ThermalSpeak – building efficiency and thermal bridging

by Jay Bowen
BPI Energy Analyst
ASNT NDT Level 3 Thermographer

Building conversations in training this week brought up the subject of thermal bridging in the structure. Defined by a material of less insulation or greater conductivity. This material bridges or short circuits the better insulation and reduces its effectiveness. This reduces the overall R value rating of the whole assembly. The following website,  http://web.ornl.gov/sci/roofs+walls/AWT/home.htm , addresses this whole wall rating concept. This isn’t a debate of test methods or approaches. It is enough to say that anything put in the wall that reduces the energy of that wall should be addressed in the construction process.

My input on this subject is finding these, before construction begins and discussion with a builder, methods to reduce or eliminate these bridges. Verification of the design or confirmation as building progresses would be where a thermographer can enter the process and provide proof that the building project has conformed to this concept of design.

The images from a qualified thermographer provide detailed analysis of construction detail typically hidden from the inspectors or builders.

Taking a set of images to compare this approach can clearly see the advantage of reducing the vertical framing typical of stick built construction. Even the wide wall of 2x6 framing to increase the side wall dimensions for increased insulation. This is an increase in the R value but the thermal bypass of the stud is still there.

Using SIP to reduce bridging	Standard stick frame wall

Looking at a SIP wall can visualize the reduction in the bypasses by reducing the vertical framing. The corners and joints are still present but can be reduced by the same methods of stick built using exterior insulated sheathing.

Making the bridge elimination or reduction a conscious thought in the whole building process can have substantial rewards in energy saving in the home.

Can I Inspect Stud Walls when the Weather is Warm?

"Am I right in thinking that the weather/ delta T are not in my favor for stud wall analysis during the summer?"
 
 
This is a question many new thermographers have. If you think about the heat transfer, the answer is you should be able to get good images from an external wall providing you have sufficient Delta T.  

Right now it is hot outside.  So, if you are observing the exterior wall of an air conditioned residence, you should have a Delta T of 10°C or 18°F or better, if the interior temperature is about 70°F and the exterior is about 88°F or more.  Remember that the air temperature does not necessarily have to be 88°F, if the sun is heating the wall (and has been for a few hours), that will work too. If the building does not have air conditioning and the windows are all open, this would certainly make things more difficult.

Here is an infrared image of an exterior wall taken during a hot day (taken from the interior of the building). Notice the hot studs (vs. the cold studs you would see during the heating season).

You can learn more about this application in the Building IR Basics course.

Can IR cameras see through walls?

Here is a question recently asked me by a student.

"I recently purchased the FLIR SD camera and took your training course which had a lot of valuable information but I'm still unsure about a couple of things. Since these cameras are supposed to be so sophisticated, shouldn't you be able to see through the wall to the studs and would this be part of thermal tuning, and I know you talked a little bit about it, but how do you get the camera exactly tuned and focused for the best possible picture?"

These cameras are sophisticated but we are dealing with Infrared, not X-Ray. Your question deals with basic physics and has nothing to do with the cameras themselves. Walls are opaque to infrared imaging cameras. Now you may have seen some movies depicting looking through walls and seeing people inside a room with an infrared camera, this is Hollywood creative hogwash.

What we see with the infrared camera is the infrared heat radiation produced by the surface of the wall based on its temperature and emissivity. The reason we can see studs in the wall, under the right temperature conditions, is due to conductive heat transfer changing the surface temperature of the wall differently over the studs, vs the insulated cavities between studs. This requires a temperature difference on each side of the wall.

The better infrared cameras can detect and image this temperature variation with less of a temperature difference on each side of the wall because they have better thermal sensitivity. A less sensitive camera will require a larger temperature difference in order to get a clear picture. This is one of the areas thermal tuning comes into play, adjusting the temperature span and the temperature level of the picture to bring out the details as best as your camera allows. Most of the time, letting the camera do the work for you in automatic mode works pretty well. But when you have low temperature differences, or there is something very hot or cold in the image that is throwing the auto adjustment off, then you have to switch the camera to manual and thermal tune yourself.

Focusing is most easily performed by finding a sharp corner or line on the target and adjusting your lens until it is a sharp as can be. If you have a surface you are viewing that doesn't have any distinct features, then what I often do is warm up a pencil in my hands, place the point against the surface, and then focus on the pencil point so it is sharp.