’s pretty clear that the least expensive way to deal with the thermal bridge issue in an 82-story residential tower is–still–to just throw more energy at the detail than can flow through it in a given day, to accept and pay for the losses through conduction while sitting inside (sorry, inside joke here) bathed in soft light and listening to Dionne Warwick in heart-warming stereo.  Now, that may not be true fifty years from now, but that’s well past the developer’s involvement in the building, making this an economically-smart but legacy-dumb detail. fairness to Aqua, thermally-bridged concrete slabs have been the modus operandi for high-rise residential towers in Chicago since the 1920s.  Flat slab construction has been the most spatially efficient way to squeeze as many floors into as little height as possible–not a great approach for commercial construction, which relies on the hollow spaces in steel-framed floors for duct runs, but perfect for residential construction where all of those ducts are replaced by thinner pipes feeding radiators.  There is, sadly, no truth to the rumor that staff in Bertrand Goldberg’s office, located on the bottom floor of the raised office block in Marina City, had to wear winter boots during cold months to keep their feet from freezing on the concrete slab.  But that’s as good an illustration as any of the problem.

Details like this are troubling, of course, to anyone concerned with how efficiently our buildings will operate over the next fifty years as energy costs and consequences soar.  But to an historian, such details tend to reveal what the building culture of the time is actually responding to.  In this case, it’s clear that despite what we know is coming, energy is still cheaper than the labor and the materials that would have been needed to make this a more efficient detail.  I’ll leave the socio-political implications of that to the economists–to a developer that fact is a good piece of actionable data, but to humans in general it should be plenty sobering.


§ 2 Responses to thermal aqua

  • ricklightburn says:

    Lstiburek’s rant was several years ago; IIRC it appeared in the ASHRAE journal back then, and I’ve mentioned this occasionally on my CAF tours. His argument as I recall it was that the major thermal bridge comes not from the concrete, but rather from the steel rebar in it, and simply replacing that with stainless for a few inches would have ameliorated that. (In my naivete, I’m amazed at how different the thermal properties of stainless vs. carbon steel.) I see that in the diagram he’s gone a bit further in blocking that bridge. But he makes the point that these fixes are all pretty much off the shelf, and so the cost premium would be there, but not much.

    • twleslie says:

      Interesting…just for kicks I looked up the thermal conductivity of all the stuff involved here (…carbon steel is nearly three times as conductive as stainless steel, which in turn is 8-10 times as conductive as concrete. So he may have a point about the change in rebar material, but I suspect the area of concrete involved makes it the greater culprit in these details (thus the block that he’s arguing for). Stainless steel is generally about eight times the cost of carbon steel, so while it may be ‘off the shelf,’ it’s a big investment, especially for a rebar-heavy detail like he’s proposing. All of which, I guess, makes the point that the life cycle savings to a (short horizon) developer still can’t compete with the first cost of making some of these sensible changes…which suggests that we’re still not close to paying the true energy costs of many of our decisions…!

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