The Thermal Bridge

Thermal imaging is an awesome tool, it can be used to find areas of missing insulation and other temperature anomalies in the building shell.  It can help us find issues with electrical, plumbing and heating systems.  And when used in conjunction with a blower door, we can often “see” the air leaks.  The photo below was taken without the assistance of a blower door, I was at this new home to conduct a blower door, but the test hadn’t begun at the time this photo was taken, can you say thermal bridge!

What exactly is a thermal bridge?  It’s an area of a wall, ceiling or sometimes a floor with lesser insulation value than other parts of the wall or ceiling.  Typically, this is the wood or metal framing of a building compared to the area of cavity insulation.  This conductive flow of heat moves more readily through the wood or steel framing, which has a lower resistance, or R-value.  This photo was taken with an outside temperature of around -5°F and an indoor temp of 60°F, a delta tee, or difference between inside and outside of 65°F.  That’s a pretty big difference, but fairly common for my area during the heating season.  (I’ve seen delta tees in excess of 110°F.)

Can we quantify the difference between cavity insulation and framing?  Yep, we can calculate the transmission heat flow in BTU’s by using the formula Q = U x A x ΔT.

Q is the total hourly rate of heat loss, in this case, through the framing lumber.

U is the U-factor for the material in question, in this case, the material is a 2 x 6 which has an R-value of 6.88.  We have to change the R-value to a U-factor by the following formula, 1/R = U-factor, so the 2 x 6 (R-6.88) has a U-0.145.

A is the net area of the surface in ft².

ΔT is the difference between inside and outside temperature.

If we take an 8-foot 2 x 6 and cut it into 1-foot pieces, we end up with a surface area of 1-foot by 1-foot.  That’s our surface area.  We can use the delta tee of the conditions from the above photo, so Δ65 and our U-factor is U-0.145.  Plug those numbers into the formula and we get:

Q = 0.145 x 1 x Δ65

Q = 9.43 BTU’s per square foot per hour

Now let’s look at the BTU loss per square foot of insulation, in our case the insulation is R-20 fiberglass.  The U-factor for R-20 is U-0.05.  Our new equation is:

Q = 0.05 x 1 x Δ65

Q = 3.25 BTU’s per square foot per hour

What we did was simply quantify the difference in heat movement between the framing lumber and insulation at the specific Δ of 65°F, that same ratio (9.43 BTU for framing and 3.25 BTU for insulation) exists in both the R-value (R-6.88 for the framing and R-20 for the insulation) and U-factor (U-0.145 for the framing and U-0.05 for the insulation).  The ratio is roughly 1/3.  This calculation only accounts for the conductive movement of heat, convective heat flow is a different calculation.

I’ve read a few articles that suggest thermal bridging can account for up to 30% of the heat load of a home.  30% will vary from home to home depending on the ratio of framing to insulation.  Windows and doors also have a lesser insulation value and more thermal bridging than the opaque cavity insulation areas.

So, how do we limit the effects of thermal bridging?  The most common method in new construction today is either to add continuous exterior insulation or use a double wall system for all exterior walls.  I’ve written several blog posts on continuous exterior insulation, here are a couple of my favorites:

Construction Design-The Anatomy of a Well-Built Wall – Northern Built

Construction Design-Continuous Exterior Insulation – Northern Built

I have yet to have the opportunity to work with a double-stud wall assembly, but I do have a friend that uses it often, Ben Bogie.  A great article on the topic was written by Dan Kolbert, Ben’s former employer.

A Case for Double-Stud Walls – Fine Homebuilding

The reduction of thermal bridging has been addressed in the building codes since 2012, when the first requirement of continuous exterior insulation was introduced.  Many areas of the country are not enforcing or have written out this requirement.  My home state of Minnesota is one example.  We are one of the coldest states and would benefit from the use of exterior insulation.  Builders are pushing back against its use because of the added costs and complexity, and because of past experience in using foam insulations with polyethylene sheeting on the warm in winter side of the wall assembly.  This experience is based on a time before we had good strategies to control interior humidity and bulk water leaks on the exterior.  Both can be big problems when walls aren’t designed to dry, whether exterior insulation is added or not.

I’ll leave you with one more cool thermal image, another thermal bridge.

I took this photo a while ago, what’s cool is the two darker purple spots near the middle of the pic are the screws used to fasten the drywall to the framing.  If you look closely at the first photo, you can also see a few fasteners in that photo as well.