the path of least resistance

As the fourth floor finishes up, and a roof over head is actually in sight, we think its a good time to talk about, in architect's terms - the envelope - or the exterior wall (and roof) of the building. Althoughthe least sexy and often overlooked, the wall assembly behind the finishes is the most critical aspect in the creation of a sustainable, energy efficient building.  Over the past several months we have focused on the specifics of insulation, thermal bridging, air sealing, and structural detailing, all of which go hand in hand.  Since this is a LEED for Homes project, we are required to undergo an energy analysis - more details on that in a future post - which has helped us review and clarify our envelope's strengths and identified possible weaknesses to reddress.

A quick overview of our structural systems, which for NYC are pretty standard -  8" concrete masonry unit (CMU) bearing and shear walls, with light gauge steel floor joists (a whole post just on our joists is here) and a composite metal deck and concrete subfloor.  Diverging from the standard NYC floor section, the finish floors will also be poured and polished concrete (yeah! we are very excited about having exposed concrete finished floors in our apartments), but this layer will be thermally and acoustically isolated from the subfloor, as they will contain the tubing for radiant heat (more on that to come).  The front and rear facades are framed with light gauge metal studs.

Among the several limitations imposed on a building by its location within an urban context, the one that has the greatest effect on energy performance is the inability to site the building to maximize passive heating and cooling strategies.  Unless it’s a fortunate coincidence, the building cannot orient toward the south for winter heat gain, nor toward the wind for ventilation/cooling. Therefore the building must rely on a high performance envelope, with lots of insulation and a continuous airtight barrier to minimize the transfer of heat and air, respectively.  The most effective placement of these layers is on the outside surface of the building so that they create a continuous “overcoat”, thus eliminating any thermal bridging.  

So what is thermal bridging? - it is a path within a building envelope along which heat energy is transferred. To prevent thermal bridging insulation and thermal breaks are installed to break these paths of least resistance.  Ideally a continuous layer of insulation creates a thermos effect through the "overcoat" concept- keeping in the heat during the cooler months, and out during the warmer months.  The most typical parts of a building where this transfer is greatest is along metal studs, metal joists, and steel connections, where they attach to the CMU bearing walls, easily finding paths to the exterior. In the trout house, we are using a redundancy of insulation and air barriers, placing them on the exterior to achieve the overcoat concept, as well as within and on the interior of the building structure.  As described above, we have two different types of wall construction – CMU at the sides of the building, and steel studs at the front and back.  Although more insulation can be placed in the cavity of the steel studs, giving a greater R-value or total insulation value compared to the CMU wall, the effective, actual performance of the two wall assemblies will be comparable, due to the high conductivity of the steel studs, and the ability of the CMU to absorb and slow down heat transfer process.

In recent years with more and more people writing, blogging, and talking about sustainable, net-zero, energy efficient design, the general principles of a well sealed building have started to take root. But before insulation or the air-sealing, during the structural phase addressing thermal bridging - which can be an even more powerful energy drain - is even more critical.  When building ground-up, we have the advantage of getting it right the first time around.

Below is an image that shows one example of where thermal bridging needs to be controlled - at balcony connections.  Although not apparent in the photos, the steel framing for the balcony and the steel framing for the floor are independent and separated structurally.  Therefore when the concrete is poured over the metal decks, the floor slabs will also be isolated from each other.   However, we still need to go a step further. The image on the left shows an improper installation that needs to be remedied : concrete infill in the center creates a themal bridge between the steel of the floor and the steel of the balcony.  This will be chopped out, and a line of insulation will be installed to create a break.  The image on the right was installed correctly, leaving a gap where insulation can fit in.

Insulation and air-sealing are additional important construction methods for ensuring energy efficiency.  While its important that the architectural drawings and details are clear and well designed - the execution in the field is the Achilles heel.  Short cuts, not understanding the basic principles, mistakes, and language barriers all add to construction that does not meet energy efficiency standards.  Recently Urban Green, the local chapter of the USGBC launched G|PRO to help contractors and maintenance crews - important but overlooked professions - get to understand their role, and techniques for building green.  More details to come when the insulation gets installed…….soon.