Thermal bypassing (also called thermal bridging) is increasing heat loss or heat gain through low-insulation areas of an otherwise efficient building envelope. A huge amount of heat may leak through these insulation gaps relative to the rest of the building envelope, with increasing heat flow as the rest of the building envelope becomes more efficient.
We conceptualize heat as vibrating atoms. A material that is hotter has atoms that are vibrating more. Since each atom that vibrates more will take up more space in conjuction with neighboring atoms, a material that is hotter expands in size, referred to as thermal expansion.
Heat transfer is the transfer of heat from one location to another. This can happen in three ways:
Conduction is the direct contact of two materials of different heat levels (different temperatures). The vibrating atoms of the hotter material cause the atoms of the cooler material to vibrate, making the previously cooler material less cool. The material that was hotter becomes less hot its atoms vibrate less after transferring some of the vibration to atoms of the other material.
Convection is the movement of gas or liquid. For example, natural convection is when warmer gas or liquid atoms rise above cooler atoms of the gas or liquid (e.g., hot air rises).
Radiation is the release of photons from excited atoms, in the infra-red (IR) spectrum of electromagnetic waves. The amount of radiation emitted is proportional to the heat of the material. The hotter a given material becomes, the more IR radiation it emits. That makes it possible to measure how hot a material is by measuring how much IR radiation the material is emitting.
Infra-red wavelengths are not visible to the human eye. However, an infra-red camera can record an image of infra-red waves, and display the image in wavelengths that are visible to the human eye, for example as a grayscale or false color image. This is called infra-red thermography.
The figures above show an IR camera used in home energy audits, and a grayscale image from that camera.
The grayscale image shows an exterior wall from inside a wood frame house in winter. An electric outlet box is shown to be a thermal bypass, even with an insulating cover plate over the outlet. This happens in legacy wood frame construction, because the walls are too thin to have insulation where an electric outlet box is installed.
The insulated cover plate does help, but heat is still leaking through the outlet itelf, through the screw in the middle of the cover plate, and through the wall area surrounding the outlet (where insulation was not installed to make room for the outlet inside the wall).
The following figures show IR images of a different house taken with a different IR camera.
IR thermography has limitations, requiring training to use. For example, angle of view and reflections must be considered. And metals emit radiation differently than other building materials, making comparison between metals and other materials difficult. Nevertheless, IR thermography is a useful tool, when taking its limitations into account.
Notice in the color IR image above, taken inside the house, that the external walls leak more heat where the walls meet, and where walls meet the ceiling. That is because in this legacy framing there is no insulation at those junctions, just wood framing (single walls). In addition, at corners, there are more directions for heat to escape through, than mid-wall.
To reduce those heat losses, corners should be filled in with insulation, and junction of walls and ceilings should also be insulated.
Thermal transmittance depends on the thermal conductivity of the materials through which heat is transmitting. Example thermal conductivities of common materials are given in lists like the following:
Notice that metals have high thermal conductivity: they transmit more heat than other materials. That is why metals are avoided in designs of building envelopes.
It is possible to use building simulation to get ideas about thermal transmittance. Following are screen shots from the THERM software program that is available for free from Lawrence Berkeley Lab (see References below).
THERM displays each building material with a user-defined color, not necessarily the actual color of the material. Specifying user-defined color is how THERM allows you to tell different materials apart, in the programs default display.
Note that along the wall where a stud is, the inside wall temperature will be lower and the outside temperature higher, compared to surrounding wall, hinting that more heat is flowing through that part of the wall.
And, in that figure, the contour lines further apart in the stud also hints that heat is transferring faster through each stud than through the insulation, which would result in more heat transferring.
Much worse is to have metal studs. Following is modelling of an exterior wall with metal studs in THERM, with the outside of the building on the left (instead of right):
The contour lines are even further apart, and wall surface temperature along a stud are much cooler inside and much warmer outside, all indicating more heat transfer through the wall in this bridge (bypass) across the insulation.
Changing the THERM display options shows the same drawing in infrared false color, as if it is a thermograph.
Another THERM display option, called Color Flux Magnitude, shows that, as expected, there is much more thermal flow (heat flux) through that bypass:
The quantity of heat flow through a solid material (denoted Q), for a given amount of time (denoted Δt), is calculated with the following formula:
where ΔT is the temperature difference of both sides of the material, k is the thermal conductivity of the material (see above), A is the width and height of the material (surface area), and Δx is the thickness of the material.
The quantity of heat that flows increases in proportion to increasing temperature difference between one side and the other side of the material, which is multiplied by the thermal conductivity, making the increase go up faster (accelerate) if the thermal conductivity is higher. That is why use of highly conductive materials, in thermal envelopes, needs to be avoided.
Thermal Bypass Calculations
THERM Web Site
Modeling Steel Studs with THERM
(LBL tutorial video, 17 MB)
LBNL THERM For Energy Modeling
(Mike Sealander, Maine)
A Troubled Tiny House
(Journal of Light Construction)