A thermal bridge too far: As much as 30% of heat loss can be caused by bad design
I often shake my head looking at the typical North American house, thinking that they have been consciously designed to maximize jogs, surface area, potential leak locations and of course, heat loss. That if it doesn't look good, just add another gable. Every time you make one of these jogs and gables, it adds what are called thermal bridges. They are pretty much a fact of life; turning a corner means more wood studs and less insulation.
On his terrific website about Passivhaus design (Don't know what Passivhaus is? Learn more here), British architect Elrond Burrell describes these as geometric thermal bridges- they are the inevitable result of design decisions about the geometry of the building. They include:
- External wall corners.
- The eaves junction.
- The ground floor and external wall junction.
- Around window and door openings.
Geometric thermal bridging is unavoidable. However, geometric thermal bridging increases with the complexity of the building form. Therefore, it can be minimised by keeping the building form simple.
© Trent Bell/ GOlogic passive house
That's why Passive Houses or Passivhaus designs tend to be simpler; each of these geometric thermal bridges are accounted for. Every one of those jogs on the silly McMansion creates a thermal bridge, almost all of which are avoided in GOLogic's wonderful Go Home passive house. Unfortunately it is often harder for an architect to make a simple design look beautiful; they have to rely on proportion and scale. It takes skill and a good eye.
Passive House expert Bronwyn Barry has a hashtag for it: #BBB or "boxy but beautiful."
Elrond then goes on to describe the even more egregious Construction Thermal Bridges that happen every time a designer adds a decorative eave or bay or pointless projection, along with every tiny window and other little architectural detail.
© Builder House plans/ how many thermal bridges do you see?The above is really a catalogue of design details of the typical American home, most of which can be eliminated through good design and careful construction. And every one of them can cause problems, the most obvious being heat loss or heat gain, cold spots, possible condensation and mould, and comfort.
A construction thermal bridge is where there literally is a physical material, a gap or a component that passes through the insulation. The material or component conducts heat better than the insulation and therefore effectively forms a bridge allowing heat to transfer between the inside and the outside. Some examples of this include:
- Rafters that pass through the thermal envelope to support the eaves (or for decoration!)
- Timber studs or joists within the insulation zone.
- Cantilevered structure passing through the thermal envelope.
- Lintels that interrupt cavity insulation.
- Gaps left between insulation boards.
Thermal bridges can have a serious impact on the energy efficiency and comfort of high-performance buildings. Thermal bridges also increase a number of undesirable risks than can damage the building fabric.Elrond Burrell believes we should aim for thermal bridge free design. He has noted that it can be a problem, thinking about this; he now sees thermal bridges everywhere.
I used to enjoy the rhythm of rafter ends projecting out around the eaves of a house. I admired timber and steel beams apparently gliding smoothly through external walls or floor to ceiling glazing. No more! I can’t help but see the thermal bridging these details create, the resultant heat loss, material degradation risks and mould risks.I am much like Elrond now, looking at jogs and details and plans and thinking about thermal bridges and comfort. But then I never knew that they were such a big deal. Some websites claim that the losses through thermal bridges can be has high as 30%; that's a lot for something that you can't see. And it's not like saving that energy is something we have to pay for, like more insulation; it's either FREE or actually has a negative cost, since every jog and gable and eave projection costs real money. It's a problem that is solved by good design, not more stuff.We should all think about this more.Read Elrond Burrell's What is Thermal Bridge Free Construction?