What Is a Thermal Bypass? Another Thing for Builders to Worry About

British architect Mark Siddall's firm is called LEAP for Lovingly Engineered Architectural Practice–he knows how buildings work. He is known to Treehugger for his Larch Corner Passivhaus and his concept of the Forever Home. Siddall recently wrote a paper for the Passivhaus Trust, the UK Passive House organization, titled "Thermal Bypass Risks: A Technical Review."

I had never heard the term "thermal bypass" before and wondered if it was a Britishism; a Google search turned up pages of thermal bypass valves and an American definition of it as "an opening between a conditioned and unconditioned space that heated or cooled air can move through, therefore violating the airtightness of the building envelope." It turns out that it is a term developed by researchers at Princeton University in 1979. The easiest way to wrap your head around it is to recognize that it is any form of air movement that undermines the thermal performance of a building. A thermal bypass, as Siddall and other British sources describe it, occurs when heat is transferred through or around insulation, reducing its effectiveness. 

 There could be:

(a) closed loops, where gaps in the insulation or between the insulation and the warm inside wall let air circulate;
(b) open loops, where "wind washing" occurs, with the wind blowing through the insulation;
(c) an open loop that would show up on a blower door test, where holes and leaks in the air barrier allow warm air to flow out, and
(d) the mess we get in typical construction, a combination of all the above. 

The astonishing thing is how much heat is lost due to air gaps between the insulation and the inside warm wall material. For a given thickness of insulation, a 7.5 mm gap could result in a 203% increase in heat loss; a 15mm gap could be a 520% increase. 

These kinds of gaps are common in British cavity wall construction but also happen in North American wood frame construction. Years ago, the real Green Curmudgeon, Carl Seville, suggested with tongue only partly in cheek that batt insulation should be banned because of lousy installations, and Allison Bailes of Energy Vanguard showed photos of awful installations where there were more thermal bypasses than there was insulation. These bypasses become thermal conveyor belts, moving heat away from the interior; the result could be discomfort and drafts, and possibly mold on the interior, or moisture condensing in the insulation, leading to the possible rotting of the structure. 

We have often talked about the importance of air tightness and the importance of blower door tests, described by building expert Mark Wille as "the red door of truth" because you can't hide the fact that your building leaks air. But it won't catch a thermal bypass behind a tight air barrier; there is no simple test for it. That's why careful installation and inspection matter: "In all cases good detailing and careful workmanship are required to prevent the formation of interlinking cavities, air gaps and joints which contribute to the risk of natural convection." 

 Wind Washing and the Permeance of Surfaces

"Wind Washing" was another term I was not familiar with, but at least the North American definition is the same as the British, with the Building Science Corporation defining it:

"The phenomenon of air movement driven by wind pressures wind passing through or behind the thermal insulation within enclosures, causing significant loss of heat flow control and potentially causing condensation. Typically occurs at exposed building edges, such as at the outside corners and roof eaves because of the large pressure gradients at these locations. This can be thought of as the "wind blowing through the insulating sweater" effect."

It can be particularly bad at corners and projections, where there are often multiple studs, gaps, and greater opportunities for poor workmanship. 

The solution here is to have a proper wind barrier on the exterior, which is what Tyvek is supposed to be doing. In an interview with Siddall by writer Kate de Selincourt, she described it as only a British writer could with my new favorite term: "Making a building windtight is like putting a cagoule over a woolly jumper on a windy day. It doesn't add any more insulation, but it ensures the insulation in the jumper can work."

I knew that a jumper is a sweater, but I had to look up cagoule: "a lightweight, hooded, waterproof jacket." 

In North American houses with vented attics, the wind can be blowing right through the insulation, so wind deflectors are suggested.

In the section on designing out thermal bypass risks, Siddall recommends that you lightly overstuff the insulation when building with frame construction.

"Encapsulate the insulation between the wind and air barrier. To achieve this, voids should be designed out i.e. the insulation must completely fill the cavity. This may be achieved by ensuring that insulation is 10 mm to 15 mm thicker than is technically required by U-value calculations. This will help ensure that the insulation is lightly compressed across the entire surface area by the encapsulating material." 

He also suggests insulating from the outside in: "This would help to be doubly certain that voids are not formed on the warm side of the insulation." This is how it is done in many factory-built homes, where the drywall is installed first and the insulation is installed from the outside. It always resulted in a much better job. 

Here, once again, I would make a case for simplicity. Every jog, bump, and dormer that designers love to add creates more opportunities for thermal bridges and bypasses. And I would once again pitch for Passivhaus. As Siddall notes:

"The evidence base considered in this paper clearly demonstrates that failures in design and construction do not have to be the norm and that they can, with the right knowledge, intent and skill, be addressed in practice and on the building site. As has been demonstrated in the literature, Passivhaus buildings clearly demonstrate how performance gaps can be closed reliably."

In the end, Siddall calls for good design, careful installation, and thorough inspection.

"If thermal performance gaps are to remain within an acceptable tolerance, then continuous insulation should be encapsulated on all sides by uninterrupted, unbroken air and wind barriers. Only once this is achieved will buildings deliver the energy savings, carbon emissions, comfort, health and well-being that owners, investors, occupants and future generations rightly expect." 

Not that long ago, nobody knew what a thermal bridge was, and we used to have apartment floor slabs exposed to the great outdoors. Now we know that they lead to big conductive heat losses, and good designers and builders try to avoid them.

A thermal bypass is a convective heat loss, with moving air carrying heat away. It's clear from Mark Siddall's report that everyone should know about these too.

Learn more from the Passivhaus Trust.