The Thermal Bridge

by Thomas Hare, April 2011

“A thermal bridge, also called a cold bridge, is created when materials that are poor thermal insulators come into contact, allowing heat to flow through the path created, although nearby layers of material separated by airspace allow little heat transfer. Insulation around a bridge is of little help in preventing heat loss or gain due to thermal bridging; the bridging has to be eliminated, rebuilt with a reduced cross-section or with materials that have better insulating properties, or with an additional insulating component called a thermal break.”

From Wikipedia, the free encyclopedia      http://en.wikipedia.org/wiki/Thermal_bridge

Example of thermal bridging in a structure with steel studs

http://www.rensolutions.co.uk/thermal_bridge.php

 

The concepts of thermal bridging were known for quite some time to building designers and long time builders.  Designers and architects are required by profession to learn R-value, porosity and conductivity of various building materials in order to use products more efficiently.  Long time builders however see the effects of thermal bridging largely during renovations and through their constant interface with building envelopes.  Why weren’t the issues of heat loss/cold gain via thermal conductivity addressed before? In fact, they were.  Large commercial buildings are the first to be analyzed and have new technologies tested on them due to the scale of loss.  Spray foam insulation is more and more popular for residential applications now but has been around in commercial applications for over 20 years. Today the additional costs associated with constructing building envelopes that do not conduct thermal energy can put the project out of reach financially for some but should not be ruled out due to energy costs of today.   As heating and cooling costs increase there is a corresponding reduction in return on investment duration for new building constructs and renovations.  This is fuelling higher interest and creating more awareness about the effects of thermal bridging.  A steel insulating building can lose up to 20% of its heat through thermal bridging alone.  These losses add up very quickly and increase the operating costs of the building. 

In a typical residential wall the wall studs act as thermal bridges, conducting heat from inside the home to the outside.  The heat travels from the room, into the drywall or finished interior wall covering (which is attached to the wall studs), through the walls studs, into the exterior sheathing (which is attached to the wall studs from the outside), into the exterior wall covering and then dispersed outside.  This is how thermal bridging works in a nutshell. 

Cutaway view of wall assembly

http://www.concretethinker.com/Content/ImageLib/woodframewall.jpg

How to solve thermal bridging issues

Thermal bridging can be solved through a variety of ways.  The main one is the alteration of construction techniques and processes.  The construction of a residential wall section is typically the stacking of materials horizontally.  Hence there is large quantity or area of intimate contact of building materials and fasteners.  Solving this problem requires an alteration of building envelope construction procedure.  Incorporating an “air gap” or gap between the interior building skin and the exterior building skin will eliminate thermal bridging and reduce building losses.  There are several ways of doing this and one of the most effective ways is through what is called “double wall” construction.  Two walls are created in the exact mirror image of each other and are placed side by each with a gap between the two walls of approximately 9 inches.

Example of double wall construction:

After this double wall is constructed, insulation, vapour barrier and normal construction practices follow for each the interior and exterior building skins.  Adjustments need to be made at all window and door openings to accommodate the now greater wall thickness (reductions in door and window swing radius) and adjustments made to finishes in these areas to provide a satisfactory visuals and weather protection.  There is a tendency to confuse the building practices of “Super Insulated Homes” with those that address and avoid thermal bridging when in fact the super insulation method of building, which does involve thicker building envelopes, does not address thermal bridging.  Construction methods addressing thermal bridging inherently create super insulated building envelopes simultaneously. 

At first glance double wall construction might seem overly expensive but let’s take a look at an 8 foot long framing section of Super Insulated Wall and an 8 foot long section of wall framing built to address thermal bridging.

Considering that a wooden 16 inch tall wooden “I beam” wall using advanced framing techniques would cost a total of $ 310.00 in materials alone and a similar wall using same framing technique for thermal bridging would cost $27.58 it becomes quickly apparent that building using the double wall technique with advanced framing is much less costly then building for a hyper insulated home.  It could even be argued that the elimination of thermal bridging could offset the increased thickness or increased r-value found in super insulated structures thus resulting in a building structure that performs just as well as a hyper insulated home for much less.  This argument will be left to the architects and designers to resolve.

Further a double wall construction method can lower building costs by reducing the labour costs often incurred by the trades.  In a double wall building envelope the electricians and plumbers can run their wires and water delivery without drilling quite so many holes and reduces their time on site.  The reduction in holes drilled into the building envelope to accommodate the building infrastructure and delivery systems also reduces heating/cooling losses.  Reductions can be increased and building performance can be improved further by the utilization of closed cell spray foam insulation in a double wall construct.  This eliminates the movement of all air and temperature gradients between the interior and exterior of the home. 

Overall there might be a slight increase in labour costs in the framing and finishing stages of building construction; those are offset and sometimes reduced by lower trade costs and displaced entirely by operating costs.   Addressing the issues created by thermal bridging will result in a structure that costs less to operate, reduces operating costs and downtime for HVAC systems, improve efficiency for the appliances within the home and drastically improve the comfort levels of building occupants.