Thermal Bridge Calculations can be tedious, challenging and many times have a relatively small impact on the building performance – HOWEVER, if not done, and done correctly can lead to major issues!
Thermal Bridge Calculations tell us 2 important things.
Have we over or underestimated the energy loss in our energy model (sometimes knows as a PSI-Value)
What is the minimum interior surface temperature. (sometimes known as fRsi or hygiene criteria)
Because, after all we don’t want puddles in our house…
Rochester Passive House Consulting was recently brought into a house to investigate a “wet floor”. The customer was claiming that when it was super cold outside they would get puddles at the outer edge of their new house, and it would lead to some mold growth on the baseboards and in the bathroom.
THE HISTORY
A little background on the house:
Its a small (1350 sqft) Slab on grade ranch.
It was built about 2-3 years ago.
The house is equipped with a commissioned Zehnder ERV.
Running in normal speed would give the house 1 full air exchange every 2.5 hours (0.4 air changes per hour)
Running in high speed would give the house 1 full air exchange every 1 hour and 40 minutes (0.6 air changes per hour)
The house is really tight – 0.8 ACH50 with the blower door
There are 3 occupants and a dog.
The builder was able to share with us the footer detail:
We decided to run a few – you guessed it – THERMAL BRIDGE CALCULATIONS using FLIXO to determine what an estimated interior surface temperature might be
IN A PERFECT WORLD
If we were in a situation where we were designing this from scratch, the detail would look something similar to this
Preparing for the Site Visit
The morning we were to go to the house to investigate, we saw the outside temperature was going to be 37F, so we decided to rerun the model with actual outdoor temperatures – so we could confirm was an IR Cameral on site.
THE SITE VISIT
When we got to the house we verified the Zehnder air flows, and did a blower door test – and verified these were correct.
I love it when the simulation matches the real life situation!
THE FIX: (We ran 5 scenarios)
It is pretty clear that the slab edge detail needs to be addressed, but how? Using FLIXO we are able to iterate this detail to see what material, thickness, and depth we need to protect against this from happening again
Lets review the current detail – this time showing “streamlines” These lines show the direction of heat flow through the detail. Think of streamlines like cars on a highway. Where they are all following the path of least resistance to get inside as fast as possible.
Using a product called WallGUARD we can install an exterior insulation at the slab edge, and down the stem wall. Now we just need to know how large and thick we need to go.
Attempt #1 – 1″ thick x 12″ deep
Attempt #2 – 1.5″ thick x 12″ deep
Attempt #3 – 1″ thick x 24″ deep
Attempt #4 – 1.5″ thick x 24″ deep
Attempt #5 – 1.5″ thick x down to top of footer
By doing these quick iterations, we were able to give the builder some direction as to how far to take this solution. (We decided to go with “attempt #4” While the "we can try this, and if that doesn't work try that" philosophy might work most of the time, I have a feeling we only want to dig up around the foundation once, and make sure the solution we have come up with will work to help minimize future call backs.