Thermal Bridging

Building insulation is intended to reduce heat transfer through a building envelope in order to reduce energy waste, energy costs and the potential for condensation within the building.  A few of the chief concerns regarding insulation are

• Selecting the right product
• It’s location within a wall or roof assembly
• It’s adjacency to other important elements such as air barriers and vapor retarders
• It’s continuity across the entire envelope

Heat transfer follows the path of least resistance; i.e. if a conductive metal is placed in the same plane as insulation in a wall assembly, heat transfer will occur at the metal as it is less resistant to heat flow.  This undesirable transfer is known as thermal bridging.  One common problem occurs in metal stud walls when batt insulation is placed in between the studs, creating a thermal bridge at the studs. 

Some building codes have begun to include requirements for continuous insulation.  In the case of metal stud walls, Massachusetts has included prescriptive requirements for continuous insulation in addition to insulation between the studs.

In stud construction, it’s important to also evaluate insulation at slab/floor edges.  An insulated slab edge can act as a significant thermal bridge.

Windows and curtain walls also require care in detailing for thermal resistance as they can be significant bridges.  Windows and curtain walls already have less thermal resistance than the average opaque wall assembly.  Insulating Glass Units and low-conductivity thermal breaks are utilized to reduce heat flow.  In these assemblies, even small thermal bridges at the perimeter framing can have significant effects on heat loss and condensation.  To mitigate these problems, the insulating components must be continuous with that of adjacent assemblies.  For example, starter sills at the perimeter of window assemblies can often be thermal bridges.  Perimeter clips and starter sills should avoid spanning across thermal breaks.

Brick veneer walls often utilize steel lintels at window heads to support masonry above.  In such circumstances, the window should be located inboard of the lintel to prevent the lintel from becoming a thermal bridge.

Many roof assemblies offer continuity by locating the insulation above the structural framing.  However, thermal bridging can often occur at penetrations for roof mounted equipment. 

Another rooftop concern is the system of fasteners used to secure insulation to the substrate.  The many locations of metal fasteners can act as small thermal bridges.  In the winter time, this can be evidenced by snow melting on the roof in the grid-like pattern of where the fasteners are located.  This can seem like an acute thermal bridge, but in a typical roof such bridges can reduce the roof’s thermal performance by about 15%.  A system of staggered furring above the roof framing can prevent the fasteners from penetrating the thermal envelope.  However, this effort is often not justified by the associated energy savings.  Where suitable, utilizing adhered insulation can eliminate this problem.

Parapets can also act as a thermal bridge.  Even if the walls and roof are continuously insulated, an un-insulated parapet can act as a bridge for heat transfer.  Reducing parapet height can improve this problem.  If possible, constructing parapets with a thermal break can also mitigate heat loss.

While the term thermal bridge most often refers to conductive heat flow, air leakage can also cause a significant increase in heat transfer.  As an example, the heat lost through an unsealed electrical wall outlet is about the same as 30sf of solid exterior wall (brick veneer with fiberglass insulation in stud framing).

Today there is a great deal of discussion about integrated project delivery and closer workflows between design and construction parties, but combating heat transfer in the building envelope is an effort where it is most critical for these parties to work in an integrated fashion to achieve successful results.

Comments

[sustainable architects]

Without detailed plans, a project is doomed to failure. Mistakes will happen. Cost overruns will almost always occur. Change orders will be a weekly event. So plan well and hard.