The ABC's of the PHPP - Part 3a. Making Cents of Thermal Bridges

This is the first of a two-part column on Thermal Bridges.  This part will deal with what is a Thermal Bridge and why they matter.  The second part will show where you get the needed information for the PHPP using THERM as the 2D heat transfer modeling program. 

Heat flows through materials at different rates depending upon their ability to conduct heat (U value) or resist the conductance of heat (R value).  I am sure most people understand the idea that an insulated construction would be able retain the heat inside longer than the same construction without insulation.  Insulation such as dense packed cellulose does not allow heat to transfer through it as quickly as steel, glass, or wood.  But constructions are not made of just one material.  They are made of combinations of materials that serve different purposes.  Wood and Steel beams are structural, so they need to be there.  Glass allows you to look outside and that is necessary as well.  Siding helps keep the rain off the rest of the construction, sheetrock gives a finish to the interior.  Each of these materials are necessary because of their characteristics but they may allow heat to transfer through them differently and we need to understand how so that we can build the best Passive House possible.

If you were to make a “wall sandwich” of sheetrock, cellulose insulation, and sheathing you could predict how heat would transfer through that entire wall based on how each layer reacted.  But walls have other things in them - there are doors, and windows, and studs in various places.  These are going to have their own degrees of heat conductance.  Because of this we need to figure out just what is happening with heat as it travels - and particularly as it confronts a different material because in some places those interactions will cause the heat to go quite quickly and others not as much.  This is important since it could also be affecting the temperature enough in those places to allow condensation to occur.  That could lead to mold, rot, or decay.

If we know in advance of building our construction that there are areas that the heat will transfer too quickly we can make changes to preserve not only the comfort of the residents, but also the health of the construction.  Because we are talking about Passive House constructions, knowing where this heat exits more rapidly also allows us to change the construction in advance of building it so that we can maintain control of the energy usage that the construction is expected to have.

So let us look quite simply at heat flowing through a wall.  The first example is our “Wall Sandwich”, the second is a concrete wall of the same thickness.  Finally we will set them next to each other as you might expect to see in an actual building to see what changes if any occur to the heat flow arrows.

The cross sections are created in THERM and have arrows indicating the flow of heat.  The longer the arrows and the more closely packed together, the faster the heat is traveling.  When the arrows are all pointing the same way in straight lines across the assembly, the heat is traveling in one dimension.  It is this manner that the PHPP considers when it calculates the U Values of constructions.  If some of the arrows start to go a different way, grow shorter or longer, space themselves out more or closer together, then the heat is actually traveling in more than one direction, and it is this variance that we need to inspect.  This is our Thermal Bridge.  The PHPP simply does not recognize this physical property because it is not a 2D heat transfer program.  That is why we need to also calculate these areas in THERM or some other approved modeling program so that we can get an accurate picture of what is actually happening and then find what Architect and CPHC Hayden Robinson calls the “Assembly Intersection Correction Factor”.  The PHPP calls it the Psi value.  Once applied to the 1D calculations that the PHPP has done for the length of the area that this change occurs, the PHPP is better equipped to predict the energy usage that is to be expected from the construction.

Next up - Entering the values of a Thermal Bridge into the PHPP.