Material Evidence - Six of One, a Baker's Dozen of Another!

This week I traveled to Berkeley, CA to attend 2 classes taught by Passive House Academy who are based in Ireland.  Tomas O’Leary (who is a hoot and a half!) covered PHPP basics and his brilliant partner Bob Ryan taught us how to use THERM and the new PSI calculator he wrote to work with THERM results.  Both guys are great instructors and if you have an opportunity to take a class from them I highly recommend it!

One thing that both classes stressed, even though they were on totally different topics, was that you should get familiar with your software.  Change things to see what happens, become aware of what to expect so that you are equipped to “spot the mistakes” when you are checking your work.  Sometimes a decimal point gets slipped a spot or a material gets selected when in fact you wanted the one below it on the list.  Other times it is just good to know what happens if you turn the building around, or change the material from one thing to another.

With that in mind this week I did just that with THERM.  I know I promised a comparison of a good window and a worse one, but I left my request for the necessary details too late to get a THERM model done for a good window, so I will bring that to you later.  In the meantime I took a sliding aluminum window frame that the fine folks at LBNL modeled and ran it through a few different scenarios.  The original model has all aluminum for the frame so when I changed materials I kept with that theme and changed the aluminum to some materials that were better and one that was a good deal worse.

First, let’s look at the all aluminum frame.  You can see the standard section on the left and the colorful infrared model on the right.  The colors represent changes in temperature and I have added a thermometer so you can see what the range is.  Additionally, for each iteration I chose the same point to model.  I chose the tip of an inside track because that was the coldest spot on the aluminum frame.  You will see how each material affects that same point.  Normally windows don’t float in air, they are installed in a wall system, but I wanted to just show how one simple change would affect a result and not get too deep in the wall systems, over insulating frames, or how deep within an assembly the frame should be placed so I ran the calculations just on the window.

Now let’s look at another typical window option – Vinyl.  While the actual frame configuration and dimensions may change in real life, this scenario shows just what swapping out a different material can do for you - in this case a tremendous change in temperature at our coldest spot - an almost 30 degree difference (real world values will vary dependent upon design). 

Then I decided to look at something really insulating, but not something you would see as a window frame material – XPS.  Obviously I am in uncharted waters here, but the idea is to see not only what the software does, but what I can expect if I chose the wrong material.  I would think that XPS is far more insulating than Vinyl and perhaps I will get a tremendous difference.  I was surprised that it wasn’t significantly better than just a few degrees.

Finally I thought I would try steel.  With aluminum and steel both being metals, I thought they might be fairly close to the same functional insulating level for the same configuration and thickness of material.  I was right!  So I now know that if I am swapping metals the results should change, but maybe not significantly so I need to be careful when selecting materials that I choose the one I intend to.  I can expect the same with insulating materials.  But if I accidently substitute an insulating material for a conductive one, I should be able to see the difference more readily and know there is a problem. 

Now as I said, you wouldn’t use the same design for all these different materials but it was a good exercise to become more familiar with the software and to test what I thought would be right with what actually was.  It’s good to know how good your guesser is and it’s fun to put in some off the wall choices just to see what happens. 

The good window manufacturers have tried changing materials and shapes around too so they could see what happens – but with optimization in mind.  They have figured out the optimal combination of materials, gasses, glasses, voids, and shapes of the different components so that your Passive House consultant can choose a window that will work with your building design in your location.